wqp* w Surgeon General's Office «jza&j£ ELEMENTS CHEMISTRY, INCLUDING THE RECENT DISCOVERIES AND DOCTRINES OF THE SCIENCE. BY EDWARD TURNER, M.D. F.R.S. L. & E. Sec. G.S. PROFESSOR OF CHEMISTRY IN THE UNIVERSITY OF LONDON : Fellow Qf the Royal College of Physicians of Edinburgh; Corresponding Member of the Royal Society of Gottingen; Honorary Member of the Plinian Society of Edinburgh; and Member, and formerly President, of the Royal Medical Society of Edinburgh. FIFTH AMERICAN', FROM THE FIFTH LONDON EDITION. WITH NOTES AND EMENDATIONS, BY FRANKLIN BACHE, M.D. PROFESSOR OF CHEMISTRY IN THE PHILADELPHIA COLLEGE OF PHARMACY J ONE OF THE SECRETARIES OF THE AMERICAN PHILOSOPHICAL£OfltyTXy PHILADELPHIA! ' ..Owl" DESILVER, THOMAS & CO. 247, MARKET STREET. 1835. 2StttCCftJ, according to the Act of Congress, in the year 1835, by Desilver, Thomas & Co. in the Clerk's Office of the District Court for the Eastern District of Pennsylvania. I ?> ' its volume in that state. As its breadth apd thickness increase in the same proportion as its length, the expanded bar will have precisely the same figure, that is, the same proportion of its dimen- sions, as the cold one; and since, by Euclid, the solidity of similar figures is as the cube of homologous sides, it follows that v. »': : 1: (l^-- in the mercurial one. All experimenters agree that liquids expand in an increasing ratio, or that equal increments of heat cause a greater dilatation at high than at low tem- peratures. Thus, if a fluid is heated from 32° to 122°, it will not expand so much as it would do in being heated from 122° to 212°, though an equal number of degrees is added in both cases. In mercury the first expansion, according to Deluc, is to the second as 14 to 15 ; in olive oil as 13.4 to 15; in alcohol as 10.9 to 15; and in pure water as 4.7 to 15. Attempts have been made to discover a law by which this progression is regulated, and Dalton conceives that the expansion observes the ratio of the square of the tempera- ture estimated from the point of congelation, or of greatest density; but this opinion is merely hypothetical, and has been shown by Dulong and Petit to be inconsistent with the facts established by their experiments. There is a peculiarity in the effect of heat upon the bulk of some fluids; • namely, that at a certain temperature increase of heat causes them to contract, and its diminution makes them expand. This singular exception to the ge- neral effect of heat is only observable in those liquids which acquire an in- crease of bulk in passing from the liquid to the solid state, and is remarked only within a few degrees of temperature above their point of congelation. Water is a noted example of it. Ice, as every one knows, swims upon the surface of water, and, therefore, must be lighter than it, which is a con- vincing proof that water, at the moment of freezing, must expand. The in- crease is estimated by Boyle at about -y-th of its volume, which gives 900 as the specific gravity of ice, that of water being 1000. (Experiments on Cold.) Dalton estimates the specific gravity of ice at 9.42. The most remarkable circumstance attending this expansion, is the prodi- gious force with which it is effected. Boyle filled a brass tube, three inches in diameter, with water, and confined it by means of a moveable plug; the expansion, when it froze, took place with such violence as to push out the plug, though preserved in its situation by a weight equal to 74 pounds. The V D' since the volume of the same liquid is also inversely as its density, H V Consequently, the heights are directly as the volumes, or ^77=^. Since, xi V therefore, VU'+VXL so is HWI+HXgjgg-H. (H—jjJ-H. (9990-4-T' \ — —). The value of H. is, of course, found by the formula H=H' (9990 \ —■---——, )■ If, in the formula for H', we substitute for H and T' their va- yyyu-r-T// Iue as stated in the text, we shall find H'=30. ("9^~28)=30-084, which is the length of a mercurial column at 60°, having the same pressure as a column of mercury at 32°. The rate of the actual and not apparent expansion is used in these for- mulae, because the length of the mercurial column, depending on atmo- spheric pressure, is not affected by the expansion or contraction of the tube. 20 HEAT. Florentine Academicians burst a hollow brass globe, whose cavity was only an inch in diameter, by freezing the water with which it was filled; and it has been estimated that the expansive power necessary to produce such an effect was equal to a pressure of 27,720 pounds weight. Major Williams gave ample confirmation of the same fact by some experiments which he performed at Quebec in the years 1784 and 1785. (Philosophical Transac- tions of Ed. ii. 23.) But it is not merely during the act of congelation that water expands; for it begins to dilate considerably before it actually freezes. Dr. Croune no- ticed this phenomenon so early as the year 1683, and it has since been ob- served by various philosophers. It may be rendered obvious to any one by the following experiment. Fill a flask, capable of holding three or four ounces, with water at the temperature of 60°, and adapt to it a cork, through which passes a glass tube open at both ends, about the eighth of an inch wide, and ten inches long. After having filled the flask, insert the cork and tube, and pour a little water into the latter till the liquid rises to the middle of it On immersing the flask into a mixture of pounded ice and salt, the water will fall in the tube, marking contraction; but in a short time an op- posite movement will be perceived, indicating that dilatation is taking place, while the water within the flask is at the same time yielding heat to the freezing mixture in which it is immersed. To the inference deduced from this experiment, it was objected by some philosophers, that the ascent of the water in the tube did not arise from any expansion in the liquid itself, but from a contraction of the flask, by which its capacity was diminished. In fact, this cause does operate to a certain ex- tent, but it is by no means sufficient to account for the whole effect; and, accordingly, it has been proved by an elegant and decisive experiment of Dr. Hope, that water does really expand previous to congelation.* He believes the greatest density of water to be between thirty-nine and a half and forty degrees of Fahrenheit's thermometer; that is, boiling water obeys the usual law till it has cooled to the temperature of about 40°, after which the ab- straction of heat produces increase instead of diminution of volume. Ac. cording to Hallstrom, whose experiments are the most recent, and appear to have been conducted with great care, the maximum density of water is 39.39° F. (An. de Ch. et de Ph. xxviii. 90.) The cause of the expansion of water at the moment of freezing is attri- buted to a new and peculiar arrangement of its particles. Ice is in reality crystallized water, and during its formation the particles arrange themselves in ranks and lines, which cross each other at angles of 60° and 120°, and consequently occupy more space than when liquid. This may be seen by examining the surface of water while freezing in a saucer. No very satis- factory reason can be assigned for the expansion which takes place previous to congelation. It is supposed, indeed, that the water begins to arrange itself in the order it will assume in the solid state before actually laying aside the liquid form ; and this explanation is generally admitted, not so much because it has been proved to be true, but because no better one has been offered. Water is not the only liquid which expands under reduction of tempera- ture, as the same effect has been observed in a few others which assume a highly crystalline structure on becoming solid ;—fused iron, antimony, zinc, and bismuth are examples of it. Mercury is a remarkable instance of the reverse; for when it freezes, it suffers a very great contraction. As the particles of air and aeriform substances are not held together by cohesion, it follows that increase of temperature must occasion a considera- ble dilatation of them; and, accordingly, they are found to dilate from equal additions of heat much more than solids or liquids. Now, chemists are in the habit of estimating the quantity of the gases employed in their experi- ments by measuring them ; and since the volume occupied by any gas is so * Philosophical Transactions of Edinburgh, v. 379. HEAT. 21 much influenced by temperature, it is essential to accuracy that a due cor- rection be made for the variations arising from this cause; that they should know how much dilatation is produced by each degree of the thermometer, whether the rate of expansion is uniform at all temperatures, and whether that ratio is the same in all gases. This subject had been unsuccessfully investigated by several philosophers, who failed in their object chiefly because they neglected the precaution of drying the gases upon which they operated ; but at last the law of dilata- tion was detected by Dalton and Gay-Lussac nearly at the same time. Dal- ton's method of operating (Manchester Memoirs, vol. v.) was exceedingly simple. He filled with dry mercury a graduated tube, closed at one end and carefully dried; and then, plunging the open end of the tube into a mer- curial trough, introduced a portion of dry air. After having marked the bulk and temperature of the air, he exposed it to a gradually increasing heat, the exact amount of which was regulated by a thermometer, and observed the dilatation occasioned by each increase of temperature. The apparatus of Gay-Lussac (An. de Ch. v. 43) was the same in principle, but more com- plicated, in consequence of the precautions he took to avoid every possible source of fallacy. It is proved by the researches of these philosophers, that all gases undergo equal expansions by the same addition of heat, supposing them placed under the same circumstances; so that it is sufficient to ascertain the law of ex- pansion observed by any one gas, in order to know the law for all. Now it appears from the experiments of Gay-Lussac, that 100 parts of air, in being heated from 32° to 212° F., expand to 137-5 parts. The increase for 180 de- grees is, therefore, 0.375 or ^^ths of its bulk; and by dividing this number by 180, it is found that a given quantity of dry air dilates to ^-^.-g-th of the volume it occupied at 32°, for every degree of Fahrenheit's thermometer. The result of Dalton's experiments corresponds very nearly with the foregoing. This point being established, it is easy to ascertain what volume any given quantity of gas should occupy at any given temperature. Suppose a certain portion of gas to occupy 20 measures of a graduated tube at 32°, it may be desirable to determine what would be its bulk at 42°. For every degree of heat it has increased by * th of its original volume, and, therefore, since the increase amounts to ten degrees, the 20 measures will have dilated by _L°_ths. The expression will, therefore, be 20+20-JgP- =20-416. It must not be forgotten that the volume which the gas occupies at 32° is a neces- sary element in all such calculations. Thus, having 20-416 measures of gas at 42°, the corresponding bulk for 52° cannot be calculated by the formula 20-416+20-416 x-y^5 the real expression is 20.416+20 xJL0^, because the increase is only _!° ths of the space occupied at 32°, which is 20 measures.* * The following are convenient general formula for these calculations:— Let P' be the volume of gas at any temperature above 32°, T' the num- ber of degrees above that point, and P its volume at 32°. Then P'=P. f 1-r"2ah)=^>" (—7575—) > ana ^ ^ *3 unknown, its value, deduced from the last equation, may be calculated from the formula P=P'. ( V It frequently happens, in the employment of Fahrenheit's thermometer, that when P' for the above formula is known, it is not P itself which is wanted, but the volume of gas at some other temperature, as at 60° F. This value may be obtained without first calculating what P is. Thus, retain- ing the value of P' and T' as in the preceding formula, let P" be the corre- sponding quantity of gas at some other temperature, the degrees of which (480+ T") above 32° may be expressed by T". Now P"=---—-— x? J but as P 4oU 22 HEAT. A similar remark applies to the formula for estimating the effect of heat on the height of the barometer. The rate of expansion of atmospheric air at temperatures exceeding 2123 has been examined by Dulong and Petit, and the following table contains the result of their observations. (An. de Ch. et de Ph. vii. 120.) Temperature by the Mercurial Thermometer. Corresponding volumes of a given volume of air. Fahrenheit. Centigrade. — 33 32 212 302 392 482 572 Mercury boils 680 — 36 0 100 150 200 250 300 360 0.8650 1.0000 1.37.50 1.5576 1.7389 1.9183 2.0976 2.3125 /480+T"\ is unknown, let its value in P' be substituted. Thus, P"=f '---j X ( F480 V which „ives 480» P'+480 T" P' P'480(480+T")_ U80+W' Wmca glVeS r - 480^+480 T' ~"480~~(480+T') " P' (480+ T") 480+T'. Suppose, for example, a portion of gas occupies 100 divisions of a gradu- ated tube at 48°, how many will it fill at 60° F. ? Here P =100; T =48— 32, or 16; T"=60—32, or 28. The number sought, or the p//=100X508_ 102.42.* * To those who are not algebraists, the following explanation and calcu- lation may be useful. As every gas expands l-480th of the volume it would occupy at 32°, for every degree of Fahrenheit's thermometer, it is clear that it will expand l-481st part of its volume at 33°, l-432d part of its volume at 34°, and so on for each successive addition of one degree of caloric. In order to know, therefore, the fractional dilatation of a gas at any temperature above 32°, for a single degree, it is only necessary to add to the denominator of the fraction 1-480, a number of units equal to the number of degrees that the gas exceeds the temperature of 32°. Thus a gas at the temperature of 42° will expand l-490th, at 52° 1-500, of its volume, for every increment of heat equal to one degree. Knowing in this simple manner the fractional amount of expansion of a gas at any temperature for one degree, we multiply this amount by the difference between the existing temperature and the tempera- ture to which it is desired to reduce the volume. If the reduction is to a higher temperature, this product is added to the existing volume; if to a lower, subtracted. Thus, to calculate the example which Dr. Turner has selected, nnmely, 100 measures of a gas at 48°, what will be its bulk at 60°, we proceed as follows: as the existing temperature is 16° above 32°, its fractional expansion for one degree will be 1-480+16=1-496. Taking the 496th part of one hundred, the given volume, we have the actual expansion HEAT. 23 Hydrogen gas was found to expand in the same proportion; so that all gases may be inferred to expand to the same extent, for equal increments of heat, between —33° F. and 680° ; and the same law probably prevails at all temperatures.* THERMOMETERS. The influence of heat over the bulk of bodies is better fitted for estimating a change in the quantity of that agent than any other of its properties; for substances not only expand more and more as the temperature increases, but in general return exactly to their original volume when the heat is with- drawn. The first attempt to measure the intensity of heat on this principle was made early in the 17th century, and the honour of the invention is by some bestowed on Sanctorius, by others on Cornelius Drebel, and by others on the celebrated Galileo. The material used by Sanctorius was atmosphe- ric air. The construction of the thermometer itself, or thermoscope as it was sometimes called, is exceedingly simple. A glass tube is to be selected for the purpose, and one end of it is blown out into a spherical cavity, while its other extremity is left open. After expelling a small quantity of air by heat- ing the ball gently, the open end of the tube is plunged into coloured water, and a portion of the liquid is forced up into the lube by the pressure of the atmosphere, as the air within the ball contracts. In this state it indicates changes of temperature with extreme delicacy, the alternate expansion and contraction of the confined air being rendered visible by the corresponding descent and ascent of the coloured water in the stem. The material used in its construction, also, is peculiarly appropriate; because air, like all gases, expands uniformly by equal increments of heat. There are, however, two forcible objections to the general employment of this thermometer. In the first place, its dilatations and contractions are so great, that it is inconve- nient to measure them when the change of temperature is considerable; and, secondly, its movements are influenced by pressure as well as by heat, so that the instrument would be affected by variations of the barometer, though the temperature should be quite stationary. For the reasons just stated, the common air thermometer is rarely em- ployed; but a modification of it, described in 1804 by Sir J. Leslie, in his Essay on Heat, under the name of Differential Thermometer, is entirely free from the last objection, and is admirably fitted for some special purposes. This instrument was invented a century and a half ago by Sturmius, Profes- sor of Mathematics at Altdorff, who has left a description and sketch of it in his CoUerrium Curiosum, p. 54, published in the year 1676; but like other air thermometers it had fallen into disuse, till it was again brought into notice by Leslie. As now made it consists of two thin glass for one degree. This, upon calculation, will be found to be 5016, which, multiplied by 12, the difference between the actual temperature and the tem- perature of the volume sought, will give 2.419, as the actual expansion, cor- responding to 12 degrees. As the temperature of the volume sought is above the original temperature, this number must be added to the given volume. So that 100+2-419=102 419 will be the volume sought. Ed. * The law of the equable expansion or contraction of gases by equal incre- ments or decrements of heat is a very curious one ; but it becomes particularly so when viewed in connexion with a descending temperature. If gases expand or contract l-480th of the volume they occupy at the freezing point, for every alteration of temperature equal to one degree, it is obvious that a given volume of any gas at 32° will be expanded by a volume equal to itself, by having its temperature raised 480". But the converse of the proposition would seem to involve a paradox'; for by the application of the same law, a given volume of any gas at 32°, if cooled down to 480°, would be contracted by a volume equal to itself, that is, reduced to nothing. Ed. 24 HEAT. balls joined together by a tube, bent twice at a right angle, as represented in the annexed figure. Both balls contain air, but the greater part of the tube is filled with sulphuric acid coloured with carmine. It is obvious that this instrument can- not be affected by any change of temperature acting equally on both balls; for as long as the air within them expands or contracts to the same extent, the pressure on the opposite surfaces of the liquid, and consequently its position, will continue unchanged. Hence the differential thermometer stands at the same point, however different may be the temperature of the medium. But the slightest difference between the tempera- ture of the two balls will.instantly be detected ; for the elasticity of the air on one side being then greater than that on the other, the liquid will retreat towards the ball whose temperature is the lowest. Solid substances are not better suited to the con- struction of a thermometer than gases ; for while the expansion of the latter is too great, that of the former is so small that it cannot be measured ex- cept by the adaptation of complicated machinery. Liquids which expand more than the one and less than the other, are exempt from both extremes; and, consequently, we must search among them for a material with which to construct a thermometer. The principle of selection is plain. A material is required whose expan- sions arc uniform, and whose boiling and freezing points are very remote from one another. Mercury fulfils these conditions better than any other liquid. No fluid can support a greater degree of heat without boiling than mercury, and none, except alcohol and ether, can endure a more intense cold without freezing. It has, besides, the additional advantage of being more sensible to the action of heat than other liquids, while its dilatations between 32° and 212° are almost perfectly uniform. Strictly speaking, the same quantity of heat does occasion a greater dilatation at high than at low temperatures, so that, like other fluids, it expands in an increasing ratio. But it is remarkable that this ratio, within the limits assigned, is exactly the same as that of glass; and, therefore, if contained in a glass tube, the in- creasing expansion of the vessel compensates for that of the mercury. The first object in constructing a thermometer is to select a tube with a very small bore, which is of the same diameter through its whole length; and then, by melting the glass, to blow a small ball at one end of it. The mercury is introduced by rarefying the air within the ball, and then dipping the open end of the tube into that liquid. As the air cools and contracts, the mercury is forced up, entering the bulb to supply the place of the air which had been expelled from it. Only a part of the air, however, is re- moved by this means; the remainder is driven out by the ebullition of the mercury. Having thus contrived that the bulb and about one-third of the tube shall be full of mercury, the next step is to seal the open end hermetically. This is done by heating the bulb till the mercury rises very near the summit, and then suddenly darting a fine-pointed flame from a blow-pipe across the open- ing, so as to fuse the glass and close the aperture before the mercury has had time to recede from it. The construction of a thermometer is now so far complete that it affords a means of ascertaining the comparative temperature of bodies; but it is deficient in one essential point, namely, the observations made with different instruments cannot be compared together. To effect this object, the ther- mometer must be graduated, a process which consists of two parts. The HEAT. 25 first and most important is to obtain two fixed points which shall be the same in every thermometer. The practice now generally followed for this purpose was introduced by Newton, and is founded on the fact, that when a thermometer is plunged into ice that is dissolving, and into water which is boiling, it constantly stands at the same elevations in all countries, provided there is a certain conformity of circumstances. The point of congelation is easily determined. The instrument is to be immersed in snow or pound- ed ice, which is liquefying in a moderately warm atmosphere, till the mer- cury becomes stationary. To fix the boiling point is a more delicate opera- tion ; since the temperature at which water boils is affected by various cir- cumstances which will be more particularly mentioned hereafter. It is sufficient to state the general directions at present;—that the water be per- fectly pure, free from any foreign particles, and not above an inch in depth, —the ebullition brisk, and conducted in a deep metallic vessel, so that the stem of the thermometer may be surrounded by an atmosphere of steam, and thus exposed to the same temperature as the bulb—the vapour be allow- ed to escape freely—and the barometer stand at 30 inches. The second part of the process of graduation consists in dividing the in- terval between the freezing and boiling points of water into any number of equal parts or degrees, which may be either marked on the tube itself, by means of a diamond, or first drawn upon a piece of paper, ivory, or metal, and afterwards attached to the thermometer. The exact number of degrees into which the space is divided, is not very material, though it would be more convenient did all thermometers correspond in this respect. Unfor- tunately this is not the case. In Britain we use Fahrenheit's scale, and ac- cordingly in this treatise the degrees always refer to that scale, except when the contrary is directly expressed; whereas the continental philosophers employ either the centigrade, or that of Reaumur. The centigrade is the most convenient in practice; its boiling point is 100, that of melting snow is the zero, or beginning of the scale, and the interval is divided into 100 equal parts. The interval in the scale of Reaumur is divided into 80 parts, and in that of Fahrenheit into 180; but the zero of Fahrenheit is placed 32 degrees below the temperature of melting snow, and on this account the point of ebullition is 212°. It is easy to reduce the temperature expressed by one thermometer to that of another, by knowing the relation which exists between their degrees. Thus, 180 is to 100 as 9 to 5, and to 80 as 9 to 4; so that nine degrees of Fahrenheit are equal to five of the centigrade, and four of Reaumur's ther- mometer. Fahrenheit's is, therefore, reduced to the centigrade scale, by mul- tiplying by five, and dividing by nine, or to that of Reaumur, by multiplying by four instead of five. Either of these may be reduced to Fahrenheit by reversing the process; the multiplier is nine in both cases, and the divisor four in the one and five in the other. But it must be remembered in these reductions, that the zero of Fahrenheit's thermometer is 32 degrees lower than that of the centigrade or Reaumur, and a due allowance must be made for this circumstance. An example will best show how this is done. To reduce 212° F. to the centigrade, first subtract 32, which leaves 180; and this number multiplied by A, gives the corresponding expression in the cen- tigrade scale. Or to reduce 100° C. to Fahrenheit, multiply by ■§■, and then add 32. To save the trouble of such reductions, I have subjoined a table, which shows the degrees on the centigrade scale and that of Reaumur, corresponding to the degrees of Fahrenheit's thermometer. 3 26 HEAT. Fahr. Cent. Reaum. j Fahr. Cent. Reaum. Fahr. Cent. Reaum. 212 100 80 113 45 36 14 -10 - 8 203 95 76 104 40 32 5 -15 -12 194 90 72 95 35 28 - 4 -20 -16 185 85 68 86 30 24 -13 -25 -20 176 80 64 77 25 20 -22 -30 -24 167 75 60 68 20 16 -31 -35 -28 158 70 56 59 15 12 -40 -40 -32 149 65 52 50 10 8 140 6) 48 41 5 4 131 55 44 32 0 0 122 50 40 23 -5 -4 The mercurial thermometer may be made to indicate temperatures which exceed 212°, or fall below zero, by continuing the degrees abdve and below those points. But as mercury freezes at 39 degrees below zero, it cannot indicate temperatures below that point; and indeed the only liquid which has been used for such purposes is alcohol. Our means of estimating high degrees of heat are as yet very unsatisfactory. Mercury is preferable to any other liquid; but even its indications cannot be altogether relied on. For, in the first place, its expansion for equal increments of heat is greater at high than at low temperatures; and, secondly, glass expands at tempera- tures beyond 212° in a more rapid ratio than mercury, and consequently, from the proportionally greater capacity of the bulb, the apparent expansion of the metal is considerably less than its actual dilatation. Thus Dulong and Petit observed, that when the air thermometer is at 572°, the common mercurial thermometer stands at 586°; but when corrected for the error caused by the glass, it indicates a temperature of 597-5°. No liquid can be employed for temperatures which exceed 662°, since all of them are then either dissipated in vapour, or decomposed. M. Bellain has observed that mercurial thermometers slowly change their point of zero, which uniformly becomes higher than at the time of gradua- tion. This phenomenon appears owing to a diminished capacity of the bulb due to the atmosphere continually pressing on its exterior, while a vacuum exists in the interior of the tube ; for it has not been noticed either in mercurial thermometers which are unsealed, or in thermometers made with alcohol. The principal contraction ensues soon after the tube is sealed, and hence some months should be permitted to elapse between the sealing and graduation of a thermometer. (An. de Ch. et de Ph. xxi. 330.) The instruments for measuring intense degrees of heat are called py- rometers, and must be formed either of solid or gaseous substances. The former alone have been hitherto employed, though the latter, from the greater uniformity with which they expand, are better calculated for the purpose. The action of most pyrometers depends on the elongation of a metallic bar by heat; and the difficulty in their construction consists in find- ing an infusible metal of uniform expansibility, and in measuring the de- gree of expansion with exactness. This subject has for some time occupied the attention of Professor Daniell, who has at last succeeded in forming a pyrometer which, with a little practice, may be used with facility, and ap. pears susceptible of very great precision. (Phil. Trans. 1830 and 31.) This instrument consists of two parts, the Register and Scale, the former designed for exposure to the heat to be estimated, and the latter for measur- ing the exact amount of expansion. The first consists of a bar of black- lead earthenware, in which is drilled a hole 3-10ths of an inch in diameter, and 7£ inches deep. Into this hole a cylindrical bar of platinum or soft iron, of nearly the same diameter, and 6 3 ed u a. E £ >> c S o ermometer sinks to —5° Degree of Cold produced. Sea-salt % Muriate of ammonia . 1 Snow .... 5 to —12° Sea-salt . . . .10 Muriate of ammonia . 5 Nitrate of potassa . • 5 Snow . . . .24 to —18° Sea-salt • . . 5 Nitrate of ammonia . . 5 Snow .... 12 to —25° Diluted sulphuric acidt . 2 Snow .... 3 from +32° to —23° 55 degrees. Concentrated muriatic acid 5 Snow 8 from +32° to —27° 59 Concentrated nitrous acid 4 Snow . ... 7 from +32° to —30° 62 Chloride of calcium . 5 Snow . . . .4 from +32° to —40° 72 Crystallized chloride of calcium . . .3 Snow .... 2 from +32° to —50° . Fused potassa - . .4 Snow 3 from +32° to —51° 83 But freezing mixtures may be made by the rapid solution of salts, without the use of snow or ice; and the following table, taken from Walker's Essay in the Philosophical Transactions for 1795, includes the most important of them. The salts must be finely powdered and dry. * The snow should be freshly fallen, dry, and uncompressed. If snow cannot be had, finely powdered ice may be substituted for it. t Made of strong acid, diluted with half its weight of snow or distilled water. 40 HEAT. MIXTURES. Muriate of ammonia Nitrate of potassa . Water Parts by Weight. 5 5 16 Temperature falls from +50° to +10° Degree of Cold produced. 40 degrees. Muriate of ammonia Nitrate of potassa Sulphate of soda Water 5 5 8 16 from +50° to +4° 46 Nitrate of ammonia Water 1 1 from +50° to +4° 46 Nitrate of ammonia Carbonate of soda Water . 1 1 1 from +50° to —7° 57 Sulphate of soda Diluted nitrous acid* 3 2 from +50° to —3° 53 Sulphate of soda Muriate of ammonia Nitrate of potassa Diluted nitrous acid 6 4 2 4 from +50° to —10° 60 Sulphate of soda Nitrate of ammonia Diluted nitrous acid 6 5 4 from +50° to —14° 64 Phosphate of soda Diluted nitrous acid 9 4 from +50° to —12° 62 Phosphate of soda Nitrate of ammonia Diluted nitrous acid 9 6 4 from +50° to —21° 71 Sulphate of soda Muriatic acid 8 5 from +50° to 0° 50 Sulphate of soda Diluted sulphuric acid 5 • 4 from +50° to +33 47 These artificial processes for generating cold are much more effectual when the materials are previously cooled by immersion in other frigorific mixtures. One would at first suppose that an unlimited degree of cold might be thus produced; but it is found that when the difference between lhe mixture and the air becomes very great, the communication of heat from one to the other becomes so rapid, as to put a limit to the reduction. The greatest cold produced by Mr. Walker did not exceed 100 degrees be- tow the zero of Fahrenheit. Though it is unlikely that we shall ever succeed in depriving any sub- stance of all ils heat, it is presumed that bodies do contain a certain definite quantity of this principle, and various attempts have been made to calculate its amount. The mode of conducting such a calculation may be shown by the process of Dr. Irvine. That ingenious chemist proceeded on the as- sumption, that the actual quantity of heat in bodies is proportional to their specific heat, and that the specific heat remains the same at all temperatures provided no change of form takes place. Tims, as the specific heat of ice * Composed of fuming nitrous acid two parts in weight, and one of water • the mixture being allowed to cool before being used. t Composed of equal weights of strong acid and water, being allowed to cool before use. HEAT. 41 is to that of water as 9 to 10, it follows, according to the hypothesis, that ice contains l-10th less heat than water, at the same temperature. Now Dr. Black ascertained that this tenth, which is the heat of fluidity, is equal to 140 degrees; whence it was inferred that water at 32° contains 10 times 140 or 1400 degrees of heat. To be satisfied that such calculations cannot be trusted, it is sufficient to know, that the estimates made by different chemists respecting the absolute quantity of heat in water vary from 900 to nearly 8000 degrees.* Besides, did even the estimates agree with each other, the principle of the calculation would still be unsatisfactory; for, in the first place, there is no proof that the quantity of heat in bodies is in the ratio of their specific heats; and, secondly, the assumption that the specific heat of a body is the same at all temperatures, so long as it does not experience a change of form, has been proved to be erroneous by the experiments of Dulong and Petit. VAPORIZATION. Aeriform substances are commonly divided into vapours and gases. The character of the former is, that they may be readily converted into liquids or solids, either by a moderate increase of pressure, the temperature at which they were formed remaining the same, or by a moderate diminution of that temperature, without change of pressure. Gases, on the contrary, retain their elastic state more obstinately; they are always gaseous at com- mon temperatures, and, with one or two exceptions, cannot be made to change their form, unless by being subjected to much greater pressure than they are naturally exposed to. Several of them, indeed, have hitherto re- sisted every effort to compress them into liquids. The only difference be- tween gases and vapours is in the relative forces with which they resist condensation. Heat appears to be the cause of vaporization, as well as of liquefaction, and it is a general opinion that a sufficiently intense heat would convert every liquid and solid into vapour. A considerable number of bodies, how- ever, resist the strongest heat of our furnaces without vaporizing. These are said to be fixed in the fire: those which, under the same circumstances, are converted into vapour, are called volatile. The disposition of various substances to yield vapour is very different; and the difference depends doubtless on the relative power of cohesion with which they are endowed. Liquids are, in general, more easily vaporized than solids, as would be expected from the weaker cohesion of the former. Some solids, such as arsenic and sal ammoniac, pass at once into vapour without being liquefied; but most of them become liquid before assuming the elastic condition. Vapours occupy more space than the substances from which they were produced. According to the experiments of Gay-Lussac, water, at its point of greatest density, in passing into vapour, expands to 1696 times its vo- lume, alcohol to 659 times, and ether to 443 times, each vapour being at a temperature of 212° F. and under a pressure of 29.92 inches of mercury. This shows that vapours differ in density. Watery vapour is lighter than air at the same temperature and pressure, in the proportion of 1000 to 1604; or the density of air being 1000, that of watery vapour is 625. The vapour of alcohol, on the contrary, is half as heavy again as air; and that of ether is more than twice and a half as heavy. As alcohol boils at a lower tempe- rature than water, and ether than alcohol, it was conceived that the density of vapours might be in the direct ratio of the volatility of the liquids which produced them. But Gay-Lussac has shown that this law does not hold ge- nerally; since bisulphuret of carbon boils at a higher temperature than ether, and nevertheless it yields a heavier vapour. * Dalton's New System of Chemical Philosophy. 4* 42 HEAT. The dilatation of vapours by heat was found by Gay-Lussac to follow the same law as gases, that is, for every degree of Fahrenheit, they increase by ^o-th of the volume they occupied at 32°. But the law does not hold un- less the quantity of vapour continue the same. If the increase of tempera- ture cause a fresh portion of vapour to rise, then the expansion will be greater than 7}oth for each degree; because the heat not only dilates the vapour previously existing to the same extent as if it were a real gas, but augments its bulk by adding a fresh quantity of vapour. The contraction of a vapour on cooling will likewise deviate from the above law, whenever the cold converts any of it into a liquid; an effect which must happen, if the space had originally contained its maximum of vapour. The circum- stances just explained should be held in view, whenever the influence of heat over the bulk of vapours is estimated by calculation. The formula of page 21, when applied to vapours, often leads to a result which would be correct for any gas, but which may be untrue in the case of vapour, by reason of its light condensibility. Thus 100 measures of steam at 212°, and when the barometer is at 30 inches, would theoretically occupy nearly 73 measures at 32°, and at the same pressure; but this estimate is practi- cally untrue, because under the conditions specified water cannot exist in the state of vapour. The calculated result, being deduced from correct principles, is. sometimes employed in effecting other calculations. The volume of vapour varies under varying pressure according to the same law as that of gases, provided always that the gaseous state is pre- served. This law, discovered by Boyle and Mariotte, is more fully ex- plained in the section on atmospheric air, and merely expresses the fact that the volume of gaseous substances at a constant temperature is inversely as the pressure to which they are subject. If 100 measures of steam at 212°, and under the atmospheric pressure, be exposed to a pressure of two atmo- spheres, the vapour will be entirely condensed, affording an instance of fail- ure in the law in consequence of the gaseous state being entirely destroyed; but if the pressure be halved instead of doubled, the 100 measures retain- ing the gaseous form, and hence acting as a gas, will expand to 200 mea- sures. In fact, if v be the volume corresponding to any pressure p, express- ed in inches of mercury, we shall have ToV==~> ana hence r=100. — m This formula gives the change of volume due to a change of pressure from 30 to p, the temperature being supposed at 212° in both cases. To render the preceding paragraph intelligible to the young student, it should be men- tioned that pressure, in reference to the volume of gaseous matter, is usual- ly expressed by the length of a column of mercury: a mercurial column, 30 inches in length, presses on a given surface with the same force as the atmosphere in its ordinary state; and hence a 60-inch column is equal to two atmospheres, 15 inches' to half an atmosphere, and one inch to l-30th of the atmospheric pressure. Vaporization is conveniently studied under two heads,—Ebullition and Evaporation. In the first, the production of vapour is so rapid that its es- cape gives rise to a visible commotion in the liquid ; in the second, it passes off quietly and insensibly. Ebullition.—The temperature at which vapour rises with sufficient free- dom.for causing the phenomena of ebullition, is called the boiling point. The heat requisite for this effect varies with the nature of the liquid. Thus sul- phuric ether boils at 96° F., alcohol at 176°, and pure water at 212° ■ while oil of turpentine must be raised to 316°, and mercury to 662° before either exhibits marks of ebullition. The boiling point of the same liquid is constant, so long as the necessary conditions are preserved; but it is liable to be affected by several circumstances. The nature of the vessel has some influence upon it. Thus Gay-Lussac observed that pure water boils precisely at 212° in a metallic vessel, and at 214° in one of glass, owing apparently to its adhering to glass more powerfully than to a metal. It is likewise affected by the presence of foreign particles. The same accurate experimenter found HEAT. 43 that when a few iron filings are thrown into water, boiling in a glass vessel, its temperature quickly falls from 214° to 212°, and remains stationary at the latter point. But the circumstance which has the greatest influence over the boiling point of fluids is variation of pressure. All bodies upon the earth are constantly exposed to considerable pressure; for the atmosphere itself presses with a force equivalent to a weight of 15 pounds on every square inch of surface. Liquids are exposed to this pressure as well as solids, and their tendency to take the form of vapour is very much counteracted by it. In fact, they cannot enter into ebullition at all, till their particles have ac- quired such elastic force as enables them to overcome the pressure upon their surfaces; that is, till they press against the atmosphere with the same force as the atmosphere against them. Now the atmospheric pressure is variable and hence it follows that the boiling point of liquids must also vary. The pressure of the atmosphere is equal to a weight of 15 pouuds on every square inch of surface, when the barometer stands at 30 inches, and then only does water boil at 212° F. If the pressure be less, that is, if the baro- meter fail below 30 inches, then the boiling point of water, and of every other liquid, will be lower than usual; or if the barometer rise above 30 inches, the temperature of ebullition will be proportionally increased. This is the reason why water boils at a lower temperature on the top of a hill than in the valley beneath it; for as the column of air diminishes in length as we ascend, its pressure must likewise suffer a proportional diminution. The ratio between the depression of the boiling point and the diminution of the atmospheric pressure is so exact, that it has been proposed as a method for determining the height of mountains. An elevation of 530 feet makes a diminution of one degree of Fahrenheit. (Mr. Wollaston in Phil. Trans for 1817.) The influence of the atmosphere over the point of ebullition is best shown by removing its pressure altogether. The late Professor Robison found that liquids boil in vacuo at a temperature 140 degrees lower than in the open air. (Black's Lectures, p. 151.) Thus water boils in vacuo at 72°, alcohol at 36°, and ether at —44° F. This proves that a liquid is not necessarily hot, because it boils. The heat of the hand is sufficient to make water boil in a vacuum, as is exemplified by the common pulse-glass ; and ether, under the same circumstances, will enter into ebullition, though its temperature be low enough for freezing mercury. Water cannot be heated under common circumstances beyond 212° ; be- cause it then acquires such expansive force as enables it to overcome the atmospheric pressure, and fly off in the form of vapour. But if subjected to sufficient pressure, it may be heated to any extent without boiling. This is best done by heating water while confined in a strong copper vessel, called Papin's digester. In this apparatus, on the application of heat, a large quan- tity of vapour collects above the water, and checks ebullition by the pressure which it exerts upon the surface of the liquid. There is no limit to the de- gree to which water may thus be heated, provided the vessel is strong enough to confine the vapour; but the expansive force of steam under these circumstances is so enormous as to overcome the greatest resistance. In estimating the power of steam it should be remembered that vapour, if separated from the liquid which produced it, does not possess greater elasti- city than an equal quantity of air. If, for example, the digester were full of steam at 212°, no water in the liquid state being present, it might be heated to any degree, even to redness, without danger of bursting. But if water be present, then each addition of heat causes a fresh portion of steam to rise, which adds its own elastic force to that of the vapour previously existing; and, in consequence, an excessive pressure is soon exerted against the inside of the vessel. Professor Robison (Brewster's edition of his works, p. 25) found that the tension of steam is equal to two atmospheres at 244°, and to three at 270° F. The results of Mr. Southern's experiments, given in the same volume, fix upon 250-3° as the temperature at which Bteam has the 44 HEAT. force of two atmospheres, on 293.4° for four, and 343-6° for eight atmo- ^ThiTsubject has been lately examined by a commission appointed by the Parisian Academy of Sciences, and Dulong and Arago took a leading part in the inquiry. The results, which are given in the following table, were obtained by experiment up to a pressure of 25 atmospheres, and at higher pressures by calculation. (Brande's Journal, N. S. vn. 191.) Elasticity of the vapour, taking atmospheric press, as unity. H 2 3 H 4 5 5£ 6 64 7 7A 8 9 10 11 12 Temperature ac- cording to Fahrenheit. 212° 233.96 250.52 263.84 275.18 285.08 293.72 300.28 307.5 314.24 320.36 326.26 331.70 336.86 341.78 350.78 358.88 366.85 374.00 Elasticity of the vapour, taking atmospheric press, as unity. 13 14 15 16 17 18 19 20 21 22 23 24 25 30 35 40 45 50 Temperature ac- cording to Fahrenheit. 380.66° 386.94 392.86 398.48 403.82 408.92 413.78 418-46 422.96 427.28 431.42 435.56 439.34 457.16 472.73 486.59 491.14 510.60 The elasticity of steam is employed as a moving power in the steam- engine. The construction ofthis machine depends on two properties of steam, namely, the expansive force communicated to it by heat, and its ready con- version into water by cold. The effect of both these properties is well shown by a little instrument •devised by Dr. Wollaston. It consists of a cylindrical glass tube, six inches long, nearly an inch wide, and blown out into a spheri- cal enlargement at one end. A piston is accurately fitted to the cylinder, so as to move up and down the tube with freedom. When the piston is at the bottom of the tube, it is forced up by causing a portion of water, previously placed in the ball, to boil by means of a spirit-lamp. On dipping the ball into cold water, the steam which occupies the cylinder is suddenly condensed, and the piston forced down by the pressure of the air above it. By the alter- nate application of heat and cold, the same movements are reproduced, and may be repeated for any length of time. The moving power of the steam-engine is the same as in this apparatus. The only essential difference between them is in the mode of condensing the steam. In a steam-engine, the steam is condensed in a separate vessel, called the condenser, where there is a regular supply of cold water for the purpose. By this contrivance, which constitutes the great improvement of Watt, the temperature of the cylinder never falls below 212°. The formation of vapour is attended, like liquefaction, with loss of sensible heat. This is proved by the well-known fact that the temperature of steam is precisely the same as that of the boiling water from which it rises ; so that all the heat which enters into the liquid is solely employed in convert- ing a portion of it into vapour, without affecting the temperature of either in the slightest degree, provided the latter is permitted to escape with freedom. The heat which then becomes latent, to use the language of Black, is again set free when the vapour is condensed into water. The exact quantity of HEAT. 45 heat rendered insensible by vaporization, may, therefore, be ascertained by condensing the vapour in cold water, and observing the rise of temperature which ensues. From the experiments of Black and Watt, conducted on this principle, it appears that steam of 212°, in being condensed into water of 212°, gives out as much heat as would raise the temperature of an equal weight of water by 950 degrees, all of which had previously existed in the vapour without being sensible to a thermometer. The latent heat of steam and several other vapours has been examined by Dr. Ure, whose results are contained in the following table. (Phil. Trans. for 1818.) Latent heat. Vapour of water at its boiling point . . 967 Alcohol . . . • .442 Ether.....302-379 Petroleum ..... 177-87 Oil of turpentine . . • 177-87 Nitric acid ..... 531-99 Liquid ammonia . . • 837-28 Vinegar ..... 875 The disappearance of heat that accompanies vaporization was explained by Black and Irvine, in the way already mentioned under the head of liquefac- tion ; and as the objections to the views of the latter ingenious chemist were then stated, it is unnecessary to mention them on the present occasion. The variation of volume and elasticity in vapours is attended, as in gases, with a change of specific heat and a consequent variation of temperature. (Page 35.) Thus when steam, highly heated and compressed in a strong boiler, is permitted to escape by a large aperture, the sudden expansion is attended with a great loss of sensible heat: its temperature instantly sinks so much, that the hand may be held in the current of vapour without in- convenience. The same principle accounts for the fact, first ascertained by Watt, that distillation at a low temperature is not attended with any saving of fuel. For when water boils at a low temperature in a vacuum, the vapour is in a highly expanded state, and contains more insensible heat than steam of greater density. From some experiments by Mr. Sharpe in the Man- chester Memoirs, and also by Clement and Desormes, (Thenard's Chemistry, i. 79, 5th Ed.) it appears that the sum of the sensible and insensible heat contained in equal weights of steam is exactly the same at all temperatures. Thus, steam at 212-, when condensed and reduced to 32°, gives out 950 de- grees of insensible and 180 of sensible heat, the sum of which is 1130. The same weight of steam at 250°, on being condensed and cooled to 32°, gives out likewise 1130 degrees, of which 218 are sensible and 912 insensible heat; whereas at 100° its sensible heat is only 68°, and insensible 1062°, forming the constant sum of 1130. The same is found by Despretz to be true of various other vapours, such as that of alcohol, ether, and turpentine. Evaporation. Evaporation as well as ebullition consists in the formation of vapour, and the only assignable difference between them is, that the one takes place quietly, the other with the appearance of boiling. Evaporation occurs at common temperatures. This fact may be proved by exposing water in a shallow vessel to the air for a few days, when it will gradually diminish, and at last disappear entirely. Most liquids, if not all of them, are susceptible of this gradual dissipation; and it may also be observed in some solids, as for example in camphor. Evaporation is much more rapid in some liquids than in others, and it is always found that those liquids, the boiling point of which is lowest, evaporate with the greatest rapidity. Thus alcohol, which boils at a lower temperature than water, evaporates also more freely; and ether, whose point of ebullition is yet lower than that of alcohol, evaporates with still greater rapidity. The chief circumstances that influence the process of evaporation are ex- tent of surface, and the state of the air as to temperature, dryness, stillness, and density. 46 HEAT. 1. Extent of surface. Evaporation proceeds only from the surface of liquids, and, therefore, ceteris paribus, must depend upon the extent of sur- face exposed. 2. Temperature. The effect of heat in promoting evaporation may easily be shown by putting an equal quantity of water into two saucers, one of which is placed in a warm, the other in a cold situation. The former will be quite dry before the latter has suffered appreciable diminution. 3. State of the air as to dryness or moisture. When water is covered by a stratum of dry air, the evaporation is rapid even when its temperature is low. Thus in dry cold days in winter, the evaporation is exceedingly rapid; whereas it goes on very tardily, if the atmosphere contain much vapour, even though the air be very warm. 4. Evaporation is far slower in still air than in a current, and for an ob- vious reason. The air immediately in contact with the water soon becomes moist, and thus a check is put to evaporation. But if the air be removed from the surface of the water as soon as it has become charged with vapour, and its place supplied with fresh dry air, then the evaporation continues with- out interruption. 5. Pressure on the surface of liquids has a remarkable influence over eva- poration. This is easily proved by placing ether in the vacuum of an air- pump, when vapour rises so abundantly as to produce ebullition. As a large quantity of heat passes from a sensible to an insensible state during the formation of vapour, it follows that cold should be generated by evaporation. The fact may readily be proved by letting a few drops of ether evaporate from the hand, when a strong sensation of cold will be excited; or if the bulb of a thermometer, covered with lint, be moistened with ether, the production of cold will be marked by the descent of the mercury. But to appreciate the degree of cold which may be produced by evaporation, it is necessary to render it very rapid and abundant by artificial processes; and the best means of doing so, is by removing pressure from the surface of volatile liquids. Water placed under the exhausted receiver of an air-pump evaporates with great rapidity, and so much cold is generated as would freeze the water, did the vapour continue to rise for some time with the same velocity. But the vapour itself soon fills the vacuum, and retards the eva- poration by pressing upon the surface of the water. This difficulty may be avoided by putting under the receiver a substance, such as sulphuric acid, which has the property of absorbing watery vapour, and consequently of removing it as quickly as it is formed. Such is the principle of Leslie's method for freezing water by its own evaporation.* The action of the cryophorus, an ingenious contrivance of the late Dr. Wollaston, depends on the same principle. It consists of two glass balls, perfectly free from air, and joined together by a tube as here represented. One of the balls contains a portion of distilled water, while the other parts of the instrument, which appear empty, are full of aqueous vapour which checks the evaporation from the water by the pressure it exerts upon its sur- face. But when the empty ball is plunged into a freezing mixture all the vapour within it is condensed; evaporation commences from the surface of the water in the other ball, and it is frozen in two or three minutes by the cold thus produced. Liquids which evaporate more rapidly than water, cause a still greater reduction of temperature. The cold produced by the evaporation of ether * See art. Cold, in the Supplement to the Encyclopaedia Britannica. HEAT. 47 in the vacuum of the air-pump, is so intense as, under favourable circum- stances, to freeze mercury.* Scientific men have differed concerning the cause of evaporation. It was once supposed to be owing to chemical attraction between the air and water, and the idea is at first view plausible, since a certain degree of affinity does to all appearance exist between them. But it is nevertheless impossible to attribute the effect to this cause. For evaporation takes place equally in vacuo as in the air; nay, it is an established fact, that the atmosphere posi- tively retards the process, and that one of the best means of accelerating it, is by removing the air altogether. The experiments of Dalton prove that heat is the true and only cause of the formation of vapour. He finds that the actual quantity of vapour, which can exist in any given space, is de- pendent solely upon the temperature. If, for instance, a little water be put into a dry glass flask, a quantity of vapour will be formed proportionate to the temperature. If a thermometer placed in it stands at 32°, the flask will contain a very small quantity of vapour. At 40°, more vapour will exist in it; at 50° it will contain still more; and at 60°, the quantity will be still further augmented. If, when the thermometer is at 60°, the temperature of the flask be suddenly reduced to 40°, then a certain portion of vapour will be converted into water; the quantity which retains the elastic form being precisely the same as- when the temperature was originally at 40°. It matters not, with regard to these changes, whether the flask is full of air, or altogether empty; for in either case, it will eventually contain the same quantity of vapour, when the thermometer is at the same height. The only effect of a difference in this respect, is in the rapidity of evaporation. The flask, if previously empty, acquires its full complement of vapour, or, in common language, becomes saturated with it, in an instant; whereas the presence of air affords a mechanical impediment to its passage from one part of the flask to another, and, therefore, an appreciable time elapses before the whole space is saturated. Dalton found that the tension or elasticity of vapour is always the same, however much the pressure may vary, so long as the temperature remains constant, and there is liquid enough present to preserve the state of satura- tion proper to the temperature. If, for example, in a flaccid bladder contain- ing a little water, the pressure on its surface be diminished, the vapour in the interior will expand proportionally, and consequently for the moment will diminish in elasticity, because the tension of gaseous substances at a constant temperature diminishes in the same ratio as the volume increases, or, in other words, the elasticity varies inversely as the volume; but the va- pour in the bladder will speedily recover its original tension, since the water will yield an additional quantity of vapour proportional to the increase of space. Again, if the pressure on the bladder be increased so as to diminish its capacity, the temperature remaining constant, the tension of the confined vapour will still continue unchanged, because a portion of it will be con- densed proportional to the diminution of space; so that, in fact, the remain- ing space contains the very same quantity of vapour as it did originally. The same law holds good, whether the vapour is pure, or mixed with air or any other gas. The elasticity of watery vapour at temperatures below 212° was care- fully examined by Dalton, (Manchester Memoirs, vol. v.); and his results, together with those since published by Dr. Ure, in the Philosophical Trans- actions for 1818, are presented in a tabular form at the end of the volume. They were obtained by introducing a portion of water into the vacuum of a common barometer, and estimating the tension of its vapour by the extent to which it depressed the column of mercury at different temperatures. But Dalton did not confine his researches to water; he extended them to the vapour of various liquids, such as ether, alcohol, ammonia, and solution of * See a paper by the late Dr. Marcet, in Nicholson's Journal, vol. xxxiv. 48 HEAT. chloride of calcium, and he inferred from them the following law:—that the force of vapour from all liquids is the same, at equal distances above or be- low the several temperatures at which they boil in the open air. Subse- quent observations by Dr. Ure, Despretz, and others, have proved that the law is far from universal, and that it fails remarkably at temperatures dis- tant from the point of ebullition: it has, indeed been abandoned by Dalton himself. A knowledge of the influence of heat and pressure over the volume of gaseous matter is elegantly employed in calculating the density of vapour ; but before giving the mode of making the calculation, it will be useful to explain what is meant by density. This term is generally used synony- mously with specific gravity, and indicates the compactness of a substance, or the quantity of ponderable matter contained in a body compared with the space which it occupies. The density of a substance is found by dividing its weight by its volume. Thus, if d, w, v, represent the density, weight, and volume of aqueous vapour, and d', w', v', the density, weight, and volume w w/ of air, then d=—» and d'=—;. Hence, comparing these densities, d : w wr d' : : —: —; if the volumes are equal, then d : d' : : w : w'; and if the v v j x weights are equal, d : d' : :—: —. Consequently, the density of sub- stances which have an equal volume, is directly as their weight; and when the weights are equal, the densities are inversely as the volumes. Accordingly, if we weigh an equal volume of any number of substances, temperature and pressure being the same in all, the density of each respectively will be re- presented by its weight. Thus, Gay-Lussac ascertained that if a certain vo- lume of air at 212° and 30 Bar. weigh 1000 grains, an equal volume of aqueous vapour, at the same temperature and pressure, will weigh 625 grains; and, therefore, the density of steam is 625 compared to that of air as 1000. Atmospheric air is universally taken as a term of comparison for the density of gaseous substances, and pure water for that of liquids and solids. As gases expand and contract, from varying temperature and pressure, according to the same laws, it follows that the densities found at any one temperature and pressure are constant for all others. Thus if air is twice as heavy as an equal volume of a certain gas, both being weighed at 32° and 30 Bar., the same ratio will be found at 32° and 15 Bar., and at 212° and 30 Bar. The same remark applies to vapours, except when they suffer condensation. For example, the density of air and steam, both being weigh- ed at 212° and 30 Bar., is expressed by 1000 and 625: the same ratio is pre- served at 212° and at any other pressure less than 30 Bar., because in that case the vapour will expand like air; but if the pressure be increased, or the temperature diminished, condensation occurs, and the density of the vapour falls below 625. Hence it happens that the density of vapours varies with the temperature, as is exemplified by the following table, showing the greatest density of aqueous vapour at the temperatures stated, the corre- eponding elasticities agreeably to Dalton's table, and the weight of 100 cubic inches of the vapour. Weight of Density. 100 cubic inches. 5.7292 0.13716 grains. 10.3539 0.2478 1418306 0.3394 46.6697 l.in 170.61 4.084 625 14.96 In calculating these densities, it is assumed that the laws of gaseous ex- pansion by varying heat and pressure are true,—that the density of steam at Elasticity in Temp. inches of mercury. 32° F. 0.2 50° 0.375 60° 0.524 100° 1.860 150° 7.42 212° 30 : HEAT. 49 212° F. and 30 Bar. is 625, compared to air at the same temperature and pressure as 1000,—and that 100 cubic inches of air at 212° and 30 Bar. weigh 23.94 grains. The formula for the calculation is thus deduced:—If d is the density of aqueous vapour at any pressure p, then since both the density and elasticity of gaseous substances vary inversely as their volume, the density and elasticity are proportional to each other; so that d : 625 : : P p : 30, and hence d=625.—-. This gives the density of aqueous vapour at 212°, and with an elasticity equal to p. In this state the vapour is rarefied, and will admit of being cooled down to a certain point, but not lower, say to t degrees above 32°, without condensation; and when it has reached that point, its density has acquired a maximum. Its elasticity remains un- changed, because the loss of tension due to loss of heat is compensated for by diminution of volume. Its density has increased exactly in the same ratio as its volume has diminished, and, therefore, the formula of page 21 in- verted will give the increased density owing to decrease of temperature. p 480+180 Hence we shall have d=625\r— ..,- ,----• For example, if we wish to cal- 30 480+< r culate the greatest density of aqueous vapour at 100° F., then <=68, and the elasticity of that vapour by Dalton's table is 1.86. Inserting these values of i ftfi fifin t and p in the preceding formula, we shall find he same, the angle of incidence is always equal to the angle of reflection. Let ab, figure 1, represent a plane mirror, id the direction of a ray falling on ab at the point d, and dp a line perpendicular Fig. 1. to the mirror ab. Then a plane passing through r JP ~R idp will be perpendicular to ab, and, by the first \ / law, the reflected ray dr. will lie somewhere in \ / that plane. Also, by the second law, the angle \ ■ / of reflection rdp must be equal to the angle of \ J / incidence idp. Hence, as soon as the direction \'/ of the incident ray is given, that of the reflected ..........,„,„.......^,.............urnim^ ray is known also. A. D J3 These laws apply equally to convex and concave mirrors. A circle or any 56 LIGHT. curve may be viewed as a polygon with very short sides circumscribing the curve, as shown in ab, fig. 2; and on this prin- Fig. 2. ciple a tangent W at any point d of a curve ab, may be taken as identical at the touching point with the curve itself. Similarly, may a plane, tan- gent to a curved surface, be considered as part of that surface at the point of contact. The action of a curved mirror may hence be referred to that of a number of tangent planes, which will reflect £ light agreeably to the two laws above mentioned. Thus, let ab, fig. 3, be a convex mirror, being a segment of a sphere, the centre of which is c; let id, i'd' Fig. 3. be parallel rays, incident at d, d'. The dotted lines dp, d'p' will be S-R- respectively perpendicular to the tangent at d, d' ; the angles of in- cidence are idp, i'd'p' ; and pdr, p'd'r', the angles of reflection. Parallel rays falling on a convexn^r""" mirror are obviously scattered or "" ~ ~ ~ ^ B > made to diverge. " ~--if-P. On the same principle must parallel rays falling on a concave -"- spherical mirror, as represented by fig. 4, be so reflected as t* con- verge and meet together at one point f, which is called its focus for parallel rays, or its principal focus, and is situated midway between the centre c, and Fig. 4. f ;~>>c the axis of the mirror e. The dotted lines represent the perpendicular to the tangent at the respective points of incidence, d, d' d, d'. From the same figure it is obvious that the diverging rays emit- ted by a light placed in the focus of a concave mirror are rendered parallel by re- flection. If the light be pla- ced between e and f, then the rays will continue divergent after reflection. On placing the light be- tween f and c, the incident rays, diverging less rapidly than when the light was at f, will converge after reflection, and meet at some point beyond C, which point is more remote from c the nearer the light is to f. When the light is at c, all the rays are reflected back to c; since each ray will then be perpendicular to the tangent at its point of incidence. The student will easily comprehend these statements, if he will but take rule and compass, and draw a few figures for himself. The statement above made, that parallel rays are collected into one point by reflection from a concave spherical mirror, is not strictly correct. When the mirror is very flat, being a Fig. 5. small segment of a large sphere, the rays meet very nearly in one point; but they are far from doing so when the curvature of the mir- ror is considerable. This defect of spherical mirrors, which arises from their form, and is termed spherical aberration, is exhibited in fig. 5, where the rays id, i'd', near the axis, meet at f ; whereas the remoter rays id, i'd', are collected at /. The consequence of such aber- >f;i\c LIGHT. 57 ration is a confused image, a defect which is remedied by diminishing cur- vature, and cutting off by screens the rays most distant from the axis. Pa- rabolic reflectors, when accurately made, are entirely free from this incon- venience. The position in which objects arc seen after being reflected will now be Fig. 6. easily understood. Let six, fig. 6, be an arrow placed before a plane mirror ab, e the eye of an observer, mc mc rays emanating from the point of the arrow, and n/ nd rays proceed- ing from its shaft. The only reflected rays which reach the eye are those that fall be- tween the points c and d. Those issuing from any single point, m, continue to diverge at the same rate after as before reflection; and, though they are reflected and enter the eye separately, they are collected together by the refracting power of that organ, and ap- pear to the observer to issue from a point m, at which, if continued back, they would in- tersect. The same is true of rays issuing from n, and from all points intermediate between m and n. By inspecting the figure it will be seen that each part of the image mn is at the same dis- tance behind the mirror as the object mn is before it, and that the image and object have the same length; consequences which flow necessarily from the laws of reflection and the known properties of triangles. Again, let the arrow mn, fig. 7, represent a high distant object, towards Fig. 7. which a spherical mirror ab is directed. Rays emanating from m and falling on the mirror at a, e, and b, will be so reflected that they all meet at a point m; rays diverging from x, and reaching the same points of the mirror, will be collected at n; and all points intermediate between m and n will be repre- sented along the line mn, forming a small inverted image of the object. As the rays prior to reflection were divergent, their focal points will be nearer the centre c than the focus for parallel rays. The image mn, will be much smaller than the object, the ratio of their lengths being directly as their dis- tances from the mirror, a relation which the geometric reader will discover for himself by inspecting the figure: if mn be 1000 feet from the mirror, and mn at one foot, the image will be diminished in length 1000 times. Hence, as the size»and position of the image can be measured, the distance of the object may be calculated if we know its size; or its size may be inferred from a knowledge of its distance. The construction of the simple reflecting telescope depends on the prin- ciple just explained. The small size of the image is compensated for, partly by its brightness, since each point is formed by the concentration of many rays, and partly by the advantage of placing the eye close to it. In order to see the image mn, the observer may place in the focus a piece of ground- glass or tissue-paper; or, a hole being cut in the mirror at e, the image may be received on a small plane mirror placed in the focus, and be reflect- ed to the observer at e. Instead of using a plane mirror for this purpose, mn may be considered as a new object, and be reflected by a second smaller 58 LIGHT. concave mirrorplaced between win and c, and in front of AB; for the con- verging rays which meet at any point m of the image, cross each other at that point, and then diverge exactly as though the place of the image were occupied by a real arrow. The second mirror may be so placed as to mag- nify the image mn; and the second image may be still further enlarged by a convex lens. Compound reflecting telescopes are constructed on this prin- ciple. The arrangement displayed by figure 7 is exactly that of a simple reflect- ing microscope, provided mn be viewed as a small real object, and mn as its magnified inverted image. If mn were placed in the principal focus, the reflected rays would be parallel, and hence could not meet to form an image; but if situated rather beyond the principal focus, as in the figure, then the rays converge after reflection, and give an enlarged image of the small object. The ratio of the length of the object and image will, as be- fore, be as their respective distances from the mirror. REFRACTION OF LIGHT. Light traverses the same transparent medium, such as air, water, or glass, in a straight line, provided no reflection occurs, and there is no change of density; but when it passes from one medium into another, or from one part of the same medium into another of a different density, a change of direction always ensues at the place of junction of the media, except when the ray is perpendicular to that plane. For instance, let ab a'b', fig. 8, represent a vertical Fig. e. section of a vessel full of water, and pp' the perpendicular to the surface of the water at the point c. Should a ray of light enter the water perpen- dicularly to its surface, as in the line of pc, it will continue on its course J{__ to p' without deviation; but if it de- scend obliquely, as in the direction of ic, it will suffer a bend at c, and V proceed to e, instead of advancing P'Gr H along the dotted line to f. Converse- ly, were a ray of light to emanate from e and emerge at c, it would not advance to e, but take the direction of ci. By comparing the direction of the refracted ray in these two cases in relation to the vertical pp', it will be seen that the ray approaches the perpendicular in entering from air into water, and recedes from it in passing out of water into air. The same re- mark applies to the passage of light from or into air into or out of solid or liquid media in general. Bodies differ in their power of refracting light. In general, the denser a substance is, the greater is the deviation which it produces. If in fig. 8 sulphuric acid were mixed with the water, the ray ic would be refracted to some point between e and g ; and if a solid cake of glass were substituted for that liquid, the refracted ray would be bent down to cg. But this is far from universal:—alcohol, ether, and olive oil, which are lighter than water, have a higher refractive power. Observation has shown it to be a law, to which no exception is yet known, that oils and other highly inflammable bodies, such as hydrogen, diamond, phosphorus, sulphur, amber, olive oil, and camphor, have a refractive power which is from two to seven times greater than that of incombustible substances of equal density. But what- ever may be the refractive power of bodies in relation to each other, re- fraction is always governed by the two following laws, discovered in 1618, by Snell, though usually ascribed to Descartes. 1. The direction of the incident and refracted ray is always in a plane perpendicular to the surface common to the media. 59 2. The sine of the angle of incidence and the sine of the angle of refrac- tion are in a constant report for the same media. The first law is similar to the first law of reflection already explained.— (Page 55.) To explain the second law, let abe, fig. 9, be a vertical section of a refracting medium, pp' the perpendicular to it, ic a ray of light incident at c, and ce the refracted ray. Then icp is the angle of incidence, and ecp' the angle of refraction. Also from c, as a centre, with any radius ci, and in the plane of the ray ice, draw a circle; and from the points i and e, where the course of the ray cuts the circle, let fall io, ec at right angles to pp'. Then may la be consider- ed the sine of the angle of incidence, and ec the sine of the angle of refraction. The second law denotes that these lines are for each substance in a constant ratio, whatever may be the direction of the incident ray. In the figure the sine of the angle of refraction is to the sine of the angle of incidence as 1 to 2; and this ratio being once determined, each ray must conform itself to it, so that any angle of incidence being given, the direction of the refract- ed ray may be foretold. Thus, if t'c be a second ray incident at c, of which ib is the sine of the angle of incidence, the ray will be bent into such a course, that ed shall be to ib as 1 is to 2. This ratio is nearly that observed in glass made of one part of flint to three of oxide of lead. In common flint-glass the ratio is nearly as 1 to 1.6; in water it is as 1 to 1.336; in oil of cassia as 1 to 1.641; in diamond as 1 to 2.755; in phosphorus as 1 to 2.224; and in melted sulphur as 1 to 2.148. By thus representing the sine of the angle of refraction by 1, the sines of the angle of incidence in all bodies refer to the same unit of comparison, and are, therefore, at once com- parable with each other: such numbers are called indices of refraction, and indicate the degree of refractive power. For example, the index of refrac- tion, for water is 1.336; for flint-glass 1.6; and for diamond 2.755. By means of SnelPs laws of refraction, and with a knowledge of the in- dices of refraction, the course of a ray of light through any medium may be indicated, whatever may be the nature or figure of that medium, or the direction of the ray. The refracting substance most used in optics is glass, which is ground into different forms, such as prisms and lenses, according to the purpose for whieh it is designed. One of the simplest cases is the refraction of a plane glass, such as the pane of a window. Let ic, fig. 10, be a ray incident on the Fig. 10. upper side, ab, of a plane glass, and ce the refracted ray: at its exit at the under side, a'b', which is parallel to ab, it will be re- fracted to the same amount as at its en- trance, and will pass on in the direction of te, appearing to an observer at e to have come along the line i'e, parallel to its real course ic. Hence, in looking at an object through a window it is not seen in its real position; but as all the rays are similarly affected, the object is not distorted, provided the opposite sides of the glass are really parallel. In studying the influence of curved media on light the same rule is to be observed as in reflection by curved mirrors (page 56): a plane, tangent to the curved surface at each point of incidence, is to be drawn or imagined, and the direction of the ray deduced in reference to that plane. On apply- ing this rule to convex and concave lenses, it is found that the former act 60 LIGHT. like concave mirrors, and tend to collect the refracted rays together; where- as a concave lens, like a convex mirror, tends to scatter them. Figure 11 represents parallel rays falling upon a doubly convex lens, the two curved surfaces of which are shown by the vertical section ab. The ray gf, which falls perpendicularly, goes without deviation through the mid- -Qr- dle or axis of the lens. The other £'_ rays enter and quit the lens so as to j-J___ form a smaller angle on one side ^ J3 than on the other, and the acute angle obviously lies on the side towards the axis; every ray is bent towards that axis by both surfaces; and as, from the figure of the lens, the rays most distant from the axis approach the lens at the sniallest angle, they also suffer the greatest refraction. The result is, that the rays converge and meet at a point f, termed the focus of parallel rays, or the principal focus. Its distance from c varies both with the curvature of the lens and the re- fracting power of the glass with which it is made. With glass of the same quality the focal distance depends on the figure of the lens, the greatest convexity giving the shortest focal distance. As the lens in figure 11 brings parallel rays into a focus at f, it is ob- vious that rays diverging from a luminous object placed at f will be render- ed parallel by the same lens, the course of the rays being simply reversed. Were a light situated between f and c, its rays would diverge so much that the lens could not render them parallel, and they would continue divergent after refraction. On removing the light to the right of f, the incident rays have such diminished divergence that they converge after refraction, and meet at a certain distance to the left of the lens, which distance diminishes as the light recedes from f ; until at length, when the luminous object is so far on the right side of the lens that the incident rays may be considered parallel, they will be bent into a focus at f'. Convex lenses are subject to the defect called spherical aberration equally with concave mirrors (page 56), and from the same cause. The spherical figure of a convex lens causes undue refraction of the rays incident near its margin, so that such rays have a shorter focal distance than those incident near its axis. The defect is more conspicuous in lenses of considerable curva- ture than in flat ones; and it may be remedied by intercepting the marginal rays with an opaque screen, or by forming such a combination of lenses, as may augment the convergence of the rays near the axis without equally acting on those more distant from it. In the eye this evil is averted by the substance of the lens increasing in density from its margin to the axis. The action of concave lenses, Fig. 12. fig. 12, is the opposite to that of convex lenses. Drawing a tangent to any point of the curve, and constructing the sines of incidence and refrac- tion, as in figure 9, it will be found that parallel rays will be so refracted by both sur- faces of a doubly concave lens, that they will diverge as if they had emanated from a common point f be- fore the lens, termed its principal focus, the position of which depends on the refracting power of the substance of the lens, as well as on its curva- ture. Conversely, the rays d' d, d, d', converging towards the principal focus f of a doubly concave lens, will be rendered parallel by such lens : if their original convergence were less rapid, they would diverge after refrac- tion; but if their convergence were to a point between f and c, they would still converge after refraction, and meet somewhere along the axis fg, at a point less remote the greater the original convergence. Rays already di- LIGHT. 61 vergent will diverge still more after passing through a concave lens. Thus the influence of concave lenses, whether concave on both sides or on one only, is exactly opposed to that of convex lenses. The former tend to dimi- nish or destroy convergence, and to render diverging rays still more diver- gent ; whereas the latter diminish or destroy divergence, and give increased convergence to rays already convergent, pj„ 13 The refracting properties of convex lenses are extensively applied in the con- struction of refracting telescopes and micro- scopes, the object being, as in reflecting te- lescopes and microscopes, to obtain a dis- tinct small image of a large distant object, or a magnified representation of a near small ob- ject. The natureof such combinations is illus- trated by the annexed wood-cut, Fig. 13, in which ab is a doubly convex lens acting on rays from a distant object represented by the arrow mn. As the incident rays are not parallel, but divergent, the rays from each point of mn will be collected into a fo- cus at a distance behind ab, somewhat greater than the focus of parallel rays/; and an inverted image nm will be produced. The length of the image to that of the ob- ject will be directly as their respective dis- tances from the centre of the lens ab, ex- actly as in the reflecting telescope (page 57). If mn is well illuminated, its image will be bright, since each point is formed by the confluence of many rays. The image will be inverted, the rays which emanate from the upper part of the object forming the lower part of the image, and conversely. The direction in which the rays from any point m meet, may be found by drawing a straight line from m through the centre c of the lens ab; for as the ray mc enters the lens above the axis at the same distance as it quits it below the axis, the second refraction is exactly the reverse of the first, and the ray emerges as though it had passed through a plane glass (fig. 10), —moving onwards, not strictly in the same straight line, (though for convenience it is usually represented as such,) but in a line parallel to it. Figure 13 likewise exhibits the application of a convex lens in the construction of a microscope. For if nm be a small object placed a little beyond the principal focus of the lens ab, the rays will be so refracted as to form a large inverted image mn, the size of which is determined by the rule above mentioned. A convex lens fitted into the wall of a darkened chamber constitutes the arrangement of a camera obscura, the inverted images of external objects being received on a disk of paper or a white board. In the simple telescope the lens is placed at the extremity of a tube of such length that the image may be formed within the tube, and the observer looks from the other end at the image formed in the air. The eye acts on the same principle. Lu- minous rays entering the transparent parts of the eye are refracted by the cornea and crystalline lens, and are brought into a focus at the bottom of the eye, an inverted image of external objects being formed upon the retina as on the table of a camera obscura. For distinct vision it is necessary that 62 LIGHT. this image should be formed exactly on the retina. Hence were the eye an ordinary lens, having an invariable focus, our range of vision would be very narrow: an eye fitted for seeing at a distance, would be useless for near objects; and persons who could see near objects, would be blind to remote ones. Two rays emanating from a distant point cannot both fall upon so small an object as the eye, unless they are nearly parallel; for if they di- verged by even a very small angle, they would before reaching the eye sepa- rate by an interval exceeding the diameter of the cornea. On the contrary, rays in rapid divergence may enter the eye, provided the point from which they emanate be close to it; and the nearer the object, the more divergent the rays which enter. When, therefore, we observe a distant landscape, then successively notice nearer and nearer objects, and lastly cast the eyes upon the page of a book only six inches distant, we receive rays coming from a multitude of different objects, each set of rays having its own pecu- liar divergence, and requiring a separate focus ; and yet, so wonderful is the adjusting power of the eye, a single minute suffices for distinctly seeing all the objects so beheld, without the consciousness of an effort. The adjustment of the eye for different distances appears to depend on a power of increasing or decreasing the distance between the posterior part of the eye and the lens, though the mechanism by which this is accomplished is unknown. Some ascribe it to a change in the figure of the whole eye- ball, produced by the muscles which move the eye ; but Sir D. Biewster, I think with better reason, considers the position of the lens to be varied by the same contractile tissue which determines the movements of the iris and the size of the pupil. To this adjusting power, however, there is a limit. The distance at which most persons see small objects distinctly is about six inches: at shorter distances the rays are so divergent, that their focal point falls behind the retina, and indistinct vision is the consequence. Persons called long-sighted are unable to see near objects distinctly, owing to a weak refracting power of the eye, due to deficient convexity or density in the hu- mours of the eye. This is the infirmity of advancing life, and is remedied by convex glasses, which cause diverging rays to be parallel or slightly con- vergent. In short-sighted persons the refractive power, either from undue convexity or undue density of the cornea and lens, is so powerful, that all rays which do not diverge rapidly are brought to a focus before they reach the retina. Youth is the period most obnoxious to this imperfection, and assistance is derived from a concave glass, which causes parallel rays to di- verge, and thereby counteracts the refracting influence of the eye. Objects are seen erect though their images on the retina are inverted. The direction in which each point of an object is seen, may depend either on the direction of the rays which form it, or on the part of the retina which is impressed. On inspecting the image nm, figure 13, it will be seen that any point n is formed by a multitude of rays lying within the angle bna, each of which has a different direction from the others; and yet when a similar collection of rays is formed on the retina, the observer sees only one point n, situated nearly in the direction of ncx. Such and similar conside- rations justify the belief, that the direction in which a luminous point is seen depends not on the direction of the rays aw they enter the eye, but on the part of the retina which is impressed. Sir D. Brewster contends that the line of visible direction is always perpendicular to that part on which a ray falls; and that, as the eye-ball is nearly a perfect sphere, these perpendicu- lars must all pass through the centre of the eye, which he regards as the centre of visible direction. To me his arguments do not appear conclusive. AVere this opinion true, the point m' of an arrow m'n', figure 14, would be seen along the dotted line wiop, appearing at a spot very remote from its real po- sition. It seems more consistent with observation to take the centre of the crystalline lens, or rather of the collective humours of the eye regarded as one lens, as the centre of visible direction. Through that centre c, fig. 14, all the directions pass from each part of an image, and these cross each other: the lowest part of an image is the highest of the object, and the highest of the image the lowest of the object. It has been supposed that in LIGHT. 63 Fig. 14. infancy we actually see erect objects inverted, and only discover that they are not so by the habitual correction derived from experience; but this fal- lacy has been fully corrected by observation on persons born blind, who first obtained the power of vision when of an age to express what they saw. The apparent size of objects depends on their dis- tance from the eye. Let mc, nc, fig. 14, be rays from the extreme points of the arrow mn, which cross within the eye at c: then the angle mcn is termed the visual angle. Mere inspection of the figure shows that the larger that angle is, the greater will be the arc on the retina occupied by the image mn; and also the greater that image, the greater will be the angle included by the lines of visible direction. The visual angle in fact varies exactly as the arc of the image ; and as that an- gle may be found with sufficient accuracy by drawing lines from the eye to the extremity of an object, it af- fords a convenient expression for the length of the image: when the angles are small, the linear magnitudes of two objects are nearly in the same ratio as their visual an- gles. If a second arrow mn', twice as long as mn, be placed parallel to mn, and at double its distance from the eye, then, by the properties of similar triangles, their visual angles will be equal, and their apparent magni- tude identical. Conversely, if the two arrows be paral- lel, have the same visual angle or apparent magnitude, and one be twice as distant as the ether, the more re- mote one must be twice as long as the other. The ap- parent magnitude of the same object at different dis- tances may be inferred on the same principles. Thus if mn approach the eye, remaining upright all the time, the visual angle will enlarge, and at half the distance its length will appear double; or if mn recede from the eye, it will be seen under a smaller angle, and appear pro- portionally smaller, until at double the distance it will seem to be half of its original length. In fact, the ap- parent length of an object increases in the same ratio as its distance from the eye, or more strictly from the point c within the eye, decreases. A large object seems a mere speck at a great distance ; and a minute object is invisible unless brought close to the eye. To bring an object near the eye is to magnify it. A tower which ap- pears 100 feet high to a person 4 miles distant, will seem 200 feet high at 2 miles, and 400 at 1 mile; and the type of a book which at 12 inches appears a line in length, will appear two lines at 6 inches. In these cases it is the linear magnitude which varies inversely as the distance : the superficial extent, or area, will vary inversely as the square of the distance. The foregoing considerations account for many optical phenomena. Short- sighted persons see minute objects better than those who have a long sight, because, from the greater refractive power of their eyes (page 62), they can bring the object closer to the eye than those who are long-sighted, and there- fore see it under a greater angle. But byaid of a convex lens a long-sighted person may attain the same end. Let him place the object in the focus of a convex lens, and the eye at a distance behind convenient for receiving all the rays which pass through it: the diverging rays, rendered parallel by the lens, are readily formed by the eye into an image on the retina, and the ob- ject is seen under the same angle as though the eye had occupied the posi- tion of the lens. This arrangement is shown by figure 13, where mn is the object, ab the lens, and e the eye of the observer. If the focal distance of the lens be 1 inch, we gain the same advantage as though the eye itself were toS 64 LIGHT. placed at one inch; and taking 6 inches as the shortest distance of distinct vision with the unaided eye, the apparent length of the arrow will lie in- creased in the ratio of 1 to 6. With a lens of half an inch focus, the increase will be as \ to 6, or 1 to 12; and if the focus is J^th of an inch, the increase will be as _!_ to 6, or 1 to 60. Convex lenses are hence familiarly known 1 o by the name of magnifying glasses. Convex lenses are similarly employed in the construction of compound microscopes and telescopes. In figure 13, let nm represent a small object formed by the lens ab into an enlarged image mn : that image may be viewed by the eye at the distance of 6 inches; but by interposing a second lens ab of 1 inch focal distance, the effect is the same as though the eye were at 1 inch, and thus the image is further increased in the ratio of 1 to 6. The lens ab is called the eye-glass, and ab the object-glass. Again, in a telescope, a large distant object is represented as a minute image, and so far its mag- nifying power depends on the eye being able to inspect a small image at 6 inches instead of the large object at a great distance. For instance, a tower 400 feet high, formed into an image 1 foot long, is thereby shortened 400 times; but as that image can be seen distinctly at the distance of £ a foot instead of the object at 400 feet, the elongation due to this cause alone is as 1 to 800. The apparent height of the tower is thus diminished 400 times by one cause, and increased 800 times by another; so that the compound effect is, that it is doubled. But by employing a second lens with a very short focus, the image may be still further magnified to a great extent. Double Refraction.—If on a piece of paper with a black line on its sur- face we place a rhombohedron of Iceland-spar, and then look at the line through the crystal, it will be found that in a certain position the line ap- pears single as when seen through water or glass; but in other positions of the crystal two lines are visible parallel to each other, and separated by a distinct interval. The light in passing through the crystal is divided into two portions, one of which obeys the laws of refraction already explained (page 58); whereas the other portion proceeds in a wholly different direc- tion, and hence gives the appearance of two objects instead of one. The former is termed the ordinary, the latter the extraordinary ray. This phe- nomenon is known by the name of double refraction, and has been witnessed in many crystallized substances, as in minerals and artificial salts. Light, transmitted through Iceland-spar or other doubly refracting sub- stances, is found to have suffered a remarkable change. In this state it is distinguished from common light by the circumstance, that when it falls upon a plate of glass at an angle of 56- 11', it is almost completely reflected in one position of the glass, and is hardly reflected at all in another: if re- flected when the plane of reflection is vertical, no reflection ensues when the reflecting plane is horizontal, the incident angle being maintained at 56° 11'. This curious property, so different from common light, has been theoreti- cally ascribed to a kind of polarity of such sort, that each side of a ray of light is thought to have a character different from the two adjacent sides at right angles to it; and hence the origin of the term polarized light, by which this property is distinguished. Light is polarized by reflection from many substances, such as glass, water, air, ebony, mother-of-pearl, and many crys- tallized substances, provided the light is incident at a certain angle peculiar to each surface, and which is called the polarizing angle. Thus, the polar- izing angle for glass is 56° 11', and for water 53° 14'; that is, common light reflected by glass and water at the angles stated will be polarized. The phenomena of double refraction and polarized light constitute a de- partment of optics of great and increasing interest; but it is too remote from the pursuits of a chemical student to be treated of at length in this work. Those interested in such studies will find an excellent guide in Sir D. Brew- ster's Treatise on Optics in the Cabinet Cyclopedia. LIGHT. 65 DECOMPOSITION OF LIGHT. The analysis of light may be effected either by refraction or absorption. Newton, who discovered the compound nature of solar light, effected its de- composition by refraction, employing a solid piece of glass bounded by three plane surfaces, well known under the name of the prism. His mode of ope- rating consisted in admitting a ray of light ig, fig. 15, into a dark chamber Fig. 15. through a window-shutter def, and interposing the prism acb, so that the ray should pass obliquely through two surfaces, and be refracted by both. On receiving the refracted ray upon a piece of white paper lm, there ap- peared, instead of a spot of white light, an oblong coloured surface composed of seven different tints, called the prismatic or solar spectrum. On subject- ing each of these colours to refraction, no further separation was accomplished; but on causing the rays separated by one prism to pass through a second of the same power and in an inverted position cbo, the seven colours disappear- ed, and a spot of white light appeared at h, in the very position which it would have occupied had both prisms been absent. From such and similar experiments Newton inferred that white light is a mixture of seven colorific rays,—red, orange, yellow, green, blue, indigo, and violet; and that the sepa- ration of these primary or simple rays depended on an original difference of refrangibility, violet being the most refrangible and red the least so. Though a prism is the most convenient instrument for decomposing light, the separation of the coloured rays is more or less effected by refracting media in general. Lenses, accordingly, disperse the colorific rays at the same time that they refract them; and this effect constitutes one of the greatest difficulties in the construction of telescopes, in so much as the sepa- ration or dispersion, as it is termed, of these rays diminishes the distinctness of the image. The combinations by which the defect is remedied are called achromatic. Newton's analysis of light led him to explain the origin of the colours of natural objects. Of opaque bodies, those are black which absorb all the light that falls upon them, and those white which reflect it unchanged ; the various combinations of tints are the consequence of certain rays being ab- sorbed, while those alone whose intermixture produces the observed colour are reflected. The same applies to transparent media, which are colourless like pure water when the light passes through unchanged, but are coloured when some rays are transmitted and others absorbed. This absorption of cer- tain rays by coloured media, such as glass of different tints, affords another mode of decomposing light; and Sir D. Brewster has ingeniously applied it to analyze the seven colours which compose the prismatic spectrum. He has proved by such experiments, what has been maintained before, that the seven colours of the spectrum are occasioned not by seven but by three simple or primary rays; namely, the red yellow, and blue. These rays are concen. 6* ��/C G6 LIGHT. trated in those parts of the spectrum where each primary colour respectively appears; but each spreads more or less over the whole spectrum, the mix- ture of red and yellow giving orange, of yellow and blue, green, and red with blue and a little yellow causing the violet. The prismatic colours, according to the experiments of Sir W. Herschel, differ in their illuminating power : the orange illuminates in a higher de- gree than the red, the yellow than the orange. The maximum of illumina- tion lies in the brightest yellow or palest green. The green itself is almost equally bright with the yellow ; but beyond the full deep green the illumi- nating power sensibly decreases. The blue is nearly equal to the red, the indigo is inferior to the blue, and the violet is the lowest on the scale. (Phil. Trans. 1800.) Solar light, both direct and diffused, possesses the property of exciting heat as well as light. This effect takes place only when the ray is absorb- ed, the temperature of transparent substances through which it passes, or of opaque ones that reflect it, remaining unchanged. Hence the burning-glass and concave reflector are themselves nearly or quite cool, though at the same time intense heat is developed at the focus. The intense coldness of the higher strata of the air arises from the same cause: the sun's rays pass on unabsorbed through the atmosphere; and its lower strata would also be very cold, did they not receive heat by contact from the earth. The absorption of light is much influenced by the nature of the surface on which it falls; and it is remarkable that those substances which absorb radiant heat most powerfully, are also the best absorbers of light. Difference of colour has still greater influence over the absorption of light than of sim- ple heat. That dark-coloured substances acquire in sunshine a higher tem- perature than light ones, may be inferred from the general preference given to the latter as articles of dress during the summer; and this practice, founded on the experience of mankind, has been justified by direct experi- ment. Dr. Hooke, and subsequently Dr. Franklin, proved the fact by placing pieces of cloth of the same texture and size, but of different colours, upon snow, and allowing the sun's rays to fall upon them. The dark- coloured specimens always absorbed more heat than the light ones, the snow beneath the former having melted to a greater extent than under the others; and it was remarked that the effect was nearly in the ratio of the depth of shade. Sir H. Davy also examined the subject, and arrived at the same conclusions. Calorific Rays of the Spectrum.—The rays of the prismatic spectrum dif- fer from each other in their heating power as well as in colour, a fact first observed by Sir W. Herschel, whose attention was attracted to it by the cir- cumstance, that, in viewing the sun with large telescopes through differently coloured glasses, he sometimes felt a strong sensation of heat with little light, and at other times he had a strong light with little heat. This obser- vation led to his celebrated researches on the heating power of the prismatic colours. (Phil. Trans. 1800.) The experiments were made by transmitting a solar beam through a prism, receiving the spectrum on a table, and placing the bulb of a very delicate thermometer successively in the different parts of it. While engaged in this inquiry, he observed not only that the red was the hottest ray, but that there was a point a little beyond the red, altogether out of the spectrum, where the thermometer stood higher than in the red itself. By repeating and varying the experiment, he found that the most intense heating power was always beyond the red ray, where there was no light at all; and that the heat progressively diminished in passing from the red to the violet, where it was least. He hence inferred that there exists in the solar beam a distinct kind of ray, which causes heat but not light.; and that these rays, from being less refrangible than the luminous ones, deviate in a smaller degree from their original direction in passing through the prism. All succeeding experimenters confirm the statement of Sir W. Herschel, that the prismatic colours differ in heating power; but they do not agree as LIGHT. 67 to the spot where the heat is greatest. Sir H. Englefield, Davy, and others affirmed with Herschel that it is beyond the red ray ; while others, and in particular Leslie, contended that it is in the red itself. The observations of the late Dr. Seebeck (Edin. Jour, of 8eiencc, i. 358), have explained these contradictory statements, by showing that the point of greatest heat varies with the kind of prism which is employed for forming the spectrum. When he used a prism of fine flint-glass, the greatest heat was uniformly beyond the red; with a prism of crown-glass, the red itself was the hottest part; and with a prism externally of glass, but containing water within, the maxi- mum heat was neither in the red itself, nor beyond it, but in the yellow. These experiments have been confirmed by Melloni, who has succeeded with a prism of rock-salt in separating the spot of maximum heat from the colour- ed part of the spectrum by a much greater interval than had been done pre- viously: his curious and surprising results appear to me to dissipate all re- maining doubt as to the existence in solar light of calorific rays distinct from those rays which produce colour. He also traces the cause of the va- rying position of the maximum heat to the unequal absorptive power of dif- ferent transparent media. Rock-salt is freely transmissible to the least re- frangible calorific rays in solar light, but absorbs a greater proportion of those which are more refrangible; whereas the latter pass more easily through flint-glass, yet more readily through crown-glass, and with still greater freedom through water. Hence in successively employing a prism of these four substances in the order stated, the spot of greatest heat is found to be far beyond the red, then approaches the red, next in the red itself, and lastly is in the yellow part of the spectrum.—The preceding facts go far to prove that most, if not all, of the heating power ascribed to light is due, not to the absorption of luminous rays, but to that of the heat by which they are accompanied. Chemical Rays.—It has long been known that solar light is capable of producing powerful chemical changes. One of the most striking instances of it, is its power of darkening the white chloride of silver, an effect which takes place slowly in the diffused light of day, but in the course of two or three minutes by exposure to sunshine. This effect was once attributed to the influence of the luminous rays; but it appears from the observations of Ritter and Wollaston, that it is owing to the presence of certain rays that excite neither heat nor light, and which, from their peculiar agency, are termed chemical rays. It is found that the greatest chemical action is ex- erted just beyond or at the verge of the violet part of the prismatic spec- trum ; that the spot next in energy is the violet itself; and that the property gradually diminishes in advancing to the green, beyond which it seems wholly wanting. It hence follows that the chemical rays are still more re- frangible than the luminous ones, in consequence of which they are dis- persed in part over the blue, indigo, and violet, but in the greatest quantity at the extreme" border of the latter. Magnetizing Rays.—The more refrangible rays of light have been thought to possess the property of rendering steel and iron magnelic. The existence of this property was first asserted by Dr. Morichini of Rome. Other ob- servers subsequently failed in obtaining the same results; but in the year 1826 the fact appeared to be decisively established by the learned and ac- complished Mrs. Somerville, in an essay published in the Transactions of the Royal Society. Since that period the subject has been re-examined by MINI. Riess and Moser. They object to Mrs. Somerville's results, that her method of ascertaining the magnetic state of the needles used in the experi- ments was not sufficiently precise : they found that the duration of the oscil- lations of needles is exactly the same whether they are made to oscillate in the shade or underexposure to the concentrated violet ray of the spectrum, a result which could not occur had even a feeble degree of magnetism been excited; and they accordingly deny the supposed magnetizing power of light. (Edin. Journ. of Science, ii. 225.) 68 LIGHT. TERRESTRIAL LIGHT. Under this head are included all kinds of artificial light. The common method of obtaining such light is by the combustion of inflammable matter, which gives out so much heat that the burning substance is rendered lumi- nous in the act of being burned. All bodies begin to emit light when heat is accumulated within them in great quantity; and the appearance of glow- ing or shining, which they then assume, is called incandescence. The tem- perature at which solids in general begin to shine in the dark is between 600° and 700° F; but they do not appear luminous in broad daylight till they are heated to about 1000°. The colour of incandescent bodies varies with the intensity of the heat. The first degree of luminousness is an ob- scure red. As the heat augments, the redness becomes more and more vivid, till at last it acquires a full red glow. If the temperature still increase, the character of the glow changes, and by degrees it becomes white, shining with increasing brilliancy as the heat augments. Liquids and gases likewise become incandescent when strongly heated; but a very high temperature is required to render a gas luminous, more than is sufficient for heating a solid body even to whiteness. The different kinds of flame, as of the fire, candles, and gas light, are instances of incandescent gaseous matter. Artificial lights differ in colour, and accordingly exhibit different appear- ances when transmitted through a prism. The white light of incandescent charcoal, which is the principal source of the light from candles, oils, and the illuminating gases, contains the three primary colorific rays, the red, yellow, and green. The dazzling light emitted by lime intensely heated, first proposed by Lieut. Drummond for the trigonometrical survey (Phil. Trans. 1830), and of late so successfully applied by Messrs. Cooper and Carey for their gas microscope, gives the prismatic colours almost as bright as in the solar spectrum. The light emitted by iron feebly incandescent consists principally of the blue and red rays, as does the red light obtained by means of strontia and lithia ; that from ignited boracic acid is such a mixture of the blue and yellow rays as constitutes green ; and incandescent soda emits a yellow light, almost wholly free from the rays which cause the red and blue colours. Artificial light differs from solar light in containing heat in two states. It contains simple radiant heat like that radiated from a body not luminous, and which may be separated by transmission through a plate of moderately thick glass; but the light so purified still heats any body which absorbs it, and is thus believed to possess calorific rays associated with its luminous rays like those in solar light, and like them to be susceptible of refraction by transparent media. Thus, Mr. Daniell found that the rays from incandes- cent lime were concentrated by convex lenses, and set fire to phesphorus placed in the focus. (Phil. Mag. N. S. ii. 59.) The heat excited under such circumstances may of course be ascribed to the absorption of luminous rays ; but the experiments of Herschel with solar light suggest the idea that calori- fic rays capable of transmission through glass may also exist in artificial light, and this opinion has been verified by the late researches of Melloni. Nothing could show the fact in a more distinct point of view than the disco- very, by this ingenious experimenter, that transparent media are unequally permeable by colorific and calorific rays: substances almost opaque in re- gard to light, were found to give free passage to calorific rays; while others of the most perfect transparency are little permeable to heat. Melloni has consequently introduced a distinct name, diathermanous (from JW and 9-fg. juttivw) to denote free permeability to heat, just as diaphanous is formed from Sid and . Rock-salt is remarkably diathermanous or transcalent, greatly more so than glass of far greater transparency. Chloride of sulphur of a reddish-brown tint is more diathermanous than nut and olive oil of a light yellow colour, and these than the purest ether, alcohol, and water, the trans. parency of these substances being in the opposite order. A great many facts LIGHT. 69 of a like kind are enumerated in Melloni's essay, which is highly deserving of attention. The source of heat was an argand oil-lamp, and the tempera- ture measured by the thermo-multiplier (page 11). (An. de Ch. et de Ph. liii. 5.) The chemical agency of artificial light is analogous to that from the sun. In general the former is too feeble for producing any visible effect; but light of considerable intensity, such as that from ignited lime, darkens chloride of silver, and seems capable of exerting the same chemical agencies as solar light, though in a degree proportionate to its inferior brilliancy. (An. of Phil, xxvii. 451.) Light is emitted by some substances either at common temperatures or at a degree of heat disproportioned to the effect, giving rise to an appearance which is called phosphorescence. This is exemplified by a composition termed Canton's phosphorus, made by mixing three parts of calcined oyster-shells with one of the flowers of sulphur, and exposing the mixture for an hour to a strong heat in a covered crucible. The same property is possessed by chloride of calcium (Homberg's phosphorus), anhydrous nitrate of lime (Baldwin's phosphorus), some carbonates and sulphates of baryta, strontia, and lime, the diamond, some varieties of fluor-spar called chlorophane, apatite, boracic acid, borax, sulphate of potassa, sea-salt, and by many other sub- stances. Scarcely any of these phosphori act unless they have been pre- viously exposed to light: for some, diffused day-light or even lamp-light will suffice; while others require the direct solar light, or the light of an electric discharge. Exposure for a few seconds to sunshine enables Canton's phosphorus to emit light visible in a dark room for several hours afterwards. Warmth increases the intensity of light, or will renew it after it has ceased ;— but it diminishes the duration. When the phosphorescence has ceased it may be restored, and in general for any number of times, by renewed expo- sure to sunshine; and the same effect may be produced by passing electric discharges through the phosphorus. Some phosphori, as apatite and chloro- phane, do not shine until they are gently heated ; and yet if exposed to a red heat, they lose the property so entirely that exposure to solar light does not restore it. Mr. Pearsall has remarked that in these minerals the phospho- rescence, destroyed by heat, is restored by electric discharges; that speci- mens of fluor-spar, not naturally phosphorescent, may be rendered so by electricity; and that this agent exalts the energy of natural phosphori in a very remarkable degree. (R. Inst. Journal, N. S. i.)—The theory of these phenomena, like that of light itself, is very obscure. They have been attri- buted to direct absorption of light, and its subsequent evolution ; but the fact, that the colour of the light emitted is more dependent on the nature of the phosphorescent body than on the colour of the light to which it was exposed, seems inconsistent with this explanation. Chemical action, is not connected with the phenomena; for the phosphori shine in vacuo, and in gases which do not act on them, and some even under water. Another kind of phosphorescence is observable in some bodies when strongly heated. A piece of lime, for example, heated to a degree which would only make other bodies red, emits a brilliant white light of such in- tensity that the eye cannot support its impression. A third species of phosphorescence is observed in the bodies of some ani- mals, either in the dead or living state. Some marine animals, and particu- larly fish, possess it in a remarkable degree. It may be witnessed in the body of the herring, which begins to phosphoresce a day or two after death, and before any visible sign of putrefaction has set in. Sea-water is capable of dissolving the luminous matter; and it is probably from this cause that the waters of the ocean sometimes appear luminous at night when agitated. This appearance is also ascribed to the presence of certain animalcules, which, like the glow-worm of this country, or the fire-fly of the West Indies, are naturally phosphorescent. Light is sometimes evolved during the process of crystallization. This is exemplified by a tepid solution of sulphate of potassa in the act of crystal. 70 LIGHT. lizing; and it has been likewise witnessed under similar circumstances in a solution of fluoride of sodium and nitrate of strontia. Another instance of the kind is afforded by the sublimation of benzoic acid. Allied to this phe- nomenon is the phosphorescence which attends the sudden contraction of porous substances. Thus, on decomposing by heat the hydrates of zirconia, peroxide of iron, and green oxide of chromium, the dissipation of the water is followed by a sudden increase of density suited to the changed state of the oxide, and a vivid glow appears at the same instant. The essential conditions are, that a substance should be naturally denser after decomposition than it was previously, and that the transition from one mechanical state to the other should be abrupt. It is sometimes of importance to measure the comparative intensities of light, and the instrument by which this is done is called a Photometer. The only photometer which is employed for estimating the relative strength of the sun's light is that of Leslie. It consists of his differential thermometer, with one ball made of black glass. The clear ball transmits all the light that falls upon it, and, therefore, its temperature is not affected; they are all ab- sorbed, on the contrary, by the black ball, and by heating and expanding the air within, cause the liquid to ascend in the opposite stem. The whole in- strument is covered with a case of thin glass, the object of which is to pre- vent the balls from being affected by currents of cold air. The action of this photometer depends on the heat produced by the absorption of light. Leslie conceives that light when absorbed is converted into heat; but accord- ing to the experiments already referred to, the effect must be attributed, not so much to the light itself, as to the absorption of the calorific rays by which it is accompanied. Sir J. Leslie recommended his photometer also for determining the relative intensities of artificial light, such as that emitted by candles, oil, or gas. This application of it differs from the foregoing, because light proceeding from terrestrial sources contains heat under two forms. One portion is ana- logous to that emitted by a hot body which is not luminous; the other is similar to that which accompanies solar light. It is presumed that the first form of heat will not prove a source of error; that these rays are wholly in- tercepted by the outer case of glass; or that, should a few penetrate into the interior, they will be absorbed equally by both balls, and will therefore heat them to the same extent. It is probable that this reasoning is not wide of the truth; and, consequently, the photometer will give correct indications so far as regards the new element—non-luminous heat. But it is not applicable to lights which differ in colour, because the relation between the heating and illuminating power of such lights is exceedingly variable. Thus, the light emitted by burning cinders or red-hot iron, even after passing through glass, contains a quantity of calorific rays, which is out of all proportion to the luminous ones; and, consequently, they may and do produce a greater effect on the photometer than some lights whose illuminating powers are far stronger. The second kind of photometer is on a totally different principle. It de- termines the comparative strength of lights by a comparison of their sha- dows. This instrument was invented by Count Rumford, and is described by him in his Essays. It is susceptible of great accuracy when employed with the requisite care;* but, like the foregoing, its indications cannot be trusted when there is much difference in the colour of the lights. In this case, the best mode of obtaining an approximative result, is by observing the distance from each light at which any given object, as a printed page, ceases to be distinctly visible, i The illuminating power of the lights so com- pared is as the squares of their distances. * See an Essay on the construction of Coal Gas Burners, &c. in the Edin- burgh Philosophical Journal for 1825. ELECTRICITY. 71 SECTION III. ELECTRICITY. Elementary Facts.—When certain substances, such as amber, glass, seal. ing-wax and sulphnr, are rubbed with dry silk or cloth, they arc found to have acquired a property, not Observable in their ordinary state, of causing contiguous light bodies to move towards them ; or if the substances so rub- bed be light and freely suspended, they will move towards contiguous bodies. After a while this curious phenomenon ceases; but it may be renewed an indefinite number of times by friction. The principle thus called into action is known by the name of electricity, from the Greek word xxiKrgov, amber, because the electric property was first noticed in it. The same term is ap- plied to the science which treats of the phenomena of electricity. When a substance by friction or any other means acquires the property just stated, it is said to be electrified, or to be electrically excited; and its motion towards other bodies, or of other bodies towards it, is ascribed to a force called electric attraction. But its influence, on examination, will be found to be not merely attractive; on the contrary, light substances, after touching the electrified body, will be disposed to recede from it just as ac- tively as they approached it before contact. This is termed electric repulsion. By aid of the electrical machine these phenomena of electric attraction and repulsion may be displayed by a great variety of amusing and instructive experiments, showing how readily an invisible power is called into operation, and how wonderfully inert matter is subject to its control. But the student may witness these effects quite satisfactorily by very simple apparatus. Let him suspend a thread of white sewing silk from the back of a chair so that one end may hang freely, taking the precaution to moisten that end slightly by holding it between the fingers, while the rest of the thread is carefully dried by the fire; and let him then place near the free end a piece of sealing- wax previously rubbed on the sleeve of his coat. The silk will move towards it; but after touching the excited wax two or three times, it will recede from it. When an electrified body touches another which is not electrified, the electric property is imparted by the former to the latter. Thus, on touching the free end of the suspended silk thread with the excited wax, the silk will itself be excited, as shown by its moving towards a book, a knife, or other unexcited object placed near it. But though electricity is always imparted by an excited to an unexcited body by contact, the latter does not always exhibit electric excitement. If, for example, the suspended silk be welted along its whole length, it will be strongly attracted by the excited wax, but after contact it will not evince the least sign of being itself electrified. Never- theless, electricity is commmunicated to the silk in both cases, only it is retained by silk when dry, and is lost as soon as received by wet silk. Such observations led to the discovery that electricity passes with great ease over the surface of some substances, and with difficulty over that of others, and hence to the division of bodies into conductors and non-conductors of elec- tricity. If electricity be imparted to one end of a conductor, such as a cop- per wire, the other extremity of which touches the ground, or is held by a person standing on the ground, the electricity will pass along its whole length and escape in an instant, though the wire were several miles long; whereas excited glass and resin, which are non-conductors, may be freely handled without losing any electricity except at the parts actually touched. To this class of conductors belong the metals, charcoal, plumbago, water, and aqueous solutions, and substances generally which are moist or contain water in its 72 ELECTRICITY. liquid state, such as animals and plants, and the surface of the earth. These, however, differ in their conducting power: of the metals, Mr. Harris found silver and copper to be the best conductors, and after these follow gold, zinc, platinum, iron, tin, lead, antimony, and bismuth. (Phil. Trans. 1827, Part i. 21.) Mr. Forbes has lately called attention to the fact that the foregoing order is very nearly the same as that expressive of their conducting power for heat. Aqueous solutions of acids and salts conduct much better than pure water. To the list of non-conductors belong glass, resins, sulphur, dia- mond, dried wood, precious stones, earth and most rocks when quite dry, silk, hair, and wool. Air and gases in general are non-conductors if dry, but act as conductors when saturated with moisture. This knowledge is of continual application in electrical experiments. When it is wished to collect electricity on a metallic surface, the metal must be insulated, that is, cut off from contact with the earth, and with conductors touching the ground, by means of some non-conductor; an object commonly effected either by supporting it on a handle of glass, or by placing it on a stool made with glass feet. Another mode of insulating is to suspend a sub- stance by silk threads. But such insulators must be dry ; since they begin to conduct as soon as they grow damp, and conduct well, as in the experiment above described, when wet. Again, electrical experiments are very apt to fail in damp weather, because the moisture both carries off electricity directly, and by being deposited on the glass supports, destroys the insulation. To diminish this inconvenience it is usual to keep the insulators warm, and to coat them with a varnish made by dissolving the resin called shell-lac in alcohol, this resinous matter being much less prone to attract moisture from the air than glass. The same principles account for'an error once preva- lent that a metal cannot be excited by friction : if held in the hand, indeed, it exhibits no sign of excitement when rubbed, because the electricity is carried off as soon as excited; but if, while carefully insulated, it is rubbed with a dry cat's fur, excitement readily ensues. On comparing the electric properties manifested by glass and sealing-wax when both are rubbed by a woollen or silk cloth, they will be found essen- tially different; and hence it is inferred that there are two kinds or states of electricity, one termed vitreous, because developed on glass, and the other resinous electricity, from being first noticed on resinous substances. These two kinds of electricity, one or other of which is possessed by every elec- trified substance, are also termed positive and negative, the terms vitreous and positive being used synonymously, as are resinous and negative. The mode of distinguishing between positive and negative electricity is founded on the circumstance, that if two electrified substances are both positive or both negative, they are invariably disposed to recede from each other, that is, to exhibit electric repulsion; but if one be positive, and the other negative, their mutual action is as constantly attractive. The end of a silk thread, after contact with an electrified stick of sealing-wax, is repelled by the wax, because both are in the same electric state; but if a dry warm wine-glass be rubbed with cloth or silk, and then presented to the thread, attraction will ensue. A silk thread, in a known electric state, thus indicates the kind of electricity possessed by other substances: a convenient mode of doing this, is to draw a thread of white silk rapidly through a fold of coarse brown pa- per previously warmed, by which means its whole length will be rendered positive. When two substances are rubbed together so as to electrify one of them, the other, if in a state to retain electricity, will be excited also, one being always negative and the other positive. It is easy to be satisfied of this by very simple experiments. Rub a stick of sealing-wax on warm coarse brown paper, and the paper will be found to repel a positively excited thread of silk, while the wax will attract it; if a warm wine-glass be rubbed on the brown paper, the glass will be positive, as shown by its repelling the positive thread, while the same thread will be attracted by the negative paper; friction of ELECTRICITY. 73 sealing-wax on a silk riband renders the wax negative and the riband positive, but with glass the riband is negative. If two silk ribands, one white and the other black, be made quite warm, placed in contact, and then drawn quickly through the closed fingers, they will be found on separation to be highly attractive to each other, the white being positive and the black ne- gative. The back of a cat is positive to all substances with which it has been tried, and smooth glass is positive to all except the back of a cat. Seal- ing-wax is negative to all the substances just enumerated, but becomes po- sitive by friction with most of the metals. The reader will perceive from these facts that the same substance may acquire both kinds of electricity, becoming positive by friction with one body, and negative with another. THEORIES OF ELECTRICITY. The nature of electricity, like that of heat and light, is at present involved in obscurity. All these principles, if really material, are so light, subtile, and diffusive, that it has hitherto been found impossible to recognize in them the ordinary characteristics of matter; and, therefore, electric phenomena might be referred, not to the agency of a specific substance, but to some property or state of common matter, just as sound is produced by a vibrating medium. But the effects of electricity are so similar to those of a me- chanical agent; it appears so distinctly to emanate from substances which contain it in excess, and rends asunder all obstacles in its course so exactly like a body in rapid motion, that the impression of its existence as a distinct material substance sui generis forces itself irresistibly on the mind. All nations, accordingly, have spontaneously concurred in regarding electricity as a material principle; and scientific men give a preference to the same view, because it offers an easy explanation of phenomena, and suggests a natural language easily intelligible to all. Theory of Two Electric Fluids.—This theory, the fundamental facts of which were supplied partly by Dufay, and partly by Symmer, is founded on the assumed existence of two electric fluids, which Dufay distinguished by the terms vitreous and resinous electricity. In order to account for electric phenomena by this supposition, the two fluids are assumed to possess the following properties:—They are both equally subtile and elastic, universally diffused, and, therefore, present in all bodies, possessed of the most perfect fluidity, each highly repulsive to its own particles, and as highly attractive to those of the opposite kind; these attractive and repulsive forces being ex- actly equal at the same distance, and both varying inversely as the square of the distance varies. Electric quiescence is ascribed to these fluids being combined and neutralized with each other; and electric excitation is the con- sequence of either fluid being in excess. Their combination is destroyed by several causes, of which friction is one. The application of these principles is as follows. Two unexcited contiguous bodies, a and b, are electrically in- different to each other; for, though each electricity in a repels the electricity of the same name in b, attraction to precisely the same extent is exerted between the opposite electricities, and no change results. If a and b are rubbed together, a portion of the combined electricities in both is decomposed, and the separated resinous fluid is transferred to one of them, suppose to a, and the vitreous to b, each being electrified to the same degree, though op- positely. The free particles of resinous electricity in a tend by their repul- sion to recede from each other, and would quit a altogether, unless their pas- sage were impeded by a non-conductor: the atmosphere, if dry, cuts off the retreat, and by its pressure confines the resinous fluid to the surface of a. The same happens to the vitreous fluid on the surface of b. But the opposite electricities fixed on a and b exert a strong mutual attraction, and may suc- ceed either in forcing their way across the intervening stratum of air, or of actually drawing a and b into contact. In either case the free electricities reunite, and the electric equilibrium is restored. On the contrary, if a and 74" ELECTRICITY. b are similarly electrified, that is, possess the same kind of free electricity, the effort of the electric fluid to escape in opposite directions causes the sub- stances themselves to fly asunder, if the repulsive force exceed their weight, and thus produces electric repulsion. This theory, as commonly stated, takes little or no cognizance of any at- traction between the electric fluids and other material substances. But it would be against all analogy to suppose no such influence to exist; and in- deed the supposition of an attractive force acting at insensible distances seems necessary to account for the impediment caused by non-conductors to the free movement of the electric fluids. Theory of a Single Fluid.—The celebrated American philosopher, Frank- lin, proposed a different theory, founded on the supposition of a single elec- tric fluid, the particles of which are conceived to repel each other with a force diminishing as the squares of the distance, and to be attracted by matter iu general according to the same law. Material substance in its unelectric state is regarded as a compound of electricity and matter, saturated and neu- tralized with each other. It is also an assumption, shown to be necessary by jEpinus and Cavendish, that ponderable bodies repel each other with the same force and according to the same law as the particles of electricity. From the nature of these postulates it will be easy to anticipate their appli- cation. Unelectric bodies are such as have their natural quantity of elec- tricity, which precisely suffices to saturate and neutralize the matter of which they consist. They are then electrically indifferent; because the repulsion exerted between the electricity and matter of contiguous bodies is exactly counteracted by the attraction of the electric fluid in each for the matter of the other. Electrical excitement is occasioned either by increase or diminu- tion of the natural quantity of electricity. On rubbing a tube of glass with a woollen cloth, the electric condition of both is disturbed : the glass ac- quires more electricity than it naturally possesses, or is overcharged with electric fluid ; and the cloth, losing what the glass gained, contains less than its natural supply, or is under-charged. These opposite states are denoted by the algebraic terms positive and negative, the former corresponding to the vitreous, the latter to the resinous electricity of Dufay. Bodies, positively excited, repel each other by means of the repulsion among the particles of the electricity witli which they are surcharged; and the equal tendency of negatively excited bodies to separate is ascribed to the mutual repulsion among the particles of matter. The electric equilibrium in excited sub- stances is restored by the electricity escaping from those where it is in ex- cess, and passing to those which are under-charged. To the theory of Franklin it is objected that it involves an assumption at variance with the laws of gravitation, namely, that of matter being repulsive to itself; but in fact this assumption, if admitted, would not satisfactorily explain the unequal distribution of the electric energy over the surface of tire electrified bodies, as well negative as positive, dependent on their form. To account for this phenomenon the theory requires a repulsive fluid superadded to matter, and freely moveable among its particles, a sort of resident electric fluid, capable of performing all the functions ascribed to resinous electricity. With such addition, however, the theory of Franklin would virtually cease to be that of a single electric flidd, and would still, I suspect, be less generally applicable than the theory of two fluids. I feel it necessary, accordingly, to adopt the latter, substituting, however, agreeably to present usage, the terms positive and negative for vitreous and resinous electricity." * The chief objection which has been urged against the Franklinian theory of one electric fluid is, that it fails to explain the repulsion of two negatively electrified bodies. It is alleged that the condition of two bodies, the essence of which consists in the absence of a subtle principle called elec- tricity, cannot cause them to repel each other; for this would be attributing to a negation the possession of a positive property, which is absurd. It was ELECTRICITY. 75 CAUSES OF ELECTRIC EXCITEMENT. Friction.—This cause of electric excitement having been already men- fcioned, it here only remains to state the usual modes of developing electricity by friction. A supply of negative electricity is easily obtained by rubbing a stick of sealing-wax, or a glass tube covered with sealing-wax, with silk or woollen cloth; and positive electricity is freely developed when a dry glass tube is rubbed with silk, brown paper, or flannel, the surface of which is co- vered with a little amalgam. But for obtaining an abundant supply of elec- tricity it is necessary to employ an electrical machine, which is a mechani- cal contrivance for exposing a large surface of glass to continuous friction. As now constructed, it is formed either with a cylinder or plate of glass which is made to revolve upon an axis, and pressed during rotation by cush- ions or rubbers made of leather stuffed with flannel, and covered usually with silk. On the rubber is spread an amalgam of tin and zinc, rendered adhesive by admixture with a small quantity of lard or tallow. To prepare the amalgam, melt in a Hessian crucible one ounce of tin and three of zinc, then add two ounces of mercury heated to near its boiling point, stir briskly with a stick for a few minutes, and pour the mixture on a clean dry stone: when cold pulverize and sift, and preserve the fine powder in a well-corked dry phial. Another essential part of the machine is the prime conductor, which is an insulated conductor, commonly made of brass, placed in such immediate proximity to the revolving glass, that the electric state of the one is instantly imparted to the other. The electricity developed by the electrical machine is due partly to fric- tion, which disunites the combined electric fluids of the glass and rubber, but principally to the oxidation of the amalgam. The positive fluid is trans- ferred to the glass, from it to the contiguous prime conductor, and thence to any system of conductors connected with the prime conductor; and similarly the negative fluid collects upon the rubber, whence it is distributed to one or more conductors with which the rubber may be in connexion. Thus all insulated conductors in contact with the prime conductor are positive, and those attached to the rubber are negative. When once the glass and rubber are excited, it is necessary that the electric equilibrium of both should be restored before a second development can occur; and accordingly it is found that very little electricity is obtained when the prime conductor and rubber's conductor are both insulated. On taking positive electricity from the prime conductor, the rubber should communicate with the ground, that its negative electricity might escape; and when negative electricity is taken from the rubber's conductor, the prime conductor is connected with the ground. The same object may be accomplished by connecting the prime conductor with the rubber's conductor, though in experiments it is commonly inconvenient to employ this arrangement. Change of Temperature.—The operation of this cause of electric excite- ment was first noticed in certain minerals, such as tourmalin and boracite, not possessed of that symmetric arrangement of parts commonly observed in crystals, and which are electrified by the application of heat. But a far more general principle was detected by the late Dr. Seebeck, who found that the electric equilibrium is disturbed in certain metallic rods or wires when one extremity has a different temperature from that of the other, whether this apparent difficulty which induced ^Epinus and Cavendish to assert that the theory of Franklin required the assumption that matter was repulsive to itself. Loaded with this postulate, the theory may, indeed, be untenable; but the question arises, whether, in point of fact, the postulate mentioned is ne- cessary to the Franklinian theory. We think it is not; but are inclined, with Dr. Hare, to refer the apparent repulsion of negatively electrified bodies to an attraction between such bodies and the contiguous air, electrified positively by induction.—Ed. 76 ELECTRICITY. the difference be effected by the application of heat or cold. This observa- tion has been since shown by Professor Cumming to be true of all metals. (An. of Phil. N. S. v. 427.) See also the recent experiments by Mr. Pri- deaux. (Phil. Mag. iii.) The experiment is usually made by heating or cooling the point of junction of two metallic wires, which are soldered toge- ther; but M. Becquerel has proved that the contact of different metals is not essential. (An. de Ch. et de Ph. xli. 353.) Chemical Action.—Another, and very fertile source of electricity, is che- mical action. This was strongly denied by the late Sir H. Davy in his Bake- rian lecture for 1826; but the experiments of Becquerel, De la Rive, and Pouillet, afford decisive proof that chemical union and decomposition are both attended with electrical excitement. (An. de Ch. et de Ph. T. 35, 36, 37, 38, and 39.) Pouillet, in particular, has demonstrated that the gas aris- ing from the surface of burning charcoal is positive, while the charcoal itself is negative; and he has proved that similar phenomena are produced by the combustion of hydrogen, alcohol, oil, and other inflammables of the same kind. In all these instances the combustible, in the act of burning, renders contiguous particles negative; while the oxygen imparts positive electricity to the products of combustion. The fact, with respect to charcoal, was ori- ginally noticed by Volta, La Place, and Lavoisier, but was subsequently de- nied by Saussure and Sir H. Davy. Pouillet has reconciled these conflicting statements by showing that the result depends on the mode in which the experiment is conducted. For if the carbonic acid be completely removed from the burning mass at the instant of its formation, both are found to be electrical; but if the carbonic acid subsequently flow over the surface of the charcoal, the equilibrium will instantly be restored, and no sign whatever of excitement be perceptible. Decisive evidence of the same kind is supplied by the amalgam of the electrical machine, the influence of which is proportional to the degree of chemical action, and which ceases to be useful as soon as the metals are oxidized. Thus, Wollaston found that amalgams of silver and platinum, which are indisposed to oxidize, are of no use when applied to the rubber; and that an amalgam of zinc and tin, which is the most oxidable, is also the best amalgam for exciting the machine. He observed that a ma- chine in good action ceased to act when surrounded with carbonic acid, but instantly recovered its action on readmitting the air. (Phil. Trans. 1801.) On such facts is founded the foregoing statement, that the energy of the electrical machine is much more owing to chemical action than to fric. tion. Contact.—Another reputed source of electricity is contact of different sub- stances, especially of metals; a source originally suggested by Volta, who founded on it a theory of galvanism. The facts on which Volta rested his opinion were of this nature. Well-cleaned plates of zinc and copper were furnished with glass handles, by which they could be both supported and insulated : the zinc plate, held by its glass handle, was laid repeatedly on the copper, which at the time need not be insulated, and after each contact the zinc was made to touch the instrument, shortly to be described, called the condenser. A positive charge was gradually accumulated ; and on operating in the same manner with the insulated plate of copper, it was found to com- municate a negative charge. He also stated that if one end of a zinc plate communicate with the condenser, while the zinc at its other end is in con- tact with a plate of copper, a positive charge is communicated; and that ne- gative electricity is indicated when a copper plate, in contact with zinc at one end, rests at its other upon the condenser. From such experiments it was inferred, that the contact of zinc and copper disturbs the electric equili- brium in both metals, the latter acquiring an excess of negative and the for- mer of positive electricity. The quantity of electricity developed by contact is confessedly so small, that it requires for its detection the aid of very delicate instruments and of very careful manipulation; and the opinion is daily gaining ground that mere contact is incapable of causing electric excitation. The phenomena ELECTRICITY. 77 referred by Volta to contact, are ascribed by others to chemical action and to friction. De la Rive of Geneva contends (An. de Ch. et de Ph. xxxix. 297,) that the feeble charge commonly observed from the contact of zinc and cop- per, is due to slight oxidation caused by moisture and oxygen of the air act- ing on the plate of zinc. When he prevented such oxidation by operating in an atmosphere of hydrogen or nitrogen, no electric excitement fol- lowed ; and when he purposely increased chemical action, as by exposing the zinc to acid fumes, or by substituting for zinc a more oxidable metal, such as potassium, the electrical effects observable on contact with copper were greatly augmented. Electric excitation and chemical action were ob- served to be strictly proportional to each other. Again, Parrot of St. Peters- burgh (An. de Ch. et de Ph. xlvi. 361,) not only confirms the statements of De la Rive, but shows that in those instances where electric excitement has been witnessed under circumstances which appear to exclude chemical action, the phenomenon may be ascribed to friction of the metals. He gives as the result of numerous experiments made with strict care, that the contact of zinc and copper, if unattended by friction or chemical action, causes not the least developement of electricity. The opposite evidence adduced by Volta and others must, therefore, I apprehend, be rejected; and the only remain- ing facts in favour of Volta's opinion are derived from certain chemical agencies evinced by metals during contact, a subject which will be discussed in the section on galvanism. Changes of Form.—The changes of form caused in a substance by varia- tions of temperature, such as liquefaction and solidification, the formation and condensation of vapour, constitute another reputed source of electricity. On liquefying sulphur in a glass vessel, and removing the cake after cool- ing, the sulphur is found to be negative and the glass positive; and on pour- ing water into a hot iron vessel or on a hot coal communicating with a delicate electrometer, the rapid evaporation of the water is attended with decisive indications of electrical excitement. To processes of this nature, continually taking place in the atmosphere, the electricity of the clouds is generally ascribed. But the opinion is questioned by Pouillet, who has shown that in most of the experiments adduced in its favour, chemical ac- tions ensue at the same time, and that the greatest part of the effect is due to such changes. If, for example, evaporation be accompanied by chemical decomposition, as when saline solutions are evaporated, the water being separated from the salt with which it was previously united, or if the vessel consist of iron or other easily oxidable material, which is more or less chemically attacked by the evaporating water, then the development of electricity is very decisive; but he contends that pure water, evaporated in a clean platinum vessel, gives rise to no electrical excitement whatever. From such experiments Pouillet concludes that the electricity, hitherto re- ferred to changes of form, is entirely owing to the chemical action by which they are generally attended; and these phenomena, of which evaporation from the ocean, rivers, and the surface of the earth, affords an instance, pure water being thereby separated from its saline impregnation, as also the chemical changes attendant on the growth and nutrition of plants, he re- gards as a fertile source of atmospheric electricity. (An. de Ch. et de Ph. xxxv. 401, and xxxvi. 5. In these views there is much truth. I have re- peatedly noticed free electric excitement on pouring a solution of chloride of sodium or sulphate of soda into a heated platinum crucible, and also when pure water was dropped on red-hot iron or a glowing cinder; but I have as constantly failed of procuring any indication when pure water was evaporating on platinum. Mr. Harris, however, informs me that with an apparatus of unusual delicacy, he finds evaporation of pure water from pla- tinum to be attended with distinct development of electricity. Proximity to an Electrified Body.—It is a direct consequence of the at- tractive and repulsive powers ascribed to the electric fluids, that an unelectri- fied conductor must be excited by the vicinity of an electrified body, Let ■ 7* 78 ELECTRICITY. ab, fig. 1, be an unexcited conductor, sup- Fig-1. ported on an insulating glass rod be; and ,< a ^ let c, containing free positive electricity, i . I r--------j ~\32 and similarly insulated, be placed near it on ■ ~t~ .1 v.-------fa------■'-+- the side a. The free positive electricity on c will both repel the positive fluid of ab, and attract its negative fluid, and the result of these concurring forces is instantly to de- \ indicated by the arrows in the figure. A current ^* of negative electricity, agreeably to the theory of two electric fluids, ought to traverse the apparatus in a direction precisely reversed; but for the sake of simplicity I shall hereafter indicate the course of the positive current only. It matters not, so far as voltaic action is concerned, at what part the plates of fig. 1 touch each other. A current takes place, whether contact between the plates is made below where covered with liquid, above where uncovered, or along the whole length of the plates, provided both plates are immersed in the same vessel of dilute acid. Immersion of one plate only in the acid solution, however contact between the plates may be made, does not excite voltaic action; nor does it suffice to have one plate in one vessel, and the other plate in another vessel. A plate of zinc soldered to one of cop- per, and plunged into dilute acid, gives a current passing from the zinc through the fluid rouncj^o the copper: but if the soldered plates are cement- ed into a box with a wooden bottom and metallic sides, Fig. 2. so as to form two separate cells, as shown in a vertical ;#>—*■ section by figure 2, then the introduction of dilute acid I to the cells will not excite a current, unless the fluid | of the cells be made to communicate by means of moistened fibres of twine, cotton, or some porous mat- ter, or, as in the figure, by wires, a b, soldered to the metallic sides which contain the dilute acid, or dipping into the acid itself. Then the positive current circu- latcs in the direction shown by the arrows, GALVANISM. 89 Instead of a pair of plates being soldered to- Fig. 3. gether, they may be connected by a wire, and plunged into separate cells, a e, b e, figure 3, in which d e acts as a partition ; provided the positive current issuing from the zinc plate z, is conveyed by a wire, / g A i, or some conducting medium, into the cell, b e, in which the copper-plate, c, is immersed. A simple voltaic circle may be formed of one metal and two liquids, provided the liquids are such that a stronger chemical action is induced on one side Fig. 4. than on the other. Thus, on cementing a plate of zinc, z, into a box, figure 4, and putting a solution of salt into the , j cell, b b', and dilute nitric acid into the cell, a a', a positive ""1 current will be excited in the direction of the arrows, pro- vided the circuit be completed by a wire, a b, attached to the metallic sides of the box, or dipped into the liquid of the cells. Nay, the same acid solution may occupy both ' cells, provided some condition be introduced which shall cause one side of the zinc to be more rapidly dissolved than the other; as by the plate being rough on one side and polished on the other, or by the acid being hot in one cell and cold in the other. In this case, however, the re- sult is the same as though two different liquids were used. An interesting kind of simple voltaic circle is afforded by commercial zinc. This metal, as sold in the shops, contains traces of tin and lead, with rather more than one per cent of iron, which is mechanically diffused through its substance: on immersion in dilute sulphuric acid, these small particles of iron and the adjacent zinc form numerous voltaic circles, transmitting their currents through the acid which moistens them, and disengaging a large quantity of hydrogen gas. Pure distilled zinc is very slowly acted on by dilute sulphuric acid of sp. gr. ranging from 1.068 to 1.215; but if fused with about 2 per cent, or rather less, of iron filings, it is as readily dissolved as commercial zinc. In like manner, pure iron or steel is less readily acted on by dilute sulphuric acid than the same substances after fusion with small quantities of platinum or silver. Mr. Sturgeon has remarked that commercial zinc, with its surface amalgamated, which may be done by dip- ping a zinc plate into nitric acid diluted with two or three parts of water, and then rubbing it with mercury, resists the action of dilute acid fully as well as the purest zinc. This fact, of which Faraday in his late researches has made excellent use, appears due to the mercury bringing the surface of the zinc to a state of perfect uniformity, preventing those differences be- tween one spot and another, which are essential to the production of minute currents; one part has the same tendency to combine with electricity as an- other, and cannot act as a discharger to it (Faraday). While the current formed by the contact of two metals gives increased effect to the affinity of one of them for some element of the solution, the ability of the other metal to undergo the same change is proportionally diminished. Thus, when plates of zinc and copper touch each other in dilute acid, the zinc oxidizes more, and the copper less, rapidly than without contact. This principle was beautifully exemplified by the attempt of Davy to preserve the copper sheathing of ships. A sheet of copper immersed in sea-water, or a solution of chloride of sodium, in an open vessel, undergoes rapid cor- rosion; and a green powder commonly termed submuriate of copper, but which is really an oxy-chloride, is formed: atmospheric oxygen dissolved in sea-water unites both with copper and sodium, the latter yields its chlorine to another portion of copper, and the oxide and chloride of copper unite. But if the copper be in contact with zinc, or some metal more electro-posi- tive than itself, the zinc undergoes the same change as the copper did, and 90 GAL.VAN1SM. the latter is preserved. Davy found that the quantity of zinc required thus to form an efficient voltaic circle with copper was very small. A piece of zinc as large as a pea, or the head of a small round nail, was found fully adequate to preserve 40 or 50 square inches of copper; and this wherever it was placed ; whether at the top, bottom, or middle of the sheet of copper, or under whatever form it was used. And when the connexion between different pieces of copper was completed by wires, or thin filaments of the 40th or 50th of an inch in diameter, the effect was the same; every side, every surface, every particle of the copper remained bright, whilst the iron or the zinc was slowly corroded. Sheets of copper defended by 1-40 to l-lOOOth part of their surface of zinc, malleable and cast iron, were exposed during many weeks to the flow of the tide in Portsmouth harbour, and their weight ascertained before and after the experiment. When the metallic protector was from l-40th to l-150th, there was no corrosion nor decay of the copper; with smaller quantities, such as l-200th to 1.460th, the copper underwent a Toss of weight which was greater in proportion as the protector was smaller; and as a proof of the universality of the principle, it was found that even 1-1000th part of cast-iron saved a certain proportion of the copper (Phil. Trans. 1824). Unhappily, for the application of this principle in practice, it is found that unless a certain degree of corrosion takes place in the copper, its surface becomes foul from the adhesion of sea-weeds and shell-fish. The oxy-chlo- ride of copper, formed when the sheathing is unprotected, is probably inju- rious to these plants and animals, and thus preserves the copper free from foreign bodies. Simple voltaic circles may be formed of very various materials; but the combinations usually employed consist either of two perfect and one imper- fect conductor of electricity, or of one perfect and two imperfect conductors. The substances included under the title of perfect conductors are metals and charcoal, and the imperfect conductors are water and aqueous solutions. It is essential to the operation of the first kind of circle, that the imperfect conductor act chemically on one of the metals;,and in case of its attacking both, the action must be greater on one metal than on the other. It is also found generally, if not universally, that the metal most attacked is positive with respect to the other, or bears to it the same relation as zinc to copper in figures 1, 2, and 3. Davy, in his Bakerian lecture for 1826 (Phil. Trans.), gave the following list of the first kind of arrangements, the imperfect con- ductor being either the common acids, alkaline solutions, or solutions of metallic sulphurets, such as sulphuret of potassium. The metal first men- tioned is positive to those standing after it in the series. With common acids.—Potassium and its amalgams, barium and its amalgams, amalgam of zinc, cadmium, tin, iron, bismuth, antimony, lead, copper, silver, palladium, tellurium, gold, charcoal, platinum, iridium, rho- dium. With alkaline solutions.—The alkaligenous metals and their amalgams, zinc, tin, lead, copper, iron, silver, palladium, gold, and platinum. With solutions of metallic sulphurets.—Zinc, tin, copper, iron, bismuth, silver, platinum, palladium, gold, charcoal. Mr. Faraday has shown that the presence of water is not essential. A battery may be composed of other liquid compounds, such as a fused metallic chl-ride, iodide, or fluoride, provided it is decomposable by galvan- ism, and acts chemically on one metal of the circle more powerfully than on the other. The following table of voltaic circles of the second kind is from Davy's Elements of Chemical Philosophy :*— GALVANISM. 91 Solution of sulphuret of potassium ■----------potassa ----------soda Copper Silver Lead Tin Zinc Other metals Charcoal Nitric acid Sulphuric acid Hydrochloric acid Any solutions con- taining acid. The most energetic of these combinations is that in which the metal is chemically attacked on one side by sulphuret of potassium, and on the other by an acid. The experiment may be made by pouring dilute nitric acid into a cup of copper or silver, which stands in another vessel containing sulphuret of potassium. The following arrangements may also be employ- ed :—Let two pieces of thick flannel be moistened, one with dilute acid and the other with the sulphuret, and then placed on opposite sides of a plate of copper, completing the circuit by touching each piece of flannel with a con- ducting wire: or, take two discs of copper, each with its appropriate wire, immerse one disc into a glass filled with dilute acid, and the other into a separate glass with alkaline solution, and connect the two vessels by a few threads of amianthus or cotton moistened with a solution of salt. A simi- lar combination may be disposed in this order: let one disc of copper be placed on a piece of glass or dry wood; on its upper surface lay in succes- sion three pieces of flannel, the first moistened with dilute acid, the second with solution of salt, and the third with sulphuret of potassium, and then cover the last with the other disc of copper. Metallic bodies are not essential to the production of galvanic phenome- na. Combinations have been made with layers of charcoal and plumbago, of slices of muscle and brain, and beet-root and wood; but the force of these circles, though accumulated by the union of numerous pairs, is extremely feeble, and they are very rarely employed in practice. Of the simple voltaic circles above described, the only one used for ordi- nary purposes is that composed of a pair of zinc and copper plates excited by an acid solution arranged as in figure 1. The form and size of the ap- paratus are exceedingly various. Instead of actually immersing the plates in the solution, a piece of moistened cloth may be placed between them. Sometimes the copper plate is made into a cup for con- taining the liquid, and the zinc is fixed between its two sides, as shown by the accompanying transverse verti- cal section, figure 5; care being taken to avoid actual contact between the plates, by interposing pieces of wood, cork, or other imperfect conductor of electricity. Another contrivance, which is much more convenient, because the zinc may be removed at will and have its surface cleaned, is that represented by the annexed wood-cut (Fig. 6.) C is a cup made with two cylinders of sheet copper, of unequal size, placed one within the other, and soldered together at bottom, so as to leave an intermediate space a a a, for containing the zinc cylinder z and the acid solution. The small copper cups b b are useful appendages; for by filling them with mercury, and inserting the ends of a wire, the vol- taic circuit may be closed or broken with ease and expedition. This appa- ratus is very serviceable in experiments on electro-magnetism. Another kind of circle may be formed by coiling a sheet of zinc and cop- per round each other, so that each surface of the zinc may be opposed to one of 92 GALVAMSM. copper, and separated from it by a small interval. The London Institution possesses a very large apparatus of this sort, made under the direction of Mr. Pepys, each plate of which is 60 feet long and two wide. The plates are prevented from coming into actual contact by interposed ropes of horsehair; and the coil, when used, is lifted by ropes and pulleys, and let down into a tub containing dilute acid. The contrivance of opposing one large connect- ed surface of zinc to a similar surface of copper, originated with Dr. Hare of Philadelphia, who, from its surprising power of igniting metals, gave it the name of Calorimotor. Compound Voltaic Circles.—This expression is applied to voltaic arrange- ments which consist of a series of simple circles. The first combinations of the kind were described by Volta, and are now well known under the names of voltaic pile and crown of cups. The voltaic pile is made by placing pairs of zinc and copper, or zinc and silver plates, one above the other, as shown in figure 7, each pair separated from those adjoining by pieces of cloth, rather smaller than the plates, and moistened with a saturated solution of salt. The relative position of the metals in each pair must be the same in the whole series; that is, if the zinc be placed below the copper in the first pair, the same order should be observed in all the others. Without such pre- caution the apparatus would give rise to opposite currents, which would neutralize each other more or less according to their relative forces. The pile, which may consist of any con- venient number of combinations, should be contained in a frame formed of glass pillars fixed into a piece of thick dry wood, by which it is both supported and insulated. Any num- ber of these piles may be made to act in concert by establish- ing metallic communication between the positive extremity of each pile and the negative extremity of the pile immediately following. The voltaic pile is now rarely employed, because we possess other modes of forming galvanic combinations which are far more powerful and conve- nient. The galvanic battery proposed by Mr. Cruickshank, consists of a trough of baked wood, about 30 inches long, in which are placed at equal distances 50 pairs of zinc and copper plates previously soldered together, and so arranged that the same metal shall always be on the same side. Each pair is fixed in a groove cut in the sides and bot- Fig. 8. torn of the box, the points of junction being made water-tight by cement. The apparatus thus con- structed is always ready for use, and is brought into action by filling the cells left between the pairs of plates with some convenient solution, which serves the same purpose as the moistened cloth in the pile of Volta. By means of the ac- companying wood-cut the mode in which the plates are arranged will easily be understood. Other modes of combination arc now in use, which facilitate the employ- ment of the voltaic apparatus and increase its energy. Most of these may be regarded as modifications of the crown of cups. In this ap- paratus the exciting solution is contained in separate cups or glasses, disposed circularly or in a line: each glass contains a pair of plates; and each zinc plate is attached to the copper of the next pair by a metallic wire, as represented in figure 9. Instead of glasses, it is more convenient in practice to employ a trough of baked wood or glazed earthenware, divided into separate cells by partitions of the same material; and in order that the plates may be im- mersed into and taken out of the liquid conveniently and at the same moment, they arc all attached to a bar of dry wood, the necessary connexion between Fig. e. GALVANISM. 93 the zinc of one cell and the copper of the adjoining Fig. 10. one being accomplished, as shown in figure 10, by a slip or wire of copper. A material improvement in the foregoing appa- ratus was suggested by Dr. Wollaston (Mr. Chil- dren's Essay in Phil. Trans, for 1815), who recom- mended that each cell should contain one zinc and two copper plates, so that both surfaces of the" for- mer metal might be opposed to one of the latter. The two copper plates communicate with each other, and the zinc between them with the copper of the adjoining cell. An increase of one-half the power is said to be obtained by this method. A variation of this contrivance, which appears to me advantageous, has been suggested by Mr. Hart of Glasgow, who proposes to have the double copper plates of the preceding battery made with sides and bottoms, so that, as in figure 5, they may contain the exciting liquid. The plates are attached, as in figure 10, to a bar of wood, and supported above the ground by vertical columns of the same material, by which they are insulated. The cells are filled by dipping the whole battery into a trough of the same form, full of the exciting liquid. (Brewster's Journal, iv. 19.) The size and number of the plates may be varied at pleasure. The largest battery ever made is that of Mr. Children, described in the essay above re- ferred to, the plates of which were six feet long, and two feet eight inches broad. The common and most convenient size for the plates is fouror six inches square; and when great power is required, a number of different bat- teries are united by establishing metallic communication between the posi- tive extremity or pole of one battery and the negative pole of the adjoining one. A very effective battery was described by Dr. Hare under the name of the Deflagrator, which consisted of 80 zinc plates, 9 inches by 6 in size, and 80 copper plates, 14 inches by 6, coiled together, and so connected that the whole could be immersed into the exciting liquid, or removed from it, at the same instant (An. of Phil. xvii. 329). The great battery of the Royal Institution, with which Davy made his celebrated discovery of the compound nature of the alkalies, was composed of 2000 pairs of plates, each plate having 32 square inches of surface. It is now recognized, however, that such large compound batteries are by no means necessary. Increasing the number of plates beyond a very moderate limit gives, for most purposes, no proportion- ate increase of power ; so that a battery of 50 or 100 pair of plates, thrown into vigorous action, will be just as effective as one of far greater extent. The electrical condition of compound voltaic arrangements is similar to that of the simple circle. In the broken circuit no electric current can be traced; but in the closed circuit, that is, when the wires from the opposite ends of the battery are in contact, the galvanometer indicates a positive electric current through the battery itself and along the wires, as shown by the arrows in figures 8 and 9. The direction of the current appears at first view to be different from that of the simple circle; since in the latter the positive elec- tric current flows from the zinc through the liquid to the copper, while in the compound circle its direction is from the extreme copper through the battery to the extreme zinc plate. This apparent difference arises from the compound circle being usually terminated by two superfluous plates. The extreme copper and extreme zinc plate of figure 8 are not in contact with the exciting fluid, and therefore contribute nothing to the galvanic action : re- moving these superfluous plates, which are solely conductors, there will re- main four simple circles, namely, the 3 pair of soldered plates marked 2, 3, 4, which act as in figure 2, and the then extreme plates, 1,1, which are related to each other as the plates in fig. 1. When thus arranged, the direction of the current will be seen to correspond with that of the simple circle. During the action of a simple circle, as of zinc and copper, excited by di- 94 GALVANISM. lute sulphuric acid, all of the hydrogen developed in the voltaic process is evolved at the surface of the copper. This fact is not apparent when com- mon zinc plates are used, owing to the numerous currents which form on the surface of the zinc (page 89); but when a plate of amalgamated zinc and another of platinum are introduced into dilute sulphuric acid of sp. gr. 1.068 or a little higher, no gas whatever appears until contact between the plates is made, and then hydrogen gas rises solely from the platinum, while zinc is tranquilly dissolved. On weighing the amalgamated plate before and after the action has continued for half an hour or an hour, and collect- ing the hydrogen gas evolved during that interval, the weight of the hydro- gen set free and of zinc dissolved will be as 1 to 32.3, being the ratio of their chemical equivalents. Mr. Faraday, who has lately proved this, has also shown that in a compound voltaic circle, say of 10 amalgamated zinc plates and 10 of platinum, each of the former during a given period of action loses exactly the same weight, and from each of the latter an equivalent quantity of hydrogen gas is evolved. This separation of one ingredient of the excit- ing solution at one plate, while the element previously combined with it unites with the other plate, seems essential to voltaic action. It is in some way connected with the passage of the current across the exciting liquid. Oxygen in a free state may, by oxidizing zinc, cause electric excitement; but the voltaic current is not established, unless the oxygen formed part of a previous liquid compound in contact or communication with both the plates. Among the different kinds of voltaic apparatus is usually placed the elec- tric column of De Luc, which is formed of successive pairs of silver and zinc, or silver and Dutch-metal leaf, separated by pieces of paper, arranged as in a voltaic pile. It is remarkable for its power of exhibiting attractions and repulsions like common electricity, but cannot produce chemical decom- position or any of the effects most characteristic of a voltaic current, and is rather an electrical than a voltaic instrument. It is quoted as a proof of elec- tric development by contact, since it will continue in action for years with- out being cleaned or taken to pieces. True it is that the more oxidable metal of the column is slowly corroded, and that no electricity is excited when the paper is quite or nearly free from hygrometric moisture, the pre- sence of which is necessary to the oxidation of the zinc and copper; but at the same time the quantity of electricity excited seems so disproportioned to the corrosion, that the one can scarcely be assigned as the cause of the other. THEORIES OF GALVANISM. Of the theories proposed to account for the development of electricity in voltaic combinations, three in particular have attracted the notice of philoso- phers. The first originated with Volta, who conceived that electricity is set in motion, and the supply kept up, solely by contact or communication be- tween the metals (page 87). He regarded the interposed solutions merely as conductors, by means of which the electricity developed by each pair of plates is conveyed from one part of the apparatus to the other. Thus, in the pile or ordinary battery, represented by the following series, 3 2 1 + zinc copper fluid zinc copper fluid zinc copper — Volta considered that contact between the metals occasions the zinc in each pair to be positive, and the corresponding copper plate to be negative; that the positive zinc in each pair except the last, being separated by an interven- ing stratum of liquid from the negative copper of the following pair, yields to it its excess of electricity; and that in this way each zinc plate communi- GALVANISM. 95 cates, not only the electricity developed by its own contact with copper, but also that which it had received from the pair of plates immediately before it. Thus, in the three pairs of plates contained in brackets, the second pair re- ceives electricity from the first only, while the third pair draws a supply from the first and second. Hence electricity is most freely accumulated at one end of the battej-y, and is proportionally deficient at the opposite extre- mity. The intensity is, therefore, greatest in the extreme pairs, gradually diminishes in approaching the centre, and the central pair itself is neither positively nor negatively excited. In batteries constructed on the principle of the crown of cups (fig. 9), the electro-motion, as Volta called it, is ascribed to metallic communication between the zinc of one glass and the copper of the adjoining one. The second is the chemical theory, proposed by Wollaston. Volta attached little importance to the chemical changes which never fail to occur in every voltaic circle, whether simple or compound, considering them as casual or unessential phenomena, and therefore neglected them in the construction of his theory. The constancy of their occurrence, however, soon attracted no- tice. In the earlier discussions on the cause of spasmodic movements in the frog (page 87), Fabroni contended, in opposition to Volta, that the effect was not owing to electricity at all, but to the stimulus of the metallic oxide form- ed, or of the heat evolved during its production. More extended researches soon proved the fallacy of this doctrine; but Fabroni made a most ingenious use of the facts within his knowledge, and paved the way to the chemical theory of Wollaston. The late Dr. Wollaston, fully admitting electricity as the voltaic agent, assigned chemical action as the cause by which it is excited. The repeti- tion and extension of Volta's experiments by the English chemists, speedily detected the error he had committed in overlooking the chemical phenomena which occur within the pile. It was observed that no sensible effects are pro- duced by a combination of conductors which do not act chemically on each other; that the action of the pile is always accompanied by the oxidation of the zinc; and that the energy of the pile in general is proportional to the activity with which its plates are corroded. Observations of this nature in- duced Wollaston to conclude that the process begins with the oxidation of the zinc,—that oxidation, or in other terms, chemical action was the primary cause of the development of electricity,—that the fluid of the circle served both to oxidize the zinc and to conduct the electricity which was excited,— and that contact between the plates served only to conduct electricity, and thereby complete the circuit. The third theory of the pile was proposed by Davy, and is intermediate between the two former. He adduced many experiments in support of the fact originally stated by Volta, that the electric equilibrium is disturbed by the contact of different substances, without any chemical action taking place between them. He acknowledged, however, with Wollaston, that the che- mical changes contribute to the general result; and he maintained that, though not the primary movers of the electric current, they are essential to the continued and energetic action of every voltaic circle. The electric ex- citement was begun, he thought, by metallic contact, and maintained by chemical action. The progress of inquiry since these theories first came into notice, has gradually given more and more support to the views of Wollaston, and has at last, I apprehend, established it to the entire exclusion of the theory of Volta. The very fundamental position, that electricity is excitable as a pri- mary result by the contact of different substances, is warmly contested, and, as some think with strong reason, has been disproved (page 76); but admit- ting, for the sake of argument, that a small effect, which is all that can now be contended for, may thus be produced, it is altogether insignificant when contrasted with the astonishing phenomena exhibited by a voltaic circle. The experiments of A. De la Rive, in reference to this question, appear irrecon- 96 GALVANISM. cilable with the theory of Volta (An. de Ch. et de Ph. xxxviii. 225). This ingenious philosopher contends that the direction of a voltaic current is not determined by metallic contact, nor even by the nature of the metals rela- tively to each other, but by their chemical relation to the exciting liquid. As the result of his inquiries he states, that of two metals composing a vol- taic circle, that one which is most energetically attacked will be positive with respect to the other. Thus, when tin and copper are placed in acid so- lutions, the former, which is most rapidly corroded, gives a positive current through the liquid to the copper, as the zinc does in the circle in fig. 1; but, if they are put into a solution of ammonia, which acts most on the copper, the direction of the current will be reversed. Copper is positive in relation to lead in strong nitric acid, which oxidizes the former most freely; whereas in dilute nitric acid, by which the lead is most rapidly dissolved, the lead is positive. Even two plates of copper immersed in solutions of the same acid, but of different strength, will form a voltaic circle, the plate on which che- mical action is most free giving a current of positive electricity to the other: nay, it is possible to construct a compound circle solely with zinc plates and one acid solution (page 89), provided the same side of each plate be more rapidly oxidized than the other. Conclusive evidence against the theory of Volta has very recently been obtained by Faraday. And here, to prevent repetition and frequent reference, I may at once state that the Philosophical Transactions for 1833 and 1834 contain a succession of essays on voltaic electricity from the pen of Mr. Fa- raday, in which numerous errors have been exposed and new views of deep interest established. It is much to affirm, but not more, I conceive, than is strictly true, that these researches, whether viewed in reference to their in- trinsic value to science, or to the energy and talent displayed by the author, are equal to any of his most successful contributions. In respect to the present question, Faraday proves metallic contact not to be essential to voltaic action, by procuring that action quite characteristically without con- Fig. 11. tact. A plate of zinc, a, fig. 11, about 8 inches long by £ an inch wide, was cleaned and bent at a right angle: and a plate of platinum, of the same width and 3 inches long, was soldered to a platinum wire, b s x, the point of which, x, rested on a piece of bibulous paper lying upon the zinc, and moistened with a solution of iodide of potassium. On intro- ducing the plates into a vessel, c, filled with dilute sulphuric and nitric acid, a positive electric current instantly ensued in the direction of the arrow, as testified by the hydrogen evolved at the plate a, by the decomposed iodide of potas- sium, and by a galvanometer. Wc have thus a simple circle of the same construction and action as in figure 1, ex- cept in the absence of metallic contact. Another proof, aptly cited by Faraday, of electric excitement being inde- pendent of contact, is afforded by the spark which appears, when the wires of a pair of plates in vigorous action are brought into contact. The spark is occasioned by the passage of electricity across a thin stratum of air, and, therefore, its production proves that electro-motion really occurred while the wires were yet separated by a thin stratum of air, which permitted the elec- tric current to pass, and anterior to their actual contact. The arrangement of figure 11, however, though good for establishing a prin- ciple, is not adapted for ordinary practice. The moist paper at x is a much less perfect conductor than a metal, and thus obstructs the passage of the current; nay, it does more, for it tends to establish an opposite current. In fact, on remov- ing the dilute acid from c, and putting the zinc plate, a, in contact with the plate of platinum, an ordinary simple circle would be formed, in which a positive current would flow from the zinc at x through the solution to and along the wire x 8 b. This current, in Faraday's experiment, was so feeble compared with the one excited by the acid solution, that its influence was scarcely appre- GALVANISM. 97 :i? <_ \ ^ \ 1 '\ ciable; but if the opposed currents had been of the same force, no action would have ensued. To illustrate this still fur- Fig 12 ther, Faraday fixed a plate of platinum, p, figure 12, parallel and near to a plate of amalgamated zinc, z. On placing a drop of dilute sulphuric acid at y, and making metallic contact between ^ ^ the plate at z r, a positive electric current flowed in the direction of the ar- rows. If in the same plates, fig. 13, the acid be Fig. 13. introduced at x, and metallic contact made at p z, the current, passing as before from zinc through the liquid to the platinum, has a direction op- posed to that of figure 2, owing to the reversed position of the acid. If, then, in the same plates, figure 14, a drop of acid be introduced at y and at x, the conditions are ob- Fi". 14. viously fulfilled for producing two opposite cur- ° rents of positive electricity, each fluid acting as y— -*- , ^ a substitute for metallic contact in conducting y\ \---- —I K the current which the other tends to generate. If 7 Z X these opposing currents happen to be equal, they will annihilate the effects each separately would produce; and if unequal, the stronger current, as in figure 11, will annihilate the weaker, and, though with diminished power, impress its character on the circuit. These considerations, made in reference to a simple circle, lead at once to the theory of the compound circle. For if, in figure 14, a drop of dilute acid, which acts solely on the zinc, be introduced at y, and a different liquid at x, capable of corroding platinum and not zinc, then the chemical action at y will cause a positive current from zinc to platinum, and that at a; a similar cur- rent from platinum to zinc. The two currents tend to circulate in the same direction, and each promotes the progress of the other. The same state of things exists in the batteries represented by figures 8 and 9. Chemical ac- tion taking place on the zinc of each pair of plates, there is a tendency to establish an equal number of positive currents all in the same direction; and the simultaneous effort of all urges on the current with a force which it could not derive from a single pair of plates. It is now, also, apparent that all the zinc plates should have their surfaces towards one side, and those of copper towards the other: one reversed pair tends to establish a counter-current, which enfeebles the influence of the rest. On the same principle, the exciting liquid of a voltaic circle should act exclusively on one of the plates: if the copper is oxidized as fast as the zinc, opposite currents will be excited, which more or less completely counteract each other. For this reason, platinum and zinc act better than copper and zinc, especially when nitric acid is em- ployed. LAWS OF THE ACTION OF VOLTAIC CIRCLES. Electricians distinguish between quantity and intensity in galvanism as in ordinary electricity. Quantity, in reference to a voltaic circle, signifies the quantity of electric fluid set in motion; and by tension or intensity is meant the energy or effort with which a current is impelled. In the broken circuit there is a strain to establish an electric current as a condition necessary for oxidation: there exists between the exciting fluid and the zinc a desire, as it were, for chemical action, which cannot be gratified until, by closing the circuit, a door or exit is opened for the escape and circulation of electricity. This strain or tension is great, according as the affinity between the exciting fluid and zinc is great, and the current derives a character from this tension. Nitric acid, from its great oxidizing power, causes a greater tendency to chemical action, and, therefore, to the formation of electric currents, than sulphuric acid, and this acid than a solution of salt. Currents of high tension 9 98 GALVANISM. are urged forward with greater impetuosity than feeble ones, more readily overcoming obstacles to their passage, whether derived from the small size of conducting wires, or the imperfect conduction of the liquids of the battery. The current of a single pair of plates, though variable in intensity accord- ing as the nature and strength of the exciting liquid varies, never attains a high tension. For if the plates are far apart, the current is unable to pass at all, or at most but feebly, owing to the quantity of interposed liquid; and if close, the obstacle to be overcome in passing through a thin stratum of fluid is too small to admit of the tension being considerable. The condition which causes high intensity is an extended liquid conductor to be traversed, along the whole line of which, as in a compound circle, are ranged successive pairs of plates, each acting chemically on the exciting liquid, and urging on a current in the same direction. Under these circumstances the tension be- comes very high, and the free circulation of electricity thereby occasioned enables the quantity set in motion to be also increased; but Faraday has given sufficient reasons for believing that the quantity of electricity trans- mitted along the wires of a closed compound circle is exactly equal in amount to that which passes through one of its cells. A compound circle does not act by directly increasing the quantity of electricity, but by giving impetus or tension to that which is excited. The energy of a voltaic circle is usually estimated either by the deflection which it causes on a magnetic needle, or by its power of chemical decomposi- tion. Using the former, Dr. Ritchie has obtained some interesting numerical results, of which the principal are as follows (Phil. Trans. 1832-33.) 1. The power of a single pair of plates in deflecting the magnetic needle is directly proportional to the surface of the plates which is covered with dilute acid; that is, a given deflection, produced by covering one square inch of each plate with liquid, will be doubled when two square inches are immersed. 2. A plate of zinc introduced into a rectangular cup of copper, as in figure 5, page 91, deflects the needle twice as much as when one side of the zinc and the adjacent surface of copper are protected by a coating of cement from the action of the acid solution.—The varying conditions of the experiments were calculated to affect the quantity of electricity set in motion, without changing the intensity ; and, therefore, the results, proving the deflection to depend on quantity and not on tension, entirely conform to general experience. 3. The deflection produced by a pair of plates, in an acid solution of uni- form strength, varies inversely as the square root of the distance between them,—a law previously established by Professor Cumming. Thus, if a plate of zinc be placed successively at one, four, and nine inches from a plate of copper, the deflecting powers will be in the ratio of 3, 2, and 1; that is, only twice as great at one inch as at four, and only three times as great at one inch as at nine inches. 4. The same law, as previously deduced by Professors Cumming and Bar- low, applies to variations in the length of the wire by which the zinc and copper plate are connected. If, all other circumstances being uniform, the conducting wire varies from four feet to one foot in length, the deflecting power will vary in the ratio of 1 to 2. Dr. Ritchie informs me that with short metallic wires, the deflection varies inversely as the square root of the length of the whole circuit, that is, of the solid and liquid conductors taken together. The wire in these experiments must be single, and not coiled as in the multiplier of Schweigger: large wires should be used capable of freely conveying all the electricity which is developed. Ritchie has also shown, agreeably to general observation, that the deflect- ing power of a compound circle is not increased by increasing the number of its plates. A single pair of plates with a good conducting liquid within the cell, and supplied with large conducting wires capable of carrying off the whole quantity of electricity set in motion, deflects the needle nearly or quite as much as a battery composed of several pairs of plates of the same size. This is another proof that the main influence of a number of plates is to in- GALVANISM. 99 crease the intensity and not the quantity of electricity; for the prevailing opinion that the magnetic needle takes no cognizance of intensity is fully borne out by the experiments of Faraday. The magnetic needle may be viewed as an exact measure of the quantity of electricity in motion, without any reference to its tension. Chemical decomposition depends on quantity and intensity together, and affords a criterion of the increased tension of a compound circle due to an increase in the number of its plates. The quantity of hydrogen gas evolved by a compound circle in a given time does not vary in the simple ratio of the number of plates; that is, the volume of the gas is not doubled when the number of plates is doubled: the effect increases at a slower rate, owing to the additional obstacle to the current caused by the increased length of the circuit. Ritchie considers the ratio to be as the square root of the number of plates; so that when the number varies as 1 to 4, the gas evolved is as 1 to 2. In discussing the preceding theoretical questions, I have gone on the as- sumption of electricity, as explained in the last section, being an indepen- dent principle susceptible of rapid motion from one body to another; and I have here supposed that the condition of a voltaic conducting wire is similar to that of a wire leading from the ground to the prime conductor of an elec- trical machine, or which connects the inner and outer surface of a charged Leyden phial, except that the voltaic current moves slowly, owing to its lower tension and the interposed imperfect conductor. Some conceive that what is called an electric current is not an actual transfer of anything, but a process of induction among the molecules of a conductor passing progres- sively along it. Others, denying independent materiality to electricity, may ascribe it to a wave of vibrating matter, just as the phenomena of optics are explained by the undulatory theory. But whatever theory of the nature of electricity may be adopted, it seems necessary, after the experiments of Fa- raday on the identity of voltaic and common electricity, that the nature of an electric and voltaic current is essentially the same. EFFECTS OF GALVANISM. When a zinc and copper plate are immersed in dilute acid, and the wire attached to the former is connected with a gold-leaf electrometer of suffi- cient delicacy, the leaves diverge with negative electricity; and on testing the wire of the copper plate in a similar manner, divergence from positive electricity is obtained. The effect is so feeble with a single pair of plates, as to be scarcely appreciable; but with a battery of many pairs it is very distinct, though never powerful. This would of course be expected, since two oppositely electrified conductors, immersed in the same liquid, would necessarily neutralize each other, unless their intensity was too feeble to find a passage through the solution. The condition of a battery which gives the greatest divergency to an electrometer is that of numerous plates; small plates an inch square being just as effectual as large ones. The free elec- tricity on the wires is apparently an effect of the tension of the unbroken circuit, and is proportional to it, though the mode in which the effect arises is of difficult explanation. It is due, perhaps, to a disturbed electric equili- brium in the zinc plate, the chemical relation of which to the acid renders that metal positive at the expense of the attached wire; while the copper plate, induced by the contiguous zinc, becomes negative at the expense of its wire, which is thus rendered positive. A Leyden jar may be charged from either wire of an unbroken circuit, provided the battery be in a state to supply a large quantity of electricity of high tension, as when formed of numerous four-inch plates excited by dilute acid. When the wires from such a battery are brought near each other a spark is seen to pass between them; and on establishing the communication by means of the hands, previously moistened, a distinct shock is perceived, 100 GALVANISM. On sending the current through fine metallic wires or slender pieces of plumbago or compact charcoal, these conductors become intensely heated, the wires even of the most refractory metals are fused, and a vivid white light appears at the points of the charcoal, equal if not superior in intensity to that emitted during the burning of phosphorus in oxygen gas; and as this phe- nomenon takes place in an atmosphere void of oxygen, or even under the surface of water, it manifestly cannot be ascribed to combustion. If the communication be established by metallic leaves, the metals burn with vivid scintillations. Gold-leaf burns with a white light tinged with blue, and yields a dark brown oxide; and the light emitted by silver is exceedingly- brilliant, and of an emerald-green colour. Copper emits a blueish-white light attended with red sparks, lead a beautiful purple light, and zinc a bril- liant white light inclining to blue, and fringed with red. (Singer.) In burn- ing metallic leaf, fusing wire, and igniting charcoal, a large quantity of elec- tricity is the only requisite. The phenomena seem to arise from the cur- rent passing along these substances with difficulty; a circumstance which, as they are perfect conductors, can only happen when the quantity to be transmitted is out of proportion to the extent of surface over which it has to pass. It is, therefore, an object to excite as large a quantity of electricity in a given time as possible, and for this purpose a few large plates answer bet- ter than a great many small ones. A strong acid solution should also be used; since energetic action, though of short continuance, is more import- ant than a moderate one of greater permanance. A mixture of ten or twelve parts of water to one of nitric acid is applicable; or, for the sake of economy, a mixture of one part of nitric to two parts of sulphuric acid may be sub- stituted for pure nitric acid. The large battery of Mr. Children, though ca- pable of fusing several feet of platinum wire, had an electric tension so fee- ble, that it did not affect the gold leaves of the electrometer, gave a shock scarcely perceptible even when the hands were moist, communicated no charge to a Leyden jar, and could not produce chemical decomposition.* Among the effects of galvanism, produced like most of the former by a closed circuit only, and caused by electric currents, none are more surpris- ing or so important as its chemical and magnetic effects. These will be studied successively under separate heads. For deflecting a magnetic nee- dle, the only required condition is quantity of electricity, the amount of which is exactly measured by the degree of deflection; but for chemical de- composition quantity and tension must be combined: for the former a single pair of large plates is best fitted; for the latter a compound circle is almost essential. Most of the effects of galvanism above described are so similar to those * Dr. Hare has broached a very ingenious theory to account for the heat excited by galvanic action. He does not consider it probable that the heat extricated by galvanic combinations is the effect of the current of electricity passing with difficulty along conductors, in consequence of the quantity to be transmitted being out of proportion to the extent of the surfaces over which it has to pass. On the contrary, he believes that caloric, like electri- city, is an original product of galvanic action. According to his views, the relative proportion of the two principles evolved depends upon the construc- tion of the apparatus; the caloric being in proportion to the extent of the generating surface, and the electricity to the number of the series. In the case of batteries, in which the size and number of the plates are very con- siderable, both electricity and caloric are presumed, by him to be generated in large quantities. When the number of the plates is very great, and their size insignificant, as in De Luc's column, electricity is the sole product; and conversely, where the size is very great and the number of the series small, caloric is abundantly produced, and the electrical effects arc nearly null—£. connect it with a closed circle; ^^^^ and a b are large platinum elec- '/i I ^s;^. trodes, placed close together, NrH^ ^^=5=;5= but prevented from contact by ^-Ifi--^ ^ interposed beads of glass. The K—I_i7tt tube when prepared for use, is J \ filled up to the bend with dilute / \ sulphuric acid of sp. gr. 1.336; i^^^ ^S^h and the gas evolved, escaping ^$^=====2^'^ from the open extremity c, is collected in a tube and measured. Theory of Electrochemical Decomposition.—The most celebrated attempt to explain the phenomena of galvanism was made by Davy in his essay on Some Chemical Agencies of Electricity (Phil. Trans. 1807), by means of an hypothesis which has received the appellation of the electro-chemical theory. The views of Davy, which in some form or other have been adopted by most persons who have speculated on this subject, are founded on the assumption, now rendered so much more plausible than in his day, that electrical and chemical attractions are owing to one and the same agent. He considered chemical substances to be endowed with natural electric energies ; meaning thereby, that a certain electric condition, either positive or negative, is natu- ral to the atoms or combining molecules of bodies; that chemical union is the result of electrical attraction taking place between oppositely excited atoms, just as masses of matter when oppositely excited are mutually attracted ; and that ordinary chemical decomposition arises from two combined atoms being drawn asunder by the electric energies of other atoms more potent than those by which they were united. Electro-chemical decomposition was at once explained by Davy on the same principles. He regarded the metallic ter- minations or poles of a voltaic circle (page 102) as two centres of electrical power, each acting repulsively to particles in the same electric state as itself, and by attraction on those which were oppositely excited. The necessary result was, that if the electric energy of the battery exceeded that by which the elements of any compound subject to its action were held together, de- composition followed, and each element was transferred bodily to the pole by which it was attracted, passing through solutions not containing the ori- ginal compound, and refusing to unite with substances with which under other circumstances it would have combined. Substances which appeared at the positive pole, such as oxygen, chlorine, and acids, were termed electro- negative substances; and those electro-positive bodies, which were separated at the negative pole. The views of Davy, both in his original essay and his subsequent expla- nations (Phil. Trans. 1826), were so generally and obscurely expressed, that chemists have never fully agreed, as to some points of the doctrine, about GALVANISM. 107 his real meaning. If he meant that a particle of free oxygen or free chlorine is in a negatively excited state; then his opinion is contrary to the fact, that neither of those gases affect an electrometer with negative or any kind of electricity, any more than hydrogen gas or potassium alone exhibit any evi- dence of positive excitement. If sulphur unites with oxygen because it has a positive electric energy, why should it unite with potassium, which con- fessedly is far more positive than itself? The only mode in which such facts as these seem reconcilable with the electro-chemical theory, is to sup- pose all bodies in their uncombined state to be electrically indifferent, but that they have a natural appetency to assume one state in preference to an- other. Electro-negative bodies are such as assume negative excitement under a certain approximation to others which at the same time become positively excited, chemical union being the consequence. On this supposition, it is intelligible that sulphur may be positive in relation to oxygen, and negative to potassium, just as black silk is positively electrified by friction with seal- ing-wax, and negatively by white silk. It is obvious, from the following ta- ble constructed by Berzelius, that this chemist takes the same view ot the electric energies of bodies as that just given. He has given it as approxi- mative only, and not as rigidly representing the exact electrical relations of the elements. Nitrogen and hydrogen scarcely occupy their true position in the series, the former being electro-negative in a lower degree than chlorine and fluorine, while hydrogen, I think, should be in a prominent station among the electro-positive elements. All the bodies enumerated in the first column are negative to those of the second. In the first column each sub- stance is negative to those below it; and in the second, each element is po- sitive with reference to those which occupy a lower place in the series. 1. Negative Electrics. Oxygen. Sulphur. Nitrogen. Chlorine. Iodine. Fluorine. Phosphorus. Selenium. Arsenic. Chromium. Molybdenum. Tungsten. Boron. Carbon. Antimony. Tellurium. Columbium. Titanium. Silicium. Osmium. Hydrogen. Positive Electrics. Potassium. Sodium. Lithium. Barium. Strontium. Calcium. Magnesium. Glucinium. Yttrium. Aluminium. Zirconium. Manganese. Zinc. Cadmium. Iron. Nickel. Cobalt. Cerium. Lead. Tin. Bismuth. Uranium. Copper. Silver. Mercury. Palladium. Platinum. Rhodium. Iridium. Gold* * The statements made in the text are, perhaps, not expressed with suf- 108 GALVANISM. It requires but little reflection on the facts described in this section, to perceive that they are inconsistent with the electro-chemical theory as un- derstood by Davy. It gives not the shadow of a reason for a voltaic battery which can decompose the protochloride or protiodide of a metal, being powerless with the perchloride and periodide of the same metal. The fact itself was not contemplated bv Davy, and his theory was designed to show why all such compounds should be decomposed. Moreover, there is no proof that the poles of a battery do exert attractive or repulsive forces. There is no need of a metallic conductor in contact with the decomposing body (page 104); nor do the elements reach the poles at all, unless they happen to be in contact with the substance under decomposition (page 104). When hydrogen reaches the negative electrode, it is freely disengaged as gas, the electrode evincing no tendency whatever to retain it: the combina- tion of the elements of a decomposed body with the matter of the electrodes, does not prove attraction, but may, and I presume does, arise from the sub- stances being presented in a state favourable for chemical union (page 103). But while Davy's theory fails, there is no other which can render a reason for all the phenomena. Faraday has done much by showing the fallacy of the former theory, and by stating the facts of the case as they are. He con- tends that, between the electrodes and acting in right lines, there is an axis of power which urges the electro-negative element of an electrolyte in the direction the positive current moves, and gives an opposite impulse to the electro-positive element. He adopts the opinion of Grotthuss, that the de- composing influence is not exerted on any single particle of the electrolyte, but that rows of particles lying between the electrodes are equally subject to its action. When a particle of oxygen is evolved at the positive electrode, the hydrogen with which it had been combined is not transferred at once to the opposite electrode, but unites with the oxygen of a contiguous particle of water, on the side towards which the positive current is moving ; the second particle of hydrogen decomposes a portion of water still nearer to the nega- tive electrode; and the same process of decomposition and reproduction of water continues until it reaches the water in immediate contact with the ne- gative electrode, the hydrogen of which is disengaged. This operation, de- scribed as commencing at one electrode, takes place simultaneously at both : a row of particles of oxygen suddenly lose their affinity for the hydrogen situated on the side next the negative electrode, in favour of those respec- tively adjacent to each on the other side; while the affinity of a similar row of particles of hydrogen is diminished for the oxygen on the side of the posi- tive electrode, and is increased for those on their opposite side. Hence, as ficient clearness for the comprehension of the student. The doctrine laid down by Dr. Turner is, that substances, considered singly, are neither posi- tive nor negative ; or in other words, that they are in a neuter state like the earth. Nevertheless, they are capable of exciting each other by being first brought in contact, and then separated. If two substances touch each other, and are then separated, one will become positive and the other negative ; but the result is not conclusive as to the electric energy of either, because the electric state of each may possibly be reversed by contact with some other substance. These positions are rigidly exact with respect to all the simple substances, except oxygen and potassium; for, as the former yields electri- city to all other substances, it must always be negative, and as the latter takes electricity from all other substances, it must be invariably positive. Thus it is plain that the electric energy of none of the simple bodies is abso- lute, except that of oxygen and potassium ; while the electric energy of the remaining simple bodies is relative, and is either positive or negative, ac- cording to circumstances. It is for these reasons that 1 have thought that the arrangement of bodies into negative and positive electrics, as Dr. Turner has done, after Berzelius, is objectionable, as leading the student into the error of supposing that each group was in its own nature either negative or "positive.—Ed. GALVANISM. 109 is the fact, for the elimination of the elements of an electrolyte at the elec- trode, it is essential that the electrolyte itself should occupy the space be- tween the electrodes, and be in contact with them. The theory, however, is at present incomplete : it affords no reason for the disturbed order of affini- ties in the elements of an electrolyte ; nor is it apparent how the chemical changes between the electrodes are so essential as they seem to be (page 104) to the passage of the currents. Magnetic Effects of Galvanism.—The power of lightning in destroying and reversing the poles of a magnet, and in communicating magnetic pro- perties to pieces of iron which did not previously possess them, was noticed at an early period of the science of electricity, and led to the supposition that similar effects may be produced by the common electrical and voltaic appa- ratus. Attempts were accordingly made to communicate the magnetic vir- tue by means of electricity and galvanism; but no results of importance were obtained till the winter of 1819, when Professor Oersted of Copenha- gen made his famous discovery, which forms the basis of a new branch of science. (Annals of Philosophy, xvi. 273.) The fact observed by Professor Oersted was, that the metallic wire of a closed voltaic circle, and the same is true of charcoal, saline fluids, and any conducting medium which forms part of a closed circle, causes a magnetic needle placed near it to deviate from its natural position, and assume a new one, the direction of which depends upon the relative position of the needle and the wire. On placing the wire above the magnet and parallel to it, the pole next the negative end of the battery always moves westward ; and when the wire is placed under the needle, the same pole goes towards the east. If the wire is on the same horizontal plane with the needle, no declination whatever takes place; but the magnet shows a disposition to move in a ver- tical direction, the pole next the negative side of the battery being depressed when the wire is to the west of it, and elevated when it is placed on the east side. Ampere has suggested a useful aid for recollecting the direction of these movements. Let the observer regard himself as the conductor, and suppose a positive electric current to pass from his head towards his feet, in a direction parallel to a magnet; then its north pole in front of him will move to his right side, and its south pole to his left. The plane in which the magnet moves is always parallel to the plane in which the observer sup- poses himself to be placed. If the plane of his chest is horizontal, the plane of the magnet's motion will be horizontal; but if he lie on either side of the horizontally suspended magnet, his face being towards it, the plane of his chest will be vertical and the magnet will tend to move in a vertical plane. The extent of the declination occasioned by a voltaic circle depends upon its power, and the distance of the connecting wire from the needle. If the apparatus be powerful, and the distance small, the declination will amount to an angler of 45°. But this deviation does not give an exact idea of the real effect which may be produced by galvanism ; for the motion of the magnetic needle is counteracted by the magnetism of the earth. When the influence of this power is destroyed by means of another magnet, the needle will place itself directly across the connecting wire; so that the real tendency of a magnet is to stand at right angles to an electric current. The communicating wire is also capable of attracting and repelling the poles of a magnet This is easily demonstrated by permitting a horizontally suspended magnet to assume the direction of north and south, and placing near it the conducting wire of a closed circle, held vertically and at right an- gles to the needle, the positive.current being supposed to flow from below upwards. When the wire is exactly intermediate between the magnetic poles, no effect is observed; on moving the wire nearly midway towards the north pole, that is, to the pole which points to the north, the needle will be attracted; and repulsion will ensue when the wire is moved close to the north pole itself. Similar effects occur on advancing the wire towards the south pole. Such are the phenomena if the positive current ascend on the west side'of the needle; but they are reversed when the wire is placed vertically 110 GALVANISM. on the east side. Attractions and repulsions likewise lake place in a dip- ping needle, when the current flows horizontally across it. The discovery of Oersted was no sooner announced, than the experiments were repeated and varied by philosophers in all parts of Europe, and, as was to be expected, new facts were speedily brought to light. Among the most successful of those who early distinguished themselves were Ampere, Biot, and Arago, of Paris, and Davy and Faraday in this country. A host of other able men have since added their contributions; and their joint labours have established an altogether new science, Electro-Dynamics, which has already become one of the most important branches of physical knowledge, and still offers a rich harvest of discovery to its cultivators. Those who wish to enter deeply into the study of this subject should consult the Recueil d' Observa- tions Electro-Dynamiques by Ampere, Professor Cumming's Manual of Elec- tro-Dynamics, Mr. Murphy's Treatise on Electricity, and the second edition of Mr. Barlow's Essay on Magnetic Attractions. A less mathematical, and, therefore, more generally intelligible treatise has been drawn up with great ability by Dr. Roget, and published as part of the Library of Useful Know- ledge; and a Popular Sketch of Electro-Magnetism has been given by Mr. Watkins of Charing-cross. To these works I refer as supplying that detail of the facts and theories of electro-dynamics, which, as belonging more to the province of physics than chemistry, is unsuited to the design of this vol- ume. My object is merely to give an outline of the discoveries in electro- dynamics, and to convey an idea of the nature and present state of the Science. The phenomena of electro-dynamics are solely produced by electricity in motion. Accumulated electricity giving rise to tension, which acts so es- sential a part in experiments with the electrical machine, has no influence whatever on a magnetic needle. The passage of electricity through solid or liquid conductors is essential; and it is remarkable that the more freely the current is transmitted, that is, the more perfect the conducting substance, the more energetic is its deflecting power. In fact, a magnetic needle is a Galvanoscope, by which means the existence and direction of an electric cur- rent may be detected. It was early employed with this intention by Ampere, who found that a voltaic apparatus itself acts on a magnetic needle placed upon or near it in the same manner as the wire whieh unites its two extre- mities ; but as the deflection took place only when the opposite ends of the battery were in communication, and ceased entirely when the circuit was broken, he inferred that electricity passes uninterruptedly through the battery itself when the circuit is closed, and not at all in the interrupted circuit. But a magnetic needle will not only indicate the existence and direction of an electric current: it may even serve, by the degree of deflection, as an exact measure of its force. When used for this purpose, under the name of Galvanometer, some peculiar arrangements are required in order to ensure the requisite delicacy and precision. Experiment proves that a magnet is equally affected by every point of a conductor along which an electric cur- rent is passing, so that a wire transmitting the same current will act with more or less energy, according as the number of its parts contiguous to the needle is made to vary. On this principle is constructed the Galvanometer or Multiplier of Schweigger. A copper wire is bent into a rectangular form consisting of several coils, and in the centre of the rectangle is placed a de- licately suspended needle, as shown in figure 16. Each coil adds its in- fluence to that of the others; and as the current, in its progress along the wire, passes repeatedly above and below the needle in opposite directions their joint action is the same. In order to prevent the electricity from pass^ ing laterally from one coil to another in con- tact with it, the wire should be covered with silk. The ends of the wire, a and b, are left free for the purpose of communication with the opposite ends of the voltaic circle. When a single needle is employed, as shown in the figure, its movements are influenced partly by the earth's magnetism, and partly by the electric current. The indications are much more delicate when GALVANISM. Ill the needle is rendered astatic, that is, when its directive property is destroyed by the proximity of another needle of equal magnetic intensity, fixed paral- lel to it, and in a reversed position, each needle having its north pole adjacent to the south pole of the other: in this state the needles, neutralizing each other, are unaffected by the magnetism of the earth, while they are still sub- ject to the influence of galvanism. If, as in the last figure, the lower needle lie within the rectangle, and the upper needle just above it, the electric cur- rent flowing between will act on both in the same manner. For researches of delicacy the needle should be suspended by a slender long thread of glass, and the deflecting force measured, not by the length of the arc traversed by the needle, but by the torsion required to keep the needle at a constant dis- tance from the wire, as in the torsion electrometer of Couloumb (page 81.) A very valuable instrument on this principle has been described by my col- league Dr. Ritchie (Royal Inst. Journal, N. S. i. 31.) The mutual influence of a magnetic pole and a conducting wire changes with the distance between them. Experiment shows that the action of a magnetic pole and a continuous conductor, every point of which exerts a se- parate energy on the pole, varies inversely as the distance. This result jus- tifies the opinion that the force of a magnetic pole on a single point of a conductor varies as the square of the distance, the same law which regulates the distribution of heat and light, as well as the effects due to electricity. From some of the experiments of Oersted above mentioned, it was at first believed that a force, one while attractive and at another repulsive, acted in straight lines between the magnet and conducting wire; but on examination all the phenomena are found referable to a force acting tangentially upon the poles of a magnet, and in a plane perpendicular to the direction of the current. Place, for instance, a blank card flat on the table, and fix a wire A C upright in its centre. If, then, a positive electric current pass up or down the wire, a magnetic pole resting on the card, will be inclined to move in the plane of the card, and, therefore, at right angles,to the current,and to describe a circle round C as its centre. If a north pole be at n and n', and a south pole at s and s', and a positive current descend as shown by the arrow in figure 17, let fall from each pole a dotted line perpendicular to the wire at C, and each dotted line will be the radius of the circle in which the corresponding pole will rotate. All the north poles will move in the line of the tangent directed to the right of the radius, and will have the same course as the hands of a watch when it is placed on a table with the dial plate upwards; and the south poles will rotate in the opposite direction. Should the electric current be ascending, the rotation of each pole will be reversed. If the current move hori- zontally, the plane of rotation will be vertical; and if figure 17 be moved into this position, the positive current still flowing from A to C, the arrows on the card will still indicate the course of rotation. The movements first observed by Oersted (page 109) are referable to this principle. When a magnetic needle, movable round the middle of its axis, is acted on by a parallel current, its poles receive an equal but contrary impulse, and the needle consequently comes to rest across the direction of the current. If a vertical positive current be placed, as shown in figure 18, at w, on the right side of a hori- zontal needle N S, movable round C, the pole N will move towards it; but if the current continue at w while the magnet occupies the position of N' S', Fig. 17. A s . /1 K# \ V "^ Fig. 18. 112 GALVANISM. M'g the pole N' will recede from the cur- rent : thus there is the appearance of repulsion in one case and of attraction in the other. If a similar current were without the circle in which a horizontal mag- net moves, as at w in figure 19, then the magnet, stationary at N S, would at first have its poles impelled in op- posite directions; but when it reaches the position N' S', the force at each pole acts on the same side of the mag- net's axis. The poles also, being equi- distant from w, and having the same inclination, will be influenced by equal forces acting at the same mechanical advantage. They will, therefore, by the laws Fj 20 of equilibrium, have a resultant which will pass directly through the centre of motion. This resultant, represented as applied at d, will tend to draw the wire w and the middle of the magnetC directly towards each other. If the conducting wire w were on the right ^jj,/ /|(TJ instead of the left side of the magnet, as in figure 20; then the resultant, passing as before through the centre of motion, but in an opposite direction, tends to draw the magnet and wire directly from each other, -j^r' and to give the appearance of repulsion. The same principle accounts for the rotation of a magnetic pole round a current, discovered by Fara- day. Into the centre of the bottom of a cup, as in the vertical section, figure 21, a copper wire c d was in- serted, a cylindrical magnet n s was attached by a thread to the copper wire c, and the cup was nearly filled with mercury, so that the pole n only of the magnet projected. A conductor a b was then fixed in the mercury perpendicularly over the wire c. On connecting the conducting wires with the opposite ends of a battery, a current was transmitted from one wire through the mercury to the other. If the posi- tive current descend, the north pole of the magnet, if uppermost, will rotate round the wire a b, passing from east through the south to west like the move- cL ments in the hands of a watch; and if the current ascend, the line of rotation will be reversed. Under similar circumstances the south pole would in each case rotate in the opposite direction. If a magnetic poie rotate round a conductor, a con- ductor will be equally disposed to rotate round a mag- netic pole, just as a magnet moves towards iron or iron towards a magnet, according as one or other is free to move. Accordingly, on fixing a magnet vertically in the middle of a cup of mercury, fig. 22, and transmitting a current by the moveable conductor a b through the mercury, and along a second con- ductor d, fixed as before in the bottom of the cup, Faraday found that the free extremity b of the wire moved round the pole of the magnet in a direction similar to the last. It is obvious that the direction of rotation imparted ^9'W &~* GALVANISM. 113 11 S 18 Fig. 24. by a fixed current to the moveable pole, will be identical with that which the same pole tends to impart to the same current. Thus let w in figure 23 represent the section of a wire along which a positive electric current is descending, and n the north pole of a magnet. If Fig. 23. jo impel n towards the right side, n will give an impulse to win the opposite direction, as indicated by the arrows. Each is disposed to describe a circle round the other as a cen- tre, moving in the same direction as the hands of a watch with its dial upwards; and if w and n were equally free to move, they would act as a couple in statics, and tend to ro- tate round the middle of the dotted line which joins them. The advantage of the rectangular form in the construction of a galvanometer (page 110) will now be intelligible. A magnetic needle N S, pointing north and south, and suspended by the point C horizontally within the rectangle, figure 24, will be in. fluenced in the same manner by each of its sides. If the posi- tive electric current flow from A horizon- tally above the nee- dle from north to south, and then suc- cessively along the other three sides up to B, the separate influence of each side, agreeably to the principle above illustrated, will impel the north pole eastward, and the south to the west. The little cups A B are designed to contain mercury, and afford a ready means of connecting the rectangle with the opposite sides of a galvanic com- bination. /,.,,• If the rectangle, in the last combination, have the property of impelling the north pole of a magnet to its right side, the north pole, when placed on that side, will give an opposite impulse to the rectangle. This may be shown by an elegant apparatus of De la Rive, which consists of a circular copper wire, the extremities of which are passed through a cork, and soldered to a plate of zinc and copper. On placing the arrangement in a vessel of acidu- lated water, a positive electric current passes from the copper plate round the circle to the zinc, as shown in figure Fig. 25. 25; and as the cork renders the apparatus . buoyant, a very slight force will throw it W into motion. It will exhibit various phe- nomena of attraction and repulsion, all explicable on the principle already ex- plained, according to the relative position of the magnetic pole which is presented. The apparatus will be more powerful if the conducting wire, covered with silk, to prevent lateral communication, be formed into several circles of the same diameter, on the principle of the multiplier. A current of voltaic electricity not only determines the position of a magnet, but renders steel permanently magnetic. This was observed nearly at the same time by Arago and Davy, who found that when needles are placed at right angles to the conducting wire, permanent magnetism is communicated; Fig. 26. 114 GALVANISM. and Davy also succeeded in producing this effect even with a shock of electricity from a Leyden phial. Arago, at the suggestion of Ampere, made a voltaic conductor into the form of a helix, into the axis of which he placed a needle, as in figure 26. As in this arrangement the current nearly in every part of its course is at right angles to the needle, and as each coil adds its effect to that of the others, the united action of the helix is ex- tremely powerful. The needle was thus fully magnetized in an instant. Though soft iron does not retain magnetism, its magnetic properties while under the influence of an electric current are very surprising. A piece of soft iron about a foot long and an inch in diameter is bent into the form of a horse-shoe, a copper wire is twisted round the bar at right angles to its axis, and an armature of soft iron, to which a weight may be attached, is fitted to its extremities, as in figure 27. On connecting the ends of the wire with a simple voltaic circle, even of small size, the soft iron instantly be- comes a powerful magnet, and will support a weight of 50, 60, or even 70 pounds. In- creasing the number of coils gives a great increase of pow- er ; but as the length of wire required for that purpose di- minishes the influence of the current (page 97), the follow- ing arrangement has been suc- cessfully adopted. The total length of copper wire intend- ed to be used is cut into seve- ral portions, each of which, covered with silk or cotton thread to prevent lateral com- munication, is coiled sepa- rately on the iron. The ends of all the wires are then collected into two separate parcels, and are made to communicate with the same voltaic battery, taking care that the positive current shall pass along each wire in the same direc- tion. The current is thus divided into a number of branches, and has only a short passage from one end of the battery to the other, though it gives energy to a multitude of coils. A combination of this kind, connected with a battery of five feet square, supported 2063 pounds, or nearly a ton weight. In witnessing the influence of voltaic conductors over the directive pro- perty of magnets, and in inducing magnetism, it is difficult to divest one's self of the conviction that these conductors, while transmitting a current, are themselves magnetic. This belief was early entertained by those who repeated the experiments of Oersted, and experimental evidence of its truth was speedily adduced. Arago and Davy found that a copper wire connect- ing the end of a voltaic combination attracted iron filings, but that they in- stantly fell off as soon as the circuit was broken; and a conductor, when its movements were not imped- ed by friction or gravity, was proved by Ampere to be obe- dient, like an ordinary magnet, to the magnetic agency of the earth. Though these properties may be exhibited by a single wire, the action is more conspi- cuous when, on the principle of the multiplier, the con- ductor is twisted into a spiral, as A figure 28, or into a rect- Fig. 28. GALVANISM. 115 angular form as represented by B in the same figure. When the arrangement is connected with a floating galvanic combination as in figure 25, or is very delicately suspended, the plane of the spiral places itself east and west, the positive current ascending on the west side and descending on the east: the positive current, in fact, takes the same course as the hands of a watch, when it is held on edge with the plane of the dial lying east and west, facing the south. That side of the spiral which is towards the north, consistently with an experiment already mentioned (page 113), acts as the north pole; and the south side of the spiral has an opposite polarity. Each side is power- fully attractive to iron filings. Another convenient form of the conductor is the helix, figure 26. Each coil of the helix is a separate magnet, and tends to place itself in the same position as the spiral or rectangle ; but the multiplied effect of all the coils causes north and south polarity to be accu- mulated at the opposite ends of the helix, and, therefore, to be separated, not by the mere thickness of the wire, but by the whole length of the helix. Since, therefore, the conductors just described may be regarded as mag- nets, such magnetized conductors ought mutually to repel or attract each other, when poles of the same or a different nature are adjacent; and as the action of a whole spiral or rectangle is merely the accumulated effect of its individual parts, it is fair to presume that each small portion of a con- ductor has its opposite sides in a state of opposite polarity, and that two such contiguous portions should attract or repel each other on the same principle as the spirals of which they constitute a part. Nay, even different parts of the same conductor ought to be mutually attractive or repulsive. These inferences from the facts already detailed, were fully demonstrated by Ampere soon after the discovery of Oersted. He proved that two vol- taic conductors, or two portions of the same conductor, attract each other when the .currents have the same direction, and are mutually repulsive when they are traversed by opposite currents; which is exactly what would be anticipated from the magnetic in- fluence of conductors. Thus, in the two parallel positive currents, AB and J\ CD, figure 29, which flow in the same direction, the contiguous sides are af- n___ _—\—-----------p fected with an opposite polarity, one ^" |s being south and the other north; whereas in the two contrary currents, EFandGH, the adjacent sides have the same polarity, and therefore repel each other. H Fig. 29. K- & Similarly when two currents cross each other, as AB and CD, figure 30, it is ob- vious that at two of the four corners, AD and CB, similar poles are contiguous; while at the other corners different poles concur. Hence the wires tend to revolve round E, and place themselves parallel to the cur- rents so that both may flow in the same direction. Fig. 30. Xi t n rC E- C If a moveable conductor CD be wholly on one side of AB, as in figure 31, repulsion will ensue on one side and attraction on the other. The direction in which these forces act is indicated by the dotted arrows, e b and eg; and they give a resultant e r, which tends to draw C D to the right side. 116 GALVANISM. Fig. 31. Fig. 32. 72,4 Figure 32 shows the effect of reversing the current in C D, which will consequently be drawn by a force at right angles to itself to the left side. If in the last case the conductor Fig. 33. C D were moveable round C as a --, centre, then the resultant e r would Ir" draw D towards F, figure 33; but if /\ the current in either conductor were /^ I ^sj,/ H reversed, CD would tend to rotate 'S^f'%—]--n y^t*- towards G. Tt N\^"" J.m ^ 1% These are a few examples of the numerous facts experimentally proved by Ampere concerning the action of voltaic conductors on each other. It is to this branch of the subject the term of Electro-Dynamics, or the science of electricity in motion, is some- times restricted, while the mutual action of conductors and magnets is called Electro-Magnetism • but these two branches are so entirely parts of the same science, that I have included both under Ampere's term of Electro- Dynamics. Any one who has studied the few preceding pages with mode- rate care, cannot fail to trace a close analogy between a helix traversed by an electric current and a magnet. The former is affected by other voltaic conductors, by the poles of a magnet, and by the magnetism of the earth, in the same manner as the latter. It was this similarity, or rather identity, of action which led Ampere to his theory of magnetism. He supposes that the polarity of every magnet is solely owing to the circulation, within its substance and at its surface, of electric currents, which continually pass around all its particles in planes perpendicular to its axis. On placing a magnet in its natural position of north and south, the direction of its cur- rents is exactly the same as in the conductors of figure 28, descending on the east side, passing under the magnet from east to west, and ascending on the side next the west. In like manner are currents supposed to circulate within the earth, especially near its surface, passing from east to west in planes parallel to the magnetic equator. These terrestrial currents cause all bodies, which are freely suspended, and are possessed of electric currents, to place themselves in such a position that the current on their under side should flow in parallelism, and in the same direction, with that in the earth immediately beneath. That the existence of such currents will account for the directive property of the earth, follows from the mutual action of con- ductors; and Mr. Barlow, to render the analogy still more complete, con- structed a hollow sphere of wood, in which electric currents were made to circulate in the same direction as they are thought to do in the earth ; and by placing an astatic needle on different parts of its surface, he found that all the phenomena of terrestrial magnetism might be imitated. Observation has even supplied a cause for the existence of currents in the earth, moving in the direction which theory requires. The diurnal rotation of our planet on its axis exposes its surface to be heated in a direction passing from east to west; and the discoveries which have been made in thermo-electricity (page 75) sufficiently prove the probability of electric currents being esta- blished in the conducting matter of the earth by the successive heating of C Jp-n ■—s>r c S-^ 1% *B' h " \n s A Is \s-b GALVANISM. 117 its parts. In short, the theory of Ampere connects the facts of electro- dynamics with the phenomena of terrestrial magnetism, and affords a splendid instance of the application of mathematical analysis to physical research. Volta-electric Induction.—The development of electricity by the vicinity of an excited body, already described under the name of induced electricity (page 78), led Mr. Faraday to inquire whether electricity in motion, as well as that of tension and at rest, may not be excited by induction. Though baffled in his early attempts, he at last succeeded in laying open a new branch of electro-dynamics, which vies in interest and importance with the fundamental discovery of Oersted (Phil. Trans. 1831). A copper wire 203 feet long was passed in form of a helix round a large block of wood, and an equal length of a similar wire was wound on the same block and in the same direction, so that the coils of each helix should be interposed, but without contact, between the coils of the other. The ends of one of the helices were connected with a galvanometer, and the other with a strong galvanic battery, with the view of ascertaining whether the passage of an electric current through one helix would induce a current in the adjoining helix. It was found that the galvanometer needle indicated a current at the moment both of completing and breaking the circuit, but that in the interval no deflection took place; and similarly the induced currents readily mag- netized a sewing needle, while the electric current along the inducing helix was in the act of beginning or ceasing to flow, but at no other period. By varying the experiment the same result was obtained; an electric current transmitted from a voltaic battery through a conducting helix does not in- duce a current in an adjoining helix, except at the moment of making or breaking the voltaic circuit In the former case the direction of the in- duced eurrent is opposite to that of the inducing current, and Ifr the latter case it is the same. This phenomenon is distinguished by Faraday under the name of Volta-electric induction. The inducing power of a magnet greatly exceeds that of an electric cur- rent. A ring of soft iron was covered to nearly half its extent by several helices, the ends of which were brought together so as to constitute a com. pound helix terminating in the conductors a b, Fig. 34. figure 34; and on the other half of the ring were arranged similar helices which commu- nicated by c d with a galvanometer. The two sets of helices were thus separated from each other by portions of the ring M M', and were protected by cloth from direct contact with the ring itself. At the moment the wires a b touched the ends of a voltaic combination, the galvanometer was strongly affected: the needle then returned to its former position and remained there until the voltaic circuit was broken, when the needle was again deflected as strongly as be- fore, but in the opposite direction. The action was still greater when both compound helices were on the same part of the ring, the induction being increased apparently by the closer contiguity of the helices. Another of Faraday's arrangements, which was in several respects more convenient than the ring, consisted of a hollow cylinder of pasteboard, round which two compound helices were adjusted. On connecting one helix with a voltaic combination, the other deflected the galvanometer and magnetized a needle, as in the experiments of volta-electric induction at first described; but when a cylinder of soft iron was introduced into the pasteboard case, and a voltaic current transmitted as before, the effect on the galvanometer was much greater. The action in this last experiment and with the iron ring is dis- tinguished by the name of Magneto-electric induction. The phenomena arising from magneto-electric and volta-electric induction are manifestly owing to the same condition of the induced wire: the action on the needle, though different in force, is identical in kind. It is equally 118 GALVANISM. clear that the agent brought into operation in the induced wire is an elec- tric current, or, to dismiss the language of theory, that the induced wire is in the same electric state as the conducting wire in a closed voltaic circle. Its power in magnetizing steel and deflecting a magnet is sufficient evi- dence of this; but Faraday, by magneto-electric induction, succeeded in throwing a frog's leg into spasms by connecting it with the induced wire; and by arming the ends of that wire with points of charcoal, and separating them at the instant the galvanic circuit of the inducing wire was broken or restored, sparks of electricity were obtained. The mode in which soft iron contributes to the effect is likewise obvious. An electric current circulating round a bar of soft iron has been shown to convert it into a temporary magnet possessed of surprising power (page 114); and it is doubtless to this magnet, called into temporary existence by the electric current, most of the induced electricity is to be ascribed. Faraday reduced this to certainty by surrounding a cylinder of soft iron with one helix connected with the galvanometer, and converting the soft iron into a temporary magnet, not by a voltaic battery, but by placing at each end of the cylinder the opposite pole of a magnet. During the act of applying the magnetic poles to the iron, the galvanometer needle was deflected; and the deflection was repro- duced, but in the opposite direction, when the magnetism of the iron was ceasing by the removal of the magnet. Similarly when a helix was wound on a hollow cylinder of pasteboard, and a real magnet was introduced, the galvanometer was deflected: the needle then remained quiescent so long as the magnet was left in the cylinder; but in the act of its removal, the needle was again deflected, though as usual in the opposite direction. These singular phenomena, which establish such new and intimate rela- tions between voltaic and magnetic action, and supply additional evidence in favour of Ampere's beautiful theory of magnetism, have led to an experi- ment by which, at first view, an electric spark appeared to be derived from the magnet itself, After Faraday had announced his experiment, above mentioned, of obtaining a spark from the induced wire, other attempts were made to effect the same object with a magnet, without the aid of galvanism. The first person who succeeded in this country was Professor Forbes, who operated with a powerful loadstone which had been presented to the Univer- sity of Edinburgh by Dr. Hope. (Phil. Trans, of Ed. 1832.) A helix of copper wire was formed round the middle of a cylinder of soft iron, which was of such length that its extremities reached from one pole of the loadstone to the other. On applying and withdrawing the soft iron cylinder to and from the poles of the loadstone, magnetism was alternately created and de- stroyed within it. At these periods of transition, electric currents were in- duced in the helix surrounding the soft iron; and when, at these instants, metallic contact between the conducting wires of the helix was broken, an electric spark was visible. Mr. Forbes succeeded best by connecting one wire with a cup of mercury, and removing the other wire from contact with its surface at the instant when an assistant withdrew the armature of soft iron from the loadstone. In this experiment, therefore, the electricity was obtained from the helix, and was induced in it by the soft iron while in the act of acquiring or losing magnetism. The same experiment was perform- ed by Mr. Faraday with a loadstone belonging to Professor Daniell; and shortly before the experiment of Mr. Forbes, Nobili and Antinori succeeded with an ordinary steel magnet. Pixii in Paris afterwards performed this experiment with great effect by causing a strong horse-shoe magnet to re- volve upon an axis, its poles passing in rapid succession in front of a soft iron armature of the same form; and a still better arrangement is to cause the armature to revolve in front of the poles of a powerful magnet, as in the instrument fitted up by Mr. Saxton, and exhibited at the Adelaide-rooms, London. The voltaic currents are induced in one direction as the armature approaches the magnetic poles, and are reversed as it quits them; so that the currents change their direction twice in each revolution. On all these occasions the source of the electricity is the same, being always induced in GALVANISM. 119 the helix by a temporary magnet; and it has all the characters of a voltaic current. It produces brilliant sparks, renders platinum wire red-hot, and gives a strong shock. It readily explodes gunpowder; and Dr. Ritchie has fitted up an apparatus for exploding with it a mixture of oxygen and hydro- gen gases. It decomposes water rapidly; and though from the rapid re- versal in the direction of the currents, both gases are given off at the same wire, Pixii succeeded in collecting them separately. (An. de Ch. et de Ph. li. 72.) Intimately associated with magneto-electric induction, if not referable to the very same origin, is the induction of electric currents by movement. On introducing a magnet into a hollow helix of copper wire, as also on withdrawing the magnet after its introduction, an electric current was mo- mentarily induced in the wire; and if, the magnet being stationary, the helix were moved in its vicinity, an electric current is likewise induced. The action is not confined to magnets and copper wire; but in all solid con- ductors of electricity, when moved near the pole of a magnet, an electric current is generated, and the most perfect conductors act with the greatest effect. The direction of the movement is not immaterial: it is essential that the plane in which the conductor moves should form an angle with the axis of the magnet; and the most powerful currents were induced, when the plane of motion was at right angles to that axis, and hence parallel to the electric currents which Ampere supposes to exist in the magnet. The direction of the currents depends on the direction of motion. If the move- ment of a wire from right to left cause a certain current, an opposite cur- rent will be induced when the wire is moved from left to right. In short, with regard to the direction of an induced current, Mr. Faraday's researches establish this law, deduced by Dr. Ritchie: if a wire, conducting voltaic electricity, produce on magnets or conductors certain motions, whether re- pulsive, attractive, or rotatory; and if the battery be removed, the ends of the wires brought into metallic contact, and the same motions be produced by mechanical means, the conductor will have the same electric state induced in it as it had when connected with the battery. (Phil. Mag. 3rd series, iv. 12.) Mr. Faraday has applied this principle in a most happy manner to ex- plain the phenomena of rotation discovered by M. Arago. If a plate of copper be revolved close to a magnetic needle suspended so that it may rotate in a plane parallel to the plate, the needle will rotate in the same direction; and, reciprocally, a rotating magnet tends to give rotation to a contiguous copper plate. The same effects are produced by the rotation not only of all metals, but, according to Arago, of all bodies, whether solid, liquid, or gaseous. These effects, which Faraday has principally examined in reference to the rotation of metals, are entirely owing to electric currents induced by the rotation, and flowing at right angles to the direction of mo- tion. Suppose abd, figure 35, to be a circular metallic Fig. 35. plate, placed horizontally, and capable of revolving round its centre c; and let n be the north pole of a magnet ^rt^....^. situated above the plate near its circumference at a. If / tf^SX a positive electric current were to flow along the plate / I Wf from c to a, the pole n would be impelled at right angles I C* h in the direction indicated by the arrow; and hence if \. J the plate were made to revolve in the same direction, in- X^ ^/ dicated by the arrows at b and d, an induced positive ci -t-^ current would instantly flow from c to a. Its direction would be constantly in that line, being at right angles to the dotted arrow, which indicates the direction in which that part of the plate nearest to the pole is moving. Hence the pole, acted on by the induced current, would receive an impulse in the same direction. If motion in the vicinity of a magnet induce an electric current, the same effect would be anticipated from the magnetic influence of the earth ; and this fact has been proved by Mr. Faraday by most decisive and interesting L20 GALVANISM. experiments. When a bar of soft iron is held in the position of the dipping needle, the direction of which, in regard to terrestrial magnetism, is analo- gous to the axis of a common magnet, it acquires magnetic properties; and, accordingly, on introducing a soft iron cylinder into a hollow helix of cop- per placed in the line of the dip, a galvanometer connected with the helix was instantly affected. But the use of iron may be dispensed with altoge- ther ; for when a helix of copper wire was simply moved at right angles to the dipping needle, electric currents were induced by the magnetism of the earth. The form of a helix is not even necessary: the movement of a piece of copper wire across the line of dip developed currents in the wire. The same effect was produced by the rotation of a copper plate placed horizon- tally so as to be nearly at right angles to the line of dip; and the revo- lution of a copper globe acted in the same manner. Faraday concludes that the rotation of the earth on its axis ought similarly to influence the conduct- ing matters of its surface; and that electric currents should be thereby in- duced from the equatorial regions to either pole. He throws out the sugges- tion whether the aurora borealis and australis may not be produced by the returning currents passing from the poles of the earth into the atmosphere. r PART II. INORGANIC CHEMISTRY. PRELIMINARY REMARKS. I.\ teaching a science such as chemistry, the details of which are nume- rous and complicated, it would be injudicious to follow the order of disco- very, and proceed from individual facts to the conclusions which have been deduced from them. An opposite course is indispensable. It is necessary to discuss general principles in the first instance, in order to aid the begin- ner in remembering insulated facts, and in comprehending the explanations connected with them. I shall, accordingly, commence the second part of the work by explaining the leading doctrines of the science. One inconvenience, indeed, arises from this method. It is often necessary, by way of illustra- tion, to refer to facts of which the beginner is ignorant; and, therefore, on some occasions more knowledge will be required for understanding a sub' ject fully, than the reader may have at his command. But these instances will, it is hoped, be rarely met with; and when they do occur, the reader is advised to quit the point of difficulty, and return to the study of it when he shall have acquired more extensive knowledge of the details. To the chemical history of each substance, its chief physical characters will be added. A knowledge of these properties is not only advantageous in assisting the chemist to distinguish one body from another, but in many instances it is applied to uses still more important. The character called specific gravity, the meaning of which was explained at page 48, is of so much importance that the mode of determining it will be mentioned in this place. The process consists in weighing a body carefully, and then deter- mining the weight of an equal bulk of water, the latter being regarded as unity. If, for example, a portion of water weigh 9 grains, and the same bulk of another body 20 grains, its specific gravity is determined by this formula :—as 9 : 20 : : 1 (assumed as the specific gravity of water) to the fourth proportional 2.2222; so that the specific gravity of any substance is found by dividing its weight by the weight of an equal volume of water. It is easy to discover the weight of equal bulks of water and any other liquid by filling a small bottle of known weight with each successively, and weigh- ing them.* The method of obtaining the necessary data in case of a solid is somewhat different. The body is first weighed in air, is next suspended in water by means of a hair attached to the scale of the balance, and is then weighed again. The difference between the two weights gives the weight of a quantity of water equal to the bulk of the solid. This rule is founded on the hydrostatic law, that a solid body, immersed in any liquid, not only weighs less than it does -in air, but the difference corresponds exactly to the * Bottles are prepared for this purpose by the philosophical instrument makers. 11 122 PRELIMINARY REMARKS. weight of liquid which it displaces; and it is obvious that the liquid so dis- placed is exactly of the same dimensions as the solid. Another method is by the use of the bottle recommended for taking the specific gravity of liquids. After weighing the bottle filled with water, a known weight of the solid is put into it, which of course displaces a quantity of water precisely equal to its own volume. The exact weight of the displaced water is found by weighing the bottle again, after its outer surface is made perfectly dry. The determination of the specific gravity of gaseous substances is an ope- ration of much greater delicacy. From the extreme lightness of gases, it would be inconvenient to compare them with an equal bulk of water, and, therefore, atmospheric air is taken as the standard of comparison. The first step of the process is to ascertain the weight of a given volume of air. This is done by weighing a very light glass flask, furnished with a good stopcock, while full of air; and then weighing it a second time, after the air has been withdrawn by means of the air-pump. The difference between the two weights gives the information required. According to the observation of Dr. Prout, 100 cubic inches of pure and dry atmospheric air, at the temperature of 60° F. and when the barometer stands at 30 inches, weigh 31.0117 grains. By a similar method the weight of any other gas may be determined, and its specific gravity be inferred accordingly. For instance, suppose 100 cubic inches of oxygen gas are found to weigh 34.109 grains, its specific gravity will be thus deduced; as 31.0117 : 34.109 : : 1 (the sp. gr. of air) : 1.1025, the specific gravity of oxygen. There are four circumstances to which particular attention must be paid in taking the specific gravity of gases :— 1. The gas should be perfectly pure, otherwise the result cannot be ac- curate. 2. Due regard must be had to its hygrometric condition. If it is saturated with moisture, the necessary correction may be made by the formula of page 50 ; or it may be dried by the use of substances which have a power- ful attraction for moisture, such as chloride of calcium, quicklime, or fused potassa. 3. As the bulk of gaseous substances, owing to their elasticity and com- pressibility, is dependent on the pressure to which they are exposed, no two observations admit of comparison, unless made under the same elevation of the barometer. It is always understood, in taking the specific gravity of a gas, that the barometer must stand at thirty inches, by which means the operator is certain that each gas is subject to equal degrees of compression. An elevation of thirty inches is, therefore, called the standard height; and if the mercurial column be not of that length at the time of performing the experiment, the error arising from this cause must be corrected by calcula- tion. It has been established by careful experiment, that the bulk of gases is inversely as the pressure to which they are subject. Thus, 100 measures of air, under the pressure of a thirty-inch column of mercury, will dilate to 200 measures, if the pressure be diminished by one-half; and will be com- pressed to fifty measures, when the pressure is double, or equal to a mercu- rial column of sixty inches. The correction for the effect of pressure may, therefore, be made by the rule of three, as will appear by an example. If a certain portion of gas occupy the space of 100 measures at twenty- nine inches of the barometer, its bulk at thirty inches may be obtained by the following proportion ; as 30 : 29 : : 100 : 96.66. It is understood that the temperature of the mercurial column is constant; if not so, correction must be made agreeably to the note at page 18. For a similar reason the temperature should always be the same. The standard or mean temperature is 60°; and if the gas be admitted into the weighing-flask when the thermometer is above or below that point, the for- mula of page 21 should be employed for making the necessary correction. Chemistry is indebted for its nomenclature to the labours of four celebrated chemists, Lavoisier, Berthollet, Guyton-Morveau, and Fourcroy. The prin- PRELIMINARY REMARKS. 123 ciples which guided them in its construction are exceedingly simple and in- genious. The kuown elementary substances, and the more familiar com- pound ones, were allowed to retain the appellation which general usage had assigned to them. The newly-discovered elements were named from some striking property. Thus, as it was supposed that acidity was always owing to the presence of the vital air discovered by Priestley and Scheele, they gave it the name of oxygen, from cgue acid, and ynvtiv to generate; and they called inflammable air hydrogen, from wfaig water, and ytvvw, because it en- ters into the composition of water. Compounds, of which oxygen forms a part, were called acids or oxides, according as they do or do not possess acidity. An oxide of iron or copper signifies a combination of those metals with oxygen, which has no acid pro- perties. The name of an acid was derived from the substance acidified by the oxygen, to which was added the termination in ic. Thus, sulphuric and carbonic acids signify acid compounds of sulphur and carbon with oxygen. If sulphur or any other body should form two acids, that which contains the least quantity of oxygen is made to terminate in ous, as sulphurous acid. The termination in uret was intended to denote combinations of the simple non-metallic substances either with one another, with a metal, or with a me- tallic oxide. Sulphured and carburet of iron, for example, signify compounds of sulphur and carbon with iron. The different oxides or sulphurets of the same substance were distinguished from one another by some epithet, which was commonly derived from the colour of the compound, such as the black and red oxides of iron, the black and red sulphurets of mercury. Though this practice is still continued occasionally, it is now more customary to dis- tinguish degrees of oxidation by the use of derivatives from the Greek or Latin. Protoxide signifies the first degree of oxidation, 6inoxide the second, and teroxide the third; and the term peroxide is often applied to the highest degree of oxidation. The Latin word sesqui, one and a half, is used as an affix when the elements of an oxide are as I to 1J, or as 2 to 3. The sul- phurets, carburets, &,c. of the same substance are designated in a similar way. Compounds consisting of acids in combination with metallic oxides, or any alkaline bases, are termed salts, the names of which are so contrived as to indicate the substances contained in them. If the acidified substance contain a maximum of oxygen, the name of the salt terminates in ate ; if a minimum, the termination in ite is employed. Thus, the sulphate, phosphate, and arseniate of potassa, are salts of sulphuric, phosphoric, and arsenic acids ; while the terms sulphite, phosphite, and arsenite of potassa, denote combina- tions of that alkali with the sulphurous, phosphorous, and arsenious acids. The advantage of a nomenclature which disposes the different parts of a science in so systematic an order, and gives such powerful assistance to the memory, is incalculable. The principle has been acknowledged in all coun- tries where chemical science is cultivated, and its minutest details have been adopted in Britain. It must be admitted, indeed, that in some respects the nomenclature is defective. The erroneous idea of oxygen being the general acidifying principle, has exercised an injurious influence over the whole structure. It would have been convenient also to have had a different name for hydrogen. But it is now too late to attempt a change; for the confusion attending such an innovation would more than counterbalance its advantages. The original nomenclature has, therefore, been preserved, and such additions have been made to it as the progress of the science rendered necessary. The most essential improvement was suggested by the discovery of the laws of chemical combination. The different salts formed of the same constituents were formerly divided into neutral, super, and s«6-salts. They were called neutral if the acid and alkali were in such proportion that one neutralized the other; super-salts, if the acid prevailed; and sub-salts, if the alkali was in excess. The name is now regulated by the atomic constitution of the salt. If it is a compound of an equivalent of the acid and the alkali, the generic name of the salt is employed without any other addition ; but if two or more equivalents of the acid are attached to one of the base, or two or 124 AFFINITY. more equivalents of the base to one of the acid, a numeral is prefixed so as to indicate its composition. The two salts of sulphuric acid and potassa are called sulphate and 6isulphate; the first containing an equivalent of the acid and the alkali, and the second salt, two of the former to one of the latter. The three salts of oxalic acid and potassa are termed the oxalate, oinoxalate, and owaaroxalate of potassa; because one equivalent of the alkali is united with one equivalent of acid in the first, with two in the second, and with four in the third salt. As the numerals which denote the equivalents of the acid in a super-salt are derived from the Latin language, Dr. Thomson proposes to employ the Greek numerals, dis, tris, tetrakis, to signify the equivalents of alkali in a sub-salt; and I shall not only adopt his proposition, but give it the following extension. Since, agreeably to the electro-chemical theory, the elements of a compound may in relation to each other be considered oppo- sitely electric, I shall distinguish two or more equivalents of the negative element by Latin numerals, and apply Greek numerals to that element which is regarded as positive. Thus a fcichloride denotes a compound which con- tains two equivalents of the negative clement chlorine; whereas a dichloride indicates that one equivalent of chlorine is combined with two of some positive body. SECTION I. AFFINITY. All chemical phenomena are owing to Affinity or Chemical Attraction. It is the basis on which the science of chemistry is founded. It is, as it were, the instrument which the chemist employs in all his operations, and hence it forms the first and leading object of his study. Affinity is exerted between the minutest particles of different kinds of matter, causing them to combine so as to form new bodies endowed with new properties. It acts only at insensible distances; in other words, apparent contact, or the closest proximity, is necessary to its action. Every thing which prevents such contiguity is an obstacle to combination; and any force which increases the distance between particles already combined, tends to separate them permanently from each other. In the former case, they do not come within the sphere of their mutual attraction ; in the latter, they are removed out of it. It follows, therefore, that though affinity is regarded as a specific power distinct from the other forces which act on matter, its action may be promoted, modified, or counteracted by them; and consequently, in studying the phenomena produced by affinity, it is necessary to inquire into the conditions that influence its operation. The most simple instance of the exercise of chemical attraction is afforded by the admixture of two substances. Water and sulphuric acid, or water and alcohol, combine readily. On the contrary, water shows little disposition to unite with sulphuric ether, and still less with oil; for however intimately their particles may be mixed together, they are no sooner left at rest than the ether separates almost entirely from the water, and a total separation takes place between that fluid and the oil. Sugar dissolves very sparingly in al- cohol, but to any extent in water; while camphor is dissolved in a very small degree by water, and abundantly by alcohol. It appears, from these examples, that chemical attraction is exerted between different bodies with different de- grees of force. There is sometimes no proof of its existence at all; between some substances it acts very feebly, and between others with great energy. Simple combination of two substances is a common occurrence; of which the solution of salts in water, the combustion of phosphorus in oxygen gas, and the neutralization of a pure alkali by an acid, are instances. But the AFFINITY. 125 phenomena are often more complex. The formation of a new compound is often attended by the destruction of a pre-existing one; as when some third body acts on a compound, for one element of which it has a greater affinity than they have for one another. Thus, oil has an affinity for the volatile al- kali, ammonia, and will unite with it, forming a soapy substance called a lin- iment. But the ammonia has a still greater attraction for sulphuric acid; and hence if this acid be added to the liniment, the alkali will quit the oil, and unite by preference with the acid. If a solution of camphor in alcohol be poured into water, the camphor will be set free, because the alcohol com- bines with the water. Sulphuric acid, in like manner, separates baryta from nitric acid. Combination and decomposition occur in each of these cases;— combination of sulphuric acid with ammonia, of water with alcohol, and of baryta with sulphuric acid ;—decomposition of the compounds formed of oil and ammonia, of alcohol and camphor, and of nitric acid and baryta. These are examples of what Bergmann called single elective affinity;—elective, be- cause a substance manifests, as it were, a choice for one of two others, unit- ing with it by preference, and to the exclusion of the other. Many of the decompositions that occur in chemistry are instances of single elective af- finity. The order in which these decompositions take place has been expressed in tables, of which the following, drawn up by Geoffroy, is an example:— Sulphuric acid. Baryta, Strontia, Potassa, Soda, Lime, Ammonia, Magnesia. This table signifies, first, that sulphuric acid has an affinity for the sub- stances placed below the horizontal line, and may unite separately with each; and, secondly, that the base of the salts so formed will be separated from the acid by adding any of the alkalies or earths which stand above it in the column. Thus ammonia will separate magnesia, lime ammonia, and potassa lime; but none can withdraw baryta from sulphuric acid, nor can ammonia or magnesia decompose sulphate of lime, though strontia or baryta will do so. Bergmann conceived that these decompositions are solely deter- mined by chemical attraction, and that consequently the order of decompo- sition represents the comparative forces of affinity; and this view, from the simple and natural explanation it affords of the phenomenon, was for a time very generally adopted. But Bergmann was in error. It does not necessa- rily follow, because lime separates ammonia from sulphuric acid, that the lime has a greater attraction for the acid than the volatile alkali. Other causes are in operation which modify the action of affinity to such a degree, that it is impossible to discover how much of the effect is owing to that power. It is conceivable that ammonia may in reality have a stronger at- traction for sulphuric acid than lime, and yet that the latter, from the great influence of disturbing causes, may succeed in decomposing sulphate of ammonia. The propriety of the foregoing remark will be made obvious by the fol- lowing example. When a stream of hydrogen gas is passed over oxide of iron heated to redness, the oxide is reduced to the metallic state, and water is generated. On the contrary, when watery vapour is brought into contact with red-hot metallic iron, the oxygen of the water quits the hydrogen and combines with the iron. It follows from the result of the first experiment, according to Bergmann, that hydrogen has a stronger attraction than iron for oxygen; and from that of the second, that iron has a greater affinity for oxygen than hydrogen. But these inferences are incompatible with each 126 AFFINITY. other. The affinity of oxygen for the two elements, hydrogen and iron, must either be equal or unequal. If equal, the result of both experiments was determined by modifying circumstances; since neither of these sub- stances ought on this supposition to take oxygen from the other. But if the forces are unequal, the decomposition in one of the experiments must have been determined by extraneous causes, in direct opposition to the ten- dency of affinity. To Berthollet is due the honour of pointing out the fallacy of Bergmann's opinion. He was the first to show that the relative forces of chemical at- traction cannot always be determined by observing the order in which sub- stances separate each other when in combination, and that the tables of Geoffroy are merely tables of decomposition, not of affinity. He likewise traced all the various circumstances that modify the action of affinity, and gave a consistent explanation of the mode in which they operate. Berthollet went even a step further. He denied the existence of elective affinity as an invariable force, capable of effecting the perfect separation of one body from another; he maintained that all the instances of complete decomposition at- tributed to elective affinity are in reality determined by one or more of the collateral circumstances that influence its operation. But here this acute philosopher went too far. Bergmann erred in supposing the result of che- mical action to be in every case owing to elective affinity; but Berthollet ran into the opposite extreme in declaring that the effects formerly ascribed to that power are never produced by it. That chemical attraction is exerted between bodies with different degrees of energy is, I apprehend, indisputa- ble. Water has a much greater affinity for hydrochloric acid and ammo- niacal gases than for carbonic and hydrosulphuric acids, and for these than for oxygen and hydrogen. The attraction of lead for oxygen is greater than that of silver for the same substance. The disposition of gold and silver to combine with mercury is greater than the attraction of platinum and iron for that fluid. As these differences cannot be accounted for by the operation of any modifying causes, we must admit a difference in the force of affinity in producing combination. It is equally clear that in some in- stances the separation of bodies fron one another can only be explained on the same principle. No one, I conceive, will contend that the decomposition of hydriodie acid by chlorine, or of hydrosulphuric acid by iodine, is deter- mined by the concurrence of any modifying circumstances. Affinity is the cause of still more complicated changes than those which have been just considered. In a case of single elective affinity, three sub- stances only are present, and two affinities are in play. But it frequently happens that two compounds are mixed together, and four different affinities brought into action. The changes that may or do occur under these cir- cumstances may be studied by aid of a diagram, a method which was first employed, I believe, by Dr. Black, and has since been generally practised. Thus, in mixing together a solution of carbonate of ammonia and hydro- chlorate of lime, their mutual action may be represented in the following manner: Carbonic acid Ammonia Hydrochloric acid Lime. Each of the acids has an attraction for both bases, and hence it is possi- ble either that the two salts should continue as they were, or that an inter- change of principles should ensue, giving rise to two new compounds— carbonate of lime and hydrochlorate of ammonia. According to the views of Bergmann, the result is solely dependent on the comparative strength of affinities. If the affinity of carbonic acid for ammonia, and of hydrochloric acid for lime, exceed that of carbonic acid for lime, added to that of hydro- AFFIN1TV. 127 chloric acid for ammonia, then will the two salts experience no change whatever; but if the latter affinities preponderate, then, as does actually happen in the present example, both the original salts will be decomposed, and two new ones generated. Two decompositions and two combinations take place, being an instance of what is called double elective affinity. Mr. Kirwan applied the terms quiescent and divellent to denote the tendency of the opposing affinities, the action of the former being to prevent a change, the latter to produce it. The doctrine of double elective affinity was assailed by Berthollet on the same ground and with the same success as in the case of single elective at- traction. He succeeded in proving that the effect cannot always be ascribed to the sole influence of affinity. For, to take the example already adduced, if carbonate of ammonia decompose hydrochlorate of lime by the mere force of a superior attraction, it is manifest that carbonate of lime ought never to decompose hydrochlorate of ammonia. But if these two salts are mixed in a dry state and exposed to heat, double decomposition does take place, carbonate of ammonia and hydrochlorate of lime beir.g formed; and, therefore, if the change in the first example was produced by chemical at- traction alone, that in the second must have occurred in direct opposition to that power. It does not follow, however, because the result is sometimes determined by modifying conditions, that it must always be so. I appre- hend that the decomposition of the solid cyanuret of mercury by hydrosul- phuric acid gas, which takes place even at a low temperature, cannot be ascribed to any other cause than a preponderance of the divellent over the quiescent affinities. CHANGES THAT ACCOMPANY CHEMICAL ACTION. The leading circumstance that characterizes chemical action is the loss of properties experienced by the combining substances, and the acquisition of new ones by the product of their combination. The change of property is sometimes inconsiderable. In a solution of sugar or salt in water, and in mixtures of water with alcohol or sulphuric acid, the compound retains so much of the character of its constituents, that there is no difficulty in recog- nizing their presence. But more generally the properties of one or both of the combining bodies disappear entirely. One would not suppose from its appearance that water is a compound body; much less that it is composed of two gases, oxygen and hydrogen, neither of which, when uncombined, has ever been compressed into a liquid. Hydrogen is one of the most in- flammable substances in nature, and yet water cannot be set on fire; oxygen, on the contrary, enables bodies to burn with great brilliancy, and yet water extinguishes combustion. The alkalies and earths were regarded as simple till Sir H. Davy proved them to be compound, and certainly they evince no sign whatever of containing oxygen and a metal. Numerous examples of a similar kind are afforded by the mutual action of acids and alkalies. Sul- phuric acid and potassa, for example, are highly caustic. The former is in- tensely sour, reddens the blue colour of vegetables, and has a strong affinity for alkaline substances; the latter has a pungent taste, converts the blue colour of vegetables to green, and combines readily with acids. On adding these principles cautiously to each other, a compound results called a neutral salt, which does not in any way affect the colouring matter of plants, and in which the other distinguishing features of the acid and alkali can no longer be perceived. They appear to have destroyed the properties of each other, and are hence said to neutralize one another. The other phenomena that accompany chemical action are changes of density, temperature, form, and colour. 1. It is observed that two bodies rarely occupy, after combination, the same space which they possessed separately. In general their bulk is diminished, so that the specific gravity of the new body is greater than the mean of its components. Thus a mixture of 100 measures of water and an 128 AFFINITY, equal quantity of sulphuric acid does not occupy the space of 200 measures, but considerably less. A similar contraction frequently attends the combi- nation of solids. Gases often experience a remarkable condensation when they unite. The elements of olcfiant gas, for instance, would expand to four times the bulk of that compound, if they were suddenly to become free, and assume the gaseous form. But the rule is not without exception. The reverse happens in some metalic compounds; and there are examples of com- bination between gases without any change of bulk. 2. A change of temperature generally accompanies chemical action. Heat is evolved either when there is a diminution in the bulk of the com- bining substances without change of form, or when a gas is condensed into a liquid, or a liquid becomes solid. The heat caused by mixing sulphuric acid with water is an instance of the former; and the common process of slaking lime, during which water loses its liquid form in combining with that earth, is an example of the latter. The rise of temperature in these cases is obviously referable to diminution in the capacity of the new com- pound for heat; but intense heat sometimes accompanies chemical action under circumstances in which an explanation founded on a change of specific heat is quite inadmissible. At present it is enough to have stated the fact; its theory will be discussed under the subject of combustion. The production of cold seldom or never takes place during combination, except when heat is rendered insensible by the conversion of a solid into a liquid, or a liquid into a gas. All the frigorific mixtures act in this way. 3. The changes of form that attend chemical action are exceedingly various. The combination of gases may give rise to a liquid or a solid; solids sometimes become liquid, and liquids solid. Several familiar chemical phenomena, such as detonation, effervescence, and precipitation, are owing to these changes. The sudden evolution of a large quantity of gaseous matter causes an explosion, as when gunpowder detonates. The slower disengagement of gas produces effervescence, as when marble is put into hydrochloric acid. A precipitate is owing to the formation of a new body which happens to be insoluble in the liquid in which its elements were dissolved. 4. Chemical action is frequently attended by change of colour. No uniform relation has been traced between the colour of a compound and that of its elements. Iodine, whose vapour is of a violet hue, forms a beau- tiful red compound with mercury, and a yellow one with lead. The brown oxide of copper generally gives rise to green and blue coloured salts; while the salts of the oxide of lead, which is itself yellow, are for the most part colourless. The colour of precipitates is a very important study, as it supplies a character by which most substances may be distinguished. CIRCUMSTANCES THAT MODIFY AND INFLUENCE THE OPERATION OF AFFINITY. Of the conditions which are capable of promoting or counteracting the tendency of chemical attraction, the following are the most important;— cohesion, elasticity, quantity of matter, and gravity. To these may be added the agency of the imponderables. Cohesion.—The first obvious effect of cohesion is to oppose affinity, by impeding or preventing that mutual penetration and close proximity of the particles of different bodies, which is essential to the successful exercise of their attraction. For this reason, bodies seldom act chemically in their solid state; their molecules do not come within the sphere of attraction, and, therefore, combination cannot take place, although their affinity may in fact be considerable. Liquidity, on the contrary, favours chemical action; it permits the closest possible approximation, while the cohesive power is comparatively so trifling as to oppose no appreciable barrier to affinity. Cohesion may be diminished in two ways, by mechanical division, or by the application of heat. The former is useful by increasing the extent of AFFINITY. 129 surface; but it is not of itself in general sufficient, because the particles, however minute, still retain that degree of cohesion which constitutes solidity. Heat acts with greater effect, and never fails in promoting combi- nation, whenever the cohesive power is a barrier to it. Its intensity should always be so regulated as to produce liquefaction. The fluidity of one of the substances frequently suffices for effecting chemical union, as is proved by the facility with which water dissolves many salts and other solid bodies. But the cohesive force is still in operation; for a solid is commonly dissolved in greater quantity when its cohesion is diminished by heat. The reduction of both substances to the liquid state is the best method for ensuring chemi- cal action. The slight degree of cohesion possessed by liquids does not appear to cause any impediment to combination; for they commonly act as energetically on each other at low temperatures, or at a temperature just sufficient to cause perfect liquefaction, as when their cohesive power is still further diminished. It seems fair to infer, therefore, that very little, if any, affinity exists between two bodies which do not combine when they arc intimately mixed in a liquid state. The phenomena of crystallization are owing to the ascendancy of cohesion over affinity. When a large quantity of salt has been dissolved in water by the aid of heat, part of the saline matter generally separates as the solution cools, because the cohesive power of the salt then becomes comparatively too powerful for chemical attraction. Its particles begin to cohere together and are deposited in crystals, the process of crystallization continuing till it is arrested by the affinity of the liquid. A similar change happens when a solution made in the cold is gradually evaporated. The cohesion of the saline particles is no longer counteracted by the affinity of the liquid, and the salt, therefore, assumes the solid form. Cohesion plays a still more important part. It sometimes determines the result of chemical action, probably even in opposition to affinity. Thus, on mixing together a solution of two acids and one alkali, of which two salts may be formed, one soluble, and the other insoluble, the alkali will unite with that acid with which it forms the insoluble compound, to the total exclusion of the other. This is one of the modifying circumstances em- ployed by Berthollet to account for the phenomena of single elective attraction, and is certainly applicable to many of the instances to be found in the tables of affinity. When, for example, hydrochloric acid, sulphuric acid, and baryta, are mixed together, sulphate of baryta is formed in conse- quence of its insolubUity. Lime, which yields an insoluble salt with carbonic acid, separates that acid from ammonia, potassa, and soda, with all of which it makes soluble compounds. A similar explanation may be given of many cases of double elective attraction. On mixing together in solution four substances, a, b, c, d, of which it is possible to form four compounds, ab and cd, or ac and bd, that compound will certainly be produced, which happens to be insoluble. Thus sulphuric acid, soda, nitric acid, and baryta, may give rise either to sulphate of soda and nitrate of baryta, or to sulphate of baryta and nitrate of soda; but the first two salts cannot exist together in the same liquid, because the insoluble sulphate of baryta is instantly generated, and its formation neces- sarily causes the nitric acid to combine with the soda. In like manner, a solution of nitrate of lime is decomposed by carbonate of ammonia, in consequence of the insolubility of carbonate of lime. To comprehend the manner in which cohesion acts in these instances, it is necessary to consider what takes place when in the same liquid two or more compounds are brought together, which do not give rise to an insolu- ble substance. Thus on mixing solutions of sulphate of potassa and nitrate of soda, no precipitate ensues; because the salts capable of being formed by double decomposition, sulphate of soda and nitrate of potassa, are likewise soluble. In this case it is possible either that each acid may be confined to one base, so as to constitute two neutral salts; or that each acid may be di- vided between both bases, yielding four neutral salts. It is difficult to decide 130 AFFINITY. this point in an unequivocal manner: but judging from many chemical phenomena, there can, I apprehend, be no doubt that the arrangement last mentioned is the most frequent, and is probably universal whenever the relative -forces of affinity are not very unequal. When two acids and two bases meet together in neutralizing proportion, it may, therefore, be inferred, that each acid unites with both the bases in a manner regulated by their respective forces of affinity, and that four salts are contained in solution. In like manner the presence of three acids and three bases will give rise to nine salts; and when four of each are present, sixteen salts will be produced. This view affords the most plausible theory of the constitution of mineral waters, and of the products which they yield by evaporation. The influence of insolubility in determining the result of chemical action may be readily explained on this principle. If nitric acid, sulphuric acid, and baryta, are mixed together in solution, the base may be conceived to be at first divided between the two acids, and nitrate and sulphate of baryta to be generated. The latter being insoluble is instantly removed beyond the influence of the nitric acid, so that for an instant nitrate of baryta and free sulphuric acid remain in the liquid; but as the base left in solution is again divided between the two acids, a fresh quantity of the insoluble sulphate is generated; and this process of partition continues, until either the baryta or the sulphuric acid is withdrawn from the solution. Similar changes ensue when nitrate of baryta and sulphate of soda are mixed. The separation of salts by crystallization from mineral waters or other sa- line mixtures is explicable by a similar mode of reasoning. Thus on mixing nitrate of potassa and sulphate of soda, four salts, according to this view, are generated, namely, the sulphates of soda and potassa, and the nitrates of those bases; and if the solution be allowed to evaporate gradually, a point at length arrives when the least soluble of these salts, the sulphate of potassa, will be disposed to crystallize. As soon as some of its crystals are deposited, and thus withdrawn from the influence of the other salts, the constituents of these undergo a new arrangement, whereby, an additional quantity of sul- phate of potassa is generated; and this process continues until the greater part of the sulphuric acid and potassa has combined, and the compound is removed by crystallization. If the difference in solubility is considerable, the separation of salts may be often rendered very complete by this method. The efflorescence of a salt is sometimes attended with a similar result. If carbonate of soda and chloride of calcium are mingled together in solu- tion, the insoluble carbonate of lime subsides. But if carbonate of lime and sea-salt are mixed in the solid state, and a certain degree of moisture is present, carbonate of soda and chloride of calcium are slowly generated ; and since the former, as soon as it is formed, separates itself from the mixture by efflorescence, its production continues progressively. The efflorescence of carbonate of soda, which is sometimes seen on old walls, or which in some countries is found on the soil, appears to have originated in this manner. Elasticity.—From the obstacle which cohesion puts in the way of affinity, the gaseous state, in which the cohesive power is wholly wanting, might be expected to be peculiarly favourable to chemical action. The reverse, how- ever, is the fact. Bodies evince little disposition to unite when presented to each other in the elastic form. Combination does indeed sometimes take place, in consequence of a very energetic attraction; but examples of an opposite kind are much more common. Oxygen and hydrogen gases, and chlorine and hydrogen, though their mutual affinity is very powerful, may be preserved together for any length of time without combining. This want of action seems to arise from the distance between the particles preventing that close approximation which is so necessary to the successful exercise of affinity. Hence many gases cannot be made to unite directly, which never- theless combine readily while in their nascent state; that is, while in the act of assuming the gaseous form by the decomposition of some of their solid or fluid combinations. AFFINITY. 131 Elasticity operates likewise as a decomposing agent. If two gases, the reciprocal attraction of which is feeble, suffer considerable condensation when they unite, the compound will be decomposed by very slight causes. Chloride of nitrogen, which is an oil-like liquid, composed of the two gases chlorine and nitrogen, affords an apt illustration of this principle, being distinguished for its remarkable facility of decomposition. Slight elevation of temperature, by increasing the natural elasticity of the two gases, or contact of substances which have an affinity for either of them, produces immediate explosion. Many familiar phenomena of decomposition are owing to elasticity. All compounds that contain a volatile and a fixed principle, are liable to be decomposed by a high temperature; The expansion occasioned by heat removes the elements of the compound to a greater distance from each other, and thus, by diminishing the force of chemical attraction, favours the tendency of the volatile principle to assume the form which is natural to it. The evaporation of water from a solution of salt is an instance of this kind. Many solid substances, which contain water in a state of intimate com- bination, part with it in a strong heat, in consequence of the volatile nature of that liquid. The separation of oxygen from some metals, by heat alone, is explicable on the same principle. From these and some preceding remarks, it appears that the influence of heat over affinity is variable; for at one time it promotes chemical union, and opposes it at another. Its action, however, is always consistent. When- ever the cohesive power is an obstacle to combination, heat favours affinity either by diminishing the cohesion of a solid, or by converting it into a liquid. As the cause of the gaseous state, on the contrary, it keeps at a distance particles which would otherwise unite; or, by producing expansion, it lends to separate from one another substances which are already com- bined. There is one effect of heat which seems somewhat anomalous; namely, the combination which ensues in gaseous explosive mixtures on the approach of flame. The explanation given by Berthollet is probably correct, —that the sudden dilatation of the gases in the immediate vicinity of the flame, acts as a violent compressing power to the contiguous portions, and thus brings them within the sphere of their attraction. Some of the decompositions, which were attributed by Bergmann to the sole influence of elective affinity, may be ascribed to elasticity. If three substances are mixed together, two of which can form a compound which is less volatile than the third body, the last will, in general, be completely driven off by the application of heat. The decomposition of the salts of ammonia by the pure alkalies or alkaline earths, may be adduced as an example; and for the same reason, all the carbonates are decomposed by hydrochloric acid, and all the hydrochlorates by sulphuric acid- This explanation applies equally well to some cases of double decomposition. It explains, for instance, why the dry carbonate of lime will decompose hydro- chlorate of ammonia by the aid of heat; for carbonate of ammonia is more volatile than the hydrochlorate either of ammonia or lime. The influence of elasticity in determining the result of chemical action in these instances, seems owing to the same cause which enables insolubility to be productive of similar effects. Thus, on mixing hydrochlorate of ammonia with lime, the acid is divided between the two bases; some ammonia becomes free, which, in consequence of its elasticity, is entirely expelled by a gentle heat. The acid of the remaining hydrochlorate of ammonia is again divided between the two bases; and if a sufficient quantity of lime is present, the ammoniacal salt will be completely decomposed. In like man- ner the decomposition of potassa may be effected by iron, though the affinity of this metal for oxygen seems much inferior to that of potassium for oxygen. If potassa in the fused state be brought in contact with metallic iron at a white heat, the oxygen is divided between the two metals, and a portion of potassium set at liberty. But as potassium is volatile at a white heat, it is expelled at the instant of reduction; and thus, by its influence being withdrawn, an opportunity is given for the decomposition of an additional quantity of potassa. 132 AFFINITY. Quantity of Matter. The influence of quantity of matter over affinity is universally admitted. If one body a unites with another body b in several proportions, that compound will be most difficult of decomposition which contains the smallest quantity of b. Of the three oxides of lead, for instance, the peroxide parts most easily with its oxygen by the action of heat; a higher temperature is required to decompose the red oxide; and the protoxide will bear the strongest heat of our furnaces without losing a particle of its oxygen. The influence of quantity over chemical attraction may be further illus- trated by the phenomena of solution. When equal weights of a soluble salt are added in succession to a given quantity of water, which is capable of dissolving almost the whole of the salt employed, the first portion of the salt will disappear more readily than the second, the second than the third, the third than the fourth, and so on. The affinity of the water for the saline substance diminishes with each addition, till at last it is weakened to such a degree as to be unable to overcome the cohesion of the salt. The process then ceases, and a saturated solution is obtained. Quantity of matter is employed advantageously in many chemical opera- tions. If, for instance, a chemist is desirous of separating an acid from a metallic oxide by means of the superior affinity of potassa for the former, he frequently uses rather more of the alkali than is sufficient for neutralizing the acid. He takes the precaution of employing an excess of alkali, in order the more effectually to bring every particle of the substance to be decomposed in contact with the decomposing agent. But Berthollet has attributed a much greater influence to quantity of mat- ter. It was the basis of his doctrine, developed in the Statique Chimique, that bodies cannot be wholly separated from each other by the affinity of a third substance for one element of a compound; and to explain why a su- perior chemical attraction does not produce the effect which might be ex- pected from it, he contended that quantity of matter compensates for a weaker affinity. From the co-operation of several disturbing causes, Berthollet per- ceived that the force of affinity cannot be estimated with certainty by observ- ing the order of decomposition; and he, therefore, had recourse to another method. He set out by supposing that the affinity of different acids for the same alkali, is in the inverse ratio of the ponderable quantity of each which is necessary for neutralizing equal quantities of the alkali. Thus, if two parts of one acid a, and one part of another acid b, are required to neutralize equal quantities of the alkali c, it was inferred that the affinity of b for c was twice as great as that of a. He conceived, further, that as two parts of a produce the same neutralizing effect as one part of b, the attraction exerted by any alkali towards two parts of a ought to be precisely the same as for the one part of b ; and he hence concluded that there is no reason why the alkali should prefer the small quantity of one to the large quantity of the other. On this he founded the principle that quantity of matter compensates for force of attraction. Berthollet has here obviously confounded two things, namely, force of at- traction and neutralizing power, which are really different, and ought to be held distinct. The relative weights of hydrochloric and sulphuric acids re- quired to neutralize an equal quantity of any alkali, or, in other words, their capacities of saturation, are as 36.4 to 40, a ratio which remains constant with respect to all other alkalies. The affinity of these acids, according to Ber- thollet's rule, will be expressed by the same numbers. But in taking this estimate, wc have to make three assumptions, each of which is disputable. There is no proof, in the first place, that hydrochloric acid has a greater af- finity for an alkali, such as potassa, than sulphuric acid. Such an inference would be directly opposed to the general opinion founded on the order of de- composition ; and though that order, as we have shown, is by no means a satisfactory test of the strength of affinity, it would be improper to adopt an opposite conclusion without having good reasons for so doing. Secondly, were it established that hydrochloric acid has the greater affinity, it does not AFFINITY. 133 follow that the attraction of those acids for potassa is in the ratio of 36.4 to 40. And, thirdly, supposing this point settled, it is very improbable that the ratio of their affinities for one alkali will apply to all others; analogy would lead us to anticipate the reverse. Independently of these objections, M. Du- long has found that the principle of Berthollet is not in accord with the results of experiment. Gravity.—The influence of gravity is perceptible when it is wished to make two substances unite, the densities of which are different. In a case of sim- ple solution, a larger quantity of saline matter is found at the bottom than at the top of the liquid, unless the solution shall have been well mixed subse- quently to its formation. In making an alloy of two metals which differ from one another in density, a larger quantity of the heavier metal will be found at the lower than in the upper part of" the compound; unless great care be taken to counteract the tendency of gravity by agitation. This force obviously acts, like the cohesive power, in preventing a sufficient degree of approximation. Imponderables.—The influence which heat exerts over chemical phenomena, and the modes in which it operates, have been already discussed. The che- mical agency of galvanism has also been described. The effects of light will be most conveniently stated in other parts of the work. Electricity is frequently employed to produce the combination of gases with one another, and in some instances to separate them. It appears to act by the heat which it occasions, and, therefore, on the same principle as flame. MEASURE OF AFFINITY. As the foregoing observations prove that the order of decomposition is not always a satisfactory measure of affinity, it becomes a question whether there are any means of determining the comparative forces of chemical attraction. When no disturbing causes operate, the phenomena of decomposition afford a sure criterion; but when the conclusions obtained in this way are doubtful, assistance may be frequently derived from other sources. The surest indi- cations are procured by observing the ■ tendency of different substances to unite with the same principle, under the same circumstances, and subse- quently by marking the comparative facility of decomposition when the compounds so formed are exposed to the same decomposing agent. Thus, on exposing silver, lead, and iron, to air and moisture, the iron soon rusts, the lead is oxidized in a slight degree only, and the silver resists oxidation al- together. It is hence inferred that iron has the greatest affinity for oxygen, lead next, and silver the least. This conclusion is supported by concurring observations of a like nature, and confirmed by the circumstances under which the oxides of those metals part with their oxygen. Oxide of silver is reduced by heat only; and oxide of lead is decomposed by charcoal at a lower temperature than oxide of iron. It is inferred from the action of heat on the carbonate of potassa, baryta, lime, and oxide Of lead, that potassa has a stronger attraction for carbonic acid than baryta, baryta than lime, and lime than oxide of lead. The affinity of different substances for water may be determined in a similar manner. Of all chemical substances, our knowledge of the relative degrees of at- traction of acids and alkalies for each other is the most uncertain. Their action on one another is affected by so many circumstances, that it is in most cases impossible, with certainty, to refer any effect to its real cause. The only methods that have been hitherto devised for remedying this defect are those of Berthollet and Kirwan. Both of them are founded on the capacities of saturation, and the objections v/hich have been urged to the rule suggested by the former philosopher apply equally to that proposed by the latter. But this uncertainty is of no great consequence in practice. We know perfectly the order of decomposition, whatever may be the actual forces by which it is effected. 12 134 / LAWS OF COMBINATION. SECTION II. PROPORTIONS IN WHICH BODIES UNITE, AND THE LAWS OF COMBINATION. The study of the proportions in which bodies unite naturally resolves itself into two parts. The first includes compounds whose elements appear to unite in a great many proportions; the second comprehends those, the elements of which combine in a few proportions only. I. The compounds contained in the first division are of two kinds. In one, combination takes place unlimitedly in all proportions; in the other, it occurs in every proportion within a certain limit. The union of water with alcohol and the liquid acids, such as the sulphuric, hydrochloric, and nitric acids, affords instances of the first mode of combination; the solutions of salts in water are examples of the second. One drop of sulphuric acid may be diffused through a gallon of water, or a drop of water through a gallon of the acid; or they may be mixed together in any intermediate proportions; and nevertheless in each case they appear to unite perfectly with each other. A hundred grains of water, on the contrary, will dissolve any quantity of sea- salt which does not exceed forty grains. Its solvent power then ceases, be- cause the cohesion of the solid becomes comparatively too powerful for the force of affinity. The limit to combination is in such instances owing to the cohesive power; and but for the obstacle which it occasions, the salt would most probably unite with water in every proportion. All the substances that unite in many proportions, give rise to compounds which have this common character, that their elements are united by a fee- ble affinity, and preserve, when combined, more or less of the properties which they possess in a separate state. In a scientific point of view, these combinations are of minor importance; but they are exceedingly useful as instruments of research. They enable the chemist to present bodies to each other under circumstances peculiarly favourable for acting with effect: the liquid form is thus communicated to them ; while the affinity of the solvent or menstruum, which holds them in solution, is not sufficiently powerful to interfere with their mutual attraction. *• II. The most interesting series of compounds is produced by substances which unite in a few proportions only; and which, in combining, lose more or less completely the properties that distinguish them when separate. Of these bodies, some form but one combination. Thus there is only one com- pound of boron and oxygen, and of chlorine and hydrogen. Others combine in two proportions. For example, two compounds are formed by tin and oxygen, and by hydrogen and oxygen. Other bodies again unite in three, four, five, or even six proportions, which is the greatest 'number of com- pounds that any two substances are known to produce, except perhaps car- bon and hydrogen, and those which belong to the first division. The combination of substances that unite in a few proportions only, is re- gulated by the three following remarkable laws :— 1. The first of these laws is, that the composition of bodies is fixed and invariable. A compound substance, so long as it retains its characteristic properties, always consists of the same elements united together in the same proportion. Sulphuric acid, for example, is always composed of sulphur and oxygen in the ratio of 16 parts* of the former to 24 of the latter: no other elements can form it, nor can it be produced by its own elements in any other proportion. Water, in like manner, is formed of 1 part of hydrogen By the expression 'parts' I always mean parts by weight LAWS OF COMBINATION. 135 and 8 of oxygen; and were these two elements to unite in any other pro- portion, some new compound, different from water, would be the product. The same observation applies to all other substances, however complicated, and at whatever period they were produced. Thus, sulphate of baryta, whether formed ages ago by the hand of nature, or quite recently by the operations of the chemist, is always composed of 40 parts of sulphuric acid and 76.7 of baryta. This law, in fact, is universal and permanent. Its im- portance is equally manifest: it is the essential basis of chemistry, without which the science itself could have no existence. Two views have been proposed by way of accounting for this law. The explanation now universally given is confined to a mere statement, that sub- stances are disposed to combine in those proportions to which they are so strictly limited, in preference to any others; it is regarded as an ultimate fact, because the phenomena are explicable on no other known principle. A different doctrine was advanced by Berthollet, in his Statique Chimique, published in 1803. Having observed the influence of cohesion and elasticity in modifying the action of affinity as already described, he thought he could trace the operations of the same causes in producing the effect at present under consideration. Finding that the solubility of a salt and of a gas in water is limited, in the former by cohesion, and in the latter by elasticity, he conceived that the same forces would account for the unchangeable com- position of certain compounds. He maintained, therefore, that within cer- tain limits bodies have a tendency to unite in every proportion ; and that combination is never definite and invariable, except when rendered so by the operation of modifying causes, such as cohesion, insolubility, elasticity, quantity of matter, and the like. Thus, according to Berthollet, sulphate of baryta is composed of 40 parts of sulphuric acid and 76.7 of baryta, not be- cause those substances are disposed to unite in that ratio rather than in an- other, but because the compound so constituted happens to have great cohe- sive power. These opinions, which, if true, would shake the whole science of chemis- try to its foundation, were founded on observation and experiment, supported by all the ingenuity of that highly gifted philosopher. They were ably and successfully combated by Proust in several papers published in the Journal de Physique, wherein he proved that the metals are disposed to combine with oxygen and with sulphur only in one or two proportions, which are definite and invariable. The controversy which ensued between these emi- nent chemists, is remarkable for the moderation with which it was con- ducted on both sides, and has been properly quoted by Berzelius as a model to controversialists. How much soever opinion may have been divided upon the question at that period, the controversy is now at an end. The great va- riety of new facts, similar to those observed by Proust, which have since been established, has proved beyond a doubt that the leading principle of Berthollet is erroneous. The tendency of bodies to unite in definite propor- tions only, is indeed so great as to excite a suspicion that all substances combine in this way ; and that the exceptions thought to be afforded by the phenomena of solution, are rather apparent than real; for it is conceivable that the apparent variety of proportion, noticed in such cases, may arise from the mixture or combination of a few definite compounds with each other. 2. The second law of combination is, that the relative quantities in which bodies unite may be expressed by proportional numbers. Thus, 8 parts of oxygen unite with 1 part of hydrogen, 16 of sulphur, 35.4 of chlorine, 39.6 of selenium, and 108 parts of silver. Such are the quantities of these five bodies which are disposed to unite with 8 parts of oxygen; and it is found that when they combine with one another, they unite either in the propor- tions expressed by those numbers, or in multiples of them according to the third law of combination. Hydrosulphuric acid, for instance, is composed of 1 part of hydrogen and 16 of sulphur, and bisulphuret of hydrogen of 1 part of hydrogen to 32 of sulphur; 35.4 of chlorine unite with 1 of hydro- 136 LAWS OF COMBINATION. gen, 16 of sulphur, and 108 of silver; and 39.6 parts of selenium with 1 of hydrogen, and 16 of sulphur. From the occurrence of such proportional numbers has arisen the use of certain terms, as Proportion, Combining Proportion, Proportional, and Che- mical Equivalent, or Equivalent, to express them. The latter term, intro- duced by Dr. Wollaston, and which I shall commonly employ, was suggest- ed by the circumstance that the combining proportion of one body is, as it were, equivalent to that of another body, and may be substituted for it in combination. At page 141 will be found a table of the equivalents of ele- mentary substances. This law does not apply to elementary substances only, since compound bodies have their combining proportions or equivalents, which may likewise be expressed in numbers. Thus, since water is composed of one equivalent or 8 parts of oxygen, and one equivalent or 1 of hydrogen, its combining proportion or equivalent is 9. The equivalent of sulphuric acid is 40, be- cause it is a compound of one equivalent or 16 parts of sulphur, and three equivalents or 24 parts of oxygen ; and in like manner, the combining pro- portion of hydrochloric acid is 36.4, because it is a compound of one equiva- lent or 35.4 parts of chlorine, and one equivalent or 1 part of hydrogen. The equivalent number of potassium is 39, and as that quantity combines with 8 of oxygen to form potassa, the equivalent of the latter is 39-f-8=47. Now when these compounds unite, one equivalent of the one combines with one, two, three, or more equivalents of the other, precisely as the simple sub- stances do. The hydrate of potassa, for example, is constituted of 47 parts of potassa and 9 of water, and its equivalent is consequently 47-f-9, or 56. The sulphate of potassa is composed of 40 sulphuric acid-f-47 potassa; and the nitrate of the same alkali of 54 nitric acid-f-47 of potassa. The equivalent of the former salt is, therefore, 87, and of the latter 101. The composition of the salts affords a very instructive illustration of this subject; and to exemplify it still further, a list of the equivalents of a few acids and alkaline bases is annexed:— 18 20.7 28.5 31.3 47 51.8 76.7 It will be seen at a glance that the neutralizing power of the different alka- lies is very different; for the equivalent of each base expresses the quantity required to neutralize an equivalent of each of the acids. Thus 18 of lithia, 31.3 of soda, and 76.7 of baryta, combine with 54 of nitric acid, forming the neutral nitrates of lithia, soda, and baryta. The same fact is obvious with respect to the acids; for 35.7 of phosphoric, 40 of sulphuric, and 57.7 of arsenic acid unite with 76.7 of baryta, forming a neutral phosphate,-sul- phate, and arseniatc of baryta. These circumstances afford a ready explanation of a curious fact, first noticed by the Saxon chemist Wenzel; namely, that when two neutral salts mutually decompose each other, the resulting compounds are likewise neu- tral. The cause of this fact is now obvious. If 71.3 parts of neutral sul- phate of soda are mixed with 130.7 of nitrate of baryta, the 76.7 parts of baryta unite with 40 of sulphuric acid, and the 54 parts of nitric acid of the nitrate combine with the 3L3 of soda of the sulphate, not a particle of acid or alkali remaining in an uncombined condition. Sulphate of Soda. Nitrate of Baryta. Sulphuric acid 40 54 Nitric acid, Soda . . 31.3 76.7 Baryta, Hydrofluoric acid 19.7 Lithia Phosphoric acid 35.7 Magnesia Hydrochloric acid 36.4 Lime Sulphuric acid 40 Soda Nitric acid 54 Potassa Arsenic acid 57.7 Strontia Selenic acid 63.6 Baryta 71.3 130,7 LAWS OF COMBINATION. 137 Hydrogen 1 Oxygen 8 i 1 Do. 16(2 Do. 1 Carbon 6 Do. 8 ? 1 Do. 16 $2 Do. 6 Nitrogen 14 Do. 8\ 1 Do. 14 Do. 16 2 Do. 14 Do. 24 U Do. 14 Do. 32 1 4 Do. 14 Do. 40 J 5 It matters not whether more or less than 71.3 parts of sulphate of soda are added; for if more, a small quantity of sulphate of soda will remain in solution; if less, nitrate of baryta will be in excess; but in either case the neutrality will be unaffected. 3. The third law of combination is, that when one body a unites with another body b in two or more proportions, the quantities of the latter, united with the same quantity of the former, bear to each other a very sim- ple ratio. The progress of chemical research, in discovering new com- pounds and ascertaining their exact composition, has shown that these ratios of b may be represented by one or other of the two following series:— 1st Series, a unites with 1, 2, 3, 4, 5, &c. of b. 2d Series, a unites with, 1, 1£, 2, 2^, &c. of b. The first series is exemplified by the subjoined compounds.— Water is composed of Binoxide of hydrogen Carbonic oxide Carbonic acid Nitrous oxide Nitric oxide Hyponitrous acid Nitrous acid Nitric acid It is obvious that in all these eompounds the ratios of the oxygen are ex- pressed by whole numbers. In water the hydrogen is combined with half as much oxygen as in the binoxide of hydrogen, so that the ratio is as 1 to 2. The same relation holds in carbonic oxide and carbonic acid. The oxygen in the compounds of nitrogen and oxygen is in the ratio of 1, 2, 3, 4, and 5. In like manner the ratio of sulphur in the two sulphurets of mercury, and that of chlorine in the two chlorides of mercury, is as 1 to 2. So, in bicarbonate of potassa, the alkali is united with twice as much car- bonic acid as in the carbonate; and the acid of the three oxalates of potassa is in the ratio of 1, 2, and 4. The following compounds exemplify the second series:— Protoxide of iron consists of Iron 28 Oxygen 8 > 1 Peroxide . . Do. 28 Do. 12 (l£ Protoxide of manganese . Manganese 27.7 Do. 8 11 Sesquioxide . . Do. 27.7 Do. 12 S LJ Binoxide . Do. 27.7 Do. 16)2 Arsenious acid . . Arsenic 37.7 Do. 12 i lj Arsenic acid . . Do. 37.7 Do. 20 < 2^ Hypophosphorous acid . Phosphorus 15.7 Do. 4 i £ Phosphorous acid . . Do. 15.7 Do. 12 > l| Phosphoric acid . . Do. 15.7 Do. 20 \ 2£ Both of these series, which together constitute the third law of combi- nation, result naturally from the operation of the second law. The first series arises from one equivalent of a body uniting witli 1, 2, 3, or more equivalents of another body. The second series is a consequence of two equivalents of one substance combining with 3, 5, or more equivalents of another. Thus if two equivalents of phosphorus unite both with 3 and with 5 equivalents of oxygen, we obtain the ratio of \\ to 2£; and should one equivalent of iron combine with one of oxygen, and another compound be formed of two equivalents of iron to three of oxygen, then the oxygen united with the same weight of iron would have the ratio, as in the table, of 1 to 1£. The compounds of manganese and phosphorus with oxygen afford ex- amples of the same nature. Still more complex arrangements will be readily conceived, such as 3 equivalents of one substance to 4, 5, or more of another. But it is remarkable that combinations of the kind are very rare { and even their existence, though theoretically possible, has not been decidedly esta- 12* 138 LAWS OF COMBINATION. Wished. Even some of the compounds which are usually included in the second series belong properly to the first. The red oxide of lead, for in- stance, appears in its chemical relations not so much as a direct compound of lead and oxygen, but as a kind of salt formed by the union of the bin- oxide of lead with the protoxide of the same metal. On this supposition the two other oxides belong to the first series. The merit of establishing the first law of combination seems justly due to Wenzel, a Saxon chemist; and the second law is also deducible from his experiments on the composition of the salts. His work, entitled Lehre der Verwandtshaft, was published in 1777. Bergmann and Richter, a few years after, confirmed the observations of Wenzel, though without adding materially in the way of generalization. The late Mr. Higgins, also, in 1789, speculated on the atomic constitution of compound bodies in a man- mer which, if pursued, would have led to the discovery of Dalton. It is to the latter, science is indebted for deducing from the scattered facts which had been previously collected, a theory of chemical union, embracing the whole science, and giving it a consistency and form which before his time it had never possessed. In his hands the second law of combination first at- tained its full generality; but the discovery which is more peculiarly his own, is that part of the third law of combination which is contained in the first of the two series above mentioned. The first public announcement of his views appears to have been made to the Philosophical Society of Man- chester in 1803; and in 1808 they were explained in his New System of Chemical Philosophy. In the same year Dr. Wollaston and Dr. Thomson gave their evidence in support of the new doctrine, and other chemists have followed in the same path of inquiry. But of all who have successfully la- boured in establishing the laws of combination, the most splendid contribu- tion is that of the celebrated Berzelius. Struck with the perusal of the works of Richter, he commenced in 1807 an investigation into the laws of definite proportion. Since that period his labours in this important field have been incessant, and every department of the science has been enrich- ed by his skill and indefatigable industry. Whether we look to pneumatic chemistry, to the chemical history of the metals and of the salts, or to the composition of minerals, we are alike indebted to Berzelius. In all has he traced the laws of definite proportion, and, by a multitude of exact analyses, given to the laws of combination that certainty which accumulated facts can alone convey. The utility of being acquainted with these important laws is almost too manifest to require mention. Through their aid. and by remembering the equivalents of a few elementary substances, the composition of an extensive range of compound bodies may be calculated with facility. Thus, by know- ing that 6 is the equivalent of carbon and 8 of oxygen, it is easy to recollect the composition of carbonic oxide and carbonic acid; the first consisting of 6 parts of carbon -}- 8 of oxygen, and the second of 6 carbon -4- 16 of oxygen. The equivalent of potassium is 39; and potassa, its protoxide, is composed of 39 of potassium -4- 8 of oxygen. From these few data, we know at once the composition of carbonate and bicarbonate of potassa; the former being composed of 22 parts of carbonic acid -f- 47 potassa, and the latter of 44 carbonic acid -j- 47 potassa. This method acts as an artificial memory, the advantage of which, compared with the common practice of stating the composition in 100 parts, will be manifest by inspecting the fol- lowing quantities, and attempting to recollect them. Carbonic Oxide. Carbonic Acid. Carbon 42.86 .... 27.27 Oxygen 57.14 .... 72.73 Carbonate of Potassa. Bicarbonate of Potassa. Carbonic acid 31.43 .... 47.83 Potassa 68.57 .... 52.17 LAWS OF COMBINATION. 139 From the same data, calculations, which would otherwise be difficult or tedious, may be made rapidly and with ease, without reference to books, and frequently by a simple mental process. The exact quantities of substances required to produce a given effect may be determined with certainty, thus affording information which is often necessary to the success of chemical processes, and of great consequence both in the practice of the chemical arts, and in the operations of pharmacy. The same knowledge affords a good test to the analyst by which he may judge of the accuracy of his result, and even sometimes correct an analysis which he has not the means of performing with rigid precision. Thus a powerful argument for the accuracy of an analysis is derived from the cor- respondence of its result with the laws of chemical union. On the contrary, if it form an exception to them, we are authorized to regard it as doubtful; and may hence be led to detect an error, the existence of which might not otherwise have been suspected. If an oxidized body be found to contain one equivalent of the combustible with 7.99 of oxygen, it is fair to infer that 8, or one equivalent of oxygen, would have been the result, had the analysis been perfect. The composition of a substance may sometimes be determined by a cal- culation, founded on the laws of chemical union, before an analysis of it has been accomplished. When the new alkali lithia was first discovered, che- mists did not possess it in sufficient quantity for determining its constitution analytically. But the neutral sulphates of the alkalies and alkaline earths are known to be composed of one equivalent of each constituent, and the oxides to contain one equivalent of oxygen. If it be found, therefore, by analysis, that neutral sulphate of lithia is composed of 40 parts of sulphuric acid and 1$ of lithia, it may be inferred, since 40 is one equivalent of the acid, that 16is the equivalent for lithia; and that this oxide is formed of 8 parts of oxygen and 10 of lithium. The method of determining equivalent numbers will be anticipated from what has already been said. The commencement is made by carefully ana- lyzing a definite compound of two simple substances which possess an ex- tensive range of affinity. Thus water, a compound of oxygen and hydrogen, is found to contain * parts of the former to 1 of the latter: and if it be as- sumed that water consists of one equivalent of oxygen and one of hydrogen, the relative weights of these equivalents will be as 8 to 1. The chemist then selects for analysis such compounds as he believes to contain one equivalent of each element, in which either oxygen or hydrogen, but not both, is pre- sent. Carbonic oxide and hydrosulphuric acid are suited to his purpose : as the former consists of 8 parts of oxygen and 6 of carbon, and the latter of 1 part of hydrogen and 16 of sulphur, the equivalent of carbon is inferred to be 6, and that of sulphur 16. The equivalents of all the other elements may be determined in a similar manner. In researches on chemical equivalents there are two kinds of difficulty, one involved in the processes for ascertaining the exact composition of com- pounds, and the other in the selection of the compounds which contain sin- gle equivalents. Important general precautions in the experimental part of the subject are the following:—1, to exert scrupulous care about the purity of materials; 2, to select methods which consist of a few simple operations only; 3, to repeat experiments, and with materials prepared at different times; 4, to arrive at the same conclusion by two or more processes inde- pendent of each other. In the selection of compounds of single equivalents, there are several circumstances calculated to direct the judgment:— ]. If two substances combine in several proportions, the law of multiples usually affects the electro-negative element of a compound. Thus, in the 5 compounds of nitrogen and oxygen, in which oxygen is the negative ele- ment, 14 parts of nitrogen are united with 8, 16, 24, 32, and 40 parts of oxygen; whereas, taking the quantity of oxygen as constant, 8 parts of oxy- gen are united with 14, 7, 4.66, 3.5, and 2.8 parts of nitrogen, in which the simple ratio of the first series does not exist. This circumstance induces the 140 LAWS OF COMBINATION. , chemist always to search among the oxides of the same element for the lowest grade of oxidation, and in most cases to consider it as a compound oT single equivalents. In some instances, however, the second degree of oxidation is formed of single equivalents, while the lowest oxide consists of two equiva- lents of the positive element and one of oxygen. Such compounds are call- ed dioxides (page 124) and sometimes suboxides. 2. Metallic oxides, distinguished for strong alkalinity or for acting as strong alkaline bases, are always protoxides. Dioxides rarely, if ever, unite definitely with acids, and are remarkable for their ready conversion into protoxides with separation of metal. If the same metal yield several oxides, the protoxide is the strongest base; the highest grade of oxidation is fre- quently an acid, and the intermediate oxides are in general little distin- guished either for alkalinity or acidity. Protoxides usually resist decompo- sition more obstinately than other oxides. 3. When a metal forms two oxides, the oxygen of which is in the ratio of 1 to l£,-the first is usually the protoxide, and the second a compound of two equivalents of the metal to three of oxygen. The oxides of iron and nickel are examples. 4. If two compounds resemble each other in their modes of combination, it is a strong presumption that their constitution is similar. Alumina and the peroxide of iron are remarkably allied in their chemical relations; and hence it is inferred, since the latter consists of two eq. of iron and three eq. of oxy- gen, that the former, whose composition would otherwise be very doubtful, is composed of two eq. of aluminium and three eq. of oxygen. 5. Mitscherlich has found, as is more fully stated in the article on crys- tallization, that certain compounds which resemble each other in composi- tion and in their modes of combining, are likewise disposed in crystallizing to affect the same form. Hence it is a strong presumption that compounds which are analogous both in their crystalline figure and modes of combin- ing, are also similar in their composition. In the oxide and acid of chro- mium, the oxygen is in the ratio 1 to 2, and hence it was at first supposed that one eq. of chromium was united in the oxide with one eq. and in the acid with two equivalents of oxygen. But the chromatcs resemble the sul- phates in form and modes of combining, and the oxide of chromium bears the same analogy to alumina and peroxide of iron. The inference is that oxide of chromium consists of two eq. of chromium and three eq. of oxygen, and chromic acid of one eq. of chromium and three eq. of oxygen. 6. Another guide in these inquiries is derived from the relation traced by Dulong and Petit between the equivalent of a body and its specific heat. The coincidences pointed out at page 35 are sufficiently numerous to show an interesting relation which is sometimes useful in selecting between doubtful numbers; but the instances of failure are at present too frequent to admit of this principle being used except with much caution. 7. The ready decomposition by galvanism, observed by Mr. Faraday, of compounds which consist of single equivalents, and the resistance to the same agent of many others not so constituted, promises to become an indi- cation of great value in determining equivalent numbers. The facts as yet known respecting it will be found in the section on galvanism. 8. Great light is often thrown on the chemical constitution of a compound by a knowledge of the volumes of the substances of which it is composed. This subject, however, will be discussed in an after part of this section. Since the equivalents merely express the relative quantities of different substances which combine together, it is in itself immaterial what figures arc employed to express them. The only essential point is, that the relation should be strictly observed. Thus, the equivalent of hydrogen may be as- sumed as 10; but then oxygen must be 80, carbon 60, and sulphur 160. We may call hydrogen 100 or 1000; or, if it were desirable to perplex the subject as much as possible, some high uneven number might be selected, provided the due relation between the different numbers Were faithfully pre- served. But such a practice would effectually do away with the advantage LAWS OF COMBINATION. 141 above ascribed to the use of equivalents; and it is the object of every one to employ such as are simple, that their relation may be perceived by mere in- spection. Dr. Thomson makes oxygen 1, so that hydrogen is eight times less than unity, or 0.125, carbon 0.75, and sulphur 2. Dr. Wollaston, in his scale of chemical equivalents, estimated oxygen at 10; and hence hydrogen is 1.25, carbon 7.5, and so on. According to Berzelius, oxygen is 100. And lastly, several other chemists, such as Dalton, Davy, Henry, and others, se- lected hydrogen as their unit; and, therefore, the equivalent of oxygen is 8. One of these series may easily be reduced to either of the others by an ob- vious and simple calculation. The numbers adopted in this work refer to hydrogen as unity, and are given in the subjoined table. CHEMICAL EQUIVALENTS OF ELEMENTARY SUBSTANCES. Elements. Equivalents. Elements. Equivalents. Elements. Equivalents Aluminium 13.7 Gold . . . 199.2 Potassium 39.15 Antimony 64.6 Hydrogen . 1 Rhodium 52.2 Arsenic . 37.7 Iodine . . 126.3 Selenium 39.6 Barium . GS.7 Iridium 98.8 Silicium 7.5 Bismuth 71 Iron . . . 28 Silver . 108 Boron 10.9 Lead . . 103.6 Sodium . 23.3 Bromine 78.4 Lithium 10 Strontium 43.8 Cadmium 55.8 Magnesium 12.7 Sulphur. 16.1 Calcium 20.5 Manganese 27.7 Tellurium 32.3 Carbon 6.12 Mercury 202 Thorium 59.6 Cerium . 46 Molybdenum 47.7 Tin . . 57.9 Chlorine 35.42 i Nickel . . 29.5 Titanium 24.3 Chromium 28 Nitrogen . 14.15 Tungsten 99.7 Cobalt . 29.5 'Osmium 99.7 Vanadium 68.5 Columbium 185 i Oxygen 8 iUranium 217 Copper 31.6 Palladium . 53.3 Yttrium 32.2 Fluorine 18.68 Phosphorus 15.7 Zinc . . 32,3 Glucinium 17.7 Platinum . 98.8 (Zirconium 33.7 The preceding table is constructed principally from the published tables of Berzelius, and partly from facts supplied by my own researches. The hy- pothesis that all equivalent numbers are simple multiples of the equivalent of hydrogen, has been elsewhere shown to be untenable. (Phil. Trans. 1833, Part ii. page 523.) Whenever the experimental quantity is nearly a whole number, the last may for many purposes be used as a sufficient approxima- tion ; and, accordingly, for such elements as carbon, sulphur, nitrogen, and potassium, which are often referred to in the way of illustration, I have gen- erally adopted round numbers, as being shorter and more easily remember- ed than fractions. But on all occasions where exact calculations are con- cerned, the numbers given in the table should be employed. The useful instrument, known by the name of the Scale of Chemical Equivalents, was originally devised by Dr. Wollaston, and is a table of equivalents, comprehending all those substances which are most frequently employed by chemists in the laboratory; and it only differs from other tabular arrangements of the same kind, in the numbers being attached to a sliding rule, which is divided according to the principle of that of Gunter. From the mathematical construction of the scale, it not only serves the same purpose as other tables of equivalents, but in many instances super- sedes the necessity of calculation. Thus, by inspecting the common table of equivalents, we learn that 87 parts, or one equivalent, of sulphate of potassa contain 40 parts of sulphuric acid and 47 of potassa; but recourse must be had to calculation, when it is wished to determine the quantity of acid or alkali in any other quantity of the salt. This knowledge, on the 142 LAWS OF COMBINATION. contrary, is obtained directly by means of the scale of chemical equivalents. For example, on pushing up the slide until 100 marked upon it is in a line with the name sulphate of potassa on the fixed part of the scale, the numbers opposite to the terms sulphuric acid and potassa will give the precise quan- tity of each contained in 100 parts of the compound. In the original scale of Dr. Wollaston, for a particular account of which I may refer to the Philosophical Transactions for 1814, oxygen is taken as the standard of comparison; but hydrogen may be selected for that purpose with equal propriety, and scales of this kind have been prepared for sale by Dr. Boswell Reid of Edinburgh. A very complete scale of equivalents has been drawn up by Mr. Prideaux of Plymouth. (Phil. Mag. and Annals, viii. 430.J ATOMIC THEORY. The brief sketch which has been given of the laws of combination, will, I trust, serve to set in its true light the importance of that department of chemical science. It is founded, as may have been seen, on experiment alone; and the laws which have been stated are the mere expression of fact. It is not necessarily connected with any speculation, and may be kept wholly free from it. The error which students of chemistry are apt to commit in supposing that the laws of combination involve something uncertain or hypothetical, may easily be traced to its source. It was impossible to reflect on the regularity and constancy with which bodies obey these laws, without speculating about the cause of that regularity : and, consequently, the facts themselves were no sooner noticed than an attempt was made to explain them. Accordingly, when Dr. Dalton published his discovery of those laws, he at once incorporated the description of them with his notion of their physical cause, and even expressed the former in language suggested by the latter. Since that period, though several British chemists of eminence, and in particular Wollaston and,Davy, recommended and practised an oppo- site course, both subjects have been too commonly comprised under the head of atomic theory; and hence it has often happened that beginners have rejected the whole as hypothetical, because they could not satisfactorily distinguish those parts which are founded on fact, from those which are conjectural. All such perplexity would have been avoided, and this depart- ment of the science have been far better understood, and its value more justly appreciated, had the discussion concerning the atomic constitution of bodies been always kept distinct from that of the phenomena which it is intended to explain. When employed in this limited sense, the atomic theory may be discussed in a few words. Two opposite opinions have long existed concerning the ultimate elements of matter. It is supposed, according to one party, that every particle of matter, however small, may be divided into smaller portions, provided our instruments and organs were adapted to the operation. Their opponents contend, on the other hand, that matter is composed of certain ultimate particles or molecules, which by their nature are indivisible, and are hence termed atoms (from * not and rt/uvm to cut). These opposite opinions have from time to time been keenly contested, and with variable success, accord- ing to the acuteness and ingenuity of their respective champions. But it was at last perceived that no positive data existed capable of deciding the question, and its interest, therefore, gradually declined. The progress of modern chemistry has revived the general attention to this controversy, by affording a far stronger argument in favour of the atomic constitution of bodies than was ever advanced before, and one which I conceive is almost irresistible. We have only in fact to assume with Dalton, that all bodies are composed of ultimate atoms, the weight of which is different in different kinds of matter, and we explain at once the foregoing laws of chemical union ; and this mode of reasoning is in the present case almost decisive, because the phenomena do not appear explicable on any other supposition. According to the atomic theory, every compound is formed of the atoms LAWS OF COMBINATION. 143 of its constituents. An atom of A may unite with one, two, three, or more atoms of B. Thus, supposing water to be composed of one atom of hydro- gen and one atom of oxygen, binoxide of hydrogen will consist of one atom of hydrogen and two atoms of oxygen. If carbonic oxide is formed of one atom of carbon and one atom of oxygen, carbonic acid will consist of one atom of carbon and two atoms of oxygen. If, in the compounds of nitrogen and oxygen enumerated at page 137, the first or protoxide consist of one atom of nitrogen and one atom of oxygen, the four others will be regarded as compounds of one atom of nitrogen to two, three, four, and five atoms of oxygen. From these instances it will appear, that the law of multiple proportion is a necessary consequence of the atomic theory. There is also no apparent reason why two or more atoms of one substance may not combine with two, three, four, five, or more atoms of another; but, on the contrary, these arrangements are necessary in explanation of the not unfrequent occurrence of half equivalents, as for- merly stated. (Page 137.) Such combinations will also account for the complicated proportion noticed in certain compounds, especially in many of those belonging to the animal and vegetable kingdoms. In consequence of the satisfactory explanation which the laws of chemical union receive by means of the atomic theory, it has become customary to employ the term atom in the same sense as combining proportion or equiva- lent. For example, instead of describing water as a compound of one equivalent of oxygen and one equivalent of hydrogen, it is said to consist of one atom of each element. In like manner sulphate of potassa is said to be formed of okc atom of sulphuric acid and one atom of potassa; the word in this case denoting as it were a compound atom, that is, the smallest integral particle of the acid or alkali,—a particle which does not admit of being divided, except by the separation of its elementary or constituent atoms. The numbers expressing the proportions in which bodies unite, must like- wise indicate, consistently with this view, the relative weights of atoms; and accordingly these numbers are often called atomic weights. Thus, as water is composed of 8 parts of oxygen and I of hydrogen, it follows, on the sup- position of water consisting of one atom of each element, that an atom ot oxygen must be eight times as heavy as an atom of hydrogen. If carbonic oxide be formed of an atom of carbon and an atom of oxygen, the relative weight of their atoms is as 6 to 5; and in short the chemical equivalents of all bodies may be considered as expressing the relative weights of their atoms. . ••*■*♦ The foregoing argument in favour of the atomic constitution of matter becomes much stronger, when we trace the intimate connexion which sub- sists, among many substances, between their crystalline form and chemical composition. This subject, however, now known under the name of isomor- phism will be more conveniently discussed under the head of crystallization. Dalton supposes that the atoms of bodies are spherical; and he has invented certain symbols to represent the mode in which he conceives they may combine together, as illustrated by the following figures. 0 Hydrogen. O Oxygen. © Nitrogen. © Carbon. BINARY COMPOUNDS. O 0 Water. O Q Carbonic oxide. TERNARY COMPOUNDS. O © O Binoxide of hydrogen. O • O Carbonic acid. &c. &c. &c. 144 LAWS OF COMBINATION. All substances containing only two atoms he called binary compounds, those composed of three atoms ternary compounds, of four quaternary, and so on. There are several questions relative to the nature of atoms, most of which will perhaps never be decided. Of this nature are the questions which re- late to the actual form, size, and weight of atoms, and to the circumstances in which they mutually differ. All that we know with any certainty is, that their weights do differ, and by exact analysis the relations between them may be determined. It is but justice to the memory of the late Mr. Higgins of Dublin, to state that he first made use of the atomic hypothesis in chemical reasonings. In his " Comparative View of the Phlogistic and Antiphlogistic Theories," published in the year 1789, he observes (pages 36 and 37) that " in volatile vitriolic acid, a single ultimate particle of sulphur is inlimately united only to a single particle of dephlogisticated air; and that, in perfect vitriolic acid, every single particle of sulphur is united to two of dephlogisticated air, being the quantity necessary to saturation;" and he reasons in the same way concerning the constitution of water and the compounds of nitrogen and oxygen. These remarks of Mr. Higgins do not appear to have had the slightest connexion with the subsequent views of Dr. Dalton, who in fact seems to have never seen the work of Higgins till after he had given an ac- count of his own doctrine. The observations of Higgins, though highly creditable to his sagacity, do not affect Dalton's merit as an original ob- server. They were made, moreover, in so casual a manner, as not only not to have attracted the notice of his contemporaries, but to prove that Higgins himself attached no particular interest to them. Dalton's chief merit con- sists in having formed a complete theory of chemical union, and in the dis- covery of an essential and most important part of the doctrine, a merit which is solely and indisputably his; but in which he would have been anticipated by Higgins, had that chemist perceived the importance of his own opinions. To the student who may desire a more ample account of the doctrine of atoms than the nature and limits of this volume admit of being given here, I may recommend a small work by Dr. Daubeny on the atomic theory, which in other respects will be found well worthy of perusal. The advanced student may also consult Dr. Prout's Bridgewater Treatise, where he will find some novel speculations on the agencies which give rise to chemical union, and on the mode in which the combining molecules are arranged; speculations which may well open a path to important views, though in their present form they will scarcely receive the general assent of chemists. THEORY OF VOLUMES. Soon after the publication of the New System of Chemical Philosophy in 1808, in which work Dr. Dalton explained his views of the atomic constitution of bodies, a paper appeared in the second volume of the Memoires d'Arcueil by M. Gay-Lussac, on the " Combination of Gaseous Substances with one another." He there proved that gases unite together by volume in very simple and definite proportions. In the combined researches of himself and Humboldt, those gentlemen found that water is composed precisely of 100 measures of oxygen gas and 200 measures of hydrogen; and Gay-Lussac, being struck by this peculiarly simple proportion, was induced to examine the combinations of other gases, with the view of ascertaining if any thing similar occurred in other instances. The first compounds which he examined were those of ammoniacal gas with hydrochloric, carbonic, and fluoboric acid gases. 100 volumes of the alkali were found to combine with precisely 100 volumes of hydrochloric acid gas, and they could be made to unite in no other ratio. With both the LAWS OF COMBINATION. 145 other acids, on the contrary, two distinct combinations were possible. These are 100 Fluoboric acid gas, with 100 Ammoniacal gas. 100 do. 200 do. 100 Carbonic acid gas 100 do. 100 do. 200 do. Various other examples were quoted, both from his own experiments and from those of others, all demonstrating the same fact. Thus ammonia was found by A. Berthollet to consist of 100 volumes of nitrogen gas and 300 volumes of hydrogen; sulphuric acid contains 100 volumes of sulphurous acid and 50 volumes of oxygen ; and carbonic acid is formed by burning a mixture of 50 volumes of oxygen and 100 volumes of carbonic oxide. From these and other instances Gay-Lussac established the fact, that gaseous substances unite in the simple ratio of 1 to 1, 1 to 2, 1 to 3, &c.; and this original observation has been confirmed by such a multiplicity of experiments, that it may be regarded as one of the best established laws in chemistry. Nor does it apply to gases merely, but to vapours also. For example, hydrosulphuric, sulphurous, and hydriodic acid gases are com- posed of 600 vol. Hydrogen gas and 100 vol. vapour of Sulphur. 600 Oxvsrcn 100 . . Sulphur. 100 Hydrogen 100 . . Iodine. Another remarkable fact established by Gay-Lussac in the same essay is, that the volumes of compound gases and vapours always bear a very simple ratio to the volumes of their elements. This will appear from the follow- ing table, in which all the substances are supposed to be in the gaseous state:— Volumes of Elements. Volumes of resulting Compounds. 100 Nitrogen + 300 Hydrogen yield 200 Ammonia. 50 Oxygen + 100 Hydrogen ... 100 Water. 50 Oxygen + 100 Nitrogen . . . 100 Protoxide of nitrogen. 100 Sulphur -f~ 600 Hydrogen . . . 600 Hydrosulphuric acid. 100 Sulphur + 600 Oxygen . . . 600 Sulphurous acid. 100 Chlorine -j- 100 Hydrogen . . . 200 Hydrochloric acid. 100 Iodine -f- 100 Hydrogen ... 200 Hydriodic acid. 100 Bromine -j- 100 Hydrogen . . . 200 Hydrobromic acid. 100 Cyanogen -j- 100 Hydrogen . . . 200 Hydrocyanic acid. 100 Oxygen -\- 100 Nitrogen . . . 200 Binoxide of nitrogen. The law of multiples (page 137) is equally demonstrable by means of combining volumes as by combining weights. The annexed tabular view will justify this statement:— Volumes of Elements. 100 Nitrogen + 100 do. + 100 do. + 100 do. + 100 do. -f 100 Hydrogen + 100 do. -f 100 Carbon vapour -j- 100 do. + 50 Oxygen 100 do. 150 do. 200 do. 250 do. 50 do. 100 do. 50 do. 100 do. Resulting Compounds. yield Protoxide of nitrogen. Binoxide of nitrogen. Hyponitrous acid. Nitrous acid. Nitric acid. Water. Binoxide of hydrogen. Carbonic oxide. Carbonic acid. It thus appears that the laws of combination may equally well be deduced from the volumes as from the weights of the combining substances, and that the composition of gaseous bodies may be expressed as well by mea- sure as weight. In the subjoined table is a comparative view of equivalent 13 146 LAWS OF COMBINATION. weights and volumes, to which are added the respective specific gravities in relation both to air and hydrogen: the facts respecting the vapours are drawn from an important essay lately published by Mitscherlich. (An. de Ch. et de Ph. lv. 5.) GASES AND VAPOURS. Specific Gravities. Chemical Equivalents. Air as 1. Hydrogen as 1. By Vol. By Weight. Hydrogen 0.0689 1.00 100 1.00 Nitrogen 0.9727 14.12 100 14.15 Chlorine 2.4700 35.84 100 35.42 Carbon (hypothetical) . 0.4215 6.12 100 6.12 Iodine .... 8.7020 126.30 100 126.30 Bromine 5.4017 78.40 100 78.40 Water .... 0.6201 9.00 100 9.00 Alcohol 1.6009 23.24 100 23.24 Sulphuric ether 2.5817 37.48 100 37.48 Light carburetted hydrogen 0.5593 8.12 100 6.12 Olefiant gas 0.9808 14.24 100 14.24 Carbonic oxide 0.9727 14.12 100 14.12 Carbonic acid . 1.5239 22.12 100 22.12 Protoxide of nitrogen . 1.5239 22.12 100 22.15 Sulphurous acid 2.2117 32.10 100 32.10 Sulphuric acid (anhydrous) 2.7629 40.10 100 40.10 Cyanogen 1.8157 26.36 100 26.39 Hydrosulphuric acid 1.1782 17.10 100 17.10 Binoxide of nitrogen 1.0375 15.06 200 30.15 Mercury 6.9589 101.00 200 202.00 Ammonia 0.5897 8.56 200 17.15 Hydrochloric acid 1.2694 18.42 200 36.42 Hydriodic acid 4.3854 63.65 200 127.30 Hydrobromic acid 2.7353 39.70 200 79.40 Hydrocyanic acid 0.9423 13.68 200 27.39 Arseniuretted hydrogen 2.7008 39.20 200 78.40 Sesquichloride of arsenic . 6.3025 91.46 200 181.66 Sesquiodide of arsenic . 15.6505 227.15 200 454.30 Protochloride of mercury . 8.1939 118.92 200 237.42 Bichloride of mercury . 9.4289 136.84 200 272.84 Bromide of mercury 9.6597 140.20 200 280.40 Bibromide of mercury . 12.3606 179.40 200 358.80 Biniodide of mercury 15.6609 227.30 200 454.60 Oxygen 1.1024 16.00 50 8.00 Arscnious acid 13.6972 198.80 50 99.40 Phosphorus . 4.3269 62.80 25 15.70 Arsenic .... 10.3901 150.80 25 37.70 Sulphur 6.6558 96.60 16.66 16.10 Bisulphuret of mercury. . 5.3788 78.06 300 234.20 The observations which more immediately flow from the facts in the pre- ceding table are these:— 1. The combining volumes of substances, both elementary and compound, are either equal, or have the simple ratio of 1 to 2, 1 to 3, &c. The same simplicity rarely exists among the equivalent weights. 2. On comparing together the third and fifth columns, the corresponding LAWS OF COMBINATION 147 numbers for the 18 first substances will be found nearly or quite identical. As those substances have the same uniting volume as hydrogen, which is the assumed unit of comparison, and as the specific gravities are merely the weights of equal volumes, the numbers of the third column, were they quite exact, must coincide with those in the fifth : their want of identity indicates errors of observation. 3. The identity in the equivalent volumes of the elementary gases, hydro- gen, nitrogen, and chlorine, led to the notion that the equivalent volumes of most other elements, such as carbon, sulphur, and phosphorus, might also be identical. Assuming that identity, the specific gravity which those elements ought to have when gaseous, may easily be calculated. Thus, taking 1, 6.12, and 16.1 as the equivalents of hydrogen, carbon, and sulphur, then will their specific gravities in the gaseous state, combining volumes being supposed equal, be in the ratio of 1, 6.12, and 16.1. This method, by which the hypo- thetical specific gravity of carbon, as stated in the table, was obtained, was first indicated by Dr. Prout. (An. of Phil. vi. 321.) But though such hypo- thetical numbers may sometimes be used for the convenience of expressing the relation of uniting substances by measure, recent facts show how dan- gerous it would be to confide in them; for by the table it appears that the equivalent volume of sulphur vapour is one-sixth of that of hydrogen, which renders the specific gravity of the vapour of sulphur six times greater than the hypothetical number. Similar deviation is observable in phosphorus, arsenic, and mercury. In these cases, the real specific gravity of a vapour is as much greater than the hypothetical, as its equivalent volume is less than that of hydrogen. 4. The identity in the equivalent volumes of hydrogen, nitrogen, and chlo- rine, suggested the idea that the atoms or indivisible molecules of all the ele- ments are of the same magnitude; and this coupled with the supposition that the self-repulsive energy of these atoms is equal, led to the opinion that equal volumes of the elements in the gaseous state must contain an equal number of atoms. This hypothesis, recommended by its simplicity, and sup- ported by the fact that the volumes of gaseous substances vary according to the same law by varying temperature and pressure, was accordingly em- ployed as a mode of determining the relative weights of atoms. As water consists of 50 measures of oxygen and 100 of hydrogen gas, it was inferred to be a compound of one atom of oxygen and two atoms of hydrogen; and consequently, taking 8 as the weight of an atom of oxygen, the weight of one atom of hydrogen is £ instead of 1, as in the table; or taking hydrogen as 1, the atom of oxygen is 16- On the same principle may the numbers which in the table represent the equivalent weights of chlorine, bromine, iodine, and nitrogen, which have the same equivalent volumes as hydrogen, be considered as the weights of two equivalents. The equivalents adopted by Davy in his Elements of Chemical Philosophy, as well as those of Ber- zelius, which are now in general use on the Continent, were framed in ac cordance with these views : this the British chemist requires to bear in mind, since the same numbers which Berzelius uses for 2 equivalents of hydrogen, nitrogen, chlorine, bromine, and iodine, he considers as one equivalent. But the opinion of Davy and Berzelius must now either be abandoned, or main- tained on other principles; since the late researches of Dumas and Mitscher- lich have shown experimentally that equal volumes of the elementary gases and vapours do not contain the same number of atoms. 5. The facts contained in the last and preceding tables supply materials for calculating the sp. gravity of compound gases, by which means the ac- curacy of other conclusions respecting their composition may be verified. Thus analysis proves that ammoniacal gas is composed of 100 volumes of nitrogen, and 300 of hydrogen gases, condensed into the space of 200 vol- umes : if so, its sp. gravity will be 0.9727-f3x0.0689_1.1794_n 5g97 2 2 148 LAWS OF COMBINATION. The near agreement of this calculated number with that found by weighing the gas itself, proves that ammonia has really the constitution above assigned to it, and gives great probability that the sp. gravity of nitrogen and hydrogen gases is nearly correct. Again, hydrochloric acid gas consists of 100 volumes of hydrogen and 100 of chlorine gases, united without any change of bulk. Hence its sp. gravity ought to be 2.47+0.0689^ og01 Hydrocyanic acid vapour is formed of 100 volumes of hydrogen and 100 of cyanogen gases united without change of volume; and, therefore, its sp. gravity should be 1£1£L+M^=0.9423. 2 Considering defiant gas as a compound of 200 volumes of hydrogen gas and 200 of the vapour of carbon condensed into 100, its sp. gravity will be 2 x 0.0689+2 x 0.4215=0.1378+0.8430= 0.9808. Aqueous vapour is composed of 100 volumes of hydrogen and 50 of oxy- gen gases condensed into the space of 100 volumes; and, therefore, its sp* gravity ought to be 0.0689+0.5512 (half the sp. gr. of oxygen) =0.6201. Protoxide of nitrogen is formed of 100 volumes of nitrogen and 50 of oxy- gen gases condensed into 100 volumes, and hence its sp. gravity should be 0.9727+0.5512=1.5239. Assuming carbonic oxide to be a compound of 100 volumes of carbon va- pour and 50 of oxygen gas contracted in uniting into 100 volumes, its sp. gravity should be 0.4215+0.5512 = 0.9727. As the different sp. gravities thus calculated are very nearly those found by direct experiment, there is a strong presumption that the elements of the calculations are correct. The principle of these calculations is sufficiently obvious. The sp. gra- vities represent the weights of equal volumes of the gases: taking 100 as the standard volume of which the sp. gravity of each gas denotes the weight, then 50 volumes of a gas may be indicated by half, 25 volumes by a fourth, and 16.66 by a sixth of its specific gravity. Thus hydrosulphuric acid is a compound of 100 volumes of hydrogen gas, and 16.66 (2-jp) of the vapour of sulphur, condensed into 100 volumes, and, therefore, its sp. gravity is nnfiflo_|_6-6558_ 0.0689+1.1093=1 J782. Sulphurous acid consists of 100 volumes of oxygen gas and 16.66 of the va- pour of sulphur condensed into 100 volumes; and hence its sp. gravity is i ino'gsc from $Z; light and 31.4 or 2 eq. + 40 or 5 cq.=7l.4 2P+50 or- P Mctaphosphoric acid ) COMPOUNDS OF OXYGEN AND PHOSPHORUS. Oxide.—When a jet of oxygen gas is thrown upon phosphorus while in fusion under hot water, combustion ensues, phosphoric acid is formed, and a number of red particles collect, which have been examined by M. Pelouze, PHOSPHORUS. 201 who has shown them to be an oxide of phosphorus. The red matter left when phosphorus is burned is probably of the same nature. This, the only known oxide of phosphorus, is of a red colour, without taste or odour, and is insoluble in water, ether, alcohol, and oil. It is per- manent in the air, even at 662° F., but takes fire at a low red heat. Heated to redness in a tube, phosphorus is expelled, and metaphosphoric acid re- mains. It takes fire in chlorine gas, and is rapidly oxidized by nitric acid. It does not appear to possess any alkaline character. (An. de Ch. et de Ph. Hypophosphorous Acid.—This acid was discovered in 1816 by Dulong. (An. de Ch. et de Ph. ii.) When water acts upon the phosphuret of barium the elements of both enter into a new arrangement, giving rise to phosphu- retted hydrogen, phosphoric acid, hypophosphorous acid, and baryta. The former escapes in the form of gas, and the two latter combine with the ba- ryta, Hypophosphite of baryta, being soluble, dissolves in the water, and may consequently be separated by filtration from the phosphate of baryta, which is insoluble. On adding a sufficient quantity of sulphuric acid for pre- cipitating the baryta, hypophosphorous acid is obtained in a free state, and on evaporating the solution, a viscid liquid remains, highly acid and even crys- tallizable, which is a hydrate of hypophosphorous acid. When exposed to heat in close vessds, it undergoes the same kind of change as hydrated phosphorous acid. Hypophosphorous acid is a powerful deoxidizing agent. It unites with alkaline bases; and it is remarkable that all its salts are soluble in water. The hypophosphites of potassa, soda, and ammonia dissolve in every pro- portion in rectified alcohql; and hypophosphite of potassa is even more deli- quescent than chloride of calcium. They are all decomposed by heat, and yield the same products as the acid itself. They are conveniently prepared by precipitating hypophosphite of baryta, strontia, or lime, with the alkaline carbonates; or by directly neutralizing these carbonates with hypophospho- rous acid. The hypophosphite of baryta, strontia, and lime are formed by boiling these earths in the caustic state in water together with fragments of phosphorus. The same change occurs as during the action of water on phosphuret of barium. The composition of this acid as stated at page 200, is ' on the authority of Rose. (Poggen. Annalen ix. 367.) Phosphorous Acid.—When phosphorus is burned in air highly rarefied, imperfect oxidation ensues, and metaphosphoric and phosphorous acids are generated, the latter being obtained in the form of a white volatile powder. In this state it is anhydrous. Heated in the open air, it takes fire, and forms metaphosphoric acid; but if exposed to heat in close vessels, it is resolved into metaphosphoric acid and phosphorus. It dissolves readily in water, has a sour taste, and smells somewhat like garlic. . It unites with alkalies, and forms salts which are termed phosphites. The solution of phosphorous acid absorbs oxygen slowly from the air, and is converted into phosphoric acid. From its tendency to unite with an additional quantity of oxygen, it is a powerful deoxidizing agent; and hence, like sulphurous acid, precipi- tates mercury, silver, platinum, and gold from their saline combinations in the metallic form. Nitric acid converts it into phosphoric acid. Phosphorous acid may be procured more conveniently by subliming phos- phorus through powdered bichloride of mercury contained in a glass tube; when a limpid liquid comes over, which is a compound of chlorine and phosphorus. (Davy's Elements, p. 288.) This substance and water mu- tually decompose each other: the hydrogen of water unites with the chlo- rine, and forms hydrochloric acid; while the oxygen attaches itself to the phosphorus, and thus phosphorous acid is produced. The solution is then evaporated to the consistence of syrup to expel the hydrochloric acid; and the residue, which is hydrate of phosphorous acid, becomes a crystalline so- lid on cooling. When this hydrate is heated in close vessels, the elements of the water and acid react on each other, forming metaphosphoric acid and 202 PHOSPHORUS. a gaseous compound of hydrogen and phosphorus. The nature of this gas will be more particularly noticed in the section on phosphuretted hydrogen. Phosphorous acid is also generated during the slow oxidation of phospho- rus in atmospheric air. The product attracts moisture from the air, and forms an oil-like liquid. Dulong thinks that a distinct acid is generated in this case, which he calls phosphatic acid; but the opinion of Davy, that it is merely a mixture of phosphoric and phosphorous acids, is in my opinion per- fectly correct. Phosphoric Acid.—It was shown in the year 1827 by Dr. Clarke, now Professor of chemistry in Aberdeen, that under the term phosphoric acid had previously been confounded two distinct acids, one of which he propos- ed to distinguish by the name of pyrophosphoric acid (from Ttvgfire), to indi- cate that it is phosphoric acid modified by heat; and very lately Mr. Gra- ham has described another modification of phosphoric acid, to which he has given the provisional name of metaphosphoric (from /jura together v:ith), im- plying phosphoric acid and something besides; but this name is rather un- fortunate, since it is applied to the only one of the three modifications which can be obtained free from water. Perhaps paraphosphoric (from 5r«g«. near to) would be more appropriate. These three acids contain phosphorus and oxygen in the same ratio, and have the same equivalent, so that they may be considered as isomeric bodies (page 152); but that difference in the ar- rangement of their elements on which their peculiarities may be presumed to depend, is very slight, since they are easily convertible into each other. Mr. Graham, indeed, supposes the difference to arise solely from a disposi- tion to unite in different proportions with water and alkaline bases; but this view scarcely suffices as an explanation, because it does not account for the peculiar disposition which causes their distinctive characters. (Phil. Trans. 1833, Part ii., and Phil. Mag. 3rd Series, iv. 401.) Phosphoric acid has hitherto been obtained only in combination with wa- ter or some alkaline base. One of the best modes for procuring it, is to oxidize phosphorus by strong nitric acid; but in this process care is neces- sary, as the action is sometimes very violent, and the escape of binoxide of nitrogen gas ungovernably rapid. It is safely conducted by adding frag-- ments of phosphorus, or the so-called phosphatic acid, to strong nitric acid contained in a platinum crucible partially closed by its cover. Gentle heat is applied so as to commence, and, when necessary, to maintain moderate ef- fervescence ; and when one portion of phosphorus disappears, another is added, till the whole of the nitric acid is exhausted. The solution is then evaporated to dryness, and exposed to a red heat to expel the last traces of nitric acid. This should always be done in vessels of platinum, since phos- phoric acid acts chemically upon those of glass or porcelain, and is thereby rendered impure. In this case, as in some other instances of the oxidation of combustibles by nitric acid, water is decomposed; and while its oxygen unites with phosphorus, its hydrogen combines with nitrogen of the nitric acid. A portion of ammonia, thus generated, is expelled by heat in the last part of the process. Phosphoric acid may be prepared at a much cheaper rate from bones. For this purpose, superphosphate of lime, obtained in the way already de- scribed, should be boiled for a few minutes with excess of carbonate of am- monia. The lime is thus predpitated as a phosphate, and the solution con- tains phosphate, together with a little sulphate of ammonia. The liquid, after filtration, is evaporated to dryness, and then ignited in a platinum crucible, by which means the ammonia and sulphuric acid are expelled. In both the foregoing processes phosphoric acid exists only in solution ; for on heating to redness in order to expel ammonia in the one case and nitric acid in the other, metaphosphoric acid is generated. To reproduce the phosphoric acid the residue in the crucible requires to be dissolved in water and boiled for a few minutes. Phosphoric acid is colourless, intensely sour to the taste, reddens litmus strongly, and neutralizes alkalies; but it does not destroy the texture of the PHOSPHORUS. 203 skin like sulphuric and nitric acids. Its solution may be evaporated at a temperature of 300° without decomposition, and when thus concentrated it assumes a dark colour, is as thick as treacle when cold, and consists of 71.4 parts or one eq. of phosphoric acid, and 27 parts or three equivalents of wa- ter. Mr. Graham obtained this hydrate in thin crystalline plates, which were extremely deliquescent, by keeping it for seven days in vacuo along with sulphuric acid. On heating this hydrate for several days to 415°, it lost nearly two-thirds of an equivalent of water, and then principally consisted of pyrophosphoric acid with two equivalents of water. At a still higher tem- perature metaphosphoric acid began to be formed; and at a red heat the conversion was complete. But after ignition it still contains water, amount- ing according to Rose to 9.44 per cent, which is rather more than an equi- valent of water to one of metaphosphoric acid. Phosphoric acid is remarkable for its tendency to unite with alkaline bases, in such proportions that the oxygen of the base and of the acid is as 3 to 5; or, in other words, it is prone to form subsalts, in which one equiva- lent of acid is combined with three equivalents of base. It manifests the same character in regard to water, and ceases to be phosphoric acid unless three equivalents of water to one of acid are present: it even appears that the water acts the part of a base, hence called basic water, and that the aqueous solution is not a mere solution of phosphoric acid, but of triphos- phate of water, a sort of salt composed of one equivalent of acid and three equivalents of water. Part of this basic water enters along with soda into the constitution of two of the phosphates of soda, the water and soda to- gether forming the three equivalents of base required by one equivalent of the acid. This point will be more fully described in the history of the phos- phates. When phosphoric acid is neutralized by ammonia and mixed with nitrate of oxide of silver, the yellow phosphate of that oxide subsides, a character by which it is distinguished from pyrophosphoric and metaphosphoric acids, as well as from all other acids except the arsenious. A certain test between phosphoric and arsenious acids is, that the former is neither changed in co- lour nor precipitated when a stream of hydrosulphuric acid gas is transmit- ted through it; while the latter, with the required precautions, first acquires a yellow tint, and then yields a yellow precipitate. Pyrophosphoric Acid.—This acid is formed by exposing concentrated phosphoric acid for some time to a heat of 415°. Its general characters re- semble phosphoric acid; but when neutralized by ammonia and mixed with nitrate of oxide of silver, it yields a snow-white granular precipitate, pyro- phosphate of that oxide, by which it is distinguished from phosphoric and metaphosphoric acids. In solution with cold water pyrophosphoric acid passes gradually, and at a boiling temperature rapidly, into phosphoric acid. Its salts, while neutral, are very permanent; but when boiled with either of the stronger acids in water, they are quickly converted more or less com- pletely into phosphates. Pyrophosphoric acid is remarkable for its tendency to unite with two equivalents of a base. Its aqueous solution probably contains a dipyrophos- phate of water, that is one equivalent of the acid with two eq. of water, ex- pressed by 2H + P or H^P. This basic water is readily displaced by two equivalents of stronger bases, such as soda; or if one equivalent only of soda be added, then the soda and water together make up the two equiva- lents of base, the formula of the salt being NaHP. The readiest mode of obtaining a pyrophosphate is to heat phosphoric acid with any fixed base in the ratio of one to two of their equivalents. This was done by Dr. Clarke in the experiments by which he established the existence of pyrophosphoric acid. (Brewster's Journal, vii. 298.) Phosphate of soda is a compound of one eq. phosphoric acid, two eq. soda, one eq. basic water, and twenty-four 204 BORON. eq. water of crystallization, its formula being Na^H P + H^4 : on drying this salt its water of crystallization is expelled and there remains Na2H P, which is still a phosphate; but on heating to rednefes the basic water is ex- pelled, and Na^P, pyrophosphate of soda, remains. By being forced to unite with two equivalents of base, the acid acquires a disposition to do so on all occasions. Metaphosphoric Acid.—This acid is obtained by burning phosphorus in dry air or oxygen gas, or heating to redness a concentrated solution of phosphoric or pyrophosphoric acids. By the former method, the acid is a white solid, and anhydrous; in the latter it is a hydrate, or probably a me- taphosphate of water, composed of one eq. acid and one eq. water, its for- mula being H P. The water in this compound cannot be expelled by fire, since on attempting to do so by a violent heat, the whole is sublimed. In an open crucible, it volatilizes at a temperature by no means high. The peculiarity of this acid is to combine with one equivalent of a base. On exposing the anhydrous acid to the air it rapidly deliquesces, and at the same time acquires its basic water, which can only be replaced by an equi- valent quantity of soda or some other alkaline base. The water is also driven off by fusion with siliceous or aluminous substances, with which the acid unites and forms very fusible compounds. The pure hydrated acid is of it- self very fusible, and on cooling concretes into a transparent brittle solid, being known under the name of glacial phosphoric acid, which is highly de- liquescent, and can hence only be preserved in its glassy state in bottles carefully closed. The metaphosphoric resembles pyrophosphoric acid in the facility with which its aqueous solution passes into phosphoric acid. On the contrary, both of the other acids are converted into metaphosphates when heated to redness in contact with no more than one equivalent of certain fixed bases, such as potassa and soda. This acid when free occasions precipitates in so- lutions of the salts of baryta, and most of the earths and metallic oxides, and forms an insoluble compound with albumen. The metaphosphate of baryta and oxide of silver both fall in gelatinous flakes of a gray colour- SECTION IX. BORON. Sir H. Davy discovered the existence of boron in 1S07 by exposing bora- cic acid to the action of a powerful galvanic battery; but he did not obtain a sufficient supply of it for determining its properties. Gay-Lussac and The- nard* procured it in greater quantity in 1808, by heating boracic acid with potassium. The boracic acid is by this means deprived of its oxygen, and boron is set free. The easiest and most economical method of preparing this substance, according to Berzelius, is to decompose borofluorido of potassium or sodium by means of potassium. (Annals of Philosophy, xxvi. 128.1 Boron is a dark olive-coloured substance, which has neither taste nor smell, and is a non-conducter of electricity. It is insoluble in water, alcohol, ether, and oils. It does not decompose water whether hot or cold. It bears * Rccherches Physico-Chimiques, vol. i. BORON. 205 intense heat in close vessels, without fusing or undergoing any other change except a slight increase of density. Its specific gravity is about twice as great as that of water. It may be exposed to the atmosphere at common temperatures without change; but if heated to 600°, it suddenly takes fire oxygen gas disappears, and boracic acid is generated. It is very difficult to oxidize all the boron by burning, because the boracic acid fuses at the moment of being formed, and by glazing the surface of the unburned boron protects it from oxidation. It also passes into boracic acid when heated with nitric acid, or with any substance that yields oxygen with facility. According to the experiments of Davy and Berzelius, boron in burning unites with 200 per cent, of oxygen; and the latter, from the composition of borax, estimates the oxygen in boracic acid at 68.8 per cent. Adopting this estimate, and regarding boracic acid as a compound of one equivalent of boron to three eq. of oxygen, the equivalent of boron is inferred from the proportion, as 68.8 : 31.2 :: 24 : 10.9. In this, as in some other cases, where a combustible unites with oxygen in one proportion only, it is difficult with any certainty to assign the true atomic constitution of the compound. Boracic acid may be a compound of boron and oxygen in the ratio of one atom to one atom, in that of one to two as supposed by Dr. Thomson, or of one to three. The latter appears to me most consistent with other com- pounds of boron. Hence boracic acid is thus constituted:— Boron. Oxygen. Equiv. Formula. Boracic acid 10.9 or 1 eq. + 24 or 3 eq. = 34.9 B + 30 or B. Boracic Acid.—This is the only known compound of boron and oxygen. As a natural product it is found in the hot springs of Lipari, and in those of Sasso in the Florentine territory. It is a constituent of several minerals, among which the datolite and boracite may in particular be mentioned. It occurs much more abundantly under the form of borax, a native com- pound of boracic acid and soda. It is prepared for chemical purposes by adding sulphuric acid to a solution of purified borax in about four times its weight of boiling water, till the liquid acquires a distinct acid reaction. The sulphuric acid unites with the soda; and the boracic acid is deposited, when the solution cools, in a confused group of shining scaly crystals. It is then thrown on a filter, washed with cold water to separate the adhering sulphate of soda and sulphuric acid, and still further purified by solution in boiling water and re-crystallization. But even after this treatment it is apt to retain a little sulphuric acid; and on this account, when required to be absolutely pure, it should be fused in a platinum crucible, and once more dissolved in hot water and crystallized. Boracic acid in this state is a hydrate, which contains 43.62 per cent, of water, being a ratio of 34.9 parts or one equivalent of the anhydrous acid to 27 parts or three eq. of water. This hydrate dissolves in 25.7 times its weight of water at 60°, and in 3 times at 212°. Boiling alcohol dissolves it freely, and the solution, when set on fire, burns with a beautiful green flame; a test which affords the surest indication of the presence of boracic acid. Its specific gravity is 1.479. It has no odour, and its taste is rather bitter than acid. It reddens litmus paper feebly, and effervesces with alkaline carbonates. Mr. Faraday has noticed that it renders turmeric paper brown like the alkalies. P'rom the weakness of its acid properties, all the borates, when in solution, are decomposed by the stronger acids. When hydrous boracic acid is exposed to a gradually increasing heat in a platinum crucible, its water of crystallization is wholly expelled, and a fused mass remains which bears a white heat without being sublimed. On cool- ing, it forms a hard, colourless, transparent glass, which is anhydrous boracic acid. If the water of crystallization be driven off by the sudden application of a strong heat, a large quantity of boracic acid is carried away during the rapid escape of watery vapour. The same happens, though in a less degree, when a solution of boracic acid in water is boiled briskly, 18 206 SELENIUM. Vitrified boracic acid should be preserved in well-stopped vessels; for if exposed to the air, it absorbs water, and gradually loses its transparency. Its specific gravity is 1.803. It is exceedingly fusible, and communicates this property to the substances with which it unites. For this reason borax is often used as a flux. SECTION X. SELENIUM. This substance was discovered in 1818 by Berzelius, who called it selenium, from liKhx, the Moon, suggested by its having at first been mistaken for the metal tellurium. (An. de Ch. et de Ph. ix. 160, and An. of Phil. xiii. 401.) It has hitherto been obtained in very small quantity, and occurs for the most part in combination with some varieties of iron pyrites. Stromeyer has also detected it, as a sulphuret of selenium, among the volcanic products of the Lipari isles. It is found likewise at Clausthal in the Hartz, combined, according to Stromeyer and Rose, with several metals, such as lead, cobalt, silver, mercury, and copper. Berzelius found it in the sulphur obtained by sublimation from the iron pyrites of Fahlun. In a manufactory of sulphuric acid, at which this sulphur was employed, it was observed that a reddish-coloured matter always collected at the bottom of the leaden chamber; and on burning this substance, Berzelius perceived a strong and peculiar odour, similar to that of decayed horse-radish, which induced him to submit it to a careful examination, and thus led to the discovery of sdenium. For the extraction of selenium from the native sulphuret, Magnus proposes to mix it with eight times its weight of perox- ide of manganese, and to expose the mixture to a low red heat in a glass retort, the beak of which dips into water. The sulphur, oxidized at the expense of the manganese, escapes in the form of sulphurous acid ; while the selenium either sublimes as such or in the state of selenious acid. Should any of the latter be carried over into the water, it would there be reduced by the sulphurous acid. Selenium, at common temperatures, is a brittle opaque solid body, without taste or odour. It has a metallic lustre and the aspect of lead when in mass: but it is of a deep red colour when reduced to powder. Its specific gravity is between 4.3 and 4.32. At 212° it softens, and is then so tenacious that it may be drawn out into fine threads which are transparent, and appear red by transmitted light. It becomes quite fluid at a temperature somewhat above that of boiling water. It boils at about 650°, forming a vapour which has a deep yellow colour, but is free from odour. It may be sub- limed in close vessels without change, and condenses again into dark globules of a metallic lustre, or as a cinnabar-rcd powder, according as the space in which it collects is small or large. Berzelius at first regarded it as a metal; but, since it is an imperfect conductor of heat and electricity, it more properly belongs to the class of the simple non-metallic bodies. Selenium is insoluble in water. It suffers no change from mere exposure to the atmosphere; but if heated in the open air, it combines readily with oxygen, and two compounds, oxide of selenium and selenious acid, are generated. If exposed to the oxidizing part of the blow-pipe flame, it tinges the flame with a light blue colour, and exhales so strong an odour of decayed horse-radish, that l-50th of a grain is said to be sufficient to scent the air of a large apartment. By this character the presence of selenium, whether alone or in combination, may always be detected. Berzelius has shown that selenic acid is composed of 24 parts of oxygen and 39.6 of selenium. This substance, also, has three grades of oxidation, SELENIUM. 207 the oxygen in the two last of which is in the ratio of 2 and 3; and the highest grade, selenic acid, has in all its chemical relations a singularly close analogy to sulphuric acid. From these facts it is inferred that selenic acid is composed of one atom of selenium and three atoms of oxygen, and that the equivalent of the former is 39.6. The compounds of selenium described in this section are the following:— Selenium. Oxygen. Equiv. Formulae. Oxide of selenium (probably) 39.6 or 1 eq.+ 8 or 1 eq.= 47.6 Se+O or Se. Selenious acid . . 39.6 +16 or2eq.= 55.6 Se+20 or Se. Selenic acid . . 39.6 +24 or 3 eq.= 63.6 Se+30 or Se. Oxide of Selenium.—This compound is formed in greatest abundance by heating selenium in a limited quantity of atmospheric air, and by washing the product to separate selenious acid, which is generated at the same time. It is a colourless gas, which is very sparingly soluble in water, and docs not possess any acid properties. It is the cause of the peculiar odour which is emitted during the oxidation of selenium. Selenious Acid.—This acid is most conveniently prepared by digesting selenium in nitric or nitro-hydrochloric acid till it is completely dissolved. On evaporating the solution to dryness, a white residue is left, which is selenious acid. By increase of temperature, the acid itself sublimes, and condenses again unchanged into long four-sided needles. It attracts mois- ture from the air, whereby it suffers imperfect liquefaction. It dissolves in alcohol and water. It has distinct acid properties, and its salts are called selenites. Selenious acid is readily decomposed by all substances which have a strong affinity for oxygen, such as sulphurous and phosphorous acids. When sulphurous acid, or an alkaline sulphite, is added to a solution of selenious acid, a red-coloured powder, pure selenium, is thrown down, and the sulphurous is converted into sulphuric acid. Hydrosulphuric acid also decomposes it; and an orange-yellow precipitate subsides, which is a sul- phuret of selenium. Selenic Acid.—The preceding compound, discovered by Berzelius, was till lately the only known acid of selenium, and has been described in ele- mentary works under the name of selenic acid; but the recent discovery of another acid of selenium containing more oxygen than the other, has ren- dered necessary a change of nomenclature. The existence of selenic acid was first noticed by M. Nitzsch, assistant of Mitscherlich, and its properties have been examined and described by the Professor himself. (Edin. Journal of Science, viii. 294.) This acid is prepared by fusing nitrate of potassa or soda with selenium, a metallic seleniuret, or with selenious acid or any of its salts. Seleniuret of lead, as the most common ore of selenium, will generally be employ- ed ; but it is very difficult to obtain pure selenic acid by its means, because it is commonly associated with metallic sulphurets. The ore is first treated with hydrochloric acid to remove any carbonate that may be present; and the insoluble part, which is about a third of the mass, is mixed with its own weight of nitrate of soda, and thrown by successive portions into a red-hot crucible. The lead is thus oxidized, and the selenium converted into selenic acid, which unites with soda. The fused mass is then acted on by hot water, which dissolves only seleniate of soda, together with nitrate and nitrite of soda; while the insoluble matter, when well washed, is quite free from selenium. The solution is next made to boil briskly, when anhydrous seleniate of soda is deposited; while, on cooling, nitrate of soda crystallizes. On renewing the ebullition and subsequent cooling, fresh portions of seleniate and nitrate are procured; and these successive operations are repeated, until the former salt is entirely separated. This process is founded on the fact, that seleniate of soda, like the sulphate of the same base, is more soluble in water of about 90° than at higher or lower temperatures. The nitrite of 208 SELENIUM. soda, formed during the fusion, is purposely reconverted into nitrate by di- gestion with nitric acid. The seleniate of soda thus procured always contains a little sulphuric acid, derived from the metallic sulphurets of the ore; and it is not possible to separate this acid by crystallization. All attempts to separate it by means of baryta were likewise fruitless; and the only method of effecting this ob- ject is by reducing the selenic acid into selenium. This is done by heating a mixture of seleniate of soda with hydrochlorate of ammonia, when the sodium unites with chlorine, all the hydrogen with oxygen, and selenium and nitrogen are set free. This change will be more readily followed when stated in symbols;—thus Na + 0,Se + 30,N + 3H, and H + Cl, yield N, Se,4(H + 0), and Na+Cl. The selenium which sublimes is quite free from sulphur. It is then con- verted by nitric acid into selenious acid, which should be neutralized with soda, and fused with nitre or nitrate of soda. The pure seleniate of soda, separated from the nitrate according to the foregoing process, is subse- quently dissolved in water, and obtained in crystals by spontaneous evapo- ration. To procure the acid in a free state, seleniate of soda is decomposed by nitrate of oxide of lead. The seleniate of that oxide, which is as insoluble as the sulphate, after being well washed, is exposed to a current of hydro- sulphuric acid gas, which precipitates all the lead as a sulphuret, but does not decompose the selenic acid. The excess of the gas is driven off by heat, and pure selenic acid remains diluted with water. The absence of fixed substances may be proved by its being volatilized by heat without residue; and if free from sulphuric acid, it gives no precipitate with chloride of barium after being boiled with hydrochloric acid.* Any nitric acid which may be present is expelled by concentrating the solution by means of heat. Selenic acid is a colourless liquid, which may be heated to 536° without appreciable decomposition; but above that point decomposition commences, and it becomes rapid at 554°, giving rise to disengagement of oxygen and selenious acid. When concentrated by a temperature of 329° its specific gravity is 2.524; at 512° it is 2.60, and at 545° it is 2.625, but a little sele- nious acid is then present. When procured by the process above described, selenic acid always contains water, but it is very difficult to ascertain its precise proportion. Some acid, which had been heated higher than 536°, contained, subtracting the quantity of selenious acid present, 15.75 per cent. of water, which approximates to the ratio of one equivalent of water and one of the acid. It is certain that selenic acid is decomposed by heat before parting with all the water which it contains. Selenic acid has a powerful affinity for water, and emits as much heat in uniting with it as sulphuric acid does. Like this acid it is not decomposed by hydrosulphuric acid, and hence this gas may be employed for decom- posing seleniate of the oxides of lead or copper. With hydrochloric acid the change is peculiar; for on boiling the mixture, mutual decomposition ensues, water and selenious acid are formed, and chlorine is set free; so that the solution, like aqua regia, is capable of dissolving gold and platinum. Selenic acid dissolves zinc and iron with disengagement of hydrogen gas, and copper with formation of selenious acid. It dissolves gold also, but not platinum. Sulphurous acid has no action on solenic acid, whereas selenious acid is easily reduced by it. Consequently, when it is wished to precipitate * The necessity for this previous boiling with hydrochloric acid is to con- vert the selenic into selenious acid, without which change the chloride of barium would produce a precipitate of seleniate of baryta. The rationale of the action of hydrochloric acid is explained further on. Ed, CHLORINE. 209 selenium from selenic aeid, it must be boiled with hydrochloric acid before sulphurous acid is added. . Mitscherlich has observed, that selenic and sulphuric acids arc not only analogous in composition and in many of their properties, but that the simi- larity runs through their compounds with alkaline substances, their salts re- sembling each other in chemical properties, constitution, and form. SECTION XI. CHLORINE. The discovery of chlorine was made in the year 1774 by Scheele, while investigating the nature of manganese, and he described it under the name of dephlogisticated marine acid. The French chemists called it oxygenized muriatic acid, a term which was afterwards contracted to oxy-muriatic acid, from an opinion proposed by Berthollet that it is a compound of muriatic acid and oxygen. In 1809 Gay-Lussac and Thenard published an abstract of some experiments upon this substance, which subsequently appeared at length in their Recherches Physico-Chimiques, wherein they stated that oxy- muriatic acid might be regarded as a simple body, though they gave the preference to the doctrine advanced by Berthollet. Sir H. Davy engaged in the inquiry about the same time; and after having exposed oxy-muriatic acid to the most powerful decomposing agents which chemists possess, without being able to effect its decomposition, he communicated to the Royal Society an essay, in which he denied its compound nature; and he maintained that, according to the true logic of chemistry, it is entitled to rank with simple bodies. This view, which is commonly termed the new theory of chlorine, though strongly objected to at the time it was first pro- posed, is now universally received by chemists. The grounds of preference will hereafter be briefly stated. Chlorine gas is obtained by the action of hydrochloric acid on peroxide of manganese. The most convenient method of preparing it is by mixing concentrated hydrochloric acid, contained in a glass flask, with half its weight of finely powdered peroxide of manganese. Effervescence, owing to the escape of chlorine, takes place even in the cold; but the gas is evolved much more freely by the application of a moderate heat. It should be col- lected in inverted glass bottles filled with warm water; and when the water is wholly displaced by the gas, the bottles should be closed with a well- ground glass stopper. As some hydrochloric acid gas commonly passes over with it, the chlorine should not be considered quite pure, till after being transmitted through water. The theory of this process will be readily understood by first viewing the elements which act on each other, namely,— Manganese . 27.7 or 1 eq. Mn Chlorine . 70.84 or 2 eq. 2C1 Oxygen . 16 2eq. 2Q Hydrogen 2 or2eq. 211 Perox.ofmang. 43.7 or 1 eq. Mn+20. Hydroch. ac. 72.84 or 2eq. 2(H+C1); and then inspecting the products derived from them, namely, Manganese . . 27.7 . Hydrogen 2 0,, . oc Ao i Chlorine . . 35.42 . Oxygen 16 Chlonne 3542 °r 1 e* Chloride of mang. 63.12 Water 18. In symbols Mn+20, and 2(H+C1) yield Mn+Cl, 2(H+0), and CL 18* 210 CHLORINE; The affinities which determine these changes are the mutual attraction of oxygen and hydrogen, and of chlorine and manganese. When it is an object to prepare chlorine at the cheapest rate, as for the purposes of manufacture, the preceding process is modified in the following manner. Three parts of sea-salt are intimately mixed with one of peroxide of manganese, and to this mixture two parts of sulphuric acid, diluted with an equal weight of water, are added. By the action of sulphuric acid on sea-salt, hydrochloric acid is disengaged, which reacts as in the former case upon the peroxide of manganese; so that, instead of adding hydrochlo- ric acid directly to the manganese, the materials for forming it are em- ployed. In this process, however, the sulphates of soda and protoxide of man- ganese are generated, instead of chloride of manganese. Thus the materials which act on each other are Mn, Na + CI, and 2S; and the products Mn + S, Na +S; and CI. Chlorine (from ^xmgot, green) is a yellowish-green coloured gas, which has an astringent taste and a disagreeable odour. It is one of the most suf- focating of the gases, exciting spasm and great irritation of the glottis, even when considerably diluted with air. When strongly and suddenly com- pressed, it emits both heat and light, the latter being solely due, as in the case of air and oxygen, to the chlorine acting chemically on the oil with which the compressing apparatus is lubricated. (An. de Ch. et de Ph. xliv. 181.) According to Davy 100 cubic inches of dry chlorine, at 30 Bar. and 60° F. weigh between 76 and 77 grains. Gay-Lussac and Thenard found the density of pure and dry chlorine to be 2.47, which gives 76.5988 grains as the weight of 100 cubic inches at 60° F. and 30 Bar. Under the pressure of about four atmospheres, it is a limpid liquid of a bright yellow colour, which does not freeze at the temperature of zero, and which assumes the gaseous form with the appearance of ebullition when the pressure is re- moved. Mr. Kemp finds that this liquid is a non-conductor of electricity. Cold recently boiled water, at the common pressure, absorbs twice its vol- ume of chlorine, and yields it again when heated. The solution, which is made by transmitting a current of chlorine gas through cold water, has the colour, taste, and most of the other properties of the gas itself. When moist chlorine gas is exposed to a cold of 32°, yellow crystals are formed, which consist of water and chlorine in definite proportions. They are composed, according to Mr. Faraday, of 35.42 parts or one equivalent of chlorine, and 90 parts or ten equivalents of water. Chlorine experiences no chemical change from the action of the impon- derables. Thus it is not affected chemically by intense heat, by strong shocks of electricity, or by a powerful galvanic battery. Davy exposed it also to the action of charcoal heated to whiteness by galvanic electricity, without separating oxygen from it, or in any way affecting its nature. Light does not act on dry chlorine ; but if water be present, the chlorine de- composes that liquid, unites with the hydrogen to form hydrochloric acid, and oxygen gas is set at liberty. This change takes place quickly in sun- shine, more slowly in diffused daylight, and not at all when light is wholly excluded. Hence the necessity of keeping moist chlorine gas, or its solution, in a dark place. Chlorine unites with some substances with evolution of heat and light,and is hence termed a supporter of combustion. If a lighted taper be plunged into chlorine gas, it burns for a short time with a small red flame, and emits a large quantity of smoke. Phosphorus takes fire in it spontaneously, and burns with a pale white light. Several of the metals, such as tin, copper, arsenic, antimony, and zinc, when introduced into chlorine in the state of powder or in fine leaves, are suddenly inflamed. In all these cases the com- bustible substances unite with chlorine. Chlorine has a very powerful attraction for hydrogen; and many of the chemical phenomena, to which chlorine gives rise, are owing to this pro- perty. A striking example is its power of decomposing water by the action CHLORINE. 211 of light, or at a red heat; and most compound substances, of which hydro- gen is an element, are deprived of that principle, and, therefore, decomposed in like manner. For the same reason, when chlorine, water, and some other body which has a strong affinity for oxygen, are presented to one another, water is usually resolved into its elements, its hydrogen attaching itself to the chlorine, and its oxygen to the other body. Hence it happens that chlo- rine is, indirectly, one of the most powerful oxidizing agents which we possess. When any compound of chlorine and an inflammable is exposed to the influence of galvanism, the inflammable body goes over to the negative, and chlorine to the positive pole of the battery. This establishes a close analogy between oxygen and chlorine, both of them being supporters of combustion, and both negative electrics. Chlorine, though formerly called an acid, possesses no acid properties. It has not a sour taste, does not redden the blue colour of plants, and shows comparatively little disposition to unite with alkalies. Its strong affinity for the metals is sufficient to prove that it is not an acid ; for chemists are not acquainted with any instance of an acid combining directly in definite pro- portion with a metal. The mutual action of chlorine and the pure alkalies leads to complicated changes. If chlorine gas be passed into a solution of potassa till all alkaline reaction cease, a liquid is obtained which has the odour of a solution of chlo- rine in water. But on applying heat, the chlorine disappears entirely, and the solution is found to contain chlorate of potassa and chloride of potas- sium. From six equivalents of chlorine and six eq. of potassa are obtained five eq. of chloride of potassium and one eq. of chlorate of potassa, the oxy- gen of five eq. of potassa just sufficing to form chloric acid with one eq. of chlorine; or, stating the same in symbols, 6C1 and 6(K + 0) yield 5(K + C1), K + O, and CI +50. One of the most important properties of chlorine is its bleaching power. All animal and vegetable colours are speedily removed by chlorine; and when the colour is once discharged, it can never be restored. Davy proved that chlorine cannot bleach unless water is present. Thus dry litmus paper suffers no change in dry chlorine ; but when water is admitted, the colour speedily disappears. It is well known also that hydrochloric acid is always generated when chlorine bleaches. From these facts it is inferred that water is decomposed during the process; that its hydrogen unites with chlorine, and that decomposition of the colouring matter is occasioned by the oxygen which is liberated. The bleaching property of binoxide of hydrogen and of chromic and permanganic acids, of which oxygen is certainly the decoloriz- ing principle, leaves little doubt of the accuracy of the foregoing explanation. Chlorine is useful, likewise, for the purposes of fumigation. The expe- rience of Guyton-Morveau is sufficient evidence of its power in destroying the volatile principles given off by putrefying animal matter; and it probably acts in a similar way on contagious effluvia. A peculiar compound of chlorine and soda, the nature of which will be considered in the section on sodium, has been lately introduced for this purpose by M. Labarraque. Chlorine is in general easily recognized by its colour and odour. Chemi- cally it may be detected by its bleaching property, added to the circum- stance that a solution of nitrate of oxide of silver occasions in it a dense white precipitate (a compound of chlorine and metallic silver), which be- comes dark on exposure to light, is insoluble in acids, and dissolves com- pletely in pure ammonia. The whole of the chlorine, however, is not thrown down; for the oxygen of the oxide of silver unites with a portion of chlo- rine, and converts it into chloric acid. Those compounds of chlorine, which are not acid, are termed chlorides or chlorurets. The former expression, from the analogy between chlorine and oxygen, is perhaps the more appropriate. Berzelius inferred the equivalent of chlorine from the oxygen lost by chlo- 212 CHLORINE. rate of potassa when decomposed by heat, and the quantity of chlorine found in the residual chloride of potassium. I investigated the same subject by examining into the composition of the nitrate of the oxide and chloride of silver, of the protoxide and chloride of lead, and of the peroxide and chlo- rides of mercury. These researches concur in showing 35.42 to be the equi- valent of chlorine, and not 36, the number commonly adopted in this coun- try. The number inferred from the densities of chlorine and hydrogen gases is 35.84 (page 146); but, unfortunately, the densities of these gases are not known with the precision required for an application of this nature. The composition of the compounds described in this section is as follows: Chlorine. Equiv. Formula?. 35.42 1 eq. +Hydrogen 1 1 eq.= 36.42 H + Cl 35.42 +Oxyffen 8 1 eq.= 43.42 4eq.= 67.42 5 eq. Hydrochloric acid Protoxide of chlorine Peroxide of chlorine Chloric acid Perchloric acid* Quadrochloride of nitrogen Protochloride of carbon |- Chloride of carbon Dichloride of carbon Perchloride of carbon Dichloride of sulphur Protochloride of sulphur Sesquichloride ofphosphorus Perchloride of phosphorus Chlorocarbonic acid gas Chloral . 35.42 35.42 35.42 141.68 32 40 56 35.42 1177.1 ( 35.42 i 106.26 i 35.42 35.42 106.26 177.1 35.42 1 eq. + Hydrogen 1 +Oxygen 8 +Ditto + Ditto + Ditto 4 eq.+Nitrogen 14.15 1 eq.+Carbon 6.12 5eq. + Ditto 24.48 leq.+Ditto 12.24 3eq.+Ditto 1254 1 eq.+Sulphur 325 leq.+Ditto 16.1 3eq.+Phosph. 31.4 5eq.+Ditto 31.4 1 eq.+Carb. ox. 14.12 75.42 7eq.= 91.42 1 eq.=155.83 Cl+O C1+40 CI+ 50 Cl + 70 N+4CI 47.66 C+Cl 4C+5C1 2C+C1 118.5 2C+3C1 67.62 2S+C1 51.52 S+Cl 2 eq.= 137.66 2P+3C1 2eq.= 208.5 2P+5C1 1 eq.= 49.54 C+Cl 1 eq.= 41.54 4 eq. =201.58 2eq.= 2eq.= 2eq.= 1 eq.= Terchloride of boron 212.52 6 eq+ } Carbon 55.08 9 eq. I c 9C1+6CI ^ ' ( Oxygen 32 4 eq. \ \ +40 106.26 3eq. + Boron 10.9 1 eq.= 117.16 B+3C1 Hydrochloric Acid.—A concentrated aqueous solution of this acid has been long known under the names of spirit of salt, and of marine or muriatic acid; but in its purer form of gas, it was discovered in 1772 by Dr. Priest- ley. It may be conveniently prepared by putting an ounce of strong hydrochloric acid solution into a glass flask, and heating it by means of a lamp till the liquid boils, when the gas is freely evolved, and may be col- lected over mercury. Another method of preparing it is by the action of concentrated sulphuric acid on an equal weight of sea-salt. Brisk efferves. cence ensues at the moment of making the mixture, and on the application * Oxychloric would be a more appropriate appellation for this acid, as its adoption would prevent all ambiguity in naming its salts. This name I pro- posed for it in 1819, in my System of Chemistry for Students of Medicine ; and it may be inferred that it has the sanction of Berzelius, as he employs it in his Traiti de Chimie.—Ed. CHLORINE. 213 of heat, a large quantity of hydrochloric acid gas is disengaged. In the former process, hydrochloric acid, previously dissolved in water, is simply expelled from the solution by heat. The explanation of the latter process is more complicated. Sea-salt was formerly supposed to be a compound of hydro- chloric acid and soda; and, on this supposition, the soda was believed merely to quit the hydrochloric and unite with sulphuric acid. But the researches of Gay-Lussac, Thenard, and Davy proved that it consists of chlorine and sodium combined in the ratio of their equivalents. The nature of its action with sulphuric acid will be understood by comparing the elements concerned in the change before and after it has occurred :— Hydrous Sulp. Acid. Chloride of Sodium. Snip, of Soda. Hydrochloric Acid. Acid 40.1 Chlorine 35.42 c i S Sod. 23.3 xr , ■• Soda}Oxy. 8 Hydrogen 1 Real acid 40.1 Chlorine 35,12 Water j "yd. 1 Sodium 23.3 Or in symbols, (S+30)+(H+0), and Na+Cl, yield (Na+0)+(S+30), and H+Cl. Thus it appears that single equivalents of water, sulphuric acid, and chloride of sodium, yield sulphate of soda and hydrochloric acid. The water of the sulphuric acid is essential; so much so, indeed, that chloride of sodium is not decomposed at all by anhydrous sulphuric acid. Hydrochloric acid may be generated by the direct union of its elements. When equal measures of chlorine and hydrogen are mixed together, and an electric spark is passed through the mixture, instantaneous combination takes place, heat and light are emitted, and hydrochloric acid is generated. A similar effect is produced by flame, by a red-hot body, and by spongy platinum. Light also causes them to unite. A mixture of the two gases may be preserved without change in a dark place; but if exposed to the diffused light of day, gradual combination ensues, which is completed in the course of 24 hours. The direct solar rays produce, like flame and electricity, sudden inflammation of the whole mixture, accompanied with explosion; and, according to Mr. Brande, the vivid light emitted by charcoal intensely heated by galvanic electricity acts in a similar manner. The experiments of Davy, and Gay-Lussac and Thenard concur in proving that hydrogen and chlorine gases unite in equal volumes, and that the hydrochloric acid, which is the sole and constant product, occupies the same space as the gases from which it is formed. From these facts the composition of hydrochloric add is easily inferred. For, as Grains. 50 cubic inches of chlorine weigh . 38.2994 50 hydrogen . . 1.0683 100 cubic inches of muriatic acid gas must weigh 39.3677 These numbers are nearly in the ratio of 1 to 35.42, being that of single equivalents of chlorine and hydrogen, as stated at page 146. The specific gravity of hydrochloric acid gas calculated from the same facts (page 148) is 1.2694, which agrees closely with direct observations. Hydrochloric acid gas is colourless, and has a pungent odour and an acid taste. Under a pressure of 40 atmospheres, and at the temperature of 50°, it is liquid. It is quite irrespirable, exciting violent spasm of the glottis; but when diluted with air, it is far less irritating than chlorine. All burn- ing bodies are extinguished by it, nor is the gas itself inflammable. Hydrochloric acid gas is not chemically changed by mere heat. It is readily decomposed by galvanism, hydrogen appearing at the negative, and chlorine at the positive pole. It is also decomposed by ordinary electricity. The decomposition, however, is incomplete; for though one electric spark resolves a portion of the gas into its elements, the next shock in a great measure effects their reunion. It is not affected by oxygen under common circumstances; but if a mixture of oxygen and hydrochloric acid gases is 214 CHLORINE. electrified, the oxygen unites with the hydrogen of the acid to form water, and chlorine is set at liberty. For this and the preceding fact, we are indebted to the researches of Dr. Henry. One of the most striking properties of hydrochloric acid gas is its power- ful attraction for water. A dense white cloud appears whenever it escapes into the air, owing to its combining with the aqueous vapour of the atmo- sphere. When a piece of ice is put into a jar full of the gas confined over mercury, the ice liquefies on the instant, and the whole of the gas disappears in the course of a few seconds. On opening a long wide jar of hydrochloric acid gas under water, the absorption of the gas takes place so instantane- ously, that the water is forced up into the jar with the same violence as into a vacuum. Hydrochloric acid is much used in the form of a concentrated aqueous solution, which is made by transmitting a current of the gas into water as long as any of it is absorbed. Considerable increase of temperature takes place during the absorption, and, therefore, the apparatus should be kept cool by ice. Davy states (Elements, p. 252.) that water at the temperature of 40° absorbs 480 times its volume of the gas, and that the solution has a density of 1.2109. Dr. Thomson finds that one cubic inch of water at 69° absorbs 418 cubic inches of gas, and occupies the space of 1.34 cubic inch. The solution has a density of 1.1958, and one cubic inch of it contains 311.04 cubic inches of hydrochloric acid gas. The quantity of real acid contained in solutions of different densities may be determined by ascertain- ing the quantity of pure marble dissolved by a given weight of each. Every 50.62 grains of marble correspond to 36.42 of real acid. The following table from Dr. Thomson's "Principles of Chemistry," is constructed accord- ing to this rule. The first and second columns show the atomic constitution of each acid. Table exhibiting the Specific Gravity of Muriatic Acid of determinate Strengths. Atoms Atoms Real acid in 100 Specific Atoms Atoms Real acid in 100 Specific of acid. ofwater. of the liquid. Gravity. of acid. ofwater. of the liquid. Gravity. 1 6 40.659 1.203 1 14 22.700 1.1060 1 7 37.000 1.179 1 15 21.512 1.1008 1 8 33.945 1.162 1 16 20.442 1.0960 1 9 31.316 1.149 1 17 19.474 1.0902 1 10 29.134 1.139 ! I 18 18.590 1.0860 1 11 27.206 1.1285 | 1 19 17.790 1.0820 1 12 25.517 1.1197 1 20 17.051 1.07S0 1 13 24.026 1.1127 | 1 All the Pharmacopoeias give directions for forming hydrochloric acid. The process recommended by the Edinburgh College is practically good. The proportions they recommend are equal weights of sea-salt water, and sulphuric acid, more acid being purposely employed than is sufficient to form a neutral sulphate with the soda, so that the more perfect decompo- sition of the sea-salt may be ensured. The acid, to prevent too violent effervescence at first, is mixed with one-third of the water, and when the mixture has cooled, it is poured upon the salt previously introduced into a glass retort. The distillation is continued to dryness; and the gas, as it escapes, is conducted into the remainder of the water. The theory of the process has already been explained. The residue is a mixture of sulphate and bisulphate of soda. The specific gravity of the acid obtained by this process is 1.170. Hydrochloric acid of commerce has a yellow colour, and is always impure. Its usual impurities are nitric acid, sulphuric acid, and oxide of iron. The CHLORINE. 215 presence of nitric acid may be inferred if the hydrochloric acid has the property of dissolving gold-leaf. Iron may be detected by ferrocyanuret of potassium, and sulphuric r.cid by chloride of barium, the suspected hydro- cldoric acid being previously diluted with three or four parts ofwater. The presence of nitric acid is provided against by igniting the sea-salt, as recom- mended by the Edinburgh College, in order to decompose any nitre which it may contain. The other impurities may be avoided by employing Woulfe's Apparatus. A few drachms of water are put into the first bottle to retain the chloride of iron and sulphuric acid which pass over, and the hydrochloric acid gas is condensed in the second. A strong solution of pure hydrochloric acid is a colourless liquid, which emits white vapours when exposed to the air, is intensely sour, reddens litmus paper strongly, and neutralizes alkalies. It combines wilh water in every proportion, and causes increase of temperature when mixed wilh it, though in a much less degree than sulphuric acid. It freezes at —60° F.; and boils at 110°, or a little higher, giving off pure hydrochloric acid gas in large quantities. Hydrochloric acid is decomposed by substances which yield oxygen rea- dily. Thus several peroxides, such as those of manganese, cobalt, and lead, effect its decomposition. Chloric, iodic, bromic, nitric, and selenic acids act on the same principle. A mixture of nitric and hydrochloric acids, in the ratio of one measure of the former to two of the latter, has long been known under the name of aqua regia, as a solvent for gold and platinum. When these acids are mixed together, the solution instantly becomes yellow; and on heating the mixture, pure chlorine is evolved, and the colour of the solu- tion deepens. On continuing the heat, chlorine and nitrous acid vapours are disengaged. At length the evolution of chlorine ceases, and the residual liquid is found to be a solution of hydrochloric and nitrous acids, which is incapable of dissolving gold. The explanation of these facts is, that nitric and hydrochloric acids decompose one another, giving rise to the production of water and nitrous acid, and the separation of chlorine; while hydro- chloric and nitrous acids may be heated together without mutual decompo- sition. It is hence inferred that the power of nitro-hydrochloric acid in dissolving gold is owing to the chlorine which is liberated. (Davy in the Quarterly Journal, vol. i.) Hydrochloric acid is distinguished by its odour, volatility, and strong acid properties. With nitrate of oxide of silver it yields the same precipitate as chlorine; but no chloric acid is generated, because the oxygen of the oxide of silver unites with the hydrogen of the hydrochloric acid, and the chlorine in consequence is entirely precipitated. Notwithstanding that nitrate of oxide of silver yields the same precipitate with chlorine and hydrochloric acid, there is no difficulty in distinguishing between them; for the bleaching property of the former is a sure ground of distinction. COMPOUNDS OF CHLORINE AND OXYGEN. The leading character of these compounds is derived from the circum- stance that chlorine and oxygen, the attraction of which for most elemen- tary substances is so energetic, have but a feeble affinity for each other. These principles, consequently, are never met with in nature in a state of combination. Indeed, they cannot be made to combine directly; and when they do unite, very slight causes effect their separation. Chemists are not agreed as to the exact number of the compounds of chlorine and oxygen. The subjoined list contains those which hitherto have been generally ad- mitted; but the exis-tence of the first is very problematical, while some enumerate a fifth under the name of chlorous acid. 216 CHLORINE. By Weight. By Volume. Chi. Oxy. Chi Protoxide of chlorine . 35.42 . 8 . .2 Peroxide of chlorine . 35.42 .32 . 2 Chloric acid . . . 35.42 . 40 . .2 Perchloric acid . . 35.42 .56 .2 According to the practice of most British chemists two volumes of chlo- rine, as also two volumes of hydrogen and of nitrogen, are considered as respectively corresponding to one equivalent or one atom; whereas one volume of oxygen corresponds to one equivalent. Berzelius, with many continental chemists, considering the atoms of all elements to possess the same volume, regards the four preceding compounds as composed of two atoms or two equivalents of chlorine combined with one, four, five, and seven atoms or equivalents of oxygen. Protoxide of Chlorine.—This gas was discovered in 1811 by Davy, and was described by him in the Philosophical Transactions for that year under the name of euchlorine. It is made by the action of hydrochloric acid on chlorate of potassa; and its production is explicable by the fact, that hydro- chloric and chloric acids mutually decompose each other. When hydro- chloric acid and chlorate of potassa are mixed together, more or less of the potassa is separated by the hydrochloric from the chloric acid, and the latter being set free reacts on free hydrochloric acid. The result depends on the relative quantities of the materials. If hydrochloric acid be in excess, the chloric acid undergoes complete decomposition. For each equivalent of chloric, five eq. of hydrochloric acid are decomposed: the five eq. of oxygen, contained in the former, unite with the hydrogen of the latter, producing five eq. ofwater; while the chlorine of both acids is disengaged. If, on the contrary, chlorate of potassa is in excess, the chloric acid is deprived of part of its oxygen only ; and the products are water, protoxide of chlorine, and chlorine, the two latter escaping in the gaseous form. The best pro- portion of the ingredients is two parts of chlorate of potassa, one of strong hydrochloric acid, and one of water; and the reaction of the materials should be promoted by heat sufficient to produce moderate effervescence. The gases should be collected over mercury, which combines with the chlo- rine, and leaves the protoxide of chlorine in a pure state. Protoxide of chlorine has a yellowish-green colour similar to that of chlorine, but considerably more brilliant, which induced Davy to give it the name of euchlorine. Its odour is like that of burned sugar. Water dis- solves eight or ten times its volume of the gas, and acquires a colour ap- proaching to orange. It bleaches vegetable substances, but gives the blue colours a tint of red before destroying them. It does not unite with alka- lies, and, therefore, is not an acid. Protoxide of chlorine is explosive in a high degree. The heat of the hand, or the pressure occasioned in transferring it from one vessel to another, sometimes causes an explosion. This effect is also occasioned by phosphorus, which bursts into flame at the moment of immersion. All burning bodies, by their heat, occasion an explosion, and then burn vividly in the decomposed gas. With hydrogen it forms a mixture which explodes by flame or the electric spark, with production of water and hydrochloric acid. The best proportion is fifty measures of protoxide of chlorine to eighty of hydrogen. Protoxide of chlorine is easily analyzed by heating a known quantity of it in a strong tube over mercury. An explosion takes place; and 50 measures of the gas expand to 60 measures, of which 20 are oxygen, and 40 chlorine. From this it was considered to contain single equivalents of chlorine and oxygen. But most chemists have regarded the existence of euchlorine as problematical, suspecting it to be a mere mixture of chlorine with the per- oxide of chlorine; and M. Soubeiran, supposing his experiments correct, Oxy. 1 4 5 7 CHLORINE. 217 has proved this suspicion to be well founded. He finds that euchlorine it- self acts on mercury, and, therefore, cannot be purified by its means. He effected its purification by transmitting the gas through a tube nearly full of calomel, which absorbed the free chlorine only ; and on subsequently ex- ploding the oxide of chlorine thus purified, he obtained one volume of chlo- nne to two volumes of oxygen, being the precise composition of the per- oxide. The confirmation of these results will cause the removal of euchlo- rine from the list of definite compounds. (An. de Ch. et de Ph. xlviii. 113.) Peroxide of Chlorine.—This compound was discovered by Davy in 1815 (Phil. Trans.), and soon after by Count Stadion of Vienna. It is formed by the action of sulphuric acid on chlorate of potassa. A quantity of this salt not exceeding 50 or 60 grains, is reduced to powder, and made into a paste by the addition of strong sulphuric acid. The mixture, which ac- quires a deep yellow colour, is placed in a glass retort, and heated by warm water, the temperature of which is kept under 212° F. A bright yellowish- green gas of a still richer colour than protoxide of chlorine is disengaged, which has an aromatic odour without any smell of chlorine, is absorbed rapidly by water, to which it communicates its tint, and has no sensible action on mercury. This gas is peroxide of chlorine. The chemical changes which take place in the process are explained in the following manner. The sulphuric acid decomposes some of the chlorate of potassa, and sets chloric acid at liberty. The chloric acid, at the mo- ment of separation, resolves itself into peroxide of chlorine and oxygen; the last of which, instead of escaping as free oxygen gas, goes over to the acid of some undecomposed chlorate of potassa, and converts it into per- chloric acid. The whole products are bisulphate and perchlorate of potassa, and peroxide of chlorine. It is most probable, from the data contained in the preceding table, that every three equivalents of chloric acid yield one eq. of perchloric acid and two eq. of peroxide of chlorine. Peroxide of chlorine does not unite with alkalies. It destroys most vege- table blue colours, without previously reddening them. Phosphorus takes fire when introduced into it, and occasions an explosion. It explodes vio- lently when heated to a temperature of 212°, emits a strong light, and un- dergoes a greater expansion than protoxide of chlorine. According to Davy, whose result is confirmed by Gay-Lussac, 40 measures of the gas occupy after explosion the space of 60 measures; and of these, 20 are chlo- rine and 40 oxygen. The peroxide is, therefore, composed of 35.42 parts or one equivalent of chlorine, united with 32 or four eq. of oxygen ; and its density must be 2.3374. Chlorous Acid.—When chlorine acts on a weak solution of the pure alka- lies, or on the alkaline earths in the state of hydrates, a bleaching substance is procured which has been commonly viewed as a direct compound of chlo- rine and an alkaline base. Berzelius, however, contends that these com- pounds consist of a base in union with chlorous acid, which he believes to be identical with the peroxide of chlorine. The reason why peroxide of chlorine, acting directly on an alkali, does not combine with it, is said to be its conversion into chloric acid, while a metallic chloride is formed; but when chlorine acts on an alkali, a chlorite and chloride are thought to be generated. The bleaching property of a chlorite is attributed to its oxygen, a chloride being formed at the same time. Soubeiran supports this view with some strong facts, though part of his reasoning appears to me falla- cious. He also assigns to chlorous acid only three equivalents of oxygen, that is, a smaller quantity than is possessed by the peroxide. On the whole, I see no sufficient reason to reject the opinion hitherto entertained concern ing the nature of the bleaching compounds. Chloric Acid.—When to a dilute solution of chlorate of baryta, a quantity of weak sulphuric acid, exactly sufficient for combining with the baryta, is added, the insoluble sulphate of baryta subsides, and pure chloric acid re- mains in the liquid. This acid, the existence of which was originally ob- 19 218 CHLORINE. served by Mr. Chenevix, was first obtained in a separate state by Gay- Lussac. Chloric acid reddens vegetable blue colours, has a sour taste, and forms neutral salts, called chlorates, (formerly hyperoxymuriates), with alkaline bases. It possesses no bleaching properties, a circumstance by which it is distinguished from chlorine. It gives no precipitate in solution of nitrate of oxide of silver, and hence cannot be mistaken for hydrochloric acid. Its so- lution may be concentrated by gentle heat, till it acquires an oily consist- ence, without decomposition : in this state of highest concentration it ac- quires a yellowish tint, emits an odour of nitric acid, sets fire to paper and other dry organic matter, and converts alcohol into acetic acid. When sharply heated in a retort, part of the acid is resolved into chlorine and oxy- gen ; but another portion, acquiring oxygen from that which is decomposed, is converted into perchloric acid, and then passes over into the receiver in the form of a dense colourless liquid. (Serullas.) Chloric acid is easily de- composed by deoxidizing agents. Sulphurous acid, for instance, deprives it of oxygen, with formation of sulphuric acid and evolution of chlorine. By the action of hydrosulphuric acid, water is generated, while sulphur and chlorine are set free. The power of hydrochloric acid in effecting its decom- position has already been explained. Chloric acid is readily known by forming a salt with potassa, which crys- tallizes in tables and has a pearly lustre, deflagrates like nitre when flung on burning charcoal, and yields peroxide of chlorine by the action of concen- trated sulphuric acid. Chlorate of potassa, like most of the chlorates, gives off pure oxygen when heated to redness, and leaves a residue of chloride of potassium. By this mode Gay-Lussac ascertained the composition of chloric acid, as stated in the preceding table. (An. de Chimie, xci.) Perchloric Acid.—The saline matter which remains in the retort after forming peroxide of chlorine is a mixture of perchlorate and bisulphate of potassa; and by washing it with cold water, the bisulphate is dissolved, and the perchlorate is left. Perchloric acid may be prepared from this salt by mixing it in a retort with half its weight of sulphuric acid, diluted with one- third of water, and applying heat to the mixture. At the temperature of about 284° F. white vapours rise, which condense as a colourless liquid in the receiver. This is a solution of perchloric acid. The existence of perchloric acid was first ascertained by Count Stadion, who found it to be a compound of two volumes or one equivalent of chlo- rine and seven of oxygen; and this view of its constitution has been con- firmed by Gay-Lussac, Serullas, and Mitscherlich. (An. de Ch. et de Ph. viii. ix. xlvi. 297, and xlix. 113.) According to Serullas it is a very stable com- pound : it may be heated with hydrochloric or sulphuric acid without change, does not set fire to organic substances, and is not decomposed by alcohol. When concentrated it has a density of 1.65, in which state it emits vapour when exposed to the air, absorbs hygrometric moisture powerfully, and boils at 392° F. By admixture with strong sulphuric acid and distilling, Serullas obtained it in the solid form, both massive and in elongated prisms. It hisses when thrown into water, like red-hot iron when quenched. Of all the salts of perchloric acid, that with potassa is the most insoluble, requiring 65 times its weight of water at 60° for solution. This salt is readily and safely formed by adding chlorate of potassa, well dried and in fine powder, in small portions at a time, to an equal weight of concentrated sulphuric acid, gently warmed in an open vessel. The peroxide of chlorine escapes without danger, and the chlorate is entirely converted into perchlo- rate and bisulphate of potassa, the latter of which, being very soluble, is easily removed by cold water. Serullas finds that chlorate of potassa, when decomposed by a low heat, is converted into chloride of potassium and per- chlorate of potassa; but the temperature must be carefully managed, other- wise the perchlorate itself would be resolved into oxygen and chloride of potassium. The perchlorate thus procured is purified by solution in hot wa- ter and crystallization. It is distinguished from chlorate of potassa by not CHLORINE. 219 acquiring a yellow tint on the addition of hydrochloric acid. The primary form of its crystals, according to Mitscherlich, is a right rhornboidal prism isomorphous with permanganate of potassa. Quadrochloride of Nitrogen.—The elements of this compound have a fee- ble mutual affinity, and do not unite when presented to each other in their gaseous form. The condition which leads to their union is the decomposi- tion of ammonia by chlorine, during which hydrochloric acid is generated by chlorine combining with the hydrogen of ammonia; while the nitrogen of that alkali, in its nascent state, enters into combination with another por- tion of chlorine. A convenient mode of preparing the quadrochloride of ni- trogen is the following. An ounce of hydrochlorate of ammonia is dissolved in 12 or 16 ounces of hot water; and when the solution has cooled to the temperature of 90°, a glass bottle with a wide mouth, full of chlorine, is in- verted in it. The solution gradually absorbs the chlorine, and acquires a yel- low colour; and in about 20 minutes globules of a yellow fluid are seen floating like oil upon its surface, which, after acquiring the size of a small pea, sink to the bottom of the liquid. The drops of the chloride, as they de- scend, should be collected in a small saucer of lead, placed for that purpose under the mouth of the bottle. Quadrochloride of nitrogen, discovered in 1811 by Dulong, (An. de Ch. Ixxxvi.) is one of the most explosive compounds yet known, having been the cause of serious accidents both to its discoverer and to Davy. (Phil. Trans. 1813.) Its specific gravity is 1.653. It does not congeal in the intense cold produced by a mixture of snow and salt. It may be distilled at 160°; but at a temperature between 200° and 212° it explodes. It appears from the in- vestigation of Messrs. Porrett, Wilson, and Kirk, that its mere contact with some substances of a combustible nature causes detonation even at common temperatures. This result ensues particularly with oils, both volatile and fixed. I have never known olive oil fail in producing the effect. The pro- ducts of the explosion are chlorine and nitrogen. (Nicholson's Journal. xxxiv.) Sir H. Davy analyzed chloride of nitrogen by means of mercury, which unites with chlorine, and liberates the nitrogen. He inferred from his ana- lysis that its elements are united in the proportion of four measures of chlo- rine to one of nitrogen; and it hence follows that, by weight, it consists of four equivalents of chlorine, and one equivalent of nitrogen.* Perchloride of Carbon.—The discovery of this compound is due to Mr. Faraday. When olefiant gas (a compound of carbon and hydrogen) is mixed with chlorine, combination takes place between them, and an oil-like liquid is generated, which consists of chlorine, carbon, and hydrogen. On exposing this liquid in a vessel full of chlorine gas to the direct solar rays, the chlo- rine acts upon and decomposes the liquid, hydrochloric acid is set free, and the carbon, at the moment of separation, unites with the chlorine. (Phil. Trans. 1821.) Perchloride of carbon is solid at common temperatures, has an aromatic odour approaching to that of camphor, is a non-conductor of electricity, and refracts light very powerfully. Its specific gravity is exactly double that of water. It fuses at 320°, and after fusion it is colourless and very transpa- rent. It boils at 360°, and may be distilled without change, assuming a crys- talline arrangement as it condenses. It is sparingly soluble in water, but dissolves in alcohol and ether, especially by the aid of heat. It is soluble also in fixed and volatile oils. Perchloride of carbon burns with a red light when held in the flame of a spirit-lamp, giving out acid vapours and smoke; but the combustion ceases * Berzelius states the composition of this compound to be three volumes of chlorine to one of nitrogen, corresponding to three equivalents of the for- mer to one of the latter. These proportions, if found to be correct, will ren» der the chloride and iodide of nitrogen analogous in composition.—Ed, 220 CHLORINE. as soon as it is withdrawn. It burns vividly in oxygen gas. Alkalies do not act upon it; nor is it changed by the stronger acids, such as the hydrochlo- ric, nitric, or sulphuric acids, even with the aid of heat. When its vapour, mixed with hydrogen, is transmitted through a red-hot tube, charcoal is se- parated, and hydrochloric acid gas evolved. On passing its vapour over the peroxides of metals, such as that of mercury and copper, heated to redness, a chloride of the metal and carbonic acid are generated. Protoxides, under the same treatment, yield carbonic oxide gas and metallic chlorides. Most of the metals decompose it also at the temperature of ignition, uniting with its chlorine, and causing deposition of charcoal. The composition of the perchloride of carbon was inferred by Mr. Fara- day from the proportions of chlorine and defiant gas employed in its pro- duction, and from the quantity of chloride of copper and carbonic acid generated when its vapour was transmitted over oxide of copper at a red heat. % Chloride of Carbon.—This compound, so called to indicate the ratio of its elements, is formed by boiling chloral with a solution of potassa, lime, or baryta; and was discovered by Liebig. The elements which appear to re- act on each other are, two eq. of chloral or 2(9C + 6C1 + 40), two eq. of potassa or 2(K+0), and five eq. of water or 5H; and the products are two eq. of the chloride of carbon or 2(4C+5C1), two eq. of chloride of potas- sium or 2(K + C1), and five eq. of formic acid or 5(2C+H). The formic acid of course unites with potassa, and the chloride of carbon, being volatile, passes off in vapour. After condensation it is washed repeatedly with pure water, and obtained quite dry by admixture with strong sulphuric acid, and distillation at 212° F. It may also be more conveniently prepared by dis- tilling, from a capacious retort, a mixture of one pound of chloride of lime, three pounds of water, and two or three ounces either of alcohol or pyro- acetic spirit. This chloride is a limpid, colourless liquid, similar in odour and appear- ance to the oily fluid which chlorine forms with olefiant gas; though in density, volatility, and composition, it is quite different. Its density is 1.48. and it boils at 141° F. It is but feebly combustible, and is not changed at moderate temperatures either by acids or alkalies. It is freely dissolved by alcohol and ether, and is precipitated from them by water, in which it is quite insoluble. It may be distilled in contact with potassium without change. When its vapour is transmitted over metallic copper, charcoal is deposited, no gas whatever appears, and a metallic chlo- ride is formed. Its composition was inferred from the quantity of chloride of copper and carbonic acid which were produced, when its vapour was transmitted over oxide of copper at a red heat. (An. de Ch. et de Ph. xlix. 146.) Protochloride of Carbon.—When the vapour of the perchloride is passed through a red-hot glass or porcelain tube, filled with fragments of rock crystal to increase the quantity of heated surface, partial decomposition occurs, chlorine gas escapes, and a vapour which, analyzed by Mr. Faraday by means of oxide of copper, proved to be protochloride of carbon. At common temperatures it is a limpid colourless liquid, which has a density of 1.5526, docs not congeal at 0° F., and at 160° or 170° is converted into va- pour. It may be distilled repeatedly without change; but when exposed to a red heat, some of it is resolved into its elements. In its chemical rela- tions it is very analogous to perchloride of carbon. Dichloride of Carbon.—The only sample of this substance yet obtained was brought from Sweden by M. Julin, and is said to have been formed during the distillation of nitric acid from crude nitre and sulphate of iron. It occurs in small, soft, adhesive fibres of a white colour, which have a peculiar odour, somewhat resembling spermaceti. It fuses on the applica- tion of heat, and boils at a temperature between 350° and 450° F. At 250° it sublimes slowly, and condenses again in the form of long needles. It CHLORINE. 221 is insoluble in water, acids, and alkalies; but is dissolved by hot oil of turpentine or by alcohol, and forms acicular crystals as the solution cools. It burns with a red flame, emitting much smoke, and fumes of hydrochloric acid gas. The nature of this substance is shown by the following circumstances. \V hen its vapour is exposed to a red heat, evolution of chlorine gas ensues, and charcoal is deposited. A similar deposition of charcoal is produced by heating it with phosphorus, iron, or tin; and a chloride is formed at the same time. Potassium burns vividly in its vapour with formation of chlo- ride of potassium and separation of charcoal. On detonating a mixture of its vapour with oxygen gas over mercury, a chloride of that metal and car- bonic acid are generated. By these means Messrs. Phillips and Faraday ascertained its composition as given at page 212. (An. of Phil, xviii. 150.) Dichloride of Sulphur.—This compound was discovered in the year 1804 by Dr. Thomson,* and was afterwards examined by Berthollet.t It is most conveniently prepared by passing a current of chlorine gas over flowers of sulphur gently heated, until nearly all the sulphur disappears. Direct combination ensues, and the product, distilled off from uncombined sulphur, is obtained under the form of a liquid which appears red by reflected, and yellowish-green by transmitted light. Its density is 1.687. It is volatile below 200°, boils at 280°, yielding vapour which has a density of 4.70, and condenses again without change in cooling. When exposed to the air it emits acrid fumes, which irritate the eyes powerfully, and have an odour somewhat resembling sea-weed, but much stronger. Dry litmus paper is not reddened by it, nor does it unite with alkalies. It acts with energy on water:—mutual decomposition ensues, with formation of hydrochloric and hyposulphurous acids, and deposite of sulphur, by which the water Is ren- dered cloudy. From a recent analysis by Rose it consists of 35.42 parts or one equivalent of chlorine, and 32.2 parts or two eq. of sulphur. (Pog. Annalen xxi. 431.) Rose maintains that the preceding is the only chloride of sulphur, arguing that the chloride analyzed by Davy was merely dichloride of sulphur hold- ing chlorine in solution. Dumas, on the other hand, contends, that when sulphur is acted on by excess of chlorine, a chloride of sulphur is really obtained, which is apt to retain traces of the dichloride, and can only be purified by repeated distillation at about 140° F. This chloride is a liquid of a deep reddish-brown tint, and has a density of 1.62. It boils at 147°, and the density of its vapour is between 3.67 and 3.70. By decomposition in water, it should yield hydrochloric and hyposulphurous acids. (An. de Ch. et de Ph. xlix. 205.) Perchloride of Phosphorus.—There are two definite compounds of chlo- rine and phosphorus, the nature of which was first satisfactorily explained by Davy. (Elements, p. 290.) When phosphorus is introduced into a jar of dry chlorine, it inflames, and on the inside of the vessel a white matter col- lects, which is perchloride of phosphorus. It is very volatile, a temperature much below 212° being sufficient to convert it into vapour. Under pressure it may be fused, and it yields transparent prismatic crystals in cooling. Water and perchloride of phosphorus mutually decompose each other; and the sole products are hydrochloric and phosphoric acids. Now in order that these products should be formed, consistently with the constitution af phosphoric acid, as stated at page 200, the perchloride must consist of 31.4 parts or two equivalents of phosphorus, and 177.1 parts or five eq. of chlo- rine. One equivalent of the chloride and five eq. of water will then mutu' ally decompose each other without any element being in excess, and yield one eq. of phosphoric, and five eq. of hydrochloric acid. This proportion is not far from the truth; for according to Davy, one grain of phosphorus is united in the perchloride with six of chlorine. * Nicholson's Journal vol. vi. + Memoires d'Arcqeil, vol, j, 19* 222 CHLORINE. Sesquichloride of Phosphorus may be made either by heating the perchlo- ride with phosphorus, or by passing the vapour of phosphorus over corrosive sublimate contained in a glass tube. It is a clear liquid like water, of specific gravity 1.45; emits acid fumes when exposed to the air, owing to the decomposition of watery vapour; but when pure it does not redden dry litmus paper. On mixing it with water, mutual decomposition ensues, heat is evolved, and a solution of hydrochloric and phosphorous acids is obtained. It hence appears to consist of 31.4 parts or two equivalents of phosphorus, and 106.26 parts or three eq. of chlorine. When hydrosulphuric acid gas is transmitted through a vessel containing perchloride of phosphorus, hydrochloric acid is disengaged, and a liquid produced which, according to Serullas, is a compound of three equivalents of chlorine, one of phosphorus, and one of sulphur. (An. de Ch. et de Ph. xlii. 25.) Chlorocarbonic Acid Gas.—This compound was discovered in 1812 by Dr. Davy, who described it in the Philosophical Transactions for that year under the name of phosgene gas. (From c light, and yivvuv to produce.) It is made by exposing a mixture of equal measures of dry chlorine and carbonic oxide gases to sunshine, when rapid but silent combination ensues, and they contract to one-half their volume. Diffused daylight also effects their, union slowly; but they do not combine at all when the mixture is wholly excluded from light. Chlorocarbonic acid gas is colourless, has a strong odour, and reddens dry litmus paper. It combines with four times its volume of ammoniacal gas, forming a white solid salt; so that it possesses the characteristic pro- perty of acids. It is decomposed by contact with water. One equivalent of each compound undergoes decomposition; and as the hydrogen of the water unites with chlorine, and its oxygen with carbonic oxide, the products are carbonic and hydrochloric acids. When tin is heated in this gas, chlo- ride of tin is generated, and carbonic oxide gas set free, which occupies ex- actly the same space as the chlorocarbonic acid which was employed. A similar change occurs when it is heated in contact with antimony, zinc, or arsenic. As chlorocarbonic acid gas contains its own volume of each of its consti- tuents, it follows that 100 cubic inches of that gas at the standard tempera- ture and pressure must weigh 106.7638 grains; namely, 76.5988 of chlorine added to 30.1650 of carbonic oxide. Its specific gravity is, therefore, 3.4427, and it consists of 35.42 parts or one equivalent of chlorine, and 14.12 parts or one equivalent of carbonic oxide. Chloral.—This name, derived from the first syllable of the words chlorine and alcohol, has been applied by Liebig to a new compound of chlorine, car- bon, and oxygen, prepared by the mutual action of alcohol and chlorine. The production of the new substance more immediately depends on the union of one portion of chlorine with the hydrogen both of the defiant gas and some of the water which constitute alcohol, while the corresponding car- bon and oxygen unite with another portion of chlorine. A very large quan- tity of chlorine is, therefore, required for its production, and a large quantity of hydrochloric acid is generated. When chlorine is transmitted through common alcohol, the chloral at first formed is retained in solution by the water of the alcohol; but when the liquid becomes charged with hydrochlo- ric acid, the chloral insoluble in that solution subsides as a liquid of an oily aspect. The method recommended by Liebig is to transmit dry chlorine gas into absolute alcohol. At first the alcohol should be artificially cooled, since otherwise the action is attended with flame; but as soon as hydrochloric acid begins to accumulate, it is necessary to expel it by a continued gentle heat, since its presence protects the alcohol from decomposition, and there- by prevents the absorption of the chlorine. By acting in this manner for several successive days, Liebig succeeded in decomposing eight ounces of absolute alcohol, and obtained a liquid of syrupy consistence, which after a few days became a soft white crystalline solid. In this state the chloral is CHLORINE. 223 united with water as a hydrate of unknown composition, and also contains traces of undecomposed alcohol and hydrochloric acid. On agitation with strong sulphuric acid, it is rendered anhydrous, and rises as a liquid to the surface; and by distillation from unslaked lime or baryta, it is entirely sepa- rated from hydrochloric acid. Pure chloral is a colourless transparent liquid of a penetrating pungent odour, is oily to the touch, and has a density of 1.502. It is nearly tasteless, or at most oily. It boils at 201°, and may be distilled without change. In water, with the aid of gentle heat, it is freely dissolved without decomposi- tion. Agitated with a few drops of water, it forms a white crystalline mass which appears to be a hydrate, though its composition is unknown; for when put into water, the chloral appears oily as usual, and by heat a solu- tion having the characteristic odour and character of chloral is obtained. On keeping this crystalline hydrate for a few days, it becomes a very white flocculent matter, which is quite insoluble in water. From the ratio of its ingredients Liebig regards it also as a hydrate, formed of two equivalents of water to one of chloral. Chloral unites with iodine, bromine, and sulphur. It may be distilled from the anhydrous metallic oxides without change; but when its vapour is transmitted over anhydrous baryta, strontia, or lime, heated to 212°, instant decomposition ensues and the earth becomes incandescent. Carbonic oxide gas escapes at the same time, and a metallic chloride, intermixed with a light charcoal, is generated. Similar products are obtained, but of course more charcoal, when the vapour of chloral is transmitted over iron or cop- per at a red heat. Chloral is readily decomposed by the alkalies or alkaline earths in the state of hydrates or when dissolved in water, giving rise to a chloride of* carbon, as already described. (An. de. Ch. et de Ph. xlix. 146.) Terchloride of Boron.—Sir H. Davy noticed that recently prepared boron takes fire spontaneously in an atmosphere of chlorine, and emits a vivid light; but he did not examine the product. Berzelius remarked, that if the boron has been previously heated, whereby it is rendered more compact, the combustion does not take place till heat is applied. This observation led him to expose boron, thus rendered dense, in a glass tube to a current of dry chlorine; and to heat it gently as soon as the atmospheric air was com- pletely expelled, in order to commence the combustion. The resulting com- pound proved to be a colourless gas; and on collecting it over mercury, which absorbed free chlorine, he procured the chloride of boron in a state of purity. This gas is rapidly absorbed by water; but double decomposition takes place at the same instant, giving rise to hydrochloric and boracic acids as the sole products: from this fact is inferred the composition of the chloride as stated at page 212 ; for one eq. of terchloride of boron or B + 3C1, and three eq. ofwater or 3 (H + O), correspond to one eq. of boracic acid or B + 30, and three eq. of hydrochloric acid or 3(H+C1). The watery va- pour of the atmosphere occasions a similar change ; so that when the gas is mixed with air containing hygrometric moisture, a dense white cloud is produced. The specific gravity of the gas, according to Dumas, is 3.942. It is soluble in alcohol, and communicates to it an ethereal odour, apparently by the action of hydrochloric acid. It unites with ammoniacal gas, forming a fluid volatile substance, the nature of which is unknown.—(Annals of Phil. xxvi. 129.) Dumas finds that chloride of boron may be generated by the action of dry chlorine on a mixture of charcoal and boracic acid, heated to redness in a porcelain tube. Desprctz also appears to have invented a similar pro- cess. (Philos. Magazine and Annals, i. 469.) Chloro-nilrous Gas.—When fused chloride of sodium, potassium, or cal- dum, in powder, is treated with as much strong nitric acid as is sufficient to wet it, mutual decomposition ensues, and a new gas, composed of chlo- rine and binoxide of nitrogen, is generated. Its discoverer, Mr. E. Davy, de- scribes it as a gas of a pale reddish-yellow colour, of an odour similar to that of chlorine, though less pungent, and possessed of bleaching properties. 224 CHLORINE. It fumes on exposure to the air, and is freely absorbed by water. It is de- composed by sulphur, phosphorus, mercury, and most metals, and by sub- stances in general which have an affinity for chlorine. It consists, according to Mr. Davy, of equal volumes of chlorine and binoxide of nitrogen, united without any condensation. In the mutual decomposition of chloride of sodium and nitric acid, the products appear to be chloro-nitrous and chlorine gases, and nitrate of soda. Their formation must obviously depend on sodium being oxidized at the ex- pense of nitric acid; while part of the chlorine unites, at the moment of se- paration from the sodium, with binoxide of nitrogen. (Phil. Mag. ix. 355.) Theoretically, it should be mixed with twice its volume of chlorine, the pre- sence of which must materially obscure the properties of the new gas. ON THE NATURE OF CHLORINE. The change of opinion which has gradually taken place among chemists concerning the nature of chlorine, is a remarkable fact in the history of the science. The hypothesis of Berthollet, unfounded as it is, prevailed at one time universally. It explained phenomena so satisfactorily, and in a manner so consistent with the received chemical doctrine, that for some years no one thought of calling its correctness into question. A singular reverse, however, has taken place; and this hypothesis, though it has not hitherto been rigidly demonstrated to be erroneous, has within a short period been generally abandoned, even by persons who, from having adopted it in early life, were prejudiced in its favour. The reason of this will readily appear on comparing it with the opposite theory, and examining the evidence in fa- vour of each. Chlorine, according to the new theory, is maintained to be a simple body, because, like oxygen, hydrogen, and other analogous substances, it cannot be resolved into more simple parts. It does not indeed follow that a body is simple, because it has not hitherto been decomposed; but as chemists have no other mode of estimating the elementary nature of bodies, they must necessarily adopt this one, or have none at all. Hydrochloric acid, by the same rule, is considered to be a compound of chlorine and hydrogen. For when exposed to the agency of galvanism, it is resolved into these sub- stances ; and by mixing the two gases in due proportion, and passing an dectric spark through the mixture, hydrochloric acid gas is the product. Chemists have no other kind of proof of the composition of water, of potassa, or of any other compound. Very different is the evidence in support of the theory of Berthollet. According to that view, hydrochloric acid gas is composed of absolute mu- riatic acid and water or its elements; chlorine consists of absolute muriatic acid and oxygen; and absolute muriatic acid is a compound of a certain unknown base and oxygen gas. Now all these propositions are gratuitous. For, in the first place, hydrochloric acid gas has not been proved to contain water. Secondly, the assertion that chlorine contains oxygen is opposed to direct experiment, the most powerful deoxidizing agents having been unable to elicit from that gas a particle of oxygen. Thirdly, the existence of such a substance as absolute muriatic acid is wholly without proof, and, therefore, its supposed base is also imaginary. But this is not the only weak point of the doctrine. Since chlorine is ad- mitted by this theory to contain oxygen, it was necessary to explain how it happens that no oxygen can be separated from it. For instance, on exposing chlorine to a powerful galvanic battery, oxygen gas does not appear at the positive pole, as occurs when other oxidized bodies are subjected to its ac- tion; nor is carbonic acid or carbonic oxide evolved, when chlorine is con- ducted over ignited charcoal. To account for the oxygen not appearing under these circumstances, it was assumed that absolute muriatic acid is unable to exist in an uncombined state, and, therefore, cannot be separated from one substance except by uniting with another. This supposition was CHLORINE. 225 thought to be supported by the analogy of certain compounds, such as nitric and oxalic acids, which appear to be incapable of existing except when com- bined with water or some other substance. The analogy, however, is incom- plete ; for the decomposition of such compounds, when an attempt is made to procure them in an insulated state, is manifestly owing to the tendency of their elements to enter into new combinations. Admitting the various assumptions which have been stated, most of the phenomena receive as consistent an explanation by the old as by the new theory. Thus, when hydrochloric acid gas is resolved by galvanism into chlorine and hydrogen, it may be supposed that absolute muriatic acid attaches itself to the oxygen of the water, and forms chlorine; while the hydrogen of the water goes to the opposite pole of the battery. When chlorine and hydrogen enter into combination, the oxygen of the former -'may be said to unite with the latter; and that hydrochloric acid gas is generated by the water so formed combining with the absolute muriatic acid of the chlorine. The evolution of chlorine, which ensues on mixing hydrochloric acid and peroxide of manganese, is explained on the supposi- tion that absolute muriatic acid unites directly with the oxygen of the black oxide of manganese. It will not be difficult, after these observations, to account for the prefer- ence shown to the new theory. In an exact science, such as chemistry, every step of which is required to be matter of demonstration, there is no room to hesitate between two modes of reasoning, one of which is hypothetical, and the other founded on experiment. Nor is there, in the present instance, temptation to deviate from the strict logic of the science; for there is not a single phenomenon which may not be fully explained on the new theory, in a manner quite consistent with the laws of chemical action in general. It was supposed, indeed, at one time, that the sudden decomposition of water, occasioned by the action of that liquid on the compounds of chlorine with some simple substances, constitutes a real objection to the doctrine; but it will afterwards appear, that the acquisition of new facts has deprived this argument of all its force. While nothing, therefore, can be gained, much may be lost by adopting the doctrine of Berthollet. If chlorine is regarded as a compound body, the same opinion, though in direct opposition to the result of observation, ought to be extended to iodine and bromine; and as other analogous substances may hereafter be discovered, in regard to which a similar hypothesis will apply, it is obvious that this view, if proper in one case, may legitimately be extended to others. One encroachment on the method of strict induction would consequently open the way to another, and thus the genius of the science would eventually be destroyed. An able attempt was made some years ago by the late Dr. Murray, to demonstrate the presence of water or its elements as a constituent part of hydrochloric acid gas, and thus to establish the old theory to the subversion* of the new. The arguments which he used, though plausible and ingenious, were successfully combated by Sir H. and Dr. Davy. The only experiment which strictly bears upon the question,—that, namely, where hydrochloric acid and ammoniacal gases were mixed together, goes far to demonstrate the absence of combined water in hydrochloric acid gas, and thereby to establish the views of Davy.* * In Nicholson's Journal, vols. xxxi. xxxii. and xxxiv. Edinburgh Philos. Trans, vol. viii. and Philos. Trans, for 1818. 226 IODINE. SECTION XII. IODINE. Iodine was discovered in the year 1812 by M. Courtois, a manufacturer of salt-petre at Paris. In preparing carbonate of soda from the ashes of sea-weeds, he observed that the residual liquor corroded metallic vessels powerfully; and on investigating the cause of the corrosion, he noticed that sulphuric acid threw down a dark-coloured matter, which was converted by the application of heat into a beautiful violet vapour. Struck with its appearance, he gave some of the substance to M. Clement, who recognized it as a new body, and in 1813 described some of its leading properties in the Royal Institute of France. Its real nature was soon after determined by Gay-Lussac and Davy, each of whom proved that it is a simple non-metallic substance, exceedingly analogous to chlorine.* Iodine is frequently met with in nature in combination with potassium or sodium. Under this form it occurs in many salt and other mineral springs, both in England and on the continent. It has been detected in the water of the Mediterranean, in the oyster and some other marine molluscous animals, in sponges, and in most kinds of sea-weed. In some of these pro- ductions, such as the Fucus serratus and Fucus digitatus, it exists ready formed, and according to Dr. Fyfe (Edin. Philos. Journal, i. 254.) may be separated by the action of water; but in others it can be detected only after incineration. Marine animals and plants doubtless derive from the sea the iodine which they contain. Vauquelin found it also in the mineral kingdom, in combination with silver. (An. de Ch. et de Ph. xxix.) All the iodine of commerce is procured from the impure carbonate of soda, called kelp, which is prepared in large quantity on the northern shores of Scotland, by incinerating sea-weeds. The kelp is employed by soap- makers, for the preparation of carbonate of soda; and the dark residual liquor, remaining after that salt has crystallized, contains a considerable quantity of iodine, combined with sodium or potassium. By adding a suffi- dent quantity of sulphuric acid, hydriodic acid is first generated, and then decomposed. The iodine sublimes when the solution is boiled, and may be collected in cool glass receivers. A more convenient process is to employ a moderate excess of sulphuric acid, and then add to the mixture some per- oxide of manganese, which acts on hydriodic in the same way as on hydro- chloric acid (page 209), (Phil. Mag. L. Dr. Ure.) Another method proposed by Soubeiran, is by adding to the ley from kelp a solution made with the sulphates of protoxides of copper and iron in the ratio of 1 of the former to 2J of the latter, as long as a white precipitate appears. The diniodide of copper is thus thrown down; and it may be decomposed either by peroxide of manganese alone, or by manganese and sulphuric acid. By means of the former, the iodine passes over quite dry; but a strong heat is requisite. Iodine, at common temperatures, is a soft friable opaque solid of a bluish-black colour, and metallic lustre. It occurs usually in crystalline scales, having the appearance of micaceous iron ore; but it sometimes crys- tallizes in large rhomboidal plates, the primitive form of which is a rhombic octohedron. The crystals are best prepared by exposing to the air a solution of iodine in hydriodic acid. Its specific gravity, according to Gay-Lussac, is 4.948; but Dr. Thomson found it only 3.0844. At 225° it is fused, and enters into ebullition at 347°; but when moisture is present, it is sublimed rapidly even below the degree of boiling water, and suffers a gradual dissipation at low temperatures. Its vapour is of an exceedingly rich violet colour, a character * The original papers on this subject are in the Annales de Chimie, vols. Ixxxviii. xc. and xci.; and in the Philos. Trans, for 1814 and 1815. IODINE. 227 to which it owes the name of iodine. (From tJJiif, violet-coloured.) This vapour is remarkably dense, its specific gravity by calculation, page 146, being 8.7020, or 8.716 as directly observed by Dumas. Hence 100 cubic inches, at the standard temperature and pressure, must weigh 269.8638 grains. Iodine is a non-conductor of electricity, and, like oxygen and chlorine, is a negative electric. It has a very acrid taste, and its odour is almost exactly similar to that of chlorine, when much diluted with air. It acts energetically on the animal system as an irritant poison, but is employed medicinally in very small doses with advantage. Iodine is very sparingly soluble in water, requiring about 7000 times its weight of that liquid for solution. It communicates, however, even in this minute quantity, a brown tint to the menstruum. Alcohol and ether dis- solve it freely, and the solution has a deep reddish-brown colour. Iodine possesses an extensive range of affinity. It destroys vegetable colours, though in a much less degree than chlorine. It manifests little disposition to combine with metallic oxides; but it has a strong attraction for the pure metals, and for most of the simple non-metallic substances, pro- ducing compounds which are termed iodides or iodurets. It is not inflam- mable ; but under favourable circumstances may, like chlorine, be made to unite with oxygen. A solution of the pure alkalies acts upon it in the same manner as upon chlorine, giving rise to decomposition of water, and the formation of an iodate and iodide. Pure iodine is not influenced chemically by the imponderables. Exposure to the direct solar rays, or to strong shocks of electricity, does not change its nature. It may be passed through red-hot tubes, or over intensely ignited charcoal, without any appearance of decomposition; nor is it affected by the agency of galvanism. Chemists, indeed, are unable to resolve it into more simple parts, and consequently it is regarded as an elementary principle. The violet hue of the vapour of iodine is for many purposes a sufficiently sure indication of its presence. A far more delicate test, however, was dis- covered by Colin and Gaultier de Claubry. They found that iodine has the property of uniting with starch, and of forming with it a compound insolu- ble in cold water, which is recognized with certainty by its deep blue co- lour. This test, according to Stromeyer, is so delicate, that a liquid, con- taining 1—450,000 of its weight of iodine, receives a blue tinge from a solu- tion of starch. Two precautions should be observed to insure success. In the first place, the iodine must be in a free state ; for it is the iodine itself only, and not its compounds, which unites with starch. Secondly, the solu- tion should be quite cold at the time of adding the starch ; for hot water dis- solves the blue compound, and forms a colourless solution. Berzelius determined the equivalent of iodine by exposing fused iodide of silver to a current of chlorine gas, whereby the iodine was expelled and chloride of silver generated. Through the known composition of chloride of silver he inferred that of the iodide, and thence found that the equivalent of iodine is 126.3. The composition of the compounds of iodine described in this section is as follows:— Iodine. Equiv. Formula?. Hacia°diC I126*3 le*+1 le* hydrogens 127.3. H + I or HI. Oxide of ^ iodine ) Composition unknown. Iodous acid J Iodic acid 126.3 1 eq. +40 5 eq. oxygen =166.3. I + 50orL Periodic acid 126.3 1 eq.+56 7 eq, ditto =182.3. 1+70 or I. C1of iodTne \ ComP°sition unknown- 228 IODINE. Iodine. Equiv. Formulae. rfnitfogenl378-9 3 eq.+14.15 1 eq. nitrogen =393.05. N +31 or NK ^ofphot? j 126.3 leq.+15.7 1 eq. phosph. =142.0. P+IorPI. fd'eTpho's. (378.9 3 eq.+31.4 2 eq. do. =410.3. 2P +31 or P«I«. ^of^hos J631.5 5eq.+31.4 2 eq. do. =662.9.2P+5I orP*Is. Iodide of ? Composition unknown. sulphur ) r eri° ' ,e > Composition unknown. of carbon ( r Protiodide i „ _ ... , c , > Composition unknown. of carbon $ r Hydriodic acid.—This compound is formed by the direct union of its elements, when a mixture of hydrogen gas and iodine vapour are transmitted through a porcelain tube at a red heat. A more convenient process, and by which it is obtained in a pure state, is by the action of water on the periodide of phosphorus. Any convenient quantity of the iodide is put into a small glass retort, together with a little water, and a gentle heat is applied. Mutual decomposition ensues; the oxygen of the water unites with phosphorus, and its hydrogen with iodine, giving rise to the formation of phosphoric and hydriodic acids, the latter of which passes over in the form of a colourless gas. The preparation of the iodide re- quires care; since phosphorus and iodine act so energetically on each other by mere contact, that the phosphorus is generally inflamed, and a great part of the iodine expelled in the form of vapour. This inconvenience is avoided by putting the phosphorus into a tube, sealed at one end and about twelve inches long, displacing the air by a current of dry carbonic acid gas, then gradually adding the iodine, and promoting the action towards the close by a gentle heat. The materials should be well dried with bibulous paper, and the iodide preserved in a well stopped dry vessel; for even atmospheric humidity gives rise to copious white fumes of hydriodic acid. The propor- tions usually employed are one part of phosphorus to about twelve of iodine. Another process has been recommended by M. F. d'Arcet, which consists in evaporating hypophosphorous acid until it begins to yield phosphuretted hydrogen, mixing it with an equal weight of iodine, and applying a gentle heat. Hydriodic acid gas of great purity is then rapidly disengaged; its production depending, as in the former process, on the decomposition of water. Hydriodic acid gas has a very sour taste, reddens vegetable blue colours without destroying them, produces dense white fumes when mixed with at- mospheric air, and has an odour similar to that of hydrochloric acid gas. The salts which it forms with alkalies are called hydriodates. Like hydro- chloric acid gas, it cannot be collected over water; for that liquid dissolves it in large quantity. Hydriodic acid is decomposed by several substances which have a strong affinity for either of its elements. Thus oxygen gas, when heated with it, unites with its hydrogen, and liberates the iodine. Chlorine effects the de- composition instantly ; hydrochloric acid gas is produced, and the iodine appears in the form of vapour. With strong nitrous acid it takes fire, and the vapour of iodine is set free. It is also decomposed by mercury. The de- composition begins as soon as hydriodic acid gas comes in contact with mercury, and proceeds steadily, and even quickly if the gas is agitated, till nothing but hydrogen remains. Gay-Lussac ascertained by this method that 100 measures of hydriodic acid gas contain precisely half their volume of hydrogen. Assuming it to consist of equal volumes of hydrogen gas and iodine vapour united without any condensation, then, since IODINE. 229 Grains. 50 cubic inches of the vapour of iodine weigh . 134.9319 50 do. hydrogen gas.....1.0683 100 cubic inches of hydriodic acid gas should weigh 136.0002 These numbers are obviously in the ratio of 1 to 126.3, the equivalents of iodine and hydrogen. On the same principles the density of the gas should be 4.3854, which is probably more correct than 4.443, a number found experi- mentally by Gay-Lussac. (An. de Ch. xci. 16.) From these coincidences there is no doubt that 100 measures of hydriodic acid gas contain 50 mea- sures of hydrogen gas and 50 of the vapour of iodine. When hydriodic acid gas is conducted into water till that liquid is fully charged with it, a colourless acid solution is obtained, which emits white fumes on exposure to the air, and has a density of 1.7. It may be prepared also by transmitting a current of hydrosulphuric acid gas through water in which iodine in fine powder is suspended. The iodine, from having a greater affinity than sulphur for hydrogen, decomposes the hydrosulphuric acid; and hence sulphur is set free, and hydriodic acid produced. As soon as the iodine has disappeared, and become colourless, it is heated for a short time to expel the excess of hydrosulphuric acid, and subsequently filtered to sepa- rate free sulphur. The solution of hydriodic acid is readily decomposed. Thus, on exposure during a few hours to the atmosphere, the oxygen of the air forms water with the hydrogen of the acid, and sets iodine free. The solution is found to have acquired a yellow tint from the presence of uncombined iodine, and a blue colour is occasioned by the addition of starch. Nitric and sulphuric acids likewise decompose it by yielding oxygen, the former being at the same time converted into nitrous, and the latter into sulphurous acid. Chlo- rine unites directly with the hydrogen of the hydriodic acid, and hydrochlo- ric acid is formed. The separation of iodine in all these cases may be proved in the way just mentioned. These circumstances afford a sure test of the presence of hydriodic acid, whether free or in combination with alkalies. All that is necessary, is to mix a cold solution of starch with the liquid, pre- viously concentrated by evaporation if necessary, and then add a few drops of strong sulphuric acid. A blue colour will make its appearance if hydrio- dic acid is present. Oxide of Iodine and Iodous Acid.—M. Sementini, of Naples, states that on mixing the vapour of iodine and oxygen gas considerably heated, the violet tint of the former disappears, and a yellow matter of the consistence of solid oil is generated. This he regards as oxide of iodine; and if the sup- ply of oxygen be kept up after its formation, it is converted into a yellow liquid, which he supposes to be iodous acid. From the mode in which the process is described, there can scarcely be a doubt that some compound of iodine and oxygen is thus formed; but its composition and properties have not been satisfactorily made out. (Quarterly Journ. of Science, N. S. i. 478.) Mitscherlich has observed, that on dissolving iodine in a rather dilute so- lution of soda, until the solution began to acquire a red tint, permanent crystals were obtained by spontaneous evaporation. They had the form of a six-sided prism, and dissolved in cold water without change; but by the ac- tion of water moderately heated, or by alcohol, they were converted into iodate of soda and iodide of sodium. On the addition of an acid, iodine and iodic acid were set at liberty. From these facts the crystals were inferred to be iodite of soda. (An. de Ch. et de Ph. xxx. 84.) Iodic Acid.—This acid was discovered at about the same time by Gay- Lussac and Davy; but the latter first succeeded in obtaining it in a state of perfect purity. When iodine is brought into contact with protoxide of chlo- rine, immediate action ensues; the chlorine of the protoxide unites with one portion of iodine, and its oxygen with another, forming two compounds, a volatile orange-coloured matter, chloride of iodine, and a white solid sub. stance, which is iodic acid. On applying heat, the former passes off in va. 230 , IODINE. pour, and the latter remains. (Philos. Trans, for 1815.) Serullas has obtain- ed it, in the form of hexagonal laininee, by evaporating in a warm place its solution either in water, or in sulphuric or nitric acid. The method which he found most convenient is by forming a solution of iodate of soda in a considerable excess of sulphuric add, keeping it at a boiling temperature for twelve or fifteen minutes, and then setting it aside to crystallize. (An. de Ch. et de Ph. xliii. 216.) Iodic acid may also be formed by dissolving perchloride of iodine in water, and gradually adding a large quantity of strong sulphuric acid, a rise of temperature being at the same time prevent- ed by the application of cold. Iodic acid will then be precipitated. Another process, suggested by Mr. Connell of Edinburgh, is by boiling iodine in ni- tric acid. For this purpose a pure acid of density 1.5 should be introduced along with about a fifth of its weight of iodine into a tube sealed at one end, about an inch wide and 15 inches long, and these materials be kept at a boil- ing temperature for at least twelve hours. As the iodine rises and condenses on the sides of the tube, it should be restored to the liquid, either by agita- tion or by help of a glass rod. As soon as the iodine disappears, the nitric acid is dissipated by cautious evaporation. This compound, which was termed oxiodine by Davy, is anhydrous iodic acid. It is a white semitransparent solid, which has a strong astringent sour taste, but no odour. Its density is considerable, as it sinks rapidly in sulphuric acid. When heated to the temperature of about 500° F. it is fused, and at the same time resolved into oxygen and iodine. In a dry air it is unchanged; but in a moist atmosphere it absorbs humidity,forming the hydrated acid, and eventually deliquesces. In water it is very soluble, and the solution has a distinct acid reaction : the bleaching power ascribed to it by Davy is said by Mr. Hiley not to be a property of pure iodic acid. (Lan- cet for July 1833.) On evaporating the solution, a thick mass of the con- sistence of paste is left, which is hydrous iodic acid; and which, by the cautious application of heat, may be rendered anhydrous. It acts powerful- ly on inflammable substances. With charcoal, sulphur, sugar, and similar combustibles, it forms mixtures which detonate when heated. It enters into combination with metallic oxides, and the resulting salts are called iodates. These compounds, like the chlorates, yield pure oxygen by heat, and deflagrate when thrown on burning charcoal. Iodic acid forms with the pure alkalies, salts which are soluble in water; but with lime, baryta, strontia, and the oxides of lead and silver, it yields compounds of very sparing solubility. It is readily detected by the facility with which it is deoxidized, an effect readily produced by the sulphurous, phosphorous, hydriodic, and hydrosulphuric acids. Iodine in each case is set at liberty, and may be detected as usual by starch. Hydrochloric and iodic acids decompose each other, water and chloride of iodine being gene- rated. Davy ascertained the composition of iodic acid, as given at page 227, by determining the quantity of oxygen which the acid loses when decomposed by heat; and Gay-Lussac arrived at the same result by heating iodate of potassa, when pure oxygen was given off and iodide of potassium re- mained. Periodic Acid.—This compound has been lately discovered by Ammer- milller and Magnus. (Poggen. Annalen, xxviii. 514.) When pure soda is mixed with a solution of iodate of soda, and chlorine gas is transmitted into it to saturation, a sparingly soluble white pulverulent salt is generated, which subsides after heating, and, if necessary, concentrating the solution. This salt is periodate of soda, the production of which appears to depend on the formation of chloride of sodium, and the union of the oxygen of the soda with the iodine of the iodic acid. For each equivalent of periodic acid, two eq. of chloride of sodium should be generated; since the materials 1 + 50, 2(Na + 0), 2 CI, just suffice for yielding 1+70, and 2(Na+Cl). On dissolving the periodate of soda in dilute nitric acid, and adding nitrate of oxide of silver, the periodate of this oxide, of a greenish-yellow colour, IODINE. 231 subsides, which should be washed with water acidulated with nitric acid. This yellow salt is soluble in hot dilute nitric acid, and separates again on cooling in small shining straw-yellow crystals, which by digestion with warm water acquire, without dissolving, a reddish-brown almost black co- lour. If the nitric acid solution of the yellow salt is so far concentrated by evaporation, that it crystallizes while still warm, orange-coloured crystals subside. These three salts are readily analyzed by exposure to a red heat in a glass tube, when iodine and metallic silver remain in the tube, and oxygen gas, along with water when water is present, is expelled., Their composition is as follows:— Oxide of Silver. Periodic Acid. Water. Formula?. Yellow salt 232 2 eq. 182.3 1 eq. 27 3 eq. Ag*I + 3H. Red salt 232 2 eq. 182.3 1 eq. 18 2 eq. Ag3l+2H. Orange salt 116 1 eq. 182.3 1 eq. Ag I. The two former are, therefore, hydrated subperiodates of oxide of silver, and the latter a neutral periodate. This neutral salt has the peculiarity, that by pure cold water it is converted into the yellow subsalt, while the water takes up exactly half of its acid without a trace of silver. By this means a pure solution of periodic acid may be obtained. Periodic acid is analogous in composition to perchloric acid, and has de- cided acid properties. Its solution may be boiled without decomposition, and on evaporation the acid yields crystals, which do not change by ex- posure to the air. By hydrochloric acid it is reduced to iodic acid with disengagement of chlorine, and the same change will of course be produced by substances which decompose iodic acid. When the heat is increased be- yond 212° (the precise point is not stated) periodic acid loses oxygen, and iodic acid remains. Thus is periodic more easy of decomposition than iodic acid. Chlorides of Iodine.—Chlorine is absorbed at common temperatures by dry iodine with evolution of heat, and a solid compound of iodine and chlo- rine results, which was discovered both by Davy and Gay-Lussac. The colour of the product is orange-yellow when the iodine is fully saturated with chlorine, but is of a reddish-orange if iodine is in excess. It is con- verted by heat into an orange-coloured liquid, which yields a vapour of the same tint on increase of temperature. It deliquesces in the open air, and dissolves freely in water. Its solution is colourless, very sour to the taste, and reddens vegetable blue colours, but afterwards destroys them. From its acid properties Davy gave it the name of chloriodic acid. . Gay-Lussac, on the contrary, calls it chloride of iodine, conceiving that the acidity of its solution arises from the presence of hydrochloric and iodic acids, which he supposes to be generated by decomposition of water. From the observa- tions of Serullas and Dumas, it appears that there exist two compounds of chlorine and iodine, by the different action of which on water the discordant opinions of Davy and Gay-Lussac may be explained. The chloride is soluble in water, alcohol, and ether without change; but the perchloride is resolved by water into hydrochloric and iodic acids, the latter of which may be precipitated either by rectified alcohol or strong sulphuric acid. The substance commonly obtained by transmitting chlorine gas over iodine is a mixture of the two chlorides; and on dissolving it in water, and agitating with ether, the undecomposed chloride is removed by the ether, while the iodic acid of the decomposed perchloride is precipitated. The composition of these chlorides has not been precisely determined. They are both con- verted by alkaline solutions into hydrochloric and iodic acids. Teriodide of Nitrogen.—From the weak affinity that exists between iodine and nitrogen, these substances cannot be made to unite directly. But when iodine is put into a solution of ammonia, the alkali is decom- 232 IODINE. posed; its elements unite with different portions of iodine, and thus cause the formation of hydriodic acid and iodide of nitrogen. The latter subsides in the form of a dark powder, which is characterized, like quadrochloride of nitrogen, by its explosive property. It detonates violently as soon as it is dried; and slight pressure, while moist, produces a similar effect. Heat and light are emitted during the explosion, and iodine and nitrogen are set free. According to the experiments of M. Colin, iodide of nitrogen consists of one equivalent of nitrogen and three of iodine. It is conveniently made, according to Serullas, by saturating alcohol of 0.852 with iodine, adding a large quantity of pure ammonia, and agitating the mixture. On diluting with water, teriodide of nitrogen subsides, which should be washed by repeated affusion of water and decantation. As thus prepared it is very finely divided, and may be pressed under water without detonating; but if, subsequently to its formation, it is put in contact with pure ammonia, it will afterwards detonate with the same facility as that prepared in the usual manner. Serullas has also remarked that water and teriodide of nitrogen mutually decompose each other, giving rise to the formation of hydriodic and iodic acids and ammonia. The change takes place slowly in cold water; but it is completed in a few minutes, and with scarcely any disengagement of nitrogen, when gentle heat is applied. When a little nitric or sulphuric acid is used, ammonia and iodic acid are alone produced. (An. de Ch. et de Ph. xlii. 201.) Iodides of Phosphorus.—Iodine and phosphorus combine readily in the cold, evolving so much heat as to kindle the phosphorus, if the experiment is made in the open air; but in close vessels no light appears. One of these compounds, apparently a protiodide, is formed of 1 part of phosphorus and 7 or 8 parts of iodine. It has an orange colour, fuses at 212°, sublimes unchanged by heat, and is decomposed by water, with the elements of which it gives rise to hydriodic and phosphorous acids, while phosphorus is set free. The sesquiodide is formed by the action of 1 part of phosphorus and 12 of iodine. It appears as a dark gray crystalline mass, fusible at 84°, and yields with water hydriodic and phosphorous acids, from which circumstance its elements are supposed to be in the ratio of two eq. of phosphorus to three eq. of iodine, The periodide is prepared with 1 part of phosphorus and 20 of iodine, and is a black compound, fusible at 114°. As by the action ot water it yields hydriodic and phosphoric acids only, it is inferred to contain phosphorus and iodine in the ratio of two eq. to five eq. Thus 1 eq. periodide phos, & 5 eq. waterS 1 eq. phos. acid & 5 eq. hydriodic acid. 2P+51 5(H+0) -^ 2P+50 5(H+1) Iodide of Sulphur.—This compound is formed by heating gently 4 parts of iodine with 1 of sulphur. The product has a dark colour and radiated appearance like antimony. Its elements are easily disunited by heat. Periodide of Carbon.—When a solution of pure potassa in alcohol is mixed with an alcoholic solution of iodine, a portion of alcohol is decom- posed ; and its hydrogen and carbon, uniting separately with iodine, give rise to periodide of carbon and hydriodic acid. The latter combines with the potassa, and remains in solution. The former has a yellow colour like sulphur, and forms scaly crystals of a pearly lustre; its taste is very sweet, and it has a strong aromatic odour resembling saffron. It was discovered by Serullas, and described by him as a hydrocarburet of iodine ; but its real nature was pointed out by Mitscherlich. An. de Ch. de Ph. xxxvii. 86.) The protiodide is formed by distilling a mixture of the preceding com- pound with corrosive sublimate. It is a liquid of a sweet taste, and has a penetrating ethereal odour. BROMINE. 233 SECTION XIII. BROMINE. Bromine was discovered in 1826 by M. Balard of Montpellier. The name originally applied to it was muride, but the term brome or bromine, from /Sguiyuoc graveolentia, signifying a strong or rank odour, has since been substituted. (An. of Phil, xviii. 381.) Bromine in its chemical relations bears a close analogy to chlorine and iodine, and has hitherto been always found in nature associated with the former, and sometimes also with the latter. It exists in sea-water in the form of bromide of sodium or magnesium. Its relative quan- tity, however, is very minute; and even the uncrystallizable residue called bittern, left after chloride of sodium has been separated from sea- water by crystallization, contains it in small proportion. It may apparently be regarded as an essential ingredient of the saline matter of the ocean ; for it has been detected in the waters of the .Mediterranean, Baltic, North Sea, and Frith of Forth. It has also been found in the waters of the Dead Sea, and in a variety of salt springs in Germany.* Dr. Daubeny has detected it in several mineral springs in England ; and states that it is rarely wanting in those springs which contain much common salt, except that of Droitwich in Worcestershire. Balard found that it exists in marine plants growing on the shores of the Mediterranean, and has procured it in appreciable quantity from the ashes of sea-weeds that furnish iodine. He has likewise detected its presence in the ashes of some animals, especially in those of the Janthina violacea, one of the testaceous mollusca. At common temperatures bromine is a liquid, the colour of which is blackish-red when viewed in mass and by reflected light, but appears hyacinth- red when a thin stratum is interposed between the light and the observer. Its odour, which somewhat resembles that of chlorine, is very disagreeable, and its taste powerful. Its specific gravity is about 3. Its volatility is con- siderable : for at common temperatures it emits red-coloured vapours which are very similar in appearance to those of nitrous acid; and at ll6.5°it enters into ebullition. By a temperature between zero and —4° it is congealed, and in that state is brittle. The density was found by Mitscherlich to be 5.54, and the number calculated (page 146) from its equivalent is 5.4017: 100 cubic inches at 60° and 30 inches Bar. should weigh 167.5158 grains. Bromine is a non-conductor of electricity, and undergoes no chemical change whatever from the agency of the imponderables. It may be trans- mitted through a red-hot glass tube, and be exposed to the agency of galvanism, without evincing the least trace of decomposition. Like oxygen, chlorine, and iodine, it is a negative electric. Bromine is soluble in water, alcohol, and ether, the latter being its best solvent. It does not redden litmus paper, but bleaches it rapidly like chlorine; and it likewise discharges the blue colour from a solution of indigo. Its vapour extinguishes a lighted taper; but before going out, it burns for a few seconds with a flame which is green at its base and red at its upper part. Some inflammable substances take fire by contact with bromine, in the same manner as when introduced into an atmosphere of chlorine. It acts with energy on organic matters such as wood or cork, and corrodes the animal texture; but if applied to * Some of the salt springs of Germany furnish a good deal of bromine. The saline at Theodorshalle, near Kreuznach, contains a sufficient quantity to make its extraction profitable. A quintal (100 lbs.) of the mother-waters of this spring yields two ounces and one drachm of bromine.—Berzelius, Traite de Chimie, L 293.—Ed. 20* 234 Bromine. the skin for a short time only, it communicates a yellow stain, which is less intense than that produced by iodine, and soon disappears. To animal life it is highly destructive, one drop of it placed on the beak of a bird having proved fatal. From the close resemblance observable between chlorine and bromine, Balard was of course led to examine its relations with hydrogen, and found that these substances may readily be made to unite; the product of the combination being a gas very similar to hydrochloric and hydriodic acid gases, whence it has received the name of hydrobromic acid gas. In its action on metals, also, bromine presents the closest similarity to that which chlorine exerts on the same substances. Antimony and tin take fire by con- tact with bromine; and its union with potassium is attended with such intense heat as to cause a vivid flash of light, and often to burst the vessel in which the experiment is performed. Its affinity for metallic oxides is feeble. By the action of alkalies it is resolved into hydrobromic and bromic acids, suffering the same kind of change as chlorine or iodine, when similarly treated. Bromine is usually extracted from bittern, and its mode of preparation is founded on the property which chlorine possesses of decomposing hydro- bromic acid, uniting with its hydrogen, and setting bromine at liberty. Accordingly, on adding chlorine to bittern, the free bromine immediately communicates an orange-yellow tint to that liquid; and on heating the solution to its boiling point, the red vapours of bromine are expelled, and may be condensed by being conducted into a tube surrounded with ice. It was this change of colour produced by chlorine that led to the discovery of bromine. The method recommended by Balard for procuring this substance as well as for detecting the presence of hydrobromic acid, is to transmit a current of chlorine gas through bittern, and then to agitate a portion of sulphuric ether with the liquid. The ether dissolves the whole of the bro- mine, from which it receives a beautiful hyacinth-red tint, and on standing it rises to the surface. When the ethereal solution is agitated with caustic potassa, its colour entirely disappears, owing to the formation of bromide of potassium and bromate of potassa, the former of which is obtained in cubic crystals by evaporation. The bromine may then be set free by means of chlorine, and separated by heat.* Balard has subsequently improved the process so much, that it is now produced in considerable quantity, and sold in Paris as an article of commerce. According to all the experiments hitherto made, bromine appears to be an element. It is so very similar in most aspects to chlorine and iodine, and, in the order of its chemical relations, is so constantly intermediate between them, that Balard at first supposed it to be some unknown com- pound of these substances. There seems, however, to be no good ground for the supposition; but on the contrary, an experiment performed by De la Rive affords a very strong argument against it. He finds that when a compound of bromine and iodine is mixed with starch, and exposed to the influence of galvanism, bromine appears at the positive and iodine at the negative wire, where the starch acquires a blue tint. On making the experiment with bromine containing a little bromide of iodine, the same appearance ensues; but if iodine is not previously added, the starch does not receive a tint of blue. * According to the authorities of Berzelius and Thenard, the best mode of treating the cubic crystals of bromide of potassium in order to extract the bromine, is to mix them in a small retort with the peroxide of manganese in powder, and to act on the mixture with sulphuric acid, diluted with half its weight of water, with the assistance of heat. The beak of the retort must plunge under cold water. As the distillation proceeds, the bromine passes over in red vapours, and condenses under the water in the form of brown and heavy drops.—Berzelius, Traite de Chim. i. 293.—Ed. BROMINE. 235 Bromine is in most cases easily detected by means of chlorine; for this substance displaces bromine from its combination with hydrogen, metals, and most other bodies. The appearance of its vapour or the colour of its solution in ether will then render its presence obvious. Bromine, like chlorine, forms a crystalline hydrate when exposed to 32° F. in contact with water. The crystals are octohedral, of a beautiful red tint, and suffer decomposition at 54°. (Lowig.) Berzelius determined the equivalent of bromine in the same way as that of iodine, namely, by heating a known weight of bromide of silver in a current of chlorine gas, so as to displace the bromine and obtain chloride of silver. He thus found 78.4 to be the equivalent of bromine. The compounds of bromine described in this section are as follows:— Bromine Equiv. Formulae. Hydrobromic acid 78.4 1 eq.+Hydrogen 1 1 eq.= 79.4. H+Br. Bromic acid . 78.4 1 eq+Oxygen 40 5 eq.=118.4. Br+SOor Br. Chloride of bromine Composition uncertain. Bromides of iodine Composition uncertain. Bromide of sulphur Composition uncertain. Protobromideof phos. 78.4 1 eq.+ Phosph. 15.7 1 eq.= 94.1 P+Br. Perbromide of phos. 392 5 eq.+ do. 31.4 2 eq.=423.4 Pa Br». Bromide of carbon Composition uncertain. Hydrobromic Acid.—No chemical action takes place between the vapour of bromine and hydrogen gas at common temperatures, not even by the agency of the direct solar rays; but on introducing a lighted candle, or a piece of red-hot iron, into the mixture, combination ensues in the vicinity of the heated body, though without extending.to the whole mixture, and without explosion. The combination is readily effected by the action of bromine on some of the gaseous compounds of hydrogen. Thus on mixing the vapour of bromine with hydriodic acid, hydrosulphuric acid, or phos- phuretted hydrogen gases, decomposition ensues, and hydrobromic acid gas is generated. It may be conveniently made for experimental purposes by a process similar to that for forming hydriodic acid. A mixture of bromine and phosphorus, slightly moistened, yields, by the aid of gentle heat, a large quantity of pure hydrobromic acid gas, which should be collected either in dry glass bottles, or over mercury. Hydrobromic acid gas is colourless, has an acid taste, and pungent odour. It irritates the glottis powerfully, so as to excite cough, and, when mixed with moist air, yields white vapours, which are denser than those occasion- ed under the same circumstances by hydrochloric acid gas. It undergoes no decomposition when transmitted through a red-hot tube, either alone, or mixed wilh oxygen. It is not affected by iodine; but chlorine decomposes it instantly, with production of hydrochloric acid gas, and deposition of bromine. It may be preserved without change over mercury; but potas- sium and tin decompose it with facility, the former at common tempera- tures, and the latter by the aid of heat. Hydrobromic acid gas is very soluble in water. The aqueous solution may be made by treating bromine with hydrosulphuric acid dissolved in water, or still better by transmitting a current of hydrobromic acid gas into pure water. The liquid becomes hot during the condensation, acquires great density, increases in volume, and emits white fumes when exposed to the air. This acid solution is colourless when pure, but possesses the pro- perty of dissolving a large quantity of bromine, and then receives the tint of that substance. Chlorine decomposes the solution of hydrobromic acid in an instant. Nitric acid likewise acts upon it, though less suddenly, occasioning the dis- engagement of bromine, and probably the formation of water and nitrous acid. Nitro-hydrobromic acid is analogous to aqua regia, and possesses the property of dissolving gold. 236 BROMINE. The elements of sulphuric and hydrobromic acids react on each other in a slight degree; and hence, on decomposing bromide of potassium by sul- phuric acid, the hydrobromic is generally mixed with a little sulphurous acid gas. The composition of hydrobromic acid gas is easily inferred from the two following facts. 1. On decomposing hydrobromic acid gas by potassium, a quantity of hydrogen remains, precisely equal to half the volume of the gas employed; and, 2, when hydriodic acid gas is decomposed by bromine, the resulting hydrobromic acid occupies the very same space as the gas which is decomposed. Hence hydrobromic is analogous to hydriodic and hydrochloric acid gases, in containing equal measures of bromine vapour and hydrogen gas united without any change of volume; and since Grains. 50 cubic inches of bromine vapour weigh . . . 83.7579 50 do. hydrogen gas .... 1.0683 100 do. hydrobromic acid must weigh . 84.8262 These numbers are in the ratio of 1 to 78.4, which is the composition of the gas by weight. Its density is 2.7353. Since bromine decomposes hydriodic, and chlorine hydrobromic acid, bromine, in relation to hydrogen, is intermediate between chlorine and iodine; for it has a stronger affinity for hydrogen than iodine, and a weaker than chlorine. The affinity of bromine and oxygen for hydrogen appears nearly similar; for while oxygen cannot detach hydrogen from bromine, bromine does not decompose watery vapour. The salts of hydrobromic acid are termed hydrobromates. Like the free acid, they are decomposed, and the presence of bromine is detected, by means of chlorine. On mixing a soluble bromide with the nitrates of the protoxides of lead, silver, and mercury, white precipitates are obtained, which are very similar in appearance to the chlorides of those metals, but which are metallic bromides. On the addition of chlorine, the vapour of bromine is evolved. Bromic Acid.—The only compound yet known of bromine and oxygen is that formed by the action of bromine on potassa, when a change exactly similar to that produced by chlorine (page 211) ensues, whereby bromide of potassium and bromate of potassa are generated; and the latter, being much less soluble than the former, is readily separated by evaporation. The bro- mate of the other alkalies and alkaline earths may be prepared in a similar manner. The bromates are analogous to the chlorates and iodates. Thus bromate of potassa is converted by heat into bromide of potassium, with disengage- ment of pure oxygen gas, deflagrates like nitre when thrown on burning charcoal, and forms with sulphur a mixture which detonates by percussion. The acid of-the bromates is decomposed by deoxidizihg agents, such as sul- phurous and hydrosulphuric acids, in the same manner as the acid of the iodates. The bromates likewise suffer decomposition from the action of hy- drobromic and hydrochloric acids. Bromate of potassa is said not to precipitate the salts of lead, but to occa- sion a white precipitate with nitrate of silver, and a yellowish-white with protonitrate of mercury; characters which, if true, serve as a good test to distinguish bromate from iodate and chlorate of potassa. Bromic acid may be procured in a separate state by decomposing a dilute solution of bromate of baryta with sulphuric acid, so as to precipitate the whole of the baryta. The resulting solution of bromic acid may be con- centrated by slow evaporation until it acquires the consistence of syrup; but on raising the temperature, in order to expel all the water, one part of the acid is volatilized, and the other resolved into oxygen and bromine. A similar result took place when the evaporation was conducted in vacuo with BROMINE. 237 sulphuric acid; and accordingly all attempts to procure anhydrous bromic acid have hitherto failed. Bromic acid has scarcely any odour, but its taste is very acid, though not at all corrosive. It reddens litmus paper powerfully at first, and soon after destroys its colour. It is not affected by nitric or sulphuric acid except when the latter is highly concentrated, in which case bromine is set free, and effervescence, probably owing to the escape of oxygen gas, ensues. From the analysis of bromate of potassa, bromic acid is obviously similar in constitution to iodic, chloric, and nitric acids; that is, it consists of one equivalent of bromine united with five of oxygen. Chloride of Bromine.—This compound may be formed at common tempe- ratures by transmitting a current of chlorine through bromine, and con- densing the disengaged vapours by means of a freezing mixture. The re- sulting chloride is a volatile fluid of a reddish-yellow colour, much less intense than that of bromine; its odour is penetrating, and causes a dis- charge of tears from the eyes, and its taste very disagreeable. Its vapour is a deep yellow, like the oxide of chlorine, and it enables metals to burn as in an atmosphere of chlorine, doubtless giving rise to the formation of metallic chlorides and bromides. Chloride of bromine is soluble in water without decomposition; for the solution possesses the colour, odour, and bleaching properties of the com- pound, and discharges the colour of litmus paper without previously redden- ing it. By the action of the alkalies it is decomposed, being converted, by means of the elements of water, into hydrochloric and bromic acids. Bromide of Iodine.—These substances act readily on each other, and appear capable of uniting in two proportions. The protobromide is a solid, convertible by heat into a reddish-brown vapour, which, in cooling, con- denses into crystals of the same colour, and of a form resembling that of fern leaves. An additional quantity of bromine converts these crystals into a fluid, which in appearance is like a strong solution of iodine in hydriodic acid. This compound dissolves without decomposition in water, but with the alkalies yields hydrobromic and iodic acids.—The existence of two bromides of iodine can scarcely be regarded as satisfactorily established. Bromide of Sulphur.—On pouring bromine on sublimed sulphur, combi- nation ensues, and a fluid of an oily appearance and reddish tint is gene- rated. In odour it somewhat resembles chloride of sulphur, and like that compound emits white vapours when exposed to the air; but its colour is deeper. It reddens litmus paper faintly when dry, but strongly if water is added. Cold water acts slowly upon bromide of sulphur ; but at a boiling temperature the action is so violent that a slight detonation occurs, and three compounds, hydrobromic, hydrosulphuric, and sulphuric acids are formed. The formation of these substances is of course attributable to de- composition of water, and the union of its elements with bromine and sul- phur. Bromide of sulphur is likewise decomposed by chlorine, which unites with sulphur and displaces bromine. The composition of bromide of sulphur is unknown. It dissolves an ex- cess both of chlorine and sulphur, and its elements separate from each other so readily, that it has hitherto been impracticable to procure a definite com- pound. Bromides of Phosphorus.—When bromine and phosphorus are brought into contact in a flask filled with carbonic acid gas, they act suddenly on each other with evolution of heat and light, and two compounds are gene- rated ; one a crystalline solid, which is sublimed and collects in the upper part of the flask, and the other a fluid, which remains at the bottom. The former contains the most bromine, and the latter is supposed by Balard to consist of single equivalents of its elements. The protobromide retains its liquid form even at 52° F. It is readily convert- ed into vapour by heat, and on exposure to the air emits penetrating fumes. It reddens litmus paper faintly, an effect which is probably owing to the pre- sence of moisture. With water it acts energetically and with free disengage- 238 FLUORINE. ment of heat, hydrobromic acid gas being evolved when only a few drops of water are employed; but if a large quantity is used, the gas is dissolved, and the acid solution leaves by evaporation a residuum, which burns slightly when dried, and is converted into phosphoric acid. The perbromide is yellow in its solid state; but with gentle heat it be- comes a red-coloured liquid, which by increase of temperature is converted into a vapour of the same tint. On cooling after fusion it yields rhombic crystals; but when its vapour is condensed, the crystals are acicular. It is decomposed by metals, probably with the formation of metallic bromides and phosphurets. It emits dense penetrating fumes on exposure to the air, and with water gives rise to the production of hydrobromic and phosphoric acids. Hence its elements should be in the ratio of twoeq. of phosphorus to five eq. of bromine. Chlorine has a greater affinity for phosphorus than bromine, and decom- poses both the bromides with evolution of the vapour of bromine. These compounds are not decomposed by iodine; but, on the contrary, bromine de- composes iodide of phosphorus. Bromide of Carbon.—This compound is formed by the action of bromine on half its weight of periodide of carbon, when bromide of carbon and a sub- bromide of iodine are formed, the latter of which is removed by a solution of caustic potassa. At common temperatures it is liquid, but crystallizes at 32° F. Its taste is sweet, and it has a penetrating ethereal odour. It resem- bles protiodide of carbon in many respects; but is distinguished from it by the vapour which it emits on exposure to heat. (Serullas, in the An. de Ch. et de Ph. xxxix. 225.) SECTION XIV. FLUORINE. The substance to which this name is applied has not hitherto been ob- tained in an insulated form, and, therefore, the properties which are peculiar to it in that state are entirely unknown. From the nature of its compounds it appears to belong to the class of negative electrics, and like oxygen and chlorine to have a powerful affinity for hydrogen and metallic substances. Berzelius determined the equivalent of fluorine, by finding that 100 parts of pure fluoride of calcium yield with sulphuric acid 175 parts of sulphate of lime. From these numbers fluoride of calcium is inferred to consist of 20.5 parts or one eq. of calcium, and 18.68 parts or one eq. of fluorine. The compounds of fluorine described in this section are the following :— Fluorine. Equiv. Formulae. HacidnUOnC 118.68 1 eq.+Hydrogen 1 1 eq.= 19.68 H+F or HF. FIacid°riC j 56.04 3 eq.+Boron 10.9 1 eq.= 66.94 B+3F or BFs. Hydrofluoric Acid.—This acid was first procured in its pure state in the year 1810 by Cay-Lussac and Thenard, and described in the second volume of their Recherches Physico-Chimiques. It is prepared by acting on the min- eral called fluor spar, which is a fluoride of calcium, carefully separated from siliceous earth and reduced to fine powder, with twice its weight of concentrated sulphuric acid. The mixture is made in a leaden retort; and on applying heat, an acid and highly corrosive vapour distils over, which must bo collected in a receiver of the same metal surrounded with ice. As the materials swell up considerably during the process, owing to a quantity of vapour forcing its way through a viscid mass, the retort should be capa- FLUORINE. 239 cious. At the close of the operation pure hydrofluoric acid is found in the receiver, and the retort contains dry sulphate of lime. The chemical changes are precisely the same as in the formation of hydrochloric acid gas at page 213, fluorine being substituted for chlorine and calcium for sodium. If the oil of vitriol is of sufficient strength, all its water is decomposed, and the re- sulting hydrofluoric acid is anhydrous. Hydrofluoric acid, at the temperature of 32° F, is a colourless fluid, and remains in that state at 59° if preserved in well stopped bottles; but when exposed to the air, it flies off in dense white fumes, which consist of the acid vapour combined with the moisture of the atmosphere. Its specific gra- vity is 1.0609; but its density may be increased to 1.25 by gradual additions ofwater. Its affinity for this liquid far exceeds that of the strongest sul- phuric acid, and the combination is accompanied with a hissing noise, as when red-hot iron is quenched by immersion in water. The vapour of hydrofluoric acid is much more pungent than chlorine or any of the irritating gases. Of all known substances, it is the most destructive to animal matter. When a drop of the concentrated acid of the size of a pin's head comes in contact with the skin, instantaneous disorganization en- sues, and deep ulceration of a malignant character is produced. On this ac- count the greatest care is requisite in its preparation. This acid when concentrated acts energetically on glass. The transpa- rency of the glass is instantly destroyed, heat is evolved, and the acid boils, and in a short time entirely disappears. A colourless gas, commonly known by the name offluosilicic acid gas, is the sole product. This compound is always formed when hydrofluoric acid comes in contact with a siliceous substance. For this reason it cannot be preserved in glass; but must be prepared and kept in metallic vessels. Those of lead, from their cheapness, are often used; but vessels of silver or platinum are preferable. In consequence of its power- ful affinity for siliceous matter, hydrofluoric acid may be employed for etch- ing on glass; and when used with this intention, it should be diluted with three or four times its weight of water. Hydrofluoric acid has all the usual characters of a powerful acid. It has a strong sour taste, reddens litmus paper, and neutralizes alkalies, either forming salts termed hydrofluates, or most generally giving rise to metallic fluorides. All these compounds are decomposed by strong sulphuric acid with the aid of heat, and the hydrofluoric acid while escaping may be de- tected by its action on glass. Hydrofluoric acid acts violently on some of the metals, especially on the bases of the alkalies. Thus when potassium is brought in contact with the concentrated acid, an explosion attended with heat and light ensues ; hydro- gen gas is disengaged, and a white compound, fluoride of potassium, is ge- nerated. It is a solvent for some elementary principles which resist the ac- tion even of nitro-hydrochloric acid. Thus it dissolves silicium, zirconium, and columbium, with evolution of hydrogen gas; and when mixed with ni- tric acid, it proves a solvent for silicium which has been condensed by heat, and for titanium. Nitro-hydrofluoric acid, however, is incapable of dissolv- ing gold and platinum. Several oxidized bodies, which are not attacked by sulphuric, nitric, or hydrochloric acid, are readily dissolved by hydrofluoric acid. As examples of this fact, several of the weaker acids, such as silica or silicic acid, titanic, columbic, molybdic, and tungstic acids may be enume- rated. (Berzelius.) A different view of the compounds of fluorine was originally taken by Gay-Lussac and Thenard, and is still held by some chemists. They adopt- ed the opinion that hydrofluoric acid is a compound of a certain inflamma- ble principle and oxygen, and applied to it the name of fluoric acid, pre- viously introduced by Scheele. Fluor spar on this view is a fluate of lime, and when this salt is decomposed by oil of vitriol, the fluoric is merely dis- placed by the sulphuric acid, and the former passes off combined wilh the water of the latter. What I have described as anhydrous hydrofluoric acid is, according to this hypothesis, hydrated fluoric acid ; and when acted upon 240 FLUORINE. by potassium, this metal is oxidized at the expense of the water, and potassa thus generated unites with fluoric acid, forming, not fluoride of potassium, but fluate of potassa. The equivalent of fluoric acid, as inferred from the analysis of Berzelius, is 10.68; for 39.18 parts or one equivalent of fluor spar is supposed to contain 28.5 parts of lime (20.5 calcium and 8 oxygen), thus leaving 10.68 as the equivalent of the acid. The theory according to which fluor spar is a compound of fluorine and calcium, originated as a suggestion with M. Ampere of Paris, and was after- wards supported experimentally by Davy. It was found that pure hydro- fluoric acid evinces no sign of containing either oxygen or water. Charcoal may be intensely heated in the vapour of the acid without the production of carbonic acid. When hydrofluoric acid was neutralized with dry ammonia- cal gas, a white salt resulted, from which no water could be separated ; and on treating this salt with potassium, no evidence could be obtained of the presence of oxygen. On exposing the acid to the agency of galvanism, there was a disengagement at the negative pole of a small quantity of gas, which from its combustibility was inferred to be hydrogen; while the platinum wire of the opposite side of the battery was rapidly corroded, and became covered with a chocolate-coloured powder. Davy explained these phenomena by supposing that hydrofluoric acid was resolved into its elements ; and that fluorine, at the moment of arriving at the positive side of the battery, entered into combination with the platinum wire which was employed as a conduc- tor. Unfortunately, however, he did not succeed in obtaining fluorine in an insulated state. Indeed, from the noxious vapours that arose during the ex- periment, it was impossible to watch its progress, and examine the different products with that precision which is essential to the success of minute chemical inquiries, and which Davy has so frequently displayed oil other occasions. Though these researches led to no conclusive result, they afforded so strong a presumption in favour of the opinion of Ampere and Davy, th^t it was adopted by several other chemists. This view has received strong additional support from the experiments of M. Kuhlman. (Quarterly Journal of Science for July 1827, p. 205.) It was found by this chemist that fluor spar is not in the slightest degree decomposed by the action of anhydrous sulphuric acid, whether at common temperatures or at a red heat. The ex- periment was made both by transmitting the vapour of anhydrous sulphuric acid over fluor spar heated to redness in a tube of platinum, and by putting the mineral into the liquid acid. In neither case did decomposition ensue ; but when the former experiment was repeated with the difference of em- ploying concentrated hydrous instead of anhydrous sulphuric acid, evolution of hydrofluoric acid was produced. M. Kuhlman also transmitted dry hydro- chloric acid gas over fluor spar at a red heat, when hydrofluoric acid was disengaged, without any evolution of hydrogen, and chloride of calcium re- mained. I am aware of no satisfactory explanation of these facts, except by regarding fluor spar as a compound of fluorine and calcium, and hydrofluoric acid as a compound of fluorine and hydrogen. I shall accordingly adopt this view in the subsequent pages, and never employ the term fluoric acid, except when explaining phenomena according to the theory of Gay-Lussac. Fluoboric Acid.—The chief difficulty in determining the nature of hydro- fluoric acid arises from the water of the sulphuric acid which is employed in its preparation. To avoid this source of uncertainty, Gay-Lussac and Thenard made a mixture of vitrified boracic acid and fluor spar, and exposed it in a leaden retort to heat, under the expectation that as no water was pre- sent anhydrous fluoric acid would be obtained. In this, however, they were disappointed; but a new gas came over, to which they applied the term of fluoboric acid gas. A similar train of reasoning led Davy about the same time to the same discovery; though the French chemists had the advantage in priority of publication. Another process, given by Dr. Davy, is to mix 1 part of vitrified boracic acid and 2 of fluor spar with 12 parts of strong sulphuric acid, and heating the mixture gently in a glass flask ; (Phil. Trans. FLUORINE. 241 1812;) but the gas thus developed contains a considerable quantity of fluosi- licic acid. Fluoboric acid gas may also be formed by heating a strong solu- tion of hydrofluoric and boracic acids in a metallic retort. In the decomposition of fluor spar by vitrified boracic acid, the former and part of the latter undergo an interchange of elements. The fluorine uniting with boron gives rise to fluoboric acid gas ; and by the union of calcium and oxygen, lime is generated, which combines with boracic acid, and is left in the retort as borate of lime. Fluoboric acid gas, therefore, is composed of boron and fluorine. Those who adopt the theory of Gay-Lussac give a dif- ferent explanation, and regard this gas as a compound of fluoric and boracic acids. The lime of fluor spar is supposed to unite with one portion of boracic acid, and fluoric acid at the moment of separation with another, yielding borate of lime and fluoboric acid gas. Fluoboric acid gas is colourless, has a penetrating pungent odour, and ex- tinguishes flame on the instant. Its specific gravity, according to Dr. Thom- son, is 2.3622. It reddens litmus paper as powerfully as sulphuric acid, and „ forms salts with alkalies which are called fluoborates. It has a singularly great affinity for water. When mixed with air or any gas which contains watery vapour, a dense white cloud, a combination of water and fluoboric acid, appears, thus affording an extremely delicate test of the presence of moisture in gases. Water acts powerfully on this gas, absorbing, according to Dr. Davy, 700 times its volume, during which the water increases in tem- perature and volume. The solution is limpid, fuming; and very caustic. On the application of heat, part of the gas is disengaged ; but afterwards the whole solution is distilled. Gay-Lussac and Thenard, and Dr. Davy were of opinion that fluoboric acid gas is dissolved in water without decomposition; but Berzelius denies the accuracy of their observation. On transmitting the gas into water until the liquid acquires a sharply sour taste, but is far from being saturated, a white powder begins to subside; and, on cooling, a considerable quantity of boracic acid is deposited in crystals. It appears that in a certain state of dilution, part of the fluoboric acid and water mutually decompose each other, with formation of boracic and hydrofluoric acids. The latter unites, accord- ing to Berzelius, with undecomposed fluoboric acid, forming what he has called boro-hydrofluoric acid. On concentrating the liquid by evaporation, the boracic and hydrofluoric acids decompose each other, and the original compound is re-produced. Fluoboric acid gas does not act on glass, but attacks animal and vegetable matters with energy, converting them like sulphuric acid into a carbona- ceous substance. This action is most probably owing to its affinity for water. When potassium is heated in fluoboric acid gas, the metal takes fire, and a chocolate-coloured solid, wholly devoid of metallic lustre, is formed. This substance is a mixture of boron and fluoride of potassium, from which the latter is dissolved by water, and the boron is left in a solid state. The composition of fluoboric acid gas has not hitherto been determined by direct experiment. Dr. Davy ascertained that it unites with an equal measure of ammoniacal gas, forming a solid salt; and that it also combines with twice and three times its volume of ammonia, yielding liquid com- pounds. In the former salt the relative weights of the constituent gases are in the ratio of their specific gravities; and if the compound consists of one equivalent of each, it will be constituted of, Fluoboric acid gas - 2.3622 - 68.69 one eq. Ammoniacal gas - 0.5897 - 17.15 one eq. so that the equivalent of the acid may be assumed in round numbers to be 68. Now supposing this acid to be formed of three equivalents of fluorine and one of boron, its equivalent will be 66.94, a number which approximates to the preceding. This view is consistent with the composition of boracic 242 HYDROGEN AND NITROGEN.--AMMONIACAL GAS. acid as given at page 205, and with the conversion of fluoboric acid by water into hydrofluoric and boracic acids. ON THE COMPOUNDS OF THE SIMPLE NON-METALLIC ACI- DIFIABLE COMBUSTIBLES WITH EACH OTHER. SECTION I. HYDROGEN AND NITROGEN.—AMMONIACAL GAS. The aqueous solution of ammonia, under the name of spirit of hartshorn, has been long known to chemists; but its existence as a gas was first noticed by Priestley, who described it in his works under the title of alkaline air. It is often called the volatile alkali; but the terms ammonia and ammonia- cal gas are now usually employed. The most convenient method of preparing ammoniacal gas for the pur- poses of experiment is by applying a gentle heat to the concentrated solution of ammonia, contained in a glass vessel. It soon enters into ebullition, and a large quantity of pure ammonia is disengaged. Ammonia is a colourless gas, which has a strong pungent odour, and acts powerfully on the eyes and nose. It is quite irrespirable in its pure form, but when diluted with air, it may be taken into the lungs with safety. Burning bodies are extinguished by it, nor is the gas inflamed by their approach. Ammonia, however, is inflammable in a low degree; for when a lighted candle is immersed in it, the flame is somewhat enlarged, and tinged of a pale yellow colour at the moment of being extinguished ; and a small jet of the gas will burn in an atmosphere of oxygen. A. mixture of ammoniacal and oxygen gases detonates by the electric spark ; water being formed, and nitrogen set free. A little nitric acid is generated at the same time, except when a smaller quantity of oxygen is employed than is sufficient for com- bining with all the hydrogen of the ammonia. (Dr. Henry, Philos. Trans. 1809.) Ammoniacal gas at the temperature of 50° and under a pressure equal to 6.5 atmospheres, becomes a transparent colourless liquid. It is also liquefied, according to Guyton-Morveau, under the common pressure, by a cold of —70°; but there is no doubt that the liquid which he obtained was a solu- tion of ammonia in water. Ammonia has all the properties of an alkali in a very marked manner. Thus it has an acrid taste, and gives a brown stain to turmeric paper; though the yellow colour soon reappears on exposure to the air, owing to the volatility of the alkali. It combines also with acids, and neutralizes their properties completely. All these salts suffer decomposition by being heated with the fixed alkalies or alkaline earths, such as potassa or lime, the union of which with the acid of the salt causes the separation of its ammonia. None of the ammoniacal salts can sustain a red heat without being dissi- pated in vapour or decomposed, a character which manifestly arises from the volatile nature of the alkali. If combined with a volatile acid, such as the hydrochloric, the compound itself sublimes unchanged by heat; but when united with an acid, which is fixed at a low red heat, such as the phospho- ric, the ammonia alone is expelled. Hydrogen and nitrogen gases do not unite directly, and, therefore, che- HYDROGEN AND NITROGEN.--AMMONIACAL GAS. 243 mists have no synthetic proof of the constitution of ammonia. Its composi- tion, however, has been determined analytically with great exactness. When a succession of electric sparks is passed through ammoniacal gas, it is re- solved into its elements ; and the same effect is produced by conducting am- monia through porcelain tubes heated to redness. A. Berthollet analyzed ammonia in both ways, and ascertained that 200 measures of that gas, on being decomposed, occupy the space of 400 measures, 300 of which are hy. drogen, and 100 nitrogen. Dr. Henry has made an analysis of ammonia by means of electricity, and his experiment proves beyond a doubt that the pro- portions above given are rigidly exact. (Annals of Philosophy, xxiv. 346.) Grains. Now since 150 cubic inches of hydrogen weigh . . 3.2050 and 50 of nitrogen.....15.0825 100 cubic inches of ammonia must weigh 18.2875 and it is composed by weight of Hydrogen • 3.2050 . 3 . or three equivalents. Nitrogen . 15.0825 . 14.15 . or one equivalent Its equivalent, therefore, is 17.15. The specific gravity of ammonia, according to this calculation, is 0.5897, a number which agrees closely with those ascertained directly by Sir H. Davy and Dr. Thomson. Ammoniacal gas has a powerful affinity for water, and for this reason must always be collected over mercury. Owing to this attraction, a piece of ice, when introduced into a jar full of ammonia, is instantly liquefied, and the gas disappears in the course of a few seconds. Davy, in his Elements, stated that water at 50°, and when the barometer stands at 29.8 inches, ab- sorbs 670 times its volume of ammonia; and that the solution has a specific gravity of 0.875. According to Dr. Thomson, water at the common tempe- rature and pressure takes up 780 times its bulk. By strong compression, water absorbs the gas in still greater quantity. Heat is evolved during its absorption; and a considerable expansion, independently of the increased temperature, occurs at the same time. The concentrated solution of ammonia, commonly though incorrectly termed liquid ammonia, is made by transmitting a current of the gas, as long as it continues to be absorbed, into distilled water, which is kept cool by means of ice or moist cloths. The gas may be prepared from any salt of ammonia by the action of any pure alkali or alkaline earth ; but hydrochlo- rate of ammonia and lime, from economical considerations, are always em- ployed. The proportions to which I give the preference are equal parts of hydrochlorate of ammonia and well-burned quicklime, considerable excess of lime being taken, in order to decompose the hydrochlorate more expedi- tiously and completely. The lime is slaked by the addition of water; and as soon as it has fallen into powder, it should be placed in an earthen pan and be covered till it is quite cold, in order to protect it from the carbonic acid of the air. It is then mixed in a mortar with the hydrochlorate of ammonia, previously reduced to a fine powder; and the mixture is put into a retort or other convenient glass vessel. Heat is then applied, and the temperature gradually increased as long as a free evolution of gas continues. The ammo- nia should be conducted by means of a safety tube of Welter into a quantity of distilled water equal to the weight of the salt employed. The residue consists of chloride of calcium, and lime. The concentrated solution of ammonia, as thus prepared, is a clear co- lourless liquid, of specific gravity 0.936. It possesses the peculiar pungent odour, taste, alkalinity, and other properties of the gas itself. On account of its great volatility, it should be preserved in well-stopped bottles, a measure which is also required to prevent the absorption of carbonic acid. At a tem- perature of 1309 it enters into ebullition, owing to the rapid escape of pure 244 COMPOUNDS OF HYDROGEN AND CARBON. ammonia; but the whole of the gas cannot be expelled by this means, as at last the solution itself evaporates. It freezes at about the same temperature as mercury. The following table, from Sir H. Davy's Elements of Chemical Philoso- phy, shows the quantity of real ammonia contained in 100 parts of solutions of different densities, at 59° F. and when the barometer stands at 30 inches. The specific gravity of water is supposed to be 10,000 : Table of the Quantity of real Ammonia in Solutions of different Densities. 100 parts of Of real 100 parts of Of real sp. gravity. Ammonia. sp. gravity. Ammonia. 8750 32.5 9435 14.53 8875 .5 29.25 9476 13.46 9000 26.00 9513 c 12.40 9054 o u 25.37 9545 u 11.56 9166 22.07 9573 10.82 9255 19.54 9597 10.17 9326 17.52 9619 9.60 9385 15.88 9692 9.50 The presence of free ammoniacal gas may always be detected by its odour, by its temporary action on yellow turmeric paper, and by its form- ing dense white fumes—hydrochlorate of ammonia—when a glass rod moistened with hydrochloric acid is brought near it. SECTION II. COMPOUNDS OF HYDROGEN AND CARBON. Chemists have for several years been acquainted with two distinct com- pounds of carbon and hydrogen, viz. carburetted hydrogen and defiant gases; but late researches have enriched the science with several other com- pounds of a similar nature, to which much interest is attached. They are remarkable for their number; for supplying some instructive instances of isomerism; for their tendency to unite with and even neutralize powerful acids, without, in their uncombined state, manifesting any ordinary signs of alkalinity ; and some of thein appear to act an important part in the forma- tion of the ethers, camphor, and some other inflammable substances. The following tabular view represents the composition of those which have as yet been studied. Hydrogen. Carbon. Equiv. Formula?. Light carburetted ) 2 2cq.+ 6.12 1 eq.= 8.12 hydrogen \ M * H 2 2cq.+12.24 2eq.= 14.24 4 4 eq. +24.48 4 eq.= 28.48 I 3 3 eq.+36.72 6 eq.= 39.72 Olefiant gas Etherine Bicarburet of hydrogen Paraffine Eupionc Naphtha Naphthaline Paranaphthaline Idrialine Camphene Citrene 2H+Cor H*C. 2H+2Cor HCa 4H+4CorHO». 3H+6Cor Hsce. Same ratio of elements as in etherine, but equivalent is unknown. 5 5eq.+36.72 6 eq.= 41.72 5H+6CorHC8. 4 4 eq.+61.2 10 eq.=65.2 411 +10C or H'C'o. 6 6eq.+91.8 15 eq. =97.8 6H+15C or HeCi". Ratio of carbon to hydrogen as 3 to 1, but equiva- lent unknown. I 8 8eq.+ 61.2 10 eq.= 69.2 8H + 10C or H*C'0 COMPOUNDS OF HYDROGEN AND CARBON. 245 Light Carburetted Hydrogen.—This gas is sometimes called heavy in- flammable air, the inflammable air of marshes, and hydrocarburet. Agree- ably to the principles of chemical nomenclature, taking carbon as the elec- tro-negative clement, it is a dicarburet of hydregen; but it is generally termed light carburetted hydrogen. It is formed abundantly in stagnant pools dur- ing the spontaneous decomposition of dead vegetable matter ; and it may readily be procured by stirring the mud at the bottom of them, and collect- ing the gas as it escapes, in an inverted glass vessel. In this state it is found to contain l-20th of carbonic acid gas, which may be removed by means of lime-water or a solution of pure potassa, and l-15th or l-20th of nitrogen, This is the only convenient method of obtaining it. Light carburetted hy- drogen is tasteless and nearly inodorous, and it does not change the colour of litmus or turmeric paper. Water, according to Dr. Henry, absorbs about l-60th of its volume. It extinguishes all burning bodies, and is unable to support the respiration of animals. It is highly inflammable; and when a jet of it is set on fire, it burns with a yellow flame, and with a much stronger light than is occasioned by hydrogen gas. With a due proportion of atmospheric air or oxygen gas it forms a mixture which detonates power- fully with the electric spark, or by the contact of flame. The sole products of the explosion are water and carbonic acid. Dalton first ascertained the real nature of light carburetted hydrogen, and it has since been particularly examined by Thomson, Davy, and Henry. When 100 measures are detonated with rather more than twice their vol- ume of oxygen gas, the whole of the inflammable gas and precisely 200 measures of the oxygen disappear, water is condensed, and 100 measures of carbonic acid are produced. Now 100 measures of carbonic acid gas contain (page 187) 100 of carbon vapour and 100 of oxygen gas, just half the oxygen which had been employed; and the remaining oxygen requires 200 measures of hydrogen to form water. Hence as, at 60° F. and 30 inches Bar., Grains, 100 cubic inches of carbon vapour weigh . . 13.0714 200 do. hydrogen gas 4.2734 100 do. 1'ght carburetted hydrogen must weigh 17.3448 These weights are obviously in the ratio of 2 to 6.12 as already assigned; and the density of such a gas ought to be 0.5593, which is nearly the quan- tity found experimentally by Thomson and Henry. Light carburetted hydrogen is not decomposed by electricity, nor by being passed through red-hot tubes, unless the temperature is very intense, in which case some of the gas does suffer decomposition, each volume yield- ing two volumes of pure hydrogen gas and a deposit* of charcoal. Chlorine and light carburetted hydrogen gas do not act on each other at common temperatures, when quite dry, even if exposed to the direct solar rays. If moist, and the mixture is kept in a dark place, still no action ensues; but if light be admitted, particularly sunshine, decomposition fol- lows. The nature of the products depends upon the proportion of the gases. If four measures of chlorine and one of light carburetted hydrogen are present, carbonic and hydrochloric acid gases will be produced: two volumes of chlorine combined with two volumes of hydrogen contained in the car- buretted hydrogen, and the other two volumes of chlorine decompose sq much water as will likewise give two volumes of hydrogen, forming hydro- chloric acid; while the oxygen of the water unites with the carbon, and converts it into carbonic acid. If there are three instead of four volumes of chlorine, carbonic oxide will be generated instead of carbonic acid, because one-half less water will be decomposed. (Dr. Henry.) If a mixture of chlo- rine and light carburetted hydrogen is electrified or exposed to a red heat, hydrochloric acid is formed, and charcoal deposited. It was first ascertained by Henry (Nicholson's Journal, vol. xix.), and his conclusions have been fully confirmed by the subsequent researches of Davy, 246 COMPOUNDS OF HYDROGEN AND CARBON. that the fire-damp of coal mines consists almost solely of light carburetted hydrogen. This gas often issues in large quantity from between beds of coal, and by collecting in mines owing to deficient ventilation, gradually mingles with atmospheric air, and forms an explosive mixture. The first unprotected light which then approaches, sets fire to the whole mass, and an explosion ensues. These accidents, which were formerly so frequent and so fatal, are now comparatively rare, owing to the employment of the safety- lamp. For this invention we are indebted to Davy, who established the principles of its construction by a train of elaborate experiment and close reasoning, which may be regarded as one of the happiest efforts of his genius. (Essay on Flame.) Davy commenced the inquiry by determining the best proportion of air and light carburetted hydrogen for forming an explosive mixture. When the inflammable gas is mixed with 3 or 4 times its volume of air, it does not explode at all. It detonates feebly when mixed with 5 or 6 times its bulk of air, and powerfully when 1 to 7 or 8 is the proportion. With 14 times its volume, it still forms a mixture which is explosive; but if a larger quantity of air be admitted, a taper burns in it only with an enlarged flame. The temperature required for causing an explosion was next ascertained. It was found that the strongest explosive mixture may come in contact with iron or other solid bodies heated to redness, or even to whiteness, without detonating, provided they are not in a state of actual combustion ; whereas the smallest point of flame, owing to its higher temperature, instantly causes an explosion. The last important step in the inquiry was the observation that flame cannot pass through a narrow tube. This led to the discovery, that the power of tubes in preventing the transmission of flame is not necessarily connected with any particular length ; and that a very short one will have the effect, provided its diameter is proportionally reduced. Thus a piece of fine wire gauze, which may be regarded as an assemblage of short narrow lubes, is quite impermeable to flame : and consequently if a common oil lamp be completely surrounded with a cage of such gauze, it may be intro- duced into an explosive atmosphere of fire-damp and air, without kindling the mixture. This simple contrivance, which is appropriately termed the safety-lamp, not only prevents explosion, but indicates the precise moment of danger. When the lamp is carried into an atmosphere charged with fire-damp, the flame begins to enlarge ; and the mixture, if highly explosive, takes fire as soon as it has passed through the gauze, and burns on its inner surface, while the light in the centre of the lamp is extinguished. When- ever this appearance is observed, the miner must instantly withdraw; for though the flame should not be able to communicate with the explosive mixture on the outside of the lamp, as long as the texture of the gauze re- mains entire, yet the heat emitted during the combustion is so great, that the wire, if exposed to it for a few minutes, would suffer oxidation, and fall to pieces. The peculiar operation of small tubes in obstructing the passage of flame admits of a very simple explanation. Flame is gaseous matter heated so intensely as to be luminous; and Davy has shown that the temperature necessary for producing this effect is far higher than the white heat of solid bodies. Now when flame comes in contact with the sides of very minute apertures, as when wire gauze is laid upon a burning jet of coal gas, it is deprived of so much heat that its temperature instantly falls below the degree at which gaseous matter is luminous; and consequently though the gas itself passes freely through the interstices, and is still very hot. it is no longer incandescent. Nor does this take place when the wire is cold only; —the effect is equally certain at any degree of heat which the flame can communicate to it. For since the gauze has a large extent of surface, and from its metallic nature is a good conductor of heat, it loses heat with great rapidity. Its temperature, therefore, though it may be heated to whiteness, is always so far below that of flame, as to exert a cooling influence over COMPOUNDS OF HYDROGEN AND CARBON. 247 the burning gas, and reduce its heat below the point at which it is incan- descent. These principles suggest the conditions under which Davy's lamp would cease to be safe. If a lamp with its gauze red-hot be exposed to a current of explosive mixture, the flame may possibly pass so rapidly as not to be cooled below the point of ignition, and in that case an accident might occur with a lamp which would be quite safe in a calm atmosphere. It lias been lately shown by ."Messrs. Upton and Roberts, lamp manufacturers of this city, that flame in this way may be made to pass through the safety-lamp as commonly constructed; and I am satisfied, from having witnessed some of their experiments, that the observation is correct. This then may account for accidents in coal mines where the safety-lamp is constantly employed. An obvious mode of avoiding such an evil is to diminish the apertures of the gauze; but this remedy is nearly impracticable from the obstacle which very fine gauze causes to the diffusion of light. A better method is to surround the common safety-lamp with a glass cylinder, allow- ing air to enter solely at the bottom of the lamp through wire gauze of extreme fineness, placed horizontally, and to escape at top by a similar con- trivance. Upton and Roberts have constructed a lamp of this kind, through which I have in vain tried to cause the communication of flame, and which appears to me perfectly secure: in case an accident should break the glass, the lamp would be reduced to a safety-lamp of the common construction. Davy's iamp thus modified gives a much better light than without the glass, just as all lamps burn better with a shade than without one. Olefiant Gas.—This gas was discovered in 1796 by some associated Dutch chemists, who gave it the name of olefiant gas, from its property of forming an oil-like liquid with chlorine. It is sometimes, but very improperly, called bicarburctted or percarburetted hydrogen. The ratio of its elements being as one to one suggests the term carburet of hydrogen; but this does not indicate that two equivalents of carbon are combined with two eq. of hydrogen to form one eq. of the gas. Perhaps the expression|. carburet of hydrogen will adequately express this, a principle of nomencla- ture already adopted by some of the German chemists. Olefiant gas is prepared *by mixing in a capacious retort six measures of strong alcohol with twelve of concentrated sulphuric acid, and heating the mixture, as soon as it is made, by means of an Argand lamp. The acid soon acts upon the alcohol, effervescence ensues, and olefiant gas passes over. The chemical changes which take place are of a complicated nature, and the products numerous. At the commencement of the process, the olefiant gas is mixed only with a little ether; but in a short time the solu- tion becomes dark, the formation of ether declines, and the odour of sul- phurous acid begins to be perceptible: towards the close of the operation, though olefiant gas is still the chief product, sulphurous acid is freely dis- engaged, some carbonic acid is formed, and charcoal in large quantity depo- sited. The olefiant gas may be collected cither over water or mercury. The greater part of the ether condenses spontaneously, and the sulphurous and carbonic acids may be separated by washing the gas with lime-water, or a solution of pure potassa. The olefiant gas in this process is derived solely from the alcohol; and its production is owing to the strong affinity of sulphuric acid for water. Alcohol is composed of carbon, hydrogen, and oxygen; and from the pro- portion of its elements it is inferred to be a compound of 14 24 parts or one equivalent of olefiant gas, united with 9 parts or one equivalent of water. It is only necessary, therefore, in order to obtain olefiant gas, to deprive al- cohol of the water which is essential to its constitution; and this is effected by sulphuric acid. The formation of ether, which occurs at the same time, will be explained hereafter. The other phenomena are altogether ex- traneous. They almost always ensue when substances derived from the animal and vegetable kingdoms are subjected to the action of sulphuric 248 COMPOUNDS OF HVDROGEN AND CARBON. acid. They occur chiefly at the close of the preceding process, in conse- quence of the excess of acid which is then present. Olefiant gas is a colourless elastic fluid, which when pure has no taste and scarcely any odour. Water absorbs about one-eighth of its volume. Like the preceding compound it extinguishes flame, is unable to support the respiration of animals, and is set on fire when a lighted candle is presented to it, burning slowly with the emission of a dense white light. With a proper quantity of oxygen gas, it forms a mixture which may be kindled by flame or the electric spark, and which explodes with great violence. To burn it completely, it should be detonated with four or five times its volume of oxygen. On conducting this experiment with the requisite care, Dr. Henry finds that for each measure of olefiant gas, precisely three of oxygen disappear, deposition of water takes place, and two measures of carbonic acid are produced. From these data the proportion of its constituents may easily be deduced in the following manner. Two measures of carbonic acid contain two measures of the vapour of carbon, which must have been present in the olefiant gas, and two measures of oxygen. Two-thirds of the oxygen which disappeared are thus accounted for; and the other third must have combined with hydrogen. But one measure of oxygen requires for forming water precisely two measures of hydrogen, which must likewise have been contained in the olefiant gas. Hence, as Grains. 200 cubic inches of the vapour of carbon weigh . 26.1428 200 do. hydrogen gas weigh . . 4.2734 100 cubic inches of olefiant gas must weigh . . 30.4162 These weights are in the ratio 12.24 or two equivalents of carbon to 2 or two eq. of hydrogen, as in the table. The density of a gas so constituted (page 148) should be 0.9808: whereas the density found experimentally by Saussure is 0.9852, by Henry 0.967, and by Thomson 0.97. Olefiant gas, when a succession of electric sparks is passed through it, is resolved into charcoal and hydrogen; and the latter of course occupies twice as much space as the gas from which it was derived. It is also de- composed by transmission through red-hot tubes of porcelain. The nature of the products varies with the temperature. By employing a very low degree of heat, it may probably be converted solely into carbon and light carburetted hydrogen; and in this case no increase of volume can occur, because these two gases, for equal bulks, contain the same quantity of hy- drogen. But if the temperature is high, then a great increase of volume takes place; a circumstance which indicates the evolution of free hydrogen, and consequently the total decomposition of some of the olefiant gas. Chlorine acts powerfully on olefiant gas. When these gases are mixed together in the ratio of two measures of the former to one of the latter, they form a mixture which takes fire on the approach of flame, and which burns rapidly with formation of hydrochloric acid gas, and deposition of a large quantity of charcoal. But if the gases are allowed to remain at rest after being mixed together, a very different action ensues. The chlorine, instead of decomposing the olefiant gas, enters into direct combinaiion with it, and a yellow liquid like oil is generated. Wohler has remarked its production by the contact of olefiant gas with certain metallic chlorides, especially the perchloride of antimony. This substance is sometimes called chloric ether; but the term chloride of hydrocarbon, as indicative of its ingredients, is more appropriate. The name hydrochloride of carbon has also been ap- plied to it. Chloride of hydrocarbon was discovered by the Dutch chemists; but Thomson* first ascertained that it is a compound of olefiant gas and chlo- * Memoirs of the Wernerian Society, vol. i. COMPOUNDS OF HYDROGEN AND CARBON. 249 rine; and its nature has since been more fully elucidated by the researches of Robiquet and Colin.* When first collected it commonly contains traces of ether, hydrochloric acid, and probably some other impurity: from these it is purified and dried by being well washed with water, and then distilled from chloride of calcium; and it is rendered still purer, according to Liebig, by agitation successively with solution of potassa, pure water, and strong sulphuric acid, from the latter of which it is separated by distillation. All the impurities are thus decomposed, while the chloride of hydrocarbon passes over in a pure state. When thus purified, it is a colourless volatile liquid, of a peculiar sweetish taste and ethereal odour. Its specific gravity at 64° is 1.247. It boils at 148°, and may be distilled without change. It suffers complete decomposition when its vapour is passed through a red-hot porce- lain tube, being resolved into charcoal, light carburetted hydrogen, and hy- drochloric acid gas. Mixed with chlorine gas, and exposed either to the direct solar rays, or to a heat of nearly 148°, it is converted into perchloride of carbon with evolution of hydrochloric acid gas. (Page 219.) Exposed moist to sunshine, it is said by Pfaff to be converted into hydrochloric acid and acetic ether; but these products are generated, according to Liebig, solely when the oil is impure. It is decomposed by potassium, which unites with chlorine, and sets olefiant gas at liberty. The composition of chloride of hydrocarbon is readily inferred from the fact, that in whatever proportions olefiant gas and chlorine may be mixed together, they always unite in equal volumes. Consequently they combine by weight according to the ratio of their densities; so that chloride of hydrocarbon consists of Chlorine gas . . 2.4700 . 35.42 one equivalent. Olefiant gas . . 0.9808 . 14.24 one equivalent. 3.4508 . 49.66 Some doubt has of late been entertained as to the accuracy of this esti- mate. It was observed by M. Morin of Geneva, that hydrochloric acid is always formed when chlorine acts on olefiant gas, and he inferred that the resulting oil must, therefore, contain less hydrogen than is commonly sup- posed. These views have in some measure been supported by Liebig, who admits the constant production of hydrochloric acid, and found by analysis rather less hydrogen than the quantity above assigned. The deficiency in hydrogen, however, is confessedly so minute, as to leave no doubt of the preceding estimate bdng very near the truth; and Dumas contends that it is rigidly exact. The appearance of hydrochloric acid is probably owing to the presence of a little ether, or to the production of some compound dis- tinct from the chloride of hydrocarbon. (An. de Ch. et de Ph. xliii. 244, xlviii. 185, and xlix. 182.) Chloride of hydrocarbon forms a very dense vapour, its specific gravity, according to Gay-Lussac, being 3.4434. This is very near the united den- sities of chlorine and olefiant gas, a circumstance greatly in favour of the general opinion concerning the constitution of the chloride. Dr. Henry has demonstrated that light is not essential to the action of chlorine on olefiant gas. On this he has founded an ingenious and perfect- ly efficacious method of separating olefiant gas from light carburetted hy- drogen and carbonic oxide gases, neither of which is acted on by chlorine unless light is present. (Philos. Trans, for 1821.) Olefiant gas unites also with iodine. This compound was discovered by Mr. Faraday (Philos. Trans, for 1821) by exposing olefiant gas and iodine, contained in the same vessel, to the direct rays of the sun. Iodide of hydro- carbon, or hydriodide of carbon, is a solid white crystalline body, which has * An. de Ch. et de Ph. i. and ii. 250 COMPOUNDS OF HYDROGEN AND CARBON. a sweet taste and aromatic odour. It sinks rapidly in strong sulphuric acid. It is fused by heat, and then sublimed without change, condensing into crystals, which are either tabular or prismatic. On exposure to strong heat, it is decomposed, and iodine escapes. It burns, if held in the flame of a spirit-lamp, with evolution of iodine and some hydriodic acid. It is insolu- ble both in water and acid or alkaline solutions. Alcohol and ether dissolve it, and on evaporating the solution it crystallizes. Iodide of hydrocarbon is composed, according to the analysis of Mr. Fara- day, of 126.3 parts or one equivalent of iodine, and 14.24 parts, or one equi- valent of olefiant gas. (Quarterly Journal of Science, xiii.) Bromide of Hydrocarbon.—This compound was formed by Serullas, by adding one part of the iodide of hydrocarbon to two parts of bromine con- tained in a glass tube. Instantaneous reaction ensues, attended with dis- engagement of heat and a hissing noise, and two compounds, the bromide of iodine and a liquid bromide of hydrocarbon, are generated. By means of water the former is dissolved; while the latter, coloured by bromine, collects at the bottom of the liquid. The decoloration is then effected by means of caustic potassa. In order that the process should succeed, the iodide of hy- drocarbon must not be in excess. Bromide of hydrocarbon, after being washed with a solution of potassa, is colourless, heavier than water, very volatile, of a penetrating ethereal odour, and of an exceedingly sweet taste, which it communicates to water in which it is placed, in consequence of being slightly soluble in that liquid. It be- comes solid at a temperature between 21° and 23° F. This compound is identical with that which M. Balard formed by letting a drop of bromine fall into a flask full of olefiant gas. (An. de Ch. et de Physique, xxxiv.) Etherine.—The substance to which this name is now applied was first obtained and examined by Mr. Faraday. (An. of Phil, xxvii. 44.) In the process of compressing oil gas in strong copper globes for the supply of port- able gas, the gas is subjected to a pressure equal to 30 atmospheres, and a volatile liquid collects, derived from inflammable vapours which were dif- fused through the gas. This liquid, when recently collected, boils at 60°. On simply heating it by the hand, and conducting the vapour through tubes cooled to 0° by a freezing mixture, the etherine is obtained in the form of a highly volatile liquid, which boils by slight elevation of temperature, and before the thermometer rises to 32° is wholly reconverted into vapour. It derives its name from being considered as an essential constituent of the ethers. The vapour of etherine is highly combustible, and burns with a brilliant flame, yielding carbonic acid and water. On being cooled to 0° it is again condensed into a liquid, the density of which at 54° (being kept liquid by the pressure of its own vapour in a tube hermetically sealed) is 0.627; so that among solids and liquids it is the lightest body known. Etherine is sparingly dissolved by water; but alcohol takes it up in large quantity, and the solution effervesces by dilution with water. Alkalies and hydrochloric acid do not affect it. Sulphuric acid absorbs more than 100 times its volume of the vapour, yielding a dark-coloured solution, but with- out evolution of sulphurous acid. From the analysis of the vapour of etherine, made by detonating it with oxygen gas, Faraday infers that each volume requires six of oxygen for com- plete combustion, and yields four volumes of carbonic acid. Hence, 100 cubic inches of the vapour contain 400 of the vapour of carbon, and 400 of hydrogen gas; and since Grains. 400 cubic inches of carbon vapour weigh - 52.2856 400 " hydrogen gas - - . 8.5468 100 " etherine vapour should weigh 60.8324 These weights are in the ratio 24.48 or four equivalents of carbon to 4 or COMPOUNDS OF HYDROGEN AND CARBON. 251 four eq. of hydrogen, bdng the composition already given in the table. The density of its vapour should be 1.9616, while Mr. Faradayfound it tobe 1.91 by observation. Bicarburet of Hydrogen.—This compound was obtained by Faraday from the same oil gas liquid which yielded etherine. As soon as the more vola- tile parts are dissipated, which happens before one-tenth is thrown off, the point of ebullition rises to 100° ; and it gradually ascends to 250° before all the liquid is volatilized, indicating the presence of two or more compounds differing in volatility. It was remarked that the boiling point was more constant between 176° and 190° than at any other temperature ; and on col- lecting the liquid which came over at that "part of the process, distilling re- peatedly, and employing a cold of 0°, he succeeded in obtaining a substance of invariable character, to which he applied the name of bicarburet of hydro- gen, expressive of the ratio of its elements though not of their quantity. Bicarburet of hydrogen, at common temperatures, is a colourless transpa- rent liquid, which smells like oil gas, and has also a slight odour of almonds. Its specific gravity is nearly 0.85 at 60° F. At 32° it is congealed, and forms dendritic crystals on the sides of the glass. At zero it is transparent, brittle, and pulverulent, and is nearly as hard as loaf-sugar. When exposed to the air at the ordinary temperature it evaporates, and boils at 186°. The density of its vapour at 60°, and when the barometer stands at 29.98 inches, is nearly 2.7760. Bicarburet of hydrogen is very slightly soluble in water; but it dissolves freely in fixed and volatile oils, in ether, and in alcohol, and the alcoholic solution is precipitated by water. It is not acted on by alkalies. It is com- bustible, and burns with a bright flame and much smoke. When admitted to oxygen gas, so much vapour rises as to make a powerfully detonating mixture. Potassium heated in it does not lose its lustre. On passing its vapour through a red-hot tube, it gradually deposites charcoal, and yields carburetted hydrogen gas. Chlorine, by the aid of sunshine, decomposes it with evolution of hydrochloric acid. Two triple compounds of chlorine, carbon, and hydrogen are formed at the same time, one of which is a crys- talline solid, the other a dense thick fluid. Bicarburet of hydrogen was analyzed m two ways. In the first, its vapour was passed over oxide of copper heated to redness ; and in the second, it was detonated with oxygen gas. Carbonic acid and water were the sole pro- ducts : and as the absence of oxygen is established by the inaction of potas- sium, it follows that the bicarburet consists of carbon and hydrogen only. Mr. Faraday infers from his analyses, that 100 measures of the inflammable vapour require 750 of oxygen for complete combustion; that 150 measures of oxygen unite with 300 hydrogen; and that the remaining 600 combine with 600 of the vapour of carbon, forming 600 measures of carbonic acid gas. Hence, as Grains. 600 cubic inches of carbon vapour weigh - 78.4284 300 - - hydrogen gas - - 6.4101 100 - - bicarburet vapour should weigh 84.8385 The ratio of these weights is 36.72 or six eq. of carbon to 3 or three eq. of hydrogen as already stated. The calculated density is 2.7357, which nearly agrees with the experimental number. Paraffine.—This substance was discovered about the same time by Dr. Reichenbach in Moravia, and by Dr. Christison. The latter obtained it by distillation from the petroleum of Rangoon, and in 1831 read a notice on it before the Royal Society of Edinburgh under the name of petroline; the former procured it by distilling tar derived from the igneous decomposition of vegetable matter, especially of beech wood, and applied to it the name under which it has become known, compounded of parum affinis, little akin, 252 COMPOUNDS OF HYDROGEN AND CARBON. to denote the remarkable chemical indifference which is its characteristic feature. (An. de Ch. et de Ph. 1. 69.) In distilling beech-tar, three liquids are obtained, the heaviest of which is unctuous and contains the paraffine. It is purified in part by repeated dis- tillation, and then heated to 212° with half its weight of strong sulphuric acid, whereby its impurities are decomposed, and the paraffine collects on cooling as a cake on the surface: for its more complete separation the acid should be kept at 112° for some hours. The paraffine is then removed, washed with water, pressed within folds of blotting paper, in order to im- bibe some adhering oil, and is then dissolved in boiling very strong alcohol, out of which it crystallizes in cooling. Paraffine at common temperatures is a rather firm solid, of density 0.87, fatty in aspect, is tasteless and inodorous, and separates from its alcoholic solution in thin laminae of a lustre and appearance very like cholesterine. At 111° it fuses into a colourless oily liquid, and rises in vapour, evaporat- ing without change. It is inflammable like the fats, and burns with a pure white flame, yielding carbonic acid and water. Paraffine resists the action of all the acids, the alkalies, and chlorine, and may be fused without change. Fusion with camphor, naphthaline, and pitch causes no action; but it unites with stearine, cetine, and wax, when fused with them. Its best solvents are spirit of turpentine and naphtha, which dis- solve it even in the cold; and the fat oils take it up readily by the aid of heat. According to an analysis by J. Gay-Lussac, its sole elements are carbon and hydrogen, in the same ratio as in olefiant gas; but as neither its equivalent nor the density of its vapour are known, its atomic constitution is undetermined. Eupione.—This substance, discovered and described by Dr. Reichenbach at the same time as paraffine, derives its name from tv well, and viaiv greasy, being analogous to oils in greasiness to paper and inflammability, though a much more perfect liquid, being as limpid as alcohol. It has no taste, colour, or odour, retains its liquid form at—4°, has a density of 0.74, and boils at 339°, evaporating without change or residue. In water it is insolu- ble; but it dissolves in ether, spirit of turpentine, naphtha, almond oil, bisul- phuret of carbon, and alcohol. The latter is its best solvent: 100 parts of eupione dissolve in 33 of absolute alcohol at 63°, and in every proportion with the aid of heat. When kindled it burns with a lively flame, without smoke, and carbonic acid and water are its sole products. Eupione dissolves camphor, stearine, cetine, cholesterine, naphthaline, and paraffine, especially when heated. It also dissolves chlorine, bromine, and iodine; but they are expelled by heat, and leave the eupione unchanged. It is not altered by exposure to the air, nor is it attacked by potassium, by the strong acids, or by the pure alkalies. Eupione is associated with paraffine in animal and vegetable tar, being most abundant in the former, as paraffine is in the latter. It is best pre- pared by distillation from the tar derived from bones or horn, and is purified by repeated distillation from strong sulphuric acid. After being washed with an alkaline solution to remove adhering acid, its only remaining impu- rity is paraffine, the greater part of which crystallizes under -a cold of 0°. The separation may also be effected by means of alcohol, and by cautious distillation along with water, since eupione is more soluble in alcohol and more volatile than paraffine. (An. de Ch. et de Ph. i. 60.) The composition of eupione has not been determined. Judging from its properties it must be a compound of carbon and hydrogen, and propably differs from paraffine only in containing a smaller proportion of carbon. Naphtha.—This name, from the Greek vu^Su, is applied to a volatile lim- pid liquid, of a strong peculiar odour, and generally of a light yellow colour; but it may be rendered colourless by careful distillation. Its specific gravity, when highly rectified, is 0.753 at 61° F. It is very inflammable, and burns with a white flame mixed with much smoke. In a platinum vessel it begins COMPOUNDS OF HYDROGEN AND CARBON. 253 to boil at 158° F., but the thermometer is not stationary until it reaches 192°: its vapour has a density of 2.833. (Saussure.) It retains its liquid form at 0°. It is insoluble in water, but unites in every proportion with absolute alcohol, sulphuric ether, petroleum, and oils. By exposure to the air, it slowly absorbs oxygen, and at the same time gives out a little carbonic acid. The oxygen found in some specimens of naphtha is probably derived from this source. (An. de Ch. et de Ph. xlix. 240.) When spirit of turpentine is deprived of all absorbed oxygen by the action of potassium, and is then carefully distilled, it is found to possess the recog- nized properties of naphtha, and to consist solely of carbon and hydrogen in the ratio of six eq. to five, a result formerly obtained by Saussure, and lately confirmed by Dumas. (An. de Ch. et de Ph. 1. 238.) Hence, regarding 100 measures of naphtha vapour to contain 600 measures of carbon vapour and 500 of hydrogen gas, then as Grains. 600 cubic inches of carbon vapour weigh . . 78.4284 500 . . hydrogen gas . . 10.6835 100 . . naphtha vapour should weigh . 89.1119 A vapour so constituted should have a density of 2.8735, which accords dosely with that found by Saussure. Naphtha occurs in some parts of Italy, and on the banks of the Caspian sea: the specimen examined by Saussure was from Amiano in the duchy of Parma. It is an ingredient of the dark bituminous liquid called petroleum, and may be obtained from it by distillation. Coal-tar yields by distillation a liquid very similar to mineral naphtha, and to all appearance identical with it; and the least volatile parts of the oil-gas liquor (page 251) appear to consist principally of naphtha. Naphthaline.—This substance is one of the products of the destructive dis- tillation of coal, and is contained along wilh naphtha in coal-tar. On distilling this matter by a very gentle heat, the naphtha passes over; and afterwards the less volatile naphthaline rises in vapour, and condenses as a white crystalline solid in the neck of the retort. It was first noticed in 1820 by Mr. Garden of Oxford-street, and afterwards described by Dr. Kid and Mr. Chamberlain. (Annals of Phil. xv. 74, xix. 143, and xxii. 103.) Pure naphthaline is heavier than water, has a pungent aromatic taste, and a peculiar, faintly aromatic, odour, not unlike that of the narcissus. It is smooth and unctuous to the touch, is perfectly white, and has a silvery lustre. It fuses at 180°, and assumes a crystalline texture in cooling. It volatilizes slowly at common temperatures, and boils at 410° F., crystal- lizing as it condenses with remarkable facility in thin transparent laminae. Naphthaline is not very readily inflamed; but when set on fire it burns rapidly, and emits a large quantity of smoke. It is insoluble in cold, and very sparingly dissolved by hot water. Its proper solvents are alcohol and ether, especially the latter: its solubility is increased by heat, and it sepa- rates in a crystalline state by cooling or evaporation. It is also soluble in olive oil, spirit of turpentine, and naphtha. To test paper, it manifests nei- ther acidity nor alkalinity. Alkalies do not act upon it. The acetic and oxalic acids dissolve informing pink-coloured solutions; hot hydrochloric acid dissolves it sparingly; and when boiled with nitric acid, the naphthaline is altered, and the acid decomposed. With sulphuric acid it enters into direct combination, forming a peculiar acid, the sulphonaphthalic, discovered by Mr. Faraday. (Phil. Trans. 1826.) According to Dr. Oppermann, the elements of naphthaline are in the ratio of one eq. of hydrogen to three eq. of carbon, (Pog. Annalen, xxiii. 303,); but from the experiments on sulphonaphthalic acid by Faraday, whose results have been confirmed by Liebig and Wohler, and from a late analysis of naphthaline by M. Laurent, it consists of carbon and hydrogen in the ratio of twenty eq. to eight eq., or of ten to four. On the hypothesis that 100 measures of naph- 254 COMPOUNDS OF HYDROGEN AND CARBON. thaline vapour contain 1000 measures of carbon vapour and 400 of hydrogen gas, then, since Grains. 1000 cubic inches of carbon vapour weigh . . 130.7140 400 . . hydrogen gas . . 8.5468 100 . . naphthaline vapour should weigh 139.2608 A vapour so constituted would have a density of 4.4906; whereas the density of naphthaline, as found experimentally by Dumas, is 4.528, thus confirming the foregoing hypothesis. Sulphonaphthalic Acid.—This acid is made by melting naphthaline with half its weight of strong sulphuric acid, when a red-coloured liquid is formed, which becomes a crystalline solid in cooling. The mass is soluble in water, and the solution contains a mixture of sulphuric and sulphonaphthalic acids. On neutralizing with carbonate of baryta, the insoluble sulphate subsides, while the soluble sulphonaphthalate remains in solution; and on decompos- ing this salt by a quantity of sulphuric acid precisely sufficient for precipitating the baryta, pure sulphonaphthalic acid is obtained. The aqueous solution of the acid, as thus formed, reddens litmus paper powerfully, and has a bitter acid taste. On concentrating by heat, the liquid at last acquires a brown tint, and if then taken from the fire becomes solid as it cools. If the concentration is effected by means of sulphuric acid in an exhausted receiver, the acid becomes a soft white solid, apparently dry, and at length hard and brittle. In this state it is chemically united with water, and deliquesces on exposure to the air; but in close vessels it under- goes no change during several months. Its taste, besides being bitter and sour, leaves a metallic flavour like that of cupreous salts. When heated in a tube at temperatures below 212°, it is fused without undergoing any other change, and crystallizes from centres in cooling. When more strongly heated, water is expelled and the acid appears to be anhydrous ; but at the same time it acquires a red tint, and a faint taste of free sulphuric acid may be detected,—circumstances which indicate commencing decomposition. On raising the temperature still higher, the red colour first deepens, then passes into brown, and at length the acid is resolved into naphthaline, sulphurous acid, and charcoal; but in order thus to decompose all the acid, a red heat is requisite. Sulphonaphthalic acid is readily soluble in water and alcohol, and is also dissolved by oil of turpentine and olive oil, in proportions dependent on the quantity ofwater which it contains. By the aid of heat it unites with naph- thaline. It combines with alkaline bases, and forms neutral salts, which are called sulphonaphthalates. All these salts are soluble in water, and most of them in alcohol, and when exposed to heat in the open air, take fire, leaving sulphates or sulphurets according to circumstances. From Mr. Faraday's analysis of the neutral sulphonaphthalate of baryta, it appears that 76.7 parts or one equivalent of baryta are combined with 210.6 parts, or what may be regarded as one equivalent, of sulphonaphthalic acid. These 210.6 parts were found to consist nearly of 80.2 parts or two eq. of sulphuric acid, 122.4 parts or twenty eq. of carbon, and 8 parts or eight eq. of hydrogen. It has not been demonstrated that sulphuric acid exists as such is the compound, nor is it known how its elements are ar- ranged ; but from some interesting facts noticed by Mr. Henncl and others, to be mentioned in the section on ether, it appears very probable that sul- phonaphthalic acid is a direct compound of sulphuric acid and naphthaline. Chloride of Naphthaline.—When a current of dry chlorine gas acts at common temperatures on naphthaline, heat is emitted, the mass fuses, hydrochloric acid gas escapes, and at length, after the full action of chlorine, a semi-fluid substance is left, which contains two chlorides, one solid and the other liquid. On agitating with successive portions of cold ether, the latter is dissolved, and the former obtained in a pure state. COMPOUNDS OF HYDROGEN AND CARBON. 255 The solid chloride is insoluble in water, and nearly so in hot alcohol, but may be dissolved, though not freely, by boiling ether, from which it sepa- rates on cooling in transparent rhomboidal lamina? of a vitreous lustre. At 320° it fuses, and at a higher temperature boils and is decomposed; but cautiously heated in a current of air, it may be distilled without change. The strong acids do not act upon it; nor is it decomposed by a solution of pure potassa except when heated with it. The liquid chloride is obtained by evaporating its ethereal solution. It has the aspect of an oil, is of a light-yellow colour, and sinks in water, in which it is insoluble. Alcohol readily dissolves it, and ether still more free- ly. It may be distilled without decomposition, and suffers little from the action of acids and alkalies. The preceding facts were observed by M. Laurent. (An. de Ch. et de Ph. lii. 275.) From his analysis the solid chloride consists of 70.84 parts or two eq. of chlorine, united with a carburet of hydrogen composed of 61.2 parts or ten eq. of carbon, and 3 parts or three eq. of hydrogen; as express- ed by the formula H3 C1o+Cl-\ The liquid chloride was found to contain 35.42 parts or one eq. of chlorine, 61.2 or ten eq. of carbon, and 4 parts or four eq. of hydrogen, being the ingredients of chloride of naphthaline, of which the formula is H*Cin+Cl. Some doubt may, however, be entertain- ed of its real composition, since the liquid chloride is apt to contain some of the solid compound in solution. Paranaphthaline.—This substance is so called from n-aga. near to, because it is very closely allied to naphthaline both in chemical properties and com- position, is associated with it in coal-tar, and obtained by the same process. Being less volatile than naphthaline it comes over in the after part of the distillation. Paranaphthaline fuses at 356°, and boils at a heat beyond 572°, being partly decomposed at the same rate. It is insoluble in water, and nearly so in alcohol and ether. Its best solvent is spirit of turpentine. The density of its vapour is 6.741, and its elements are in the same ratio as in naphtha- line ; whence it is thought to consist of 1500 measures of carbon vapour and 600 of hydrogen gas condensed into 100 measures. A vapour so constituted should have a density of 6.7359. These facts were observed by Dumas and Laurent. (An. de Ch. et de Ph. 1. 187.) In a critique on their essay Reichenbach states that paranaphtha- line was obtained some years ago by Vogel, and that it is a mixture of pa- raffine and naphthaline. (Pog. Annalen, xxviii. 434.) Idrialine.—This substance was obtained by Dumas from a mineral from the quicksilver mines at Idria in Carniola, whence he applied to it the name of idrialine. In its properties it is very similar to paranaphthaline, but is less fusible and less volatile. In order to sublime it unchanged, it must be heated in a current of carbonic acid gas. It is insoluble in water, and very sparingly soluble in alcohol and ether; but boiling spirit of turpentine takes it up and deposites it again rapidly as it cools. Hot sulphuric acid dissolves it, and acquires at the same time a beautiful blue tint, like sulphate of in- digo, an appearance which is characteristic of idrialine. Respecting its composition nothing farther is known than that it consists of carbon and hydrogen in the ratio of three eq. of the former to one eq. of the latter. (An. de Ch. et de Ph. 1. 193.) Camphene.—From late researches by Dumas, it appears that several of the volatile oils consist essentially of compounds of carbon and hydrogen, which are capable, like cyanogen, of uniting successively with other sub- stances, without undergoing any change in their own constitution. The compound which he has principally examined is the basis of camphor, hence called eamphogen or camphene, which appears to exist in turpentine. From a sample of turpentine believed to have been brought from Savoy or Switzer- land, Dumas distilled off an essence or volatile oil, which he supposes to be camphene in a state of purity; it is, in fact, a very pure essence of turpen- tine, colourless, limpid, volatile, inflammable, and possessed of the character- 256 COMPOUNDS OF HYDROGEN AND CARBON. istic odour of that liquid. Its boiling point is 312° F. From the density of its vapour and the ratio of its elements, Dumas infers that 100 measures of camphene vapour contain 1000 measures of carbon vapour and 800 of hydro- gen gas; so that since Grains. 1000 cubic inches of carbon vapour weigh • • 130.7140 800 do. hydrogen gas .... 17.0936 100 cubic inches of camphene vapour should weigh 147.8076 The weights 130.7140 and 17.0936 are in the ratio of 61.2 parts or ten eq. of carbon, to 8 parts or eight eq. of hydrogen, as indicated by the formula H» Cio. The density of its vapour should be 10 X 0.4215 or 4.215 + 8 X 0.0689 or 0.5512=4.7662, which agrees with observation. Camphene unites with oxygen in two proportions, and gives rise to camphor and camphoric acid; and with hydrochloric acid it yields a perfectly neutral compound called artificial camphor. These compounds, which will be de- scribed hereafter, are thus constituted: (An. de Ch. et de Ph. 1. 225.) Camphene. Equiv. Formulae. Camphor . 69.2 1 eq.+Oxygen 8 leq.= 77.2 HsCio+O Camphoric acid 138.4 2 eq.+ ditto 40 5eq.=178.4 2HsCi°+50 Artificial camphor 69.2 1 eq.+Hyd. ac. 36.42 leq.=105.62 HsCio+HCl. Citrene.—This substance is so called from being the essential and almost sole ingredient of the volatile oil of lemons. Its elements are in exactly the same ratio as in camphene, but are one-half less condensed; so that it may be viewed as a compound of four eq. of hydrogen to five eq. of carbon, indicated by the formula H* Ce. Coal and Oil Gas.—The nature of the inflammable gases derived from the destructive distillation of coal and oil was first ascertained by Dr. Henry,* who showed, in several elaborate and able essays, that these gaseous products do not differ essentially from each other, but consist of a few well- known compounds, mixed in different and very variable proportions. The chief constituents were found to be light carburetted hydrogen and olefiant gases; but besides these ingredients, they contain an inflammable vapour, free hydrogen, carbonic acid, carbonic oxide, and nitrogen gases. The dis- coveries of Mr. Faraday have elucidated the subject still further, by proving- that there exists in oil gas, and by inference in coal gas also, the vapour of several definite compounds of carbon and hydrogen, the presence of which, for the purposes of illumination, is exceedingly important. The illuminating power of the ingredients of coal and oil gas is very un- equal. Thus the carbonic oxide and carbonic acid are positively hurtful; that is, the other gases would give more light without them. The nitrogen of course can be of no service. The hydrogen is actually prejudicial; be- cause, though it evolves a large quantity of heat in burning, it emits an ex- ceedingly feeble light. The carburets of hydrogen are the real illuminating agents, and the degree of light emitted by these is dependent on the quantity of carbon which they contain. Thus olefiant gas illuminates much more powerfully than light carburetted hydrogen; and for the same reason, the dense vapour of etherine emits a far greater quantity of light, for equal volumes, than olefiant gas. From these facts, it is obvious that the comparative illuminating power of different kinds of coal and oil gas may be estimated, approximately at least, by determining the relative quantities of the denser carburets of hy- drogen which enter into their composition. This may be done in three ways. 1. By their specific gravity. 2. By the relative quantities of oxygen required for their complete combustion. 3. By the relative quantity of * Nicholson's Journal of 1805. Phil. Trans. 1808, and 1821. COMPOUNDS OF HYDROGEN AND CARBON. 257 gaseous matter condensible by chlorine in the dark; for chlorine, when light is excluded, condenses all the hydrocarburets, excepting light carburetted hydrogen. Of these methods, the last is, I conceive, the least exception- able.* The formation of coal and oil gas is a process of considerable delicacy. Coal gas is prepared by heating coal to redness in iron retorts. The quality of the gas, as made at different places, or at the same place at different times, is very variable; the specific gravity of some specimens having been found as low as 0.42, and that of others as high as 0.700. These differences arise in part from the nature of the coal, and partly from the mode in which the process is conducted. The regdation of the degree of heat is the chief circumstance in the mode of operating, by which the quality of the gas is affected. That its quality may be influenced from this cause is obvious from the fact, that all the dense hydrocarburets are resolved by a strong red heat either into charcoal and light carburetted hydrogen, or into charcoal and hydrogen gas. Consequently the gas made at a very high temperature, though its quantity may be comparatively great, has a low specific gravity, and illuminates feebly. It is, therefore, an object of importance that the temperature should not be greater than is required for decomposing the coal effectually, and that the retorts be so contrived as to prevent the gas from passing over a red-hot surface subsequently to its formation. These remarks apply with still greater force to the manufacture of oil gas; because oil is capable of yielding a much larger quantity of the heavy hydrocarburets than coal. The quality of oil gas from the same material is liable to such great variation from the mode of manufacture, that the den- sity of some specimens has been found as low as 0.464, and that of others as high as 1.110. The average specific gravity of oil gas is 0.900, and it should never be made higher. The interest of the manufacturer is to form as much olefiant gas as possible, with only a small proportion of the heavier hydrocarburets. If the latter predominate, the quantity of gas derived from a given wdght of oil is greatly diminished; and a subsequent loss is expe- rienced by the condensation of the inflammable vapours when the gas is compressed, or while it is circulating through the distributing tubes. Coal gas, when first prepared, always contains hydrosdphuric acid, and for this reason must be purified before being distributed for burning. The process of purification consists in passing the gas under strong pressure through milk of hme, or causing it to descend through successive layers of dry hydrate of lime. The latter method has this advantage over the former, that while it deprives the gas completely of hydrosulphuric acid, there, is no loss from absorption of olefiant gas or the heavy hydrocarburets, as ensues when milk of lime is employed. But coal gas, after being thus purified, still retains some compound of sulphur, most probably, as Mr. Brande conjec- tures, sulphuret of carbon, owing to the presence of which a minute quan- tity of sulphurous acid is generated during its combustion. Oil gas, on the contrary, needs no purification; and as it is free from all compounds of sul- phur, it does not yield any sulphurous acid in burning, and is, therefore, better fitted for lighting dwelling-houses than coal gas. With respect to the relative economy of the two gases, I may observe that the illuminating power of oil gas, of specific gravity 0.900, is about double that .of coal gas of 0.600. In coal districts, however, oil gas is fully three times the price of coal gas, and, therefore, in such situations, the latter is considerably cheaper. (Essay above quoted.) A successful attempt has been made by Mr. Daniell to procure a gas, similar to that from oil in being free from sulphur, but made with cheaper materials. The substance employed for this purpose is a solution of com- * For a discussion of this and other questions relative to oil and coal gas, the reader may consult an essay by Dr. Christison and myself in the Edin- burgh Philosophical Journal of 1825. 22* 258 COMPOUNDS OF HYDROGEN AND SULPHUR. mon resin in oil of turpentine. The combustible liquid is made to drop into red-hot retorts in the same manner as oil; and the oil of turpentine, which from its volatility is driven off in vapour, is collected, and again used as a menstruum. For this process Mr. Daniell has taken out a patent, and the gas so prepared is employed for filling portable lamps. The gas, when pro- perly made, is said to be of a very superior quality, and nearly if not quite equal to oil gas. A patent has also been taken for the formation of gas from a volatile oil, prepared during the destructive distillation of resin, and a manufacture both of the oil and gas is established at Hammersmith, near London. SECTION III. COMPOUNDS OF HYDROGEN AND SULPHUR. Sulphur unites with hydrogen in at least two proportions, and the result- ing compounds are thus constituted:— Hydrogen. Sulphur. Equiv. Formula?. Hydrosulphuric acid 1 1 eq. + 16.l 1 eq.= 17.1 H + S or HS. Persulphuret of hydrogen 1 leq.+ 32.2 2 eq.=33.2 H+2S or HS*. Hydrosulphuric Acid.—This compound is also known under the name of sulphuretted hydrogen. The best method of preparing it, is by heating ses- quisulphuret of antimony in a retort, or other convenient glass vessel, with four or five times its weight of strong hydrochloric acid; when, by an inter- change of elements, sesquichloride of antimony and hydrosulphuric acid are generated, the latter of which escapes with effervescence. The elements con- cerned before and after the change, are 1 eq. sesquisulphuret of antimony and 3 eq. hydrochloric acid 2Sb+3S 3(H+C1) which yield 3 eq. hydrosulphuric acid and 1 eq. sesquichloride of antimony. 3(H+S) 2Sb+3Cl. It may also be formed by the action of sulphuric acid diluted with 3 or 4 parts of water on protosulphuret of iron: this sulphuret and water inter- change elements, hydrosulphuric acid and protoxide of iron are generated, and the latter unites with sulphuric acid, while the former in the state of gas is rapidly disengaged. Hydrochloric acid may be substituted for the sul- phuric. A sulphuret of iron may be procured for the purpose, either by ig- niting common iron pyrites, by which means nearly half of its sulphur is expelled, or by exposing to a low red heat a mixture of two parts of iron filings and rather more than one part of sulphur. The materials should be placed in a common earthen or cast-iron crucible, and be protected as much as possible from the air during the process. The sulphuret procured from iron filings and sulphur always contains some uncombined iron, and, there- fore, the gas obtained from it is never quite pure, being mixed with a little free hydrogen. This, however, for many purposes, is immaterial. Hydrosulphuric acid is a colourless gas, which reddens moist litmus pa- per feebly, and is distinguished from all other gaseous substances by its offensive taste and odour, which is similar to that of putrefying eggs, or the water of sulphurous springs. Under a pressure of 17 atmospheres, at 50°, it is compressed into a limpid liquid, which resumes the gaseous state as soon as the pressure is removed. To animal life it is very injurious. According to Dupuytren and Thenard, the presence of l-1500th of this gas is instantly fatal to a small bird'; 1-lOOOth killed a middle-sized dog; and a horse died in an atmosphere which contained l-250th of its volume. COMPOUNDS OF HYDROGEN AND SULPHUR. 259 Hydrosulphuric acid extinguishes all burning bodies; but the gas takes fire when a lighted candle is immersed in it, and burns with a pale blue flame. Water and sulphurous acid are the products of its combustion, and sulphur is deposited. With oxygen gas it forms a mixture which detonates by the application of flame or the electric spark : if 100 measures of it are exploded with 150 of oxygen, the former is completely consumed, the oxygen disap- pears, water is deposited, and 100' measures of sulphurous acid gas remain. (Dr. Thomson.) From the result of this experiment, the composition of hy- drosulphuric acid gas may be inferred; for it is clear, from the composition of sulphurous acid, (page 193,) that two-thirds of the oxygen must have combined with sulphur; and, therefore, that the remaining one-third contri- buted to the formation of water. Consequently, hydrosulphuric acid contains its own volume of hydrogen gas, and 16.66 of the vapour of sulphur; and since Grains. 16.66 cubic inches of the vapour of sulphur weigh . 34.4012 100 cubic inches of hydrogen gas weigh . . . 2.1367 100 cubic inches of hydrosulph. acid gas must weigh 36.5379 The sp. gravity of a gas so constituted should be 1.1782, which agrees with observation; and its elements are in the ratio of 1 to 16.1 as already mentioned. The accuracy of this view is confirmed by several circumstances. Thus, according to Gay-Lussac and Thenard, the weight of 100 cubic inches of hydrosdphuric acid gas is 36.33 grains. When sulphur is heated in hy- drogen gas, hydrosulphuric acid is generated without any change of volume. On igniting platinum wires in it by means of the voltaic apparatus, sulphur is deposited, and an equal volume of pure hydrogen remains; and a similar effect is produced, though more slowly, by a succession of electric sparks. (Elements of Sir H. Davy, p. 232.) Gay-Lussac and Thenard found that on heating tin in hydrosulphuric acid gas, sulphuret of tin is formed; and when potassium is heated in it, vivid combustion ensues, with formation of sulphuret of potassium. In both cases, pure hydrogen is left, which occupies precisely the same space as the gas from which it was derived. (Recherches Physico-Chimiques, vol. L) The salts of hydrosulphuric acid are called hydrosulphates, and sometimes hydrosulphurets. This acid, however, rarely unites directly with metallic oxides; but in most cases its hydrogen combines with the oxygen of the oxide, and its sulphur with the metal. All the hydrosulphates which do exist are decomposed by sulphuric or hydrochloric acid, and hydrosulphuric acid gas is disengaged with effervescence. Recently boiled water absorbs its own volume of hydrosulphuric acid, be- comes thereby feebly acid, and acquires the peculiar odour and taste of sul- phurous springs. The gas is expelled without change by boiling the water. The elements of hydrosulphuric acid may easily be separated from one another. A solution of the gas cannot be preserved in an open vessel, be- cause its hydrogen unites with the oxygen of the atmosphere, and sulphur is deposited. When mixed with sulphurous acid, both compounds are de- composed, water is generated and sulphur set free. On pouring into a bottle of the gas a little fuming nitric acid, mutual decomposition ensues, a bluish-white flame frequently appears, sulphur and nitrous acid fumes come into view, and water is generated. Chlorine, iodine, and bromine decom- pose it, with separation of sulphur, and formation of hydrochloric, hydriodic, and hydrobromic acids. An atmosphere charged with hydrosulphuric acid gas may be purified by means of chlorine in the space of a few minutes. Hydrosulphuric acid gas is readily distinguished from other gases by its odour, by tarnishing silver with which it forms a sulphuret, and by the cha- 260 COMPOUNDS OF HYDROGEN AND SULPHUR. raeter of the precipitate which it produces with solutions of arsenious acid, tartar emetic, and salts of lead. The most delicate test of its presence, when diffused in the air, is moist carbonate of oxide of lead spread on white paper. Persulphuret of Hydrogen.—Though Scheele discovered this compound, it was first specially described by Berthollet. (An. de Chimie, xxv.) When protosulphuret of potassium (or of any metal of the alkalies and alkaline earths) is mixed in solution with sulphuric acid, the oxygen of water unites with potassium and its hydrogen with sulphur, just as when protosulphuret of iron is employed, hydrosulphuric acid and sulphate of potassa being gene- rated : the elements K + S and H + O mutually interchange, and yield K+O and H + S. If the potassium be combined with two or more equi- valents of sulphur as in the so called liver of sulphur made by fusing carbo- nate of potassa with half of its weight of sulphur, then one of two events will happen, the hydrogen of the decomposed water will either unite with one eq. of sulphur and form hydrosulphuric acid, the superfluous sulphur subsiding in the form of a gray hydrate, or with two eq. of sulphur, and give rise to persul- phuret of hydrogen. Now the former of these changes always occurs when the acid is added to the persulphuret of potassium; and the latter takes place when a concentrated solution of that sdphuret is added by little and little to the acid, provided the acid is in considerable excess, and the mixture well stirred after each addition. The same phenomena ensue when hydrochloric instead of sulphuric acid is employed: but then there are two sources from which hydrogen may be supplied. It may be derived, as above, from decom- posed water, hydrochlorate of potassa being generated; or hydrochloric acid itself may be decomposed, its hydrogen uniting with sulphur and its chlorine with potassium. On all such occasions I adopt the latter view, and will give reasons for doing so in the section introductory to the study of the metals. Such are the principles to be attended to in preparing persulphuret of hydrogen. In practice it is conveniently made by boiling equal parts of re- cently slaked lime and flowers of sulpur with 5 or 6 parts of water for half an hour, when a deep orange-yellow solution is formed, which contains per- sulphuret of calcium. Let this liquid be filtered, and gradually added cold to an excess of hydrochloric acid diluted with about twice its weight ofwater, briskly stirring. A copious deposite of hydrated sulphur falls (the Sulphur Prffidpitatum of the London Pharmacopoeia), and persulphuret of hydrogen gradually subsides in the form of a yellowish semi-fluid matter like oil. The change which ensues in the formation of the yellow solution may be theo- retically represented thus:— 2 eq. lime & 6 eq. sulphur 2 1 eq. hyposulph. acid & 2 eq. bisulphuret calcium. 2(Ca-fO) 6S •« 2S+20 2(Ca+2S). The hyposulphurous acid exists in solution united with lime, and is decom- posed when hydrochloric acid is added, resolving itself into sulphurous acid and sulphur (page 197), a change not essentially connected with the produc- tion of persulphuret of hydrogen, but resulting from the mode of preparing the persulphuret of calcium. It is probable that the calcium is combined with more than two eq of sulphur, and that the deposited sulphur is derived from that source as well as from decomposed hyposulphurous acid. From the facility with which this substance resolves itself into sulphur and hydrosulphuric acid, its history is imperfect: indeed we are indebted to a recent essay by Thenard for the principal facts which are known. (An. de Ch. et de Ph. xlviii. 79.) At common temperatures it is a viscid liquid of a yellow colour, with a density of about 1.769, and a consistence varying between that of a volatile and fixed oil. It has the peculiar odour and taste of hydrosulphuric acid, though in a less degree. Its elements are so feebly united, that in the cold it gradually resolves itself into sulphur and hydrosul- phuric acid, and suffers the same change instantly by a heat considerably short of 212° F. Decomposition is also produced by the contact of most substances, HYDROGEN AND SELENIUM.—HYDROSELENIC ACID. 261 especially of metals, metallic oxides, even the alkalies, and metallic sulphurets. Thus effervescence from the escape of hydrosulphuric acid gas is produced by peroxide of manganese, silica, the alkaline earths in powder, and solutions of potassa or soda; and the oxides of gold and silver are reduced by it with such energy, that they are rendered incandescent. It is remarkable that the substance which causes the decomposition, often undergoes no chemi- cal change whatever. In these respects persulphuret of hydrogen bears a dose analogy to peroxide of hydrogen, and Thenard has traced other points of resemblance. They are both, for instance, rendered more stable by the presence of acids; they both whiten the tongue and skin when applied to them, and they are both possessed of bleaching properties. The composition of persulphuret of hydrogen has been variously stated. According to Dalton it is a bisulphuret, consisting of two equivalents of sul- phur, and one of hydrogen; and this view of its composition is corroborated by Sir John Herschel's analysis of persulphuret of calcium. (Edin. Phil. Journal, i. 13.) But Thenard found its constituents to vary; whence it is probable that hydrogen is capable of uniting with sulphur in several propor- tions. Persulphuret of hydrogen is sometimes regarded as an acid; and on this supposition it may be termed hydropersulphuric acid, and its salts hydroper- sulphates. This view is founded on the hypothesis, that the solutions formed by boiling hme or an alkali with sulphur contain hyposulphite and hydro- persulphate of lime, the hydrogen in the one acid and oxygen in the other being attributed to decomposed water, and not hyposulphite of lime and per- sdphuret of calcium as I have supposed. The latter view is more consistent with the fact that persulphuret of hydrogen in its free state has no, acidity, and exhibits no tendency to unite with alkalies. SECTION IV. HYDROGEN AND SELENIUM—HYDROSELENIC ACID. Selenium, like sulphur, forms a gaseous compound with hydrogen, which has distinct acid properties, and is termed seleniurelted hydrogen, or hydro- selenic acid. This gas is disengaged by the action of dilute sulphuric or hy- drochloric acid on a protoseleniuret of any of the more oxidable metals, such as potassium, calcium, manganese, or iron, the explanation being the same as in the formation of hydrosulphuric acid from protosulphuret of iron. Hydroselenic acid gas is colourless. Its odour is at first similar to that of hydrosulphuric acid; but it afterwards irritates the lining membrane of the nose powerfully, excites catarrhal symptoms, and destroys for some hours the sense of smelling. It is absorbed freely by water, forming a co- lourless solution, which reddens litmus paper, and gives a brown stain to the skin. The acid is soon decomposed by exposure to the atmosphere; for the oxygen of the air unites with the hydrogen of the hydroselenic acid, and selenium, in the form of a red powder, subsides. It is decomposed by nitric acid and chlorine in the same manner as hydrosulphuric acid; and like that gas it decomposes many metdlic salts, the hydrogen of the acid combining with the oxygen of the oxide, while an insoluble seleniuret of the metal is generated. According to the analysis of Berzelius, hydroselenic acid consists of 39.6 parts or one eq. of selenium, and 1 part or one eq. of hydrogen; so that its equivalent is 40.6, and its formula H + Se or HSe. 262 COMPOUNDS OF HYDROGEN AND PHOSPHORUS. SECTION V. COMPOUNDS OF HYDROGEN AND PHOSPHORUS. Most chemists admit the existence of two compounds of phosphorus and hydrogen, the phosphuretted and perphosphuretted hydrogen, the constitu- tion and properties of which have of late been closely studied by Dumas, Buff, and Rose. (An. de Ch. et de Ph. xxxi. 113, xli. 220, and li. 5.) Much uncertainty exists respecting the composition of the latter; but there is strong concurrent testimony to prove- that phosphuretted hydrogen consists of 31.4 parts or two equivalents of phosphorus, and 3 parts or three eq. of hydrogen. Its equivalent should, therefore, be 34.4, and its formula 3H + 2P, orHspz. Phosphuretted Hydrogen.—This gas, which was discovered in 1812 by Sir H. Davy, is colourless, and has a disagreeable odour, somewhat like that of garlic Water absorbs about one-eighth of its volume. It does not take fire spontaneously, as perphosphuretted hydrogen does, when mixed with air or oxygen at common temperatures; but the mixture detonates with the electric spark, or at a temperature of 300° F. Even diminished pressure causes an explosion; an effect which, in operating with a mercurial trough, is produced simply by raising the tube, so that the level of the mercury within may be a few inches higher than at the outside. Admitted into a ves- sel of chlorine it inflames instantly, and emits a white light, a property which it possesses in common with perphosphuretted hydrogen. Its specific gravity was found by Dumas to be 1.214, and by Rose 1.154. Davy prepared this gas by heating hydrated phosphorous acid in a retort (page 201); and it is also evolved from hydrous hypophosphorous acid by similar treatment. It is also formed, according to Dumas, by the action of strong hydrochloric acid on phosphuret of caldum ; and likewise by the spontaneous decomposition of perphosphuretted hydrogen. Phosphuretted hydrogen frequently contains free hydrogen gas, especially when a strong heat is used in its preparation, an impurity which may be detected by agitation with a cold saturated solution of sulphate of oxide of copper. This substance has the property of absorbing both of the compounds of phosphorus and hydrogen entirely, with production of phosphuret of cop- per, while the free hydrogen gas remains. Dumas ascertained the composition of phosphuretted hydrogen by in- troducing into a tube containing the gas a fragment of bichloride of mer- cury, applying heat so as to convert it into vapour. Mutual decomposition instantly took place: phosphuret of mercury and hydrochloric acid were generated ; and 100 measures of gas, thus decomposed, yielded 300 measures of hydrochloric acid gas, corresponding to 150 of hydrogen. The quantity of hydrogen contained in any given volume of phosphuretted hydrogen is thus found; and the weight of the former deducted from that of the latter gives the quantity of combined phosphorus. This inference is conformable to the quantity of oxygen required for the combustion of phosphuretted hy- drogen. Dr. Thomson affirms that when this gas is detonated with 1.5 of its volume of oxygen gas, the only products are water and phosphorous acid ; but that when the oxygen is in considerable excess, two volumes disappear for one of the compound, and water and phosphoric acid are generated. Now the hydrogen contained in one volume of phosphuretted hydrogen is equal to 1.5, and it unites with 0.75 of oxygen. Hence if 0.75, or \, be de- ducted from 1.5 and from 2. the remainders, | and L, represent the relative quantity of oxygen which is required to convert the same weight of phos- phorus into phosphorous and phosphoric acid. These numbers are obviously in the ratio of 3 to 5, as already stated on the authority of Berzelius. (Pago COMPOUNDS OF HVDROGEN AND PHOSPHORUS. 263 200.) The elements of the calculation have been confirmed both by Dumas and Buff. Agreeably to these views, and to the combining volume of phosphorus (page 146), 100 measures of phosphuretted hydrogen gas contain 150 of hy- drogen gas and 25 of the vapour of phosphorus; and hence, as Grains. 150 cubic inches of hydrogen gas weigh .... 3.2050 25 do. phosphorus vapour weigh . . . 33.5461 100 do. phosphuretted hydrogen gas should weigh 36.7511 The calculated density of a gas so constituted should be 1.1850, which is nearly a mean of the observations of Dumas and Rose. If the equivalent of phosphorus were 31.4 instead of 15.7, as is very far from improbable, then the combining volume of phosphorus vapour would be 50 instead of ~5 (page 146); and phosphuretted hydrogen would consist of 50 measures of phosphorus vapour and 150 of hydrogen gas condensed into 100 measures, thus agreeing in composition with ammoniacal gas.* Phosphuretted hydrogen has neither an acid nor alkaline reaction ; but in its chemical relations it inclines to alkalinity. Thus it unites with hydro- bromic and hydriodic acids, forming definite compounds which crystallize in cubes; and Rose finds that it unites with metallic chlorides, forming compounds andogous to those which ammonia forms with metallic chlorides. Perphosphuretted Hydrogen.—The gas to which this name is applied, was discovered in the year 1783 by 31. Gengembre, and has since been particu- larly examined by Dalton, Thomson, Dumas, and Rose. It may be prepared in several ways. The first method is by heating phosphorus in a strong so- lution of pure potassa; the second by heating a mixture made of small pieces of phosphorus and recently slaked lime, to which a quantity ofwater is added sufficient to give it the consistence of thick paste; and the third by the action of dilute hydrochloric acid, aided by moderate heat, on phos- phuret of calcium. In these processes, three compounds of phosphorus are generated;—phosphoric acid, hypophosphorous acid, and perphosphuretted hydrogen—all of which are produced by decomposition of water, and the union of its dements with separate portions of phosphorus. The last me- thod appears to yield the purest gas. The gas obtained by either of these processes is said by Dalton to be gen- erally, and by Dumas to be always, mixed with variable proportions of hy- drogen ; but Rose denies that free hydrogen gas is evolved, except when the heat is so great as to decompose the hypophosphite, a temperature which is never attained so long as the materials are moist. It has a peculiar odour, resembling that of garlic, and a bitter taste. Its density is estimated at 0.9027 by Thomson, at 1.1 nearly by Dalton, at 1.761 by Dumas, and 1.1935 by Rose. It has no action on test paper. Recently boiled water absorbs ^Vth of its bulk of this gas, most of which is expelled by boiling or agitation with other gases. Like phosphuretted hydrogen it is freely absorbed by a solu- tion of chloride of lime or sulphate of oxide of copper, by which means its purity may be ascertained, and the presence of hydrogen detected; and like that compound it sometimes decomposes metallic salts in the same manner as hydrosulphuric acid, giving rise to the formation of water and a phos- phuret of the metal. But if the metal have a feeble affinity for oxygen, both the phosphurets of hydrogen throw it down in the metallic state, and water and phosphoric acid are generated. This is the case, according to Rose, with solutions of gold and silver. * This statement is inaccurate. On the supposition made by Dr. Turner, phosphuretted hydrogen would consist of 50 measures of phospho- rus vapour and 300 of hydrogen gas, condensed into 200 measures. Conse- quently, it would not agree in composition with ammoniacal gas.—Ed. COMPOUNDS OF NITROGEN AND CARBON. Perphosphuretted hydrogen does not support combustion nor respiration. in most of its properties it resembles phosphuretted hydrogen; but from that gas as well as from other gases, it is distinguished by being spontaneously inflammable when mixed with air or oxygen gas. If the beak of the retort from which it issues is plunged under water, so that successive bubbles of the gas may arise through the liquid, a very beautiful appearance takes place. Each bubble, on reaching the surface of the water, bursts into flame, and forms a ring of dense white smoke, which enlarges as it ascends, and retains its shape, if the air is tranquil, until it disappears. The wreath is formed by the products of the combustion—metaphosphoric acid and water. If received in a vessel of oxygen gas, the entrance of each bubble is in- stantly followed by a strong concussion, and a flash of white light of ex- treme intensity. It is remarkable that whatever may be the excess of oxy- gen, traces of phosphorus always escape combustion; but that if the gas be previously mixed with three times its volume of carbonic acid, and be then mixed with oxygen, the combustion is perfect. Dalton observed that it may be mixed with pure oxygen in a tube three-tenths of an inch in diameter without taking fire; but that the mixture detonates when an electric spark is transmitted through it. In consequence of the combustibility of perphosphuretted hydrogen, it would be hazardous to mix it in any quantity with air or oxygen gas in close vessels. For the same reason care is necessary in the formation of this gas, lest, in mixing with the air of the apparatus, an explosion ensue, and the vessel burst. The risk of such an accident is avoided, when phosphuret of calcium is used, by filling the flask or retort entirely with dilute acid; and in either of the other processes, by causing the phosphuretted hydro- gen to be formed slowly at first, in order that the oxygen gas within the apparatus may be gradually consumed. A very simple method of averting all danger has been mentioned by Mr. Graham. It consists in moistening the interior of the retort with one or two drops of ether, the vapour of which, when mixed with atmospheric air even in small proportion, effectually pre- vents the combustion of perphosphuretted hydrogen. Great uncertainty exists respecting the composition of this gas. It seems pretty certain, from the experiments of Dumas and Rose, that 100 measures of it contain 150 of hydrogen; but the quantity of phosphorus combined with that hydrogen is a point by no means agreed on. The weight of phos- phorus united with 1 part of hydrogen is estimated by Thomson at 12, by Rose at 10.52, and at 15.9 by Dumas. Such discordant estimates are only referable to the changeable nature of the gas itself. Perphosphuretted hydrogen, as first prepared, not only often contains variable quantities of the vapour of phosphorus, of free hydrogen, and probably of phosphuretted hy- drogen, but is itself very liable to change. Not only do its elements separate when electric sparks are passed through it, or by exposure to a strong heat, but it is apt to decompose spontaneously at common temperatures, whereby the same specimen may vary in composition during the intervd of an hour. Rose, in his last essay, contends that the two compounds of phosphorus and hydrogen, are isomeric, being identical in composition, and differing in cha- racter only by one being spontaneously inflammable and the other not so. SECTION VI. COMPOUNDS OF NITROGEN AND CARBON. BICARBURET OF NITROGEN, OR CYANOGEN GAS. Cyanogen gas, the discovery of which was made in 1815 by Gay-Lussac, (An. de Ch. xcv.) is prepared by heating carefully dried bicyanuret of mer- cury in a small glass retort by means of a spirit-lamp. This cyanuret, COMPOUNDS OF NITROGEN AND CARBON. 265 which was formerly considered a compound of oxide of mercury and prussic acid, and was then called prussiate of mercury, is composed of metallic mer- cury and cyanogen. On exposure to a low red heat, it is resolved into its elements; the cyanogen passes over in the form of gas, and the metallic mercury is sublimed. The retort, at the close of the process, contains a small residue of a dark brown matter like charcoal, but which Mr. John- ston has shown to consist of the same ingredients as the gas itself Cyanogen gas is colourless, and has a strong pungent and very peculiar odour. At the temperature of 45° and under a pressure of 3.6 atmospheres, it is a limpid liquid, which 3Ir. Kemp finds to be a non-conductor of electri- city, and which resumes the gaseous form when the pressure is removed. It extinguishes burning bodies; but it is inflammable, and burns with a beautiful and characteristic purple flame. It can support a strong heat without decomposition. Water, at the temperature of 60° absorbs 4.5 times, and alcohol 23 times its volume of the gas. The aqueous solution reddens litmus paper; but this effect is not to be ascribed to the gas itself, but to the presence of acids which are generated by the mutual decomposition of cyano- gen and water. It appears from the observations of Wohler that two of the pro- ducts are cyanic acid and ammonia; which, uniting together, generate urea. (An. de Ch. et de Ph. xliii. 73.) The composition of cyanogen may be determined by mixing that gas with a due proportion of oxygen, and inflaming the mixture by electricity. Gay-Lussac ascertained in this way that 100 measures of cyanogen require 200 of oxygen for complete combustion, that no water is formed, and that the products are 200 measures of carbonic acid gas and 100 of nitrogen. Hence it follows that cyanogen contains its own bulk of nitrogen, and twice its volume of the vapour of carbon. Consequently, since Grains. 100 cubic inches of nitrogen gas weigh . . . 30.1650 200 ' do. the vapour of carbon weigh . 26.1428 100 cubic inches of cyanogen gas must weigh . . 56.3078 The ratio of its elements by weight is Nitrogen . 30.1650 . 0.9727 . • 14.15 1 eq. Carbon . 26.1428 . 0.8430(2x0.4215) 12.24 2 eq. The specific gravity of a gas so constituted is 0.9727+0.843= 1.8157, which is near 1.8064, the number found experimentally by Gay-Lussac. Cyanogen is a bicarburet of nitrogen, the formula of which is N+2C or NCS; but its most convenient name is cyanogen, proposed by its discoverer,* which may be expressed shortly by Cy. Cyanogen, though a compound body, has a remarkable tendency to com- bine with elementary substances. Thus it is capable of uniting with the simple non-metallic bodies, and evinces a strong attraction for metals. When potassium, for instance, is heated in cyanogen gas, such energetic action ensues, that the metal becomes incandescent, and cyanuret of potas- sium is generated. The affinity of cyanogen for metallic oxides, on the contrary, is comparatively feeble. It enters into direct combination with a few dkaline bases only, and these compounds are by no means permanent. From these remarks it is apparent that cyanogen has no claim to be regarded as an acid. An examination of the brown matter, left in the retort after the prepara- tion of cyanogen gas, has been made by Mr. Johnston, who by burning it with chlorate of potassa found it to contain carbon and nitrogen united in the same ratio as in cyanogen gas. It is, in fact, a solid bicarburet of nitrogen, isomeric with cyanogen, but differing from it essentially in its * From kuclvo; blue, and ynvda> I generate; because it is an essential ingredient of Prussian blue. B 23 266 COMPOUNDS OF NITROGEN AND CARBON. physical and chemical relations. On heating this solid bicarburet in the open air, several definite compounds of carbon and nitrogen may be succes- sively obtained. After considerable heating, the ratio of carbon to nitrogen is as 3 to 2; again heated, the proportion becomes as 7 to 6; and finally, after a still longer heat, the ratio of the equivalents is as 1 to 1. Thus the carbon is gradually burned away, leaving the nitrogen fixed, until a proto- carburet of nitrogen is formed. On continuing the heat after this period, both elements fly off together, and the whole is dissipated. The solid bicarburet of cyanogen is also generated, when a saturated solution of cyanogen in alcohol is kept in contact with mercury; and Mr. Johnston suggests that the carbonaceous residue after the charring of animal substances by heat, is probably in many cases a carburet of nitrogen, and not pure charcoal as is commonly thought. (Brewster's Journ. N. S. i. 75.) The composition of the compounds of cyanogen described in this section isas follows:— Cyanogen. Equiv. Formulae. Hydrocyanic acid 26.39 1 eq.+Hydrogen 1 1 eq. = 27.39 H+Cy Cyanic acid . 26.39 1 eq.+Oxygen 8 1 eq. = 34.39 Cy+O FUnic)nacidCya" |26-39 le1-+d0- 8 leq. = 34.39 'Cy+O Cyanuric acid j , ^ ,0 c . PaacaiTnufic (m7 3 eq,+ i ^.1 3 % \=130-17 cy=°'Hs ^noget °f T" ( 26-39 * eq.+Chlorine 35.42 1 eq. = 61.8.1 Cy+Cl ^anoaen?^07"!26,39 J e<*-+ da 70.84 2 eq. = 97.23 Cy+2C1 Iodide of cyanogen. Composition uncertain. Bromide of cya- ) „ n J > Composition uncertain. Hydro- I Cyano- (9fi on i i sulphocy-^ gen p0,,jy 1 e Existence doubtful. anogen . v Cyano-hydro- ( 2g 39 . , K Sulphur 32.2 2 eq. ) fin „ Cv+2HS.' sulphuric acid \ **'** 1 e*+ ) Hydros, o 2 eq. \ =60-59 2H+CyS- Hydroseleniocy- f 0 a e d i Composition uncertain. HYDROCYANIC OR PRUSSIC ACID. This acid was discovered in the year 1782 by Scheele, and Berthollet afterwards ascertained that it contains carbon, nitrogen, and hydrogen; but Gay-Lussac first procured it in a pure state, and by the discovery of cyanoo-en was enabled to determine its real nature. The substance prepared by Scheele was merely a solution of hydrocyanic acid in water. Pure hydrocyanic acid may be prepared by heating bicyanuret of mer- cury in a glass retort with two-thirds of its weight of concentrated hydro- chloric acid. By an interchange of elements 2 eq. hydrochloric acid and 1 eq. bicyanuret of mercury, 2(H+C1) Hg+2Cy, yield 2 eq. hydrocyanic acid and 1 eq. bichloride of mercury. -'(H+'Cy) Hg+2C1. The volatile hydrocyanic acid passes off in vapour along with moisture and hydrochloric acid : by transmission through a tube over fragments of marble COMPOUNDS OF NITROGEN AND CARBON. 267 it is deprived of hydrochloric acid. It is then' dried by passing through another tube filled with fused chloride of calcium in fragments, and is ultimately collected in a clean dry tube surrounded by ice. Vauquelin proposes the following process, which affords a more abundant product than the preceding. It consists in filling a narrow tube, placed horizontally, with fragments of bicyanuret of mercury, and causing a cur- rent of dry hydrosulphuric acid gas to pass very slowly along it. The instant that gas comes in contact with the bicyanuret, double decomposition ensues, and hydrocyanic acid and bisulphuret of mercury are generated. The pro- gress of the hydrosulphuric acid along the tube may be distinctly traced by the change of colour, and the experiment should be closed as soon as the whole of the bicyanuret has become black. It then only remains to expel the hydrocyanic acid by a gentle heat, and collect it in a cool receiver. This process is elegant, easy of execution, and productive. Pure hydrocyanic acid is a limpid colourless fluid, of a strong odour, similar to that of peach blossoms. It excites at first a sensation of coolness on the tongue, which is soon followed by heat; but when diluted, it has the flavour of bitter almonds. Its specific gravity at 45° is 0.7058. It is so exceedingly volatile that its vapour during warm weather may be collected over mercury. Its point of ebullition is 79°, and at zero it congeals. When a drop of it is placed on a piece of glass, it becomes solid, because the cold produced by the evaporation of one portion is so great as to freeze the remainder. It unites with water and alcohol in every proportion. Pure hydrocyanic acid is a powerfixl poison, producing in poisonous doses insensibility and convulsions, which are speedily followed by death. A sin- gle drop of it placed on the tongue of a dog causes death in the course of a very few seconds; and smdl animals, when confined in its vapour, are rapid- ly destroyed. On inspiring the vapour, diluted with atmospheric air, head- acb and giddiness supervene; and for this reason the pure acid should not be made in close apartments during warm weather. The distilled water from the leaves of the Prunus lauro-cerasus owes its poisonous quality to the presence of this acid. Its effects are best counteracted by diffusible stimulants, and of such remedies solution of ammonia appears to be the most beneficial. The aqueous solution of chlorine may be used as an anti- dote, which decomposes hydrocyanic acid instantly, with formation of hy- drochloric acid. In some experiments, recently described by MM. Persoz and Nonat, symptoms of poisoning, induced by hydrocyanic acid applied to the globe of the eye, ceased on the internal administration of chlorine. It would hence appear, that both substances were absorbed into the circulating fluids, and there reacted on each other. (An. de Ch. et de Ph. xliii. 324.) Pure hydrocyanic acid, even when excluded from air and moisture, is very liable to spontaneous changes, owing to the tendency of its elements to form new combinations. These changes sometimes commence within an hour after the acid is made, and it can rarely be preserved for more than two weeks. The commencement of decomposition is marked by the liquid acquiring a reddish-brown tinge. The colour then gradually deepens, a matter like charcoal subsides, and ammonia is generated. On analyzing the black matter, it was found by Gay-Lussac to contain carbon and nitro- gen : M. P. Boullay considers it to be a peculiar acid, composed of carbon, nitrogen, and hydrogen, very analogous to ulmic acid, and for which he proposes the name of azulmic acid; but farther observation is desirable be- fore this view can be relied on. Hydrocyanic acid may be preserved for a longer period if diluted with water, but even then it undergoes gradual de- composition. Hydrocyanic acid is one of the feeblest of the acids. It is not acid to the taste, scarcely reddens litmus paper, does not decompose the salts of carbonic acid, and is unable in any quantity to neutralize the alkaline re- action of potassa. When mixed with metallic oxides it rarely if ever enters into combination, so as to constitute hydrocyanates ; but by an interchange of elements, water and metallic cyanurets are generated. Though a solu- tion of cyanuret of potassium is alkaline, has the odour of hydrocyanic 268 COMPOUNDS OF NITROGEN AND CARBON. acid, and acts like that acid as a poison, it does not follow that potassa and hydrocyanic acid were in combination; for they may be merely developed at the moment by the influence of carbonic acid of the air, or of any sub- stance which has an affinity for potassa. Hydrocyanic acid is resolved by galvanism into hydrogen and cyanogen, the former of which appears at the negative and the latter at the positive pole. When its vapour is conducted through a red-hot porcelain tube, par- tid decomposition ensues. Charcoal is deposited, and nitrogen, hydrogen, and cyanogen gases are set at liberty; but the greater part of the acid passes over unchanged. Electricity produces a similar effect. The vapour of hydrocyanic acid takes fire on the approach of flame; and with oxygen gas it forms a mixture which detonates with the electric spark. The pro- ducts of the combustion are nitrogen, water, and carbonic acid. Gay-Lussac proved that hydrocyanic acid vapour is analogous in compo- sition to hydrochloric acid gas, consisting of equal measures of cyanogen and hydrogen gases united without change of volume; for on heating po- tassium in 100 measures of hydrocyanic acid vapour, he obtained as much cyanuret of potassium as would have been formed from 50 measures of cyanogen gas, and 50 measures of pure hydrogen remained. Hence, as Grains. 50 cubic inches of cyanogen gas weigh . . . 28.1539 50 do. hydrogen gas .... 1.0683 100 do. hydrocyanic acid vapour should weigh 295222 Also 1.0683 is to 28.1539 as 1 to 26.39, the composition given in the table. The density of a gas so constituted should be £(1.8157+ 0.069)=0.9423; whereas the number which Gay-Lussac found by weighing the gas is 0.9476. From the powerful action of hydrocyanic acid on the animal economy, this substance, in a diluted form, is sometimes employed in medicd prac- tice to diminish pain and nervous irritability. Several processes are in use for obtaining the medicind acid of uniform strength and of a qudity to keep long without decomposition. I. One of these consists in transmitting a current of hydrosulphuric acid gas through water, holding in solution a known weight of bicyanuret of mercury, until complete decomposition, which is ascertained by the filtered liquid remaining colourless when acted on by an additiond quantity of hy- drosulphuric acid. The excess of this acid is removed by agitation with carbonate of oxide of lead, after which the solution is filtered.—It admits of doubt whether the hydrocyanic acid thus obtained is uniform; since a quan- tity of its vapour, variable in different operations, is apt to be carried off by the hydrosulphuric acid gas. II. Another process, adopted at Apothecaries' Hdl, is to mix in a retort 1 part of bicyanuret of mercury, 1 part of hydrochloric acid of sp. gravity 1.15, and 6 parts of water; and to distil the mixture until a quantity of acid, equal to that of the water employed, is collected. The product has a density of 0.995. (Brande's Chemistry). A little hydrochloric acid com- monly passes over into the recipient, and may be got rid of and discovered by mixing the impure acid with a little chalk and distilling to dryness, when chloride of calcium remains. Its presence while mixed with hydro- cyanic acid cannot well be detected by nitrate of oxide of silver ; because cyanuret of silver is wry similar to the chloride both in appearance, and in severd of its leading properties. The cyanuret, however, is soluble in hot nitric acid, and yields cyanogen gas when heated to redness, by which characters it may be distinguished from chloride of silver. In a medicind point of view the presence of a little hydrochloric acid is not material, and tends probably to give greater stability to the product than it would possess jf absolutely pure. III. A third and very good process is that recommended by Dr. Clark, who mixes solutions of cyanuret of potassium and tartaric acid in such COMPOUNDS OF NITROGEN AND CARBON. 269 proportions as to form bitartratc of potassa and hydrocyanic acid. In 100 drachms of water are dissolved 8! of cyanuret of potassium, and with this is mixed a solution of 18| drachms of crystallized tartaric acid in 20 of water; and after the sparingly soluble bitartratc of potassa has subsided the clear solution is decanted for use. Its only impurity is a little bitartrate the presence of which in a medicinal point of view is not objectionable. The oxygen and hydrogen, which unite with the elements of the cyanuret of potassium, are derived from decomposed water. Mr. Laming adds to the preceding materials a portion of alcohol, which tends to promote the separation of bitartrate of potassa, and perhaps to preserve the hydrocyanic acid from change. IV. In a fourth process 20 drachms of pulverized ferrocyanuret of potas- sium are dissolved in 80 of water; and with this solution is mixed a dilute acid made previously with 18 drachms (by weight) of strong sulphuric acid and 40 drachms of water, and allowed to cool. The dilute hydrocyanic acid is then obtained by distillation, gentle ebullition being continued until the materials become rather thick, and strong succussions commence. If the amount distilled over does not weigh 120 drachms, the requisite quan- tity of distilled water should be added to give that weight. The foregoing proportions are recommended by Giese, who states that the acid so pre- pared may be kept for years. Dr. Christison tells me he has had some un- changed for 2£ years, though exposed to diffused daylight; and that it con- tains a small quantity of sulphuric acid, which is probably the preservative agent. The process ought to be conducted in a capacious matrass instead of a retort; as when the last is used, a little Prussian blue is apt to pass over. The production of hydrocyanic acid in the foregoing process is due to cyanogen uniting with the hydrogen, and potassium uniting with the oxygen of water, a change which is determined by the affinity of sulphuric acid for potassa; but besides hydrocyanic acid and sulphate of potassa, the colouring principle called Prussian blue is generated. As the quality of dilute hydrocyanic acid, however prepared, is apt to vary, owing to the volatility of the acid, and its tendency to spontaneous decomposition, it should be made only in small quantities at a time, kept in well-stopped bottles, and excluded from light. One of the best modes of estimating the strength of any solution, as Orfila suggests, is by precipita- tion with nitrate of oxide of silver. The cyanuret, after being washed and moderately dried, consists of 108 parts of silver and 26.39 of cyanogen, being an equivalent of each; and, therefore, 100 parts of the dry cyanuret correspond to 20.9 of real hydrocyanic acid. M. Pelouze has noticed that when strong hydrocyanic and hydrochloric acids are mixed together, the whole shortly becomes a solid saline mass, from which a gentle heat expels formic acid, while hydrochlorate of ammo- nia remains behind. The hydrocyanic acid, by the aid of water, is entirely resolved into ammonia and formic acid; and hydrochloric acid appears to determine the change by its affinity for ammonia. Thus 1 cq. hydrocyanic acid and 3 eq water 2 1 pq. ammonia and 1 eq. formic acid. H+(N+2C) 3(H+0) -a 3H+N 2(C + 0)+(H+0). Sulphuric acid acts on hydrocyanic acid in the same manner as the hydro. chloric; but it is apt to decompose the formic acid. It was also observed that when cyanuret of potassium is boiled in pure water, hydrocyanic acid disappears, ammonia is disengaged, and formate of potassa is left in solu- tion : thus are the same changes determined by the affinity of potassa for formic acid, as by that of hydrochloric acid for ammonia. (An. de Ch. et de Ph. xlviii. 395.) Tlio presence of free hydrocyanic acid is easily recognized by its odour. Chemically it may be detected by agitating the fluid supposed to contain it with peroxide of mercury in fine powder. Double decomposition ensues, by which water and bicyanuret of mercury are generated ; and on evaporating the solution slowly, the latter is obtained in the form of crystals. 23* 270 COMPOUNDS OF NITROGEN AND CARBON. A test of far greater delicacy, originally noticed by Scheele, is the follow- ing. To the liquid supposed to contain hydrocyanic acid, add a solution of green vitriol, throw down the protoxide of iron by a slight excess of pure potassa, and acidulate with hydrochloric or sulphuric acid, so as to redissolve the precipitate. Prussian blue will then make its appearance, if hydrocyanic acid had been originally present. The nature of the chemical change will be explained in the section on the haloid salts when describing the manufac- ture of Prussian blue. The presence of protoxide of iron is essential. As hydrocyanic acid is sometimes administered with criminal designs, the chemist may be called on to search for its presence after death. The subject has been investigated experimentally by Leuret and Lassaigne, and the process they have recommended is the following. The stomach or other substances to be examined are cut into small fragments, and introduced into a retort along with water, the liquid being slightly acidulated with sul- phuric acid. The distillation is then conducted by the heat of boiling water, the volatile products are collected in a receiver surrounded with ice, and the presence of hydrocyanic acid in the distilled matter is tested by the method above mentioned. These gentlemen found that hydrocyanic acid may be thus detected two or three days after death, but not after a longer period. In the case of a young man poisoned during the winter season by a strong dose of the poison, I detected it four days after death ; but the quantity ob- tained was very minute, though the odour of hydrocyanic acid emitted by the contents of the stomach was considerable. The disappearance of the acid appears owing partly to its volatility, and partly to the facility with which it undergoes spontaneous decomposition. (Journal de Chimie Medi- cale, ii. 561.) Cyanic Acid.—There are two compounds entitled to this appellation, one of which was discovered by Wohler and the other by Liebig ; but to prevent confusion I shall apply to the former the term cyanic acid, and that of ful- minic acid to the latter. It was stated by Gay-Lussac, in the essay already quoted, that cyanogen gas is freely absorbed by pure alkaline solutions; and he expressed his opinion that the alkali combines directly with the cyanogen. It appears, however, from the experiments of Wohler, that cyanic acid and a cyanuret are formed under these circumstances; and, consequently, that alkaline so- lutions act upon cyanogen in the same manner as on chlorine, iodine, bro- mine, and sulphur. But the salts of cyanic acid cannot conveniently be pro- cured in this way, owing to the difficulty of separating the cyanate from the cyanuret with which it is accompanied. Wohler finds that cyanate of po- tassa may be procured in large quantity by mixing ferrocyanuret of potas- sium with an equal weight of peroxide of manganese in fine powder, and exposing the mixture to a low red heat. The cyanogen and potassium of that salt receive oxygen from the manganese, and are converted into cya- nate of potassa. The ignited mass is then boiled in alcohol of 86 per cent; and as the solution cools, the cyanate is deposited in small tabular crystals resembling chlorate of potassa. The only precaution necessary in this pro- cess is to avoid too high a temperature. Cyanic acid is characterized by the facility with which it is resolved by water into carbonic acid and ammonia. This change is effected merely by boiling an aqueous solution of cyanate of potassa; and it takes place still more rapidly when an attempt is made to decompose the cyanate by means of another acid. If the acid is diluted, cyanic acid is instantly decomposed, and carbonic acid escapes with effervescence. But, on the contrary, if a con- centrated acid is employed, then the cyanic acid resists decomposition for a short time, and emits a strong odour of vinegar. Wohler and Liebig have succeeded in obtaining it in a very concentrated and pure state by placing cyanurlc acid, previously deprived of its water of crystallization by a tem- perature of 212°, in a small glass retort, and applying a heat gradually in- creasing to low redness. The vapours are collected in a receiver surrounded by a freezing mixture of salt and ice, and are thereby condensed into a lim, pid colourless liquid, which was at first supposed to be anhydrous cyanic COMPOUNDS OF NITROGEN AND CARBON. 271 acid, but which turns out to be cyanic acid with one equivalent of water. This liquid has a penetrating odour similar to the strongest acetic acid, is extremely pungent, produces a flow of tears, and when applied in minute quantity to the skin instantly causes a white vesicle to be formed, attended with severe pain. It is very volatile, giving off a vapour which is inflamma- ble and has a strong acid reaction. When this vapour is conducted into ice- cold water, it is freely dissolved without decomposition; but as soon as the temperature rises a little, carbonic acid escapes with effervescence, and the acid is entirely destroyed. When the pure hydrous acid, removed from the freezing mixture, has acquired the temperature of the air, it speedily begins to grow turbid, its temperature rises, and the liquid enters into violent ebul- lition from the heat vaporizing the acid. During this action no gas what- ever is evolved; but a white insoluble suhstance is rapidly produced, and in the course of a few minutes the whole liquid acid entirely disappears, and a dry compact matter of brilliant whiteness occupies its place. This solid will hereafter be dluded to under the name of insoluble cyanuric acid, when it will be found to contain precisely the same elements as the hydrous cyanic acid. The composition of cyanic acid, as stated at page 266, has been fully established by Wohler and Liebig. (An. de Ch. et de Ph. xx. and x.wii.) It will be understood, from the ratio of its elements and their known affinities, why a solution of cyanic acid in water is readily convertible into bicarbo- nate of ammonia; for one equivalent of cyanic acid with three equivalents of water contain exactly the same elements as one equivalent of bicarbonate. of ammonia: Thus 1 eq. cyanic acid and 3 eq. water. 2 2 eq. carbonic acid and 1 eq. ammonia. (X+2C)+0 3(H+0) -2 2(C+20) 3H+N. A great part of the carbonic acid is disengaged in a free state, because the bicarbonate formed by one portion of cyanic acid is decomposed by another; so that cyanate of ammonia (urea) is instantly generated, and remains in solution. It is dso obvious that pure hydrous cyanic acid cannot undergo the same change as its aqueous solution. Cyanic acid forms a soluble salt with baryta, but insoluble ones with the oxides of lead, mercury, and silver. A pure soluble cyanate, as that of po- tassa for instance, gives a white precipitate with nitrate of oxide of silver ; and the cyanate of that oxide is soluble without residue in dilute nitric acid. The action of cyanic acid on ammonia is peculiarly interesting. When dry ammoniacal gas is mixed with the vapour of hydrous cyanic acid, they form a white crystalline salt, which is a cyanate of ammonia with excess of alkali, probably a dicyanate, and one equivalent of water. This salt dis- solves in water, and possesses all the characters of a cyanate of ammonia: potassa displaces ammonia, and an acid expels cyanic acid. But on apply- ing a gentle heat to the dry salt, or evaporating its aqueous solution, am- monia escapes, and a substance remains composed of the same elements as cyanic acid and ammonia, but which does not evolve cyanic acid or am- monia when an acid or dkali is added; and in wliich all the properties of the animal principle urea may be recognized. The possibility of such a change is readily understood by comparing the composition of urea, and that of cyanate of ammonia with one equivalent ofwater.—Thus Carbon 12.24 2 eq.1 I Cyanic acid 34.39 leq. (N + 2C) + 0. Nitrogen 28.3 2 eq. [ § | Ammonia 17.15 1 eq. 3 H+N. Hydrogen 4 4 cq. [ £ Water 9 leq. II+ 0. Oxygen 16 2 eq. J ---- ---- 60.54 60.54 " The hydrated neutral cyanate of ammonia has exactly the same ingre- dients and the same equivalent as urea; and whenever the elements for pro- ducing the former come into contact, they invariably constitute the latter. This production of urea ensues so readily, that mere exposure of the dry dicyanate to the air, or the spontaneous evaporation of its aqueous solution, 272 COMPOUNDS OF NITROGEN AND CARBON. suffices for the change. These phenomena illustrate in a most instructive manner the subject of isomerism, and prove how possible it may be, by a difference in arrangement, to produce two different compounds with the same materials. (An. de Ch. et de Ph. xlvi. 25.) The existence of cyanic acid was suspected by Vauquelin before it was actually discovered by Wohler. The experiments of the former chemist led him to the opinion that a solution of cyanogen in water is gradually converted into hydrocyanic, cyanic, and carbonic acids, and ammonia ; and he supposed alkalies to produce a similar change. He did not establish the fact, however, in a satisfactory manner. (An. de Ch. et de Ph. vol. ix.) Fulminic Acid.—A powerfully detonating compound of mercury was described in the Philosophical Transactions for 1800 by Mr. E. Howard. It is prepared by dissolving 100 grains of mercury in a measured ounce and a half of nitric acid of specific gravity 1.3; and adding, when the solu- tion has become cold, two ounces by measure of alcohol, the density of which is 0.849. The mixture is then heated till moderately brisk efferves- cence takes place, during which the fulminating compound is generated. A similar substance may be made by treating silver in the same manner. The conditions necessary for forming these compounds are, that the silver or mercury be dissolved in a fluid which contains so much free nitric acid and alcohol, that on the application of heat nitric ether shall be freely dis- engaged. Fulminating silver and mercury bear the heat of 212° or even 260° with- out detonating; but a higher temperature, or slight percussion between two hard bodies, causes them to explode with violence. The nature of these compounds was discovered in 1823 by Liebig,* who demonstrated that they are salts composed of a peculiar acid, which he termed fulminic acid, in combination with oxide of mercury or silver. According to an analysis of fulminating silver made by Liebig and Gay-Lussac,t the acid of the salt is composed of 26.39 parts or one equivalent of cyanogen, -and 3 parts or one equivalent of oxygen. It is hence inferred to be a real cyanic acid, identi- cal in composition with the cyanic acid of Wohler, though essentially dif- ferent in its properties. The composition of this acid is differently stated by Mr. Davy of Dublin; but the known care and skill of Gay-Lussac and Liebig in such researches justify all confidence in their result. It is remarkable that the oxide of silver cannot be entirely separated from fulminic acid by means of an alkali. On digesting fulminate of that oxide in potassa, one equivalent of oxide of silver is separated, and a double ful- minate is formed, which consists of two equivalents of fulminic acid, one of oxide of silver, and one of potassa. Similar compounds may be procured by substituting other alkaline substances, such ns baryta, lime, or magnesia, for the potassa. These double fulminates are capable of crystallizing; and they all possess detonating properties. From the presence of oxide of silver in the double fulminates, it was at first imagined that this oxide actually constitutes a part of the acid; but since several other substances, such as oxide of mercury, zinc, and copper, may be substituted for that of silver, this view can no longer be admitted. Wc are indebted to Mr. Davy for a method of obtaining fulminic acid in a separate state. Into a bottle with a ground stopper are inserted one part of fulminating mercury, two parts of clean zinc filings, and about 18 parts of water; and the materials, kept at a temperature of 80°, are occasionally agitated, in order that metallic mercury should be thrown down and fulmi- nate of oxide of zinc be generated. This salt is then decomposed by a solution of pure baryta, and the resulting fulminate of baryta decomposed by a quantity of dilute sulphuric acid just sufficient to neutralize the baryta. The fulminic acid thus obtained is a colourless, transparent liquid, of a pungent odour, somewhat like strong prussic acid, and has a sweet taste followed by a particular astringency and disagreeable impression on the * An. de Ch. et de Ph. vol. xxiv. + Ibid. xxv. COMPOUNDS OF NITROGEN AND CARBON. 273 palate. It is volatile, poisonous to animals, and has an acid reaction. After a few hours it acquires a yellow tint, and undergoes partial decomposition; but the nature of the change, and the effects of heat and chemical agents in general on fulminic acid are not yet understood. Cyanuric Acid.—This compound was originally prepared by gently boil- ing the so-called bichloride of cyanogen in water. On the supposition of its discoverer Serullas, that it consists solely of cyanogen and oxygen, and that his bichloride of cyanogen has the composition implied by its name, the origin of cyanuric acid would be referable to the mutual decomposition of water and the bichloride; but since Wohler and Liebig have proved cyanuric acid to contain hydrogen as well as oxygen and cyanogen, its production by this method ceases to be intelligible, and there is consequent- ly a well-grounded suspicion of the bichloride of cyanogen having a differ- ent constitution from that stated by Serullas. (An. de Ch. et de Ph. xxxviii. 379, and xlvi. 25.) However this may be, the preceding process gives rise to cyanuric and hydrochloric acids; and on evaporating the solution, the latter is expelled in vapour, and the former deposited on cooling in oblique rhomboidal prisms. The crystals are further purified by a second solution and evaporation. As thus obtained, cyanuric acid is chemically united with two equivalents of water. The recent crystals are colourless and transparent, but become opaque by exposure to the dr, and if heated to 212°, their water of crystal- lization is wholly expelled. In cold water they are very sparingly soluble; but they are dissolved by this menstruum, as also by sulphuric, nitric, and hydrochloric acid, with the aid of heat. They have little taste, redden litmus paper, and are rather lighter than sulphuric acid. One of the most remarkable characters of the acid is its permanence. For instance, it may be boiled in strong nitric or sulphuric acid without decomposition; and by evaporating its solution in the former, it is obtained in a very white and pure state. It is volatile at a lower temperature than boiling mercury, and condenses, unchanged, in the form of acicular crystals. When heated with potassium, it is decomposed, yielding potassa and cyanuret of potassium. With metallic oxides it forms permanent salts, which do not detonate. Anhydrous cyanuric acid, first noticed by Wohler, is obtained by cooling from a hot concentrated solution of the crystals in sulphuric or hydrochloric acid. The figure of its crystals, when they are regularly formed, is that of an octohedron with a square base. When the anhydrous acid is sharply heated, part of it sublimes without change; but part is decomposed, and hydrous cyanic acid, as dready mentioned, is formed in considerable quantity. Liebig and Wohler have remarked, that the substance called pyro-uric acid, which sublimes when uric acid is decomposed by heat, is cyanuric acid. This compound is dso formed, according to Liebig, by transmitting chlorine gas through water in which cyanate of oxide of silver is suspend- ed; chloride of silver, carbonic acid, and ammonia being generated at the same time. Liebig" also states, that on heating dry uric acid in dry chlo- rine gas, a large quantity of cyanic and hydrochloric acids is generated. He adds, further, that cyanate of potassa, when heated in strong acetic acid, is converted into cyanurate of potassa. (An. de Ch. et de Ph. xli. 225, and xliii. 64) Cyanuric acid may also be obtained from urea, a mode of preparation which has lately been illustrated by the joint labours of Wohler and Liebig. When pure urea is put into a retort, and heated by a gradually increasing temperature to about 600°, it is entirely resolved into ammonia and anhy- drous cyanuric acid, the former of which escapes, while the latter, if the heat be carefully managed, is left entirely in the retort. In order to sepa- rate some ammonia which is always retained, it is dissolved in hot concen- trated sulphurie acid, and nitric acid is added drop by drop until it occasions no further effervescence, and the solution becomes colourless. After cooling it is mixed with cold water, which throws down the cyanuric acid in the form of a crystalline powder of brilliant whiteness. It may dso be puri- 274 COMPOUNDS OF NITROGEN AND CARBON. fied by transmitting a current of chlorine through water in which the im- pure acid is suspended. Very large crystals may be obtained by forming a saturated solution of the pure acid in boiling water, evaporating it to one- half of its volume on a sand-bath heated to about 150°, and allowing it to cool gradually on the sand-bath itself. The analysis of cyanuric acid by Wohler and Liebig accounts in an degant manner for its production from urea. They have established the singular fact that cyanuric acid contains the same elements, in the very same proportion, as hydrous cyanic acid. Hence the formulae of page 271, connecting the composition of urea and hydrous cyanic acid, will also serve to compare the constitution of urea and cyanuric acid: the elements of urea may be disposed into such order as to constitute either hydrated cyanate, or anhydrous cyanurate, of ammonia, though in reality they have, doubtless, a very different arrangement. The elements of water, contained in hydrous cyanic acid as water, and separable from that acid, appear to exist in cyanuric acid in a wholly different state, and to be essential to the existence of cyanuric acid. Besides, one equivalent of cyanuric acid (page 266), namely, 3Cy, 60, and 3H, contains the same elements as three equiva- lents of hydrous cyanic acid; and hence three eq. of urea are capable of supplying one eq. of cyanuric acid and three eq. of ammonia. Another interesting compound described in the important essay of Woh- ler and Liebig, is that which they have termed insoluble cyanuric acid, a compound entirely different in its chemical properties from the preceding soluble acid, and yet identical with it in the nature and ratio of its elements, and in having the same equivalent. This is the substance already mention- ed as the sole product of the spontaneous decomposition of hydrous cyanic acid, three eq. of which yield one eq. of insoluble cyanuric acid, without loss or gain of a single element. This acid, as isomeric with cyanuric acid, may appropriately be called paracyanuric acid. The preceding history sufficiently explains the origin of the name ap- plied to these acids. The term cyan-uric, is suggested by the close relation of these acids to urea and uric acid, added to the circumstance of cyanogen being an essential part of their composition. Chloride of Cyanogen.—The existence of this compound was first noticed by Berthollet, who named it oxyprussic acid, on the supposition of its con- taining prussic acid and oxygen; and it was afterwards described by Gay- Lussac, in his essay on cyanogen, under the appellation of chlorocyanic acid. It was procured by this chemist by transmitting chlorine gas into an aqueous solution of hydrocyanic acid until the liquid acquired bleaching properties, removing the excess of chlorine by agitation with mercury, and then heating the mixture, so as to expel the gaseous chloride of cyanogen. The chemical changes which take place during this process are complicated. At first the elements of hydrocyanic acid unite with separate portions of chlorine, and give rise to hydrochloric acid and chloride of cyanogen ; and when heat is applied, the elements of the chloride and water react on each other, in consequence of which hydrochloric acid, ammonia, and carbonic add are generated. Owing to this circumstance the chloride of cyanogen was always mixed with carbonic acid, and its properties imperfectly under- stood. During the year 1829 Serullas succeeded in procuring this compound in a pure state, by exposing bicyanuret of mercury, in powder and moistened with water, to the action of chlorine gas contained in a well-stopped phial. The vessel is kept in a dark place ; and after ten or twelve hours the colour of the chlorine is no longer perceptible, bichloride of mercury is found at the bottom of the phial, and its space is filled with the vapour of chloride of cyanogen. The bottle is then cooled down to zero by freezing mixtures of snow and salt, at which temperature chloride of cyanogen is solid. Some chloride of calcium is then introduced, the stopper replaced, and the bottle kept in a moderately warm situation, in order that the moisture within may be completely absorbed. The chloride of cyanogen is then again solidified by cold, the phial completely filled with dry and cold mercury, and a bent COMPOUNDS 0> NITROGEN AND CARBON. 275 tube adapted to its aperture by means of a cork. The solid chloride, which remains adhering to the inner surface of the phial, is converted into gas by gentle heat, and passing along the tube, is collected over mercury. Expo- sure to the direct solar rays interferes with the success of this process: hy- drochlorate of ammonia, together with a little carbonic acid, is then gene- rated, and a yellow liquid collects, which appears to be a mixture of chlo- ride of carbon and chloride of nitrogen. (An. de Ch. et de Ph. xxxv. 291.) Chloride of cyanogen is solid at 0°, and in congealing crystallizes in very long slender needles. At temperatures between 5° and 10.5° it is liquid, and also at G8° under a pressure of four atmospheres; but at the common pressure, and when the thermometer is above 10.5° or 11°, it is a colourless gas. In the liquid state it is as limpid and colourless as water. It has a very offensive odour, irritates the eyes, is corrosive to the skin, and highly inju- rious to animal life. Chloride of cyanogen is very soluble in water and alcohol. The former under the common pressure, and at 68° dissolves twenty-five times its vo- lume. Alcohol takes up 100 times its volume, and the absorption is effected almost with the same velocity as that of ammoniacal gas by water. These solutions are quite neutral with respect to litmus and turmeric paper, arid may be kept without apparent change. The gas may even be separated without decomposition by boiling. The chloride of cyanogen, accordingly, does not possess the characters of an acid. The changes induced by the action of alkalies do not appear to be very clearly understood. Serullas agrees with Gay-Lussac in stating, that if to a solution of chloride of cyanogen a pure alkali is added, and then an acid, effervescence ensues from the escape of carbonic acid gas. Ammonia, and probably hydrochloric and hydrocyanic acid, are also generated. According to Gay-Lussac and Serullas, 100 measures of the vapour of chloride of cyanogen contain 50 measures of cyanogen and 50 of chlorine gases. The density of a vapour so constituted should be 2.1428 and its ele- ments have the ratio assigned at page 266. Bichloride of Cyanogen.—This compound, which is said to contain twice as much chlorine as the preceding, was prepared by Serullas by the action of dry chlorine on anhydrous hydrocyanic acid, hydrochloric acid being ge- nerated at the same time. It is solid at common temperatures, and oc- curs in white acicular crystals. At 284° it fuses, and enters into ebullition at 374°. Its vapour is acrid and excites a flow of tears, and it is very de- structive to animals. Its odour somewhat resembles that of chlorine, and is very similar to that of mice. It is very soluble in alcohol and ether, and is precipitated from them by water, which dissolves it in small quantity. When boiled in water, or solution of potassa, it is converted into hydrochlo- ric and cyanuric acids. (An. de Ch. et de Ph. xxxviii. 370.) Recent re- searches throw great doubt on the composition of this substance as stated by Serullas. (Page 273.) Iodide of Cyanogen.—This compound was discovered by Daw in 1816. (Quarterly Journal of Science, i. 289.) and since studied by Serullas, who recommended for its preparation the following process. (An. de Ch. et de Ph. xxvii.) Two parts of bicyanuret of mercury and one of iodine are inti- mately and quickly mixed in a glass mortar, and the mixture is introduced into a phial with a wide mouth. On applying heat the violet vapours of iodine appear; but as soon as the bicyanuret of mercury begins to be de- composed, the vapour of iodine is succeeded by white fumes, which, if re- ceived in a cool glass receiver, condense upon its sides into flocks like cot- ton wool. The action is found to be promoted by the presence of a little water. Iodide of cyanogen, when slowly condensed, occurs in very long and ex- ceedingly slender needles, of a white colour. It has a very caustic taste and penetrating odour, and excites a flow of tears. It sinks rapidly in sulphuric acid. It is very volatile and sustains a temperature much higher than 212° without decomposition, but it is decomposed by a red heat. It dissolves in water and alcohol, and forms solutions which do not redden litmus paper. 276 COMPOUNDS OF NITROGEN AND CARBON. Alkalies act upon it in the same manner as on chloride of cyanogen, a com- pound to which it is very analogous. Sulphurous acid, when water is present, has a very powerful action on iodide of cyanogen. On adding a few drops of this acid, iodine is set free, and hydrocyanic acid produced ; but when more of the sulphurous acid is employed, the iodine disappears, and the solution is found to contain hy- driodic acid. These changes are of course accompanied with formation of sulphuric acid, and decomposition ofwater. Iodide of cyanogen has not been analyzed with accuracy; but Serullas in- fers from an approximate analysis, that it is composed of one equivalent of iodine and one of cyanogen. Bromide of Cyanogen.—This substance was prepared by Liebig by a pro- cess similar to that described for procuring iodide of cyanogen. At the bot- tom of a small tubulated retort, or a rather long tube, is placed some bicy- anuret of mercury slightly moistened, and after cooling the apparatus by cold water, or still better by a freezing mixture, a precaution which is indis- pensable in summer, half its weight of bromine is introduced. Strong reac- tion instantly ensues, and heal is so freely evolved, that a considerable quan- tity of the bromine would be dissipated, unless the temperature of the retort had been previously reduced. The new products are bromide of mercury and bromide of cyanogen, the latter of which collects in the upper part of the tube in the form of long needles. After allowing any vapour of bromine, which may have risen at the same time, to condense and fall back upon the bicyanuret of mercury, the bromide of cyanogen is expelled by a gentle heat, and collected in a recipient carefully cooled. As thus formed, the bromide is crystallized, sometimes in small regular colourless and transparent cubes, and sometimes in long and very slender needles. In its physical properties it is so very similar to iodide of cyanogen, that they may easily be mistaken for each other, especially when the crys- tals of the bromide possess the acicular form. They agree closely in odour and volatility, but the bromide is even more volatile than the iodide of cya- nogen. It is converted into vapour at 59°, and crystallizes suddenly on cool- ing. Its solubility in water and alcohol is likewise greater than that of iodide of cyanogen. By a solution of caustic potassa it is converted into cyanate of potassa and bromide of potassium. Bromide of cyanogen is highly deleterious. A grain of it dissolved in a little water, and introduced into the oesophagus of a rabbit, proved fatal on the instant, acting with the same rapidity as hydrocyanic acid. In conse- quence of the volatility and noxious qualities of the substance, experiments with it should be conducted with great circumspection. The danger from this cause, together with a deficient supply of bromine, prevented Serullas from continuing the investigation of its properties. (Edin. Journd of Science, vii. 189.) Hydrosulphocyanic Acid.—This acid was discovered in 1808 by Mr. Por- rett, who ascertained that it is a compound of sulphur, carbon, hydrogen, and nitrogen, and described it under the name of sulphuretted chyazic acid, the term chyazic being composed of the initials of carbon, hydrogen, and azote. It is now generally called sulphocyanic or hydrosulphocyanic acid. It may be procured by distilling a strong solution of sulphocyanuret of potassium with phosphoric acid; when hydrosulphocyanic acid passes over into the recipient, and phosphate of potassa remains in the retort: sulphuric acid may be substituted for the phosphoric, but in that case sulphurous acid is generated. Another process is to suspend recently formed sulphocyanuret of silver or mercury in water, and transmit through it a current of hydro- sulphuric acid gas; when, by an interchange of elements, sulphuret of silver or mercury and hydrosulphocyanic acid are produced. The solution is fil- tered, and the excess of gas expelled by a gentle heat, or a small quantity of sulphocyanuret of silver. (Berzelius.) The solution of hydrosulphocyanic acid is either colourless or has a shade of pink, and its odour somewhat resembles that of vinegar. The strongest COMPOUNDS OF NITROGEN AND CARBON. 277 solution of it which Mr. Porrett could obtain had a sp. gravity of 1 022. At 216.5° it boils, and at 54.5° it crystallizes in six-sided prisms. Though it contains the elements of hydrocyanic acid, it is quite distinct from it in its characters; it is much less poisonous, is distinctly acid to the taste and test paper, and neutralizes alkalies. Like most of the hydracids, it refuses to unite with many of the metallic oxides, because its radical unites by preference with the metal, and its hydrogen with the oxygen of the oxides. Hydrosulphocyanic acid is easily detected by a salt of the peroxide of iron, with which it gives a deep blood-red solution. With a salt of copper it yields a white precipitate, which is a sulphocyanuret of that metd. It is gradually deprived of its hydrogen by exposure to the atmosphere, and the same change is rapidly effected by chlorine or nitric acid, bisulphuret of cyanogen being precipitated. The composition of this hydracid, as given at page 266, is on the autho- rity of Porrett and Berzelius. (An. of Phil, xiii., and An. de Ch. et de Ph. xvi.) Bisulphuret of Cyanogen.—This compound is the base or radical of hy- drosulphocyanic acid, and was obtdned in a separate state by Liebig. (An. de Ch. et de Ph. xli. 187.) It was prepared by exposing fused sulphocyanu- ret of potassium to a current of dry chlorine gas. Reaction readily en- sued, and at first chloride of sulphur and bichloride of cyanogen distilled over; but at length a red vapour appeared, which collected as a red or orange-coloured substance in the upper part of the tube. In this state it contdned some free sulphur, which was in a great measure removed by heating it in dry chlorine gas; when it acquired an orange tint, and in powder was yellow. It had then so nearly the constitution of bisulphuret of cyanogen, that there can be little doubt of its being such. When heated with potassium, the action is exceedingly violent, and three compounds, sd- phocyanuret, sulphuret, and cyanuret of potassium are generated. If a solution of hydrosulphocyamc acid is exposed to the air, a yellow matter gradudly collects, which Wohler conceived to be a compound of sul- phur and hydrosulphocyanic acid, but which Liebig considers bisulphuret of cyanogen. It is formed freely by boiling sulphocyanuret of potassium with dilute nitric acid, the best proportions being 1 part of the salt, 3 of water, and 2 or 2.5 of nitric acid; for if the nitric acid be too strong or in too great excess, the yellow compound will not be formed. It is also gene- rated by the action of chlorine on a strong solution of the salt. In fact, the oxygen of the air, nitric acid, and chlorine, act upon sulphocyanic acid in the same manner as on hydriodic and hydrosulphuric acids. The yellow matter retains water with obstinacy. Cyano-hydrosulphuric Acid.—When hydrosulphuric acid gas is trans- mitted into a saturated solution of cyanogen in dcohoL the liquid acquires a reddish-brown tint, and in a short time numerous small crystals of an orange-red colour are generated. At page 266 I have represented this acid as a compound of cyanogen with hydrosulphuric acid; but Wohler and Liebig, who analyzed it, consider it rather as a hydracid differing from the hydrosulphocyanic, only in containing two equivalents of hydrogen instead of one. The red crystals consist of three eq. of the acid and one eq. of water, and dissolve in dcohol and water at a boiling temperature, but are sparingly soluble in both liquids when cold. The acid appears to unite directly with some metallic oxides; but with others a sulphocyanuret is formed. Frequently, as with acetate of oxide of lead, a sulphuret and sul- phocyanuret of the metal are formed at the same time. (An. de Ch. et de Ph. xlix. 21.) Sulphuret of Cyanogen.—Another sulphuret of cyanogen, different from that just described, was discovered in 1628 by M. Lassaigne. It was pre- pared by the action of bicyanuret of mercury in fine powder with half its weight of bichloride of sulphur, confined in a small glass globe, and ex- posed for two or three weeks to day-light. A small quantity of crystals, 278 COMPOUNDS OF SULPHUR WITH CARBON, &C. biting to the tongue and of a penetrating odour, collected in the upper part of the vessel, which formed red-coloured compounds with persalts of iron. Its constitution has not been accurately determined; and the attempts of Liebig to prepare it were unsuccessful. (An. de Ch. et de Ph. xxxix.) Hydroseleniocyanic Acid.—The radical of this acid was obtained by Berzelius in combination with potassium, but he could not obtain the acid in a separate state. It may be regarded as a hydracid, of which seleniuret of cyanogen is the radical. SECTION VII. COMPOUNDS OF SULPHUR WITH CARBON, &c. The compounds described in this section are thus constituted: Equiv. Formula. Bisulph. of carbon Carb. 6.12 + Sulp. 32.2=38.32. C + 2S or CSs. Sulph. of phosphorus. Composition uncertain. Bisulph. of selenium Selcn. 39.6 + Sulp. 32.2=71.8. Se + 2S or ScS*. Selen. of phosphorus. Composition uncertain. Bisulphuret of Carbon.—This substance was discovered accidentally in the year 1796 by Professor Lampadius, who regarded it as a compound of sulphur and hydrogen, and termed it alcohol of sulphur. Clement and De- sormes first declared it to be a sulphuret of carbon, and their statement was fully confirmed by the joint researches of Berzelius and the late Dr. Marcet. (Phil. Trans. 1813.) Bisulphuret of carbon may be obtained by heating in close vessels native bisulphuret of iron (iron pyrites) with one-fifth of its weight of well-dried charcoal; or by transmitting the vapour of sulphur over fragments of char- coal heated to redness in a tube of porcelain. The compound, as it is form- ed, should be conducted by means of a glass tube into cold water, at the bottom of which it is collected. To free it from moisture and adhering sulphur, it should be distilled at a low temperature in contact with chloride of calcium. Bisulphuret of carbon is a transparent colourless liquid, which is remark- able for its high refractive power. Its specific gravity is 1.272. It has an acid, pungent, and somewhat aromatic taste, and a very fetid odour. It is exceedingly volatile;—its vapour at 63.5° supports a column of mercury 7.36 inches long; and at 110° it enters into brisk ebullition. From its great volatility it may be employed for producing intense cold. Bisulphuret of carbon is very inflammable, and kindles in the open air at a temperature scarcely exceeding that at which mercury boils. It burns with a pale blue flame. Admitted into a vessel of oxygen gas, so much va- pour rises as to form an explosive mixture; and when mixed in like manner with binoxide of nitrogen, it forms a combustible mixture, which is kindled on the approach of a lighted taper, and burns rapidly, with a large greenish- white flame of dazzling brilliancy. It dissolves readily in alcohol and ether, and is precipitated from the solution by water. It dissolves sulphur, phos- phorus, and iodine, and the solution of the latter has a beautiful pink colour. Chlorine decomposes it, wilh formation of chloride of sulphur. The pure acids have little action upon it By nitro-hydrochloric acid it is changed into a white crystalline substance like camphor, which Berzelius regards as a compound of the hydrochloric, carbonic, and sulphurous acids. Bisulphuret of carbon is a sulphur acid, that is, unites with sulphur-bases to constitute compounds analogous to ordinary salts, and hence called sul- phur-salts. Thus bisulphuret of carbon unites with sulphuret of potassium, forming a sulphur-salt, in which the former acts as an acid and the latter as a base. The same compound is formed by the action of bisulphuret of carbon on a solution of pure potassa; but in this case sulphuret of potas- GENERAL PROPERTIES OF METALS. 279 sium is first generated by an interchange of elements with a portion of bisul- phuret of carbon, carbonic acid being produced at the samo time.—Thus 2 eq. potassa &. 1 eq. bisulph. carbon 2 2eq. sulphuret potassium & 1 eq. carb. acid. 2 (K + O) C + 2S -a 2(K+S) C+20. If the bisulphuret of carbon is in sufiident quantity, carbonic acid gas is disengaged, and a neutral compound results. Such is inferred to be the nature of the change, agreeably to the researches of Berzelius on the sul- phur-salts. The action of potassa on bisulphuret of carbon has been studied by M. Zeise, Professor of Chemistry at Copenhagen, who inferred the pro- duction of a peculiar hydracid, composed of carbon, sulphur, and hydrogen, wliich he thought was united with potassa; but the more probable explana- tion of the phenomena is that above given. (An. of Phil. xx. 241.) Sulphuret of Phosphorus.—When sulphur and fused phosphorus are brought into contact they unite readily, but in proportions which have not been precisely determined; and they frequently react on each other with such violence as to cause an explosion. For this reason the experiment should be made with a quantity of phosphorus not exceeding 30 or 40 grains. The phosphorus is placed in a glass tube, five or six inches long, and about half an inch wide; and when by a gentle heat it is liquefied, the sulphur is added in successive small portions. Heat is evolved at the mo- ment of combination, and hydrosulphuric and phosphoric acids, owing to the presence of moisture, are generated. This compound may also be made by agitating flowers of sulphur with fused phosphorus under water. The temperature should not exceed 160°; for otherwise hydrosulphuric and phosphoric acids would be evolved so freely as to prove dangerous, or at least to interfere with the success of the process. Sulphuret of phosphorus, from the- nature of its elements, is highly com- bustible. It is much more fusible than phosphorus. A compound made by Mr. Faraday with about 5 parts of sulphur and 7 of phosphorus was quite fluid at 32°, and did not solidify at 20c F. (Quarterly Journal, iv.) Bisulphuret of Selenium.—Sulphur and selenium mix together in all pro- portions by fusion, and, therefore, by such means it is difficult to procure a definite compound; but the bisulphuret of an orange colour was formed by Berzelius by precipitating a solution of selenious acid with hydrosulphuric acid. The sulphuret found by Stromeyer among the volcanic products of the Lipari isles is probably similar in composition. Bisulphuret of selenium fuses at a heat a little above 212D, and at a higher temperature may be sublimed without change. In the open air it takes fire when heated, and sulphurous, selenious, and selenic acids are the products of its combustion. The alkalies and soluble metallic sulphurets dissolve it. Nitric acid acts upon it with difficulty; but the nitro-hydrochloric converts it into sulphuric and selenious acids. (An. of Phil, xiv.) Seleniuret of Phosphorus.—This compound may be prepared in the same manner as the sulphuret of phosphorus; but as selenium is capable of uniting with phosphorus in several proportions, the compound formed by fusing them together can hardly be supposed to be of a definite nature. This seleniuret is very fusible, sublimes without change in close vessels, and is inflammable. It decomposes water gradually when digested in it, giving rise to seleniuretted hydrogen, and one of the acids of phosphorus. METALS. GENERAL PROPERTIES OF METALS. Metals are distinguished from other substances by the following proper- ties. They are all conductors of electricity and heat. When the compounds which they form with oxygen, chlorine, iodine, sulphur, and similar sub- 280 GENERAL PROPERTIES OF METALS. stances, are submitted to the action of galvanism, the metals always appear at the negative side of the battery, and are hence said to be positive elec- trics. They are quite opaque, refusing a passage to light, though reduced to very thin leaves. They are in general good reflectors of light, and pos- sess a peculiar lustre, which is termed the metallic lustre.—Every substance in which these characters reside may be regarded as a metal. The number of metals, the existence of which is admitted by chemists, amounts to forty-two. The following table contains the names of those that have been procured in a state of purity, together with the date at which they were discovered, and the names of the chemists by whom the discovery was made. Table of the Discovery of Metals. Names of Metals. Authors of the Discovery. Dates of the Discovery. Gold . . " Silver . Iron Copper . V ■ Known to the Ancients. Mercury Lead . Tin . Antimony . Described by Basil Valentine . 1490 Bismuth Described by Agricola . 1530 Zinc First mentioned by Paracelsus 16th century Arsenic . ) Cobalt . . { Brandt ..... 1733 Platinum Wood, assay-master, Jamaica . 1741 Nickel . Cronstedt..... 1751 Manganese . Gahn and Scheele .... 1774 Tungsten D'Elhuyart..... 1781 Tellurium . Muller...... 1782 Molybdenum Hiclm...... 1782 Uranium 1789 Titanium 1791 Chromium . Vauquelin ..... 1797 Columbium . Hatchett ..... 1802 Palladium . ? Rhodium . S Wollaston ..... 1803 Iridium Descotils and Smithson Tennant . 1803 Osmium Smithson Tennant .... 1803 Cerium Hisinger and Berzelius . 1804 Potassium . Sodium Barium . > Davy...... 1807 Strontium . Calcium . _, Cadmium Stromeyer..... 1818 Lithium Arfwedson ..... 1818 Silicium . ) Zirconium . $ Berzelius ..... 1824 Aluminium . i Glucinium . > Wohler...... 1828 Yttrium > Thorium Berzelius..... 1829 Magnesium . Bussy..... 18-39 Vanadium . SelstrOm..... 1830 GENERAL PROPERTIES OF METALS. 281 Most of the metals are remarkable for their great specific gravity; some of them, such as gold and platinum, which are the densest bodies known in nature, being- more than 19 times as heavy as an equal bulk of water. Great density was once supposed to be an essential characteristic of metals; but the discovery of potassium and sodium, which are so light as to float on the surface of water, has shown that this supposition is erroneous. Some metals experience an increase of density to a certain extent when hammer- ed, their particles being permanently approximated by the operation. On this account, the density of some of the metds contained in the following. table is represented as varying between two extremes. Table of the Specific Gravity of Metals at 60° F. compared to Water as Unity. Platinum Gold Tungsten Mercury Palladium Lead Silver Bismuth Uranium, Copper Molybdenum Cadmium Cobalt Nickel Manganese Iron Tin Zinc Antimony Tellurium Arsenic Titanium Sodium Potassium beaten into thin plates or leaves by hammering. The malleable metals are gold, silver, copper, tin, platinum, palladium, cadmium, lead, zinc, iron, nickel, potassium, sodium, and frozen mercury. The other metals are either malleable in a very small degree only, or, like antimony, arsenic, and bis- muth, are actually brittle. Gold surpasses all metals in malleability: one grain of it may be extended so as to cover about 52 square inches of surface. and to have a thickness not exceeding 28 210 2Otb of an inch. Nearly all malleable metds may be drawn out into wires, a property which is expressed by the term ductility. The only metals which are remarkable in this respect are gold, silver, platinum, iron, and copper. Wollaston devised a method by which gold wire may be obtained so fine that its diameter shall be only j-Q^Q-Q-th of an inch, and that 550 feet of it are required to weigh one grain. He obtained a platinum wire so small, that its diameter did not exceed 30000th of an inch. (Philos. Trans. 1813.) It is singular that the ductility and malleability of the same metal are not always in proportion to each other. Iron, for example, cannot be made into fine leaves, but it may be drawn into very small wires. The tenacity of metals is measured by ascertaining the greatest weight which a wire of a certain thickness can support without breaking. Accord- ing to the experiments of Gujton-Morveau, whose results are comprised in the following table, iron, in point of tenacity, surpasses all other metals. 24* 20.98 Brisson. 19.257 Do. 17.6 13.5G8 D'Elhuyart. Brisson. 11.3 to 11.8 Wollaston. 11.352 Brisson. 10.474 Do. 9.802 Do. 9.000 Buchholz. 8.895 Hatchett 8.615 to 8.636 . Buchholz. 8.604 8.538 8.279 Stromeyer. Hauy. Richter. 8.013 John. 7.788 Brisson. 7.291 Do. 6.861 to 7.1 Do. 6.702 Do. 6.115 5.8843 . Klaproth. Turner. 5.3 Wollaston. 0.972 i . 0.865 £ . S Gay-Lussac and ) Thenard, perty of malleabili y, that is, admit of 282 GENERAL PROPERTIES OF METALS. The diameter of each wire was 0.787th of a line. Pounds. Iron wire supports . 549.25 Copper......302.278 Platinum.....274.32 Silver ...... 187.137 Gold . . . • • 150.753 Zinc......109.54 Tin......34.63 Lead ...... 27.621 Metals differ also in hardness, but I am not aware that their exact relation to each other, under this point of view, has been determined by experiment. In the list of hard metals may be placed titanium, manganese, iron, nickel, copper, zinc, and palladium. Gold, silver, and platinum, are softer than these ; lead is softer still, and potassium and sodium yield to the pressure of the fingers. The properties of elasticity and sonorousness are allied to that of hardness. Iron and copper are in these respects the most conspicuous. Many of the metals have a distinctly crystalline texture. Iron, for exam- ple, is fibrous ; and zinc, bismuth and antimony are lamellated. Metals are sometimes obtained dso in crystals; and most of them in crystallizing, as- sume the figure of a cube, the regular octohedron, or some form allied to it. Gold, silver, and copper, occur naturally in crystals, while others crystal- lize when they pass gradually from the liquid to the solid condition. Crys- tals are most readily procured from those metals which fuse at a low tem- perature ; and bismuth, from conducting heat less perfectly than other metals, and, therefore, cooling more slowly, is best fitted for the purpose. The process should be conducted in the way dready described for forming crystds of sulphur. (Page 191.) Metals, with the exception of mercury, are solid at common tempera- tures ; but they may all be liquefied by heat. The degree at which they fuse, or their point of fusion, is very different for different metds, as ap- pears from the following table. Table of the Fusibility of different Metals. Fahr. Fusible below a red heat. Infusible below a red heat 'Mercury Potassium .Sodium [Cadmium . about [Tin .... 'Bismuth . Lead iTellurium—rather less fusible than lead. Arsenic—undetermined. Zinc Antimony—a little below a red heat. Silver Copper ,Gold Iron, cast 'Iron, malleable Manganese JCobalt—rather less fu sible than iron. Nickel—nearly the same Palladium —39° 136 190 442 442 497 612 Different chemists. i Gay-Lussac and The- \ nard. Stromeyer. Crichton. Klaproth. 773 Daniell. 1873) 1996 ( Daniell. 2016 ( 2786 ) Requiring the highest heat of a smith's forge. as cobdt. GENERAL PROPERTIES OF METALS. 283 Molybdenum"| Almost infusible, and not") Fusible be- Uranium I to be procured in but- I fore the oxy- , Tungsten [ tons by the heat of a [ hydrogen ^Chromium J smith's forge. J blowpipe. Infusible in the heat of a smith's forge, but fusible before the oxy-hydrogen blow- pipe. Infusible below a 7?itanium 1 j u t < Cerium red heat. >,-, \Osmium /iridium Rhodium Platinum sColumbium __ Metals differ also in volatility. Some are readily volatilized by heat, while others are of so fixed a nature that they may be exposed to the most intense heat of a wind furnace without being dissipated in vapour. There are seven metals, the volatility of which has been ascertained with certainty; namely, cadmium, mercury, arsenic, tellurium, potassium, sodium, and zinc. Metals cannot be resolved into more simple parts; and, therefore, in the present state of chemistry, they must be regarded as elementary bodies. It was formerly conceived that they might be converted into each other ; and this notion led to the vdn attempts of the alchemists to convert the baser metals into gold. The chemist has now learned that his art solely consists in resolving compound bodies into their elements, and causing substances to unite which were previously uncombined. One elementary principle can- not assume the properties peculiar to another. Metds have an extensive range of affinity, and on this account few of them are found in the earth native, that is, in an uncombined form. They commonly occur in combination with other bodies, especially with oxygen and sulphur, in which state they are said to be mineralized. It is a singu- lar fact in the chemical history of the metds, that they are little disposed to combine in the metallic state with compound bodies, such as an oxide or an acid. They unite readily, on the contrary, with elementary substances. Thus, they often combine with each other, yielding compounds termed al- loys, which possess all the characteristic physical properties of pure metals. They unite likewise with the simple substances not metallic, such as oxy- gen, chlorine, and sulphur, giving rise to new bodies in which the metallic character is wholly wanting. In all these combinations, the same tendency to unite in a few definite proportions is equally conspicuous as in that de- partment of the science of which I have just completed the description. The chemicd changes are regulated by the same general laws, and in describing them the same nomenclature is applicable. The order which it is proposed to follow in describing the metals has al- ready been explained in the introduction ; but before treating of each sepa- rately, some general observations may be premised, by which the study of this subject will be much facilitated. Metds are of a combustible nature; that is, they are not only susceptible of slow oxidation, but, under favourable circumstances, they unite rapidly with oxygen, giving rise to all the phenomena of real combustion. Zinc burns with a brilliant flame when heated to full redness in the open air; iron emits vivid scintillations on being inflamed in an atmosphere of oxy- gen gas; and the least oxidable metals, such as gold and platinum, scintil- late in a similar manner when heated by the oxy-hydrogen blowpipe. , The product either of the slow or rapid oxidation of a metal, when heated in the air, has an earthy aspect, and was called a calx by the older chemists, the process of forming it being expressed by the term calcination. Another method of oxidizing metals is by deflagration; that is, by mixing them with nitrate or chlorate of potassa, and projecting the mixture into a red-hot cru- cible. Most metals may be oxidized by digestion in nitric acid; and nitro- hydrochloric acid is an oxidizing agent of still greater power. Some metals unite with oxygen in one proportion only, but most of them 284 GENERAL PROPERTIES OF METALS. have two or three degrees of oxidation. Metals differ remarkably in their relative forces of attraction for oxygen. Potassium and sodium, for example, are oxidized by mere exposure to*the air; and they decompose water at all temperatures, the instant they come in contact with it. Iron and copper may be preserved in dry air without change, nor can they decompose water at common temperatures; but they are both slowly oxidized by exposure to a moist atmosphere, and combine rapidly with oxygen when heated to red- ness in the open air. Iron has a stronger affinity for oxygen than copper; "for the former decomposes water at a red heat, whereas the latter cannot produce that effect. Mercury is less inclined than copper to unite with oxy- gen. Thus it may be exposed without change to the influence of a moist atmosphere. At a temperature of 650° or 700° it is oxidized ; but at a red heat it is reduced to the metallic state, while oxide of copper can sustain the strongest heat of a blast furnace without losing its oxygen. The affinity of gold for oxygen is still weaker than that of mercury; for it will bear the most intense heat of our furnaces without oxidation. Metallic oxides suffer reduction, or may be reduced to the metallic state in several ways: 1. By heat alone. By this method the oxides of gold, silver, mercury, and platinum may be decomposed. 2. By the united agency of heat and combustible matter. Thus, by trans- mitting a current of hydrogen gas over the oxides of copper or iron heated to redness in a tube of porcelain, water is generated, and the metals are ob- tained in a pure form. Carbonaceous matters are likewise used for the pur- pose with great success. Potassa and soda, for example, may be decomposed by exposing them to a white heat after being intimately mixed with char- coal in fine powder. A similar process is employed in metallurgy for ex- tracting metals from their ores, the inflammable materials being wood, charcoal, coke, or coal. In the more delicate operations of the laboratory, charcoal and black flux are preferred. 3. By the galvanic battery. This is a still more powerful agent than the preceding; since some oxides, such as baryta and strontia, which resist the united influence of heat and charcoal, are reduced by the agency of gal- vanism. 4. By the action of deoxidizing agents on metallic solutions. Phosphorous acid, for example, when added to a liquid containing oxide of mercury, de- prives the oxide of its oxygen, metallic mercury subsides, and phosphoric acid is generated. In like manner, one metal may be precipitated by another, provided the affinity of the latter for oxygen exceeds that of the former. Thus, when mercury is .added to a solution of nitrate of the oxide of silver, metallic silver is thrown down, and oxide of mercury is dissolved by the nitric acid. On placing metallic copper in the liquid, pure mercury subsides, and a nitrate of the oxide of copper is formed; and from this solution metal- lic copper may be precipitated by means of iron. Metals, like the simple non-metallic bodies, may give rise to oxides or acids by combining with oxygen. The former are the most frequent pro- ducts. Many metals which are not acidified by oxygen may be formed into oxides; whereas one metal only, arsenic, is capable of forming an acid and not an oxide. All the other metals which are convertible into acids by oxygen, such as chromium, tungsten, and molybdenum, are also suscepti- ble of yielding one or more oxides. In these instances, the acids always contain a larger quantity of oxygen than the oxides of the same metal. Many of the metallic oxides have the property of combining with acids. In some instances all the oxides of a metal are capable of forming salts with acids, as is exemplified by the oxides of iron ; but, generally, the protoxide is the sole alkaline or salifiable base. Most of the metallic oxides are insoluble in water; but all those that are soluble have the property of giving a brown stain to yellow turmeric paper, and of restoring the blue colour of reddened litmus, GENERAL PROPERTIES OF METALS. 285 Oxides sometimes unite with each other, and form definite compounds. The most abundant ore of chromium, commonly called chromate of iron, is an instance of this kind; and the red and sesquioxide of manganese, and the red oxide of lead appear to belong to the same class of bodies. Chlorine has a powerful affinity for metallic substances. It combines readily with most metds at common temperatures, and the action is in many instances so violent as to be accompanied with the evolution of light. For example, when powdered zinc, arsenic, or antimony is thrown into a jar of dilorinegas, the metal is instantly inflamed. The attraction of chlorine for metals even surpasses that of oxygen. Thus, when chlorine is brought into contact at a red heat with pure lime, magnesia, baryta, strontia, potassa, or soda, oxygen is emitted, and a chloride of the metal is generated, the ele- ments of which are so strongly united that no temperature hitherto tried can separate them. All other metallic oxides are, with few exceptions, acted on in the same manner by chlorine, and in some cases the change takes place below the temperature of ignition. Most of the metallic chlorides are solid at common temperatures. They are fusible by heat, assume a crystalline texture in cooling, and under favourable circumstances crystallize with regularity. Several of them, such as the chlorides of tin, arsenic, antimony, and mercury, are volatile, and may be sublimed without change. They are for the most part colourless, do not possess the metallic lustre, and have the aspect of a salt. Two of the chlo- rides are insoluble in water, namely, chloride of silver and protochloride of mercury; several, such as the chlorides of antimony, arsenic, and titanium, are decomposed by that liquid; but most of them are more or less soluble. Some of the metdlic chlorides, those especially of gold and platinum, are decomposable by heat. All the chlorides of the common metals are decom- posed at a red heat by hydrogen gas, hydrochloric acid being disengaged while the metal is set free. Pure charcoal does not effect their decomposi- tion; but if moisture be present at the same time, hydrochloric and carbonic acid gases are formed, and the metal remains. They resist the action of anhydrous sulphuric acid; but dl the chlorides, excepting those of silver and mercury, are readily decomposed by hydrated sulphuric acid, with disengagement of hydrochloric acid gas. The change is accompanied with decomposition of water, the hydrogen of which combines with chlorine, and its oxygen with the metal. All chlorides, when in solution, may be recognized by yielding, with nitrate of oxide of silver, a white precipitate, which is chloride of silver. Metallic chlorides may in most cases be formed by direct action of chlo- rine on the pure metals. They are also frequently procured by dissolving • metallic oxides in hydrochloric acid, evaporating to dryness, and applying heat so long as any water is expelled. Metallic chlorides are often deposit- ed from such solutions by crystallization. Chlorine manifests a feeble affinity for metallic oxides. No combination of the kind occurs at a red heat, and no chloride of a metallic oxide can be heated to redness without decomposition. Such compounds can only be formed at low temperatures; and they are possessed of little permanency. Chlorine may combine under favourable circumstances with the dkalies and alkaline earths; and M. Grouvelle has succeeded in making it unite with the oxides of zinc, copper, and iron. (An. de Ch. et de Ph. xvii.) Of these chlorides, that of potassa may be taken as an example. If chlorine is con- ducted into a dilute and cold solution of pure potassa, the chloride of that alkdi will be produced; but the affinity which gives rise to its formation is not sufficient for rendering it permanent. It is destroyed by most sub- stances that act on either of its constituents. The addition of an acid produces this effect by combining with the alkali, and hence the chlorine is separated by the carbonic acid of the atmosphere. Animal or vegetable colouring matters are fatal to the compound by giving chlorine an opportu- 286 GENERAL PROPERTIES OF METALS. nity to exert its bleaching power, and, indeed, the colour is removed by the chloride of potassa almost as readily as by a solution of chlorine in pure water. When its solution is boiled, or even concentrated to a certain point, chloride of potassium and chlorate of potassa are generated. (Page 211.) Berzelius has published some ingenious remarks in order to prove that chlorine does not unite with metallic oxides, and that the bleaching com- pounds, supposed to be examples of such a mode of combination, are mix- tures of a metallic chloride and a chlorite of an oxide. The supposed chlo- rite, when heated, passes into a chlorate and chloride; but if colouring mat- ter or an oxidable substance be present, the chlorous acid yields its oxygen, and a metallic chloride results. The bleaching power of the compound is of course attributed to the oxygen which is set at liberty. This point is power- fully argued by Berzelius, and supported on well-contrived experiments; but since no decisive proof of the existence of such a compound as chlorous acid has as yet been given, there appears to be no sufficient reason for rejecting the explanation generally adopted by chemists. (An. de Ch. et de Ph. xxxviii. 208.) Iodine has a strong attraction for metals; and most of the compounds which it forms with them sustain a red heat in close vessels without decom- position. But in the degree of its affinity for metallic substances it is inferior to chlorine and oxygen. We have seen that chlorine has a stronger affinity than oxygen for metals, since it decomposes nearly all oxides at high temperatures; and it separates iodine also from metals under the same circum- stances. If the vapour of iodine is brought into contact with potassa, soda, protoxide of lead, or oxide of bismuth, heated to redness, oxygen gas is evolved, and the metals of those oxides will unite with iodine. But iodine, so far as is known, cannot separate oxygen from any other metal; nay, all the iodides, except those just mentioned, are decomposed by exposure to oxygen gas at the temperature of ignition. All the iodides are decomposed by chlorine, bromine, and concentrated sulphuric and nitric acids; and the iodine which is set free may be recognized either by the colour of its vapour, or by its action on starch. (Page 227.) The metallic iodides are generated under circumstances analogous to those above mentioned for procuring the chlorides. When the vapour of iodine is conducted over red-hot lime, baryta, or strontia, oxygen is not disengaged, but an iodide of those oxides, according to Gay-Lussac, is generated. The iodides of these oxides are, therefore, more permanent than the analogous compounds with chlorine. Iodine does not com- bine with any other oxide under the same circumstances; and indeed all other such iodides, very few of which exist, are, like the chlorides of oxides, possessed of little permanency, and are decomposed by a red heat. The action of iodine on metallic oxides, when dissolved or suspended in water, is precisely analogous to that of chlorine. On adding iodine to a solution of the pure dkalies or dkaline earths, an iodide and iodate are generated. Bromine, in its affinity for metallic substances, is intermediate between chlorine and iodine; for while chlorine disengages bromine from its combi- nation witli nietuls, metallic iodides are decomposed by bromine. The same phenomena attend the union of bromine with metals, as accompany the formation of metallic chlorides. Thus, antimony and tin take fire by con (act with bromine, and its action with potassium is attended with a fl h of light and intense heat. These compounds have as yet been but Darti-fiu! examined. They may be formed by the action of bromine on the n„Z metals, and often by dissolving metallic oxides in hydrobromic acid ,nrf evaporating the solution to dryness. Bromine unites with notiss, Jh, and some other oxides, constituting bleaching compounds similVr Vn Yl chlorides above described. Bromide of hme is obtained by the action of bromine on milk of hme, a yellowish solution being formed with water which bleaches powerfully. vvaicr, GENERAL PROPERTIES OF METALS. 287 As fluorine has not hitherto been obtained in a separate state, the nature of its action on the metals is unknown ; but the chief difficulty of procuring it in an insulated form appears to arise from its extremely powerful affinity for metallic substances, in consequence of which, at the moment of becom- ing free, it attacks the vessels and instruments employed in its preparation. The best mode of preparing the soluble fluorides, such as those of potas- sium and sodium, is by dissolving the carbonate of these alkalies in hydro- fluoric acid, and evaporating the solution to perfect dryness. The insoluble fluorides are easily formed by precipitation from the soluble fluorides. They are without exception decomposed by concentrated sulphuric acid with the aid of heat; and the hydrofluoric acid, in escaping, may easily be detected by its action on glass. Sulphur, like the preceding elementary substances, has a strong tendency to unite with metals, and the combination may be effected in several ways : 1. By heating the metal directly with sulphur. The metal, in the form of powder or filings, is mixed with a due proportion of sulphur, and the mix- ture heated in an earthen crucible, which is covered to prevent the access of air; or if the metal can sustain a red heat without fusing, the vapour of sulphur may be passed over it while heated to redness in a tube of porcelain. The act of combination, which frequently ensues below the temperature of ignition, is attended by free disengagement of heat, which in several in- stances is so great, that the whole mass becomes luminous, and shines with a vivid light. This appearance of combustion, which occurs quite indepen- dently of the presence of oxygen, is exemplified by the sulphurets of potas- sium, sodium, copper, iron, lead, and bismuth. 2. By igniting a mixture of a metallic oxide and sulphur. 3. By depriving the sulphate of an oxide of its oxygen by means of heat and combustible matter. Charcoal or hydrogen gas may be employed for the purpose, as will he described immediately. 4. By hydrosulphuric acid, or a soluble metallic sulphuret. Nearly all the salts of the second class of metals are decomposed when a current of hydro- sulphuric acid gas is conducted into their solutions. The salts of uranium, iron, manganese, cobalt, and nickel are exceptions; but these are precipi- tated by sulphuret of potassium. The sulphurets are opaque brittle solids, many of which, such as the sul- phurets of lead, antimony, and iron, have a metallic lustre. They are all fusible by heat, and commonly assume a crystalline texture in cooling. Most of them are fixed in the fire; but the sulphurets of mercury and arsenic are remarkable for their volatility. All the sulphurets, excepting those of the first class of metals, are insoluble in water. Most of the protosulphurets support an intense heat without decomposi- tion ; but, in general, those which contain more than one equivalent of sul- phur, lose part of it when strongly heated. They are all decomposed without exception by exposure to the combined agency of air or oxygen gas and heat; and the products depend entirely on the degree of heat and the na- ture of the metal. The sulphuret is more or less converted into the sulphate of an oxide, provided the sulphate is able to support the temperature em- ployed in the operation. If this is not the case, the sulphur is evolved under the form of sulphurous acid, and a metallic oxide is left; or if the oxide it- self is decomposed by heat, the pure metal remains. The action of heat and air in decomposing metallic sulphurets is the basis of several metallurgic processes. A few sulphurets are decomposed by the action of hydrogen gas at a red heat, the pure metal being set free and hydrosulphuric acid evolved. Rose finds that the only sulphurets which admit of being easily reduced to the metallic state in this way are those of antimony, bismuth, and silver. The sulphuret of tin is decomposed with difficulty, and requires a very high temperature. All the other sulphurets which he subjected to this treatment were either deprived of a part only of their sulphur, such as bisulphuret of iron, or were not attacked at all, as happened with the sulphurets of zinc, lead, and copper. (Poggendorff's Annalen, iv. 109.) 288 GENERAL PROPERTIES OF METALS. Many of the metallic sulphurets were formerly thought to be compounds of sulphur and a metallic oxide; an error first pointed out by Proust, who demonstrated that protosulphuret of iron, as well as the bisulphuret, are compounds of sulphur and metallic iron without any oxygen. (Journal de Physique, liii.) He proved the same of the sulphurets of other metals, such as mercury and copper. He was of opinion, however, that in some instances sulphur does unite with a metallic oxide. Thus, when sulphur and per- oxide of tin are heated together, sulphurous acid is disengaged, and the re- sidue, according to Proust, is a sulphuret of the protoxide, but in this he was in error. In 1817 Vauquelin extended these views to the compounds formed by heating an alkali or an alkaline earth with sulphur, which were previously regarded as sulphurets of a metallic oxide. He explained that the elements of the alkali unite with separate portions of sulphur, forming a metallic sul- phuret and sulphuric acid, the latter of which unites with undecomposed alkali. Thus, in preparing the so-called liver of sulphur, made by fusing carbonate of potassa with sdphur, one portion of the alkali is completely decomposed, its elements unite separately with sulphur, giving rise to sul- phuret of potassium and sulphuric acid, the latter of which combines with undecomposed potassa. These views were at the same time supported by Gay-Lussac. (An. de Ch. et de Ph. vi.) One of the chief arguments adduced by Vauquelin in support of his opinion was drawn from the action of charcoal on sulphate of potassa. When a mixture of this sdt with powdered charcoal is ignited without ex- posure to the air, carbonic oxide and carbonic acid gases are formed, and a sulphuret is left, analogous both in appearance and properties to that which may be made by igniting carbonate of potassa directly with sulphur. They are both essentially the same substance, and Vauquelin conceived from the strong attraction of carbon for oxygen, that both the sulphuric acid and po- tassa would be decomposed by charcoal at a high temperature; and that, consequently, the product must be a sulphuret of potassium. Berthier has proved that these changes do actually occur. (An. de Ch. et de Ph. xxii.) He put a known weight of sulphate of baryta into a crucible lined with a mixture of clay and charcoal, defended it from contact with the air, and exposed it to a white heat for the space of two hours. By this treatment it suffered complete decomposition, and it was found that in pass- ing into a sulphuret, it had suffered a loss in weight precisely equal to the quantity of oxygen originally contained in the acid and earth. This circum- stance, coupled with the fact that there had been no loss of sulphur, is de- cisive evidence that the baryta as well as the acid had lost its oxygen, and that a sulphuret of barium had been formed. He obtained the same results also with the sulphates of strontia, lime, potassa, and soda; but from the light fusibility of the sulphurets of potassium and sodium, their loss of weight could not be determined with such precision as in the other in- stances. The experiments of Berzelius, performed about the same time, are ex- ceedingly elegant, and still more satisfactory than the foregoing. (An. de Ch. et de Ph. xx.) He transmitted a current of dry hydrogen gas over a known quantity of sulphate of potassa, heated to redness. It was expected from the strong affinity of hydrogen for oxygen, that the sulphate would be decomposed; and, accordingly, a considerable quantity of water was form- ed, which was carefully collected and weighed. The loss of weight which the salt had experienced was precisely equivalent to the oxygen of the acid and alkali; and the oxygen of the water was exactly equal to the loss in weight. A similar result was obtained with the sulphates of soda, baryta, strontia, and lime. It is demonstrated, therefore, that the metallic bases of the alkalies and alkaline earths agree with the common metals in their disposition to unite with sulphur. It is now certain that, whether a sulphate be decomposed by hydrogen or charcoal, or sulphur ignited with an alkali or alkaline earth, a GENERAL PROPERTIES OF METALS. 289 metallic sulphuret is always the product. Direct combination between sul- phur and a metallic oxide is a very rare occurrence, nor has the existence of such a compound been clearly established. Gay-Lussac indeed states that, when an alkali or an alkaline earth is heated with sulphur in such a man- ner that the temperature is never so high as a low red heat, the product is really the sulphuret of an oxide. But the facts adduced in favour of this opinion are not altogether satisfactory, so that the real nature of the pro- duct must be decided by future observation. Several of the metallic sulphurets occur abundantly in nature. Those that are most frequently met with are the sulphurets of lead, antimony, copper, iron, zinc, molybdenum, and silver. The metallic seleniurets have so close a resemblance in their chemical relations to the sulphurets, that it is unnecessary to give a separate descrip- tion of them. They may be prepared either by bringing selenium in con- tact with the metals at a high temperature, or by the action of hydroselenic acid on metallic solutions. Cyanogen, as mentioned at page 265, has an affinity for metallic sub- stances. Nearly all the cyanurets of the second class of metals are insolu- ble in water, and may be obtained by precipitation with cyanuret of potas- sium. Most of them are decomposed by a red heat even without exposure to the air. Cyanogen unites also with some of the metallic oxides. When hydro- cyanic acid vapour is transmitted over pure baryta contained in a porcelain tube, and heated till it begins to be luminous, hydrogen gas is evolved, and cyanuret of baryta, according to Gay-Lussac, is generated. The same chemist succeeded in forming the cyanurets of potassa and soda by a simi- lar process. These compounds exist only in the dry state. A change is produced in them by the action of water, the nature of which has already been explained. (Page 270.) Respecting the preceding compounds there remains one subject, the con- sideration of which, as applying equally to all, has been purposely delayed. The non-metallic ingredient of each of these compounds is the radical of a hydracid; that is, has the property of forming with hydrogen an acid, which, like other acids, is unable to unite with metals, but appears to com- bine readily with many metallic oxides. Owing to this circumstance, a difficulty arises in explaining the action of such substances on water. Thus, when chloride of potassium is put into water, it may dissolve without suffering any other chemical change, and the liquid accordingly contain chloride of potassium in solution. But it is also possible that the elements of this compound may react on those of water, its potassium uniting with oxygen, and its chlorine with hydrogen; and as the resulting potassa and hydrochloric acid have a strong affinity for each other, the solution would of course contain hydrochlorate of potassa. A similar uncertainty attends the action of water on other metallic chlorides, and on the compounds of metals with iodine, bromine, sulphur, and similar substances; so that when iodide, sulphuret, and cyanuret of potassium are put into water, it may be doubted whether they dissolve as such, or whether they may not be converted, by decomposition of water, into hydriodate, hydrosulphate, and hydroeyanate of potassa. This question would at once be decided, could it be ascertained whether water is or is not decomposed during the process of solution; but this is the precise point of difficulty, since, from the operation of the laws of chemical union, no disengagement of gas does or can take place, by which the occurrence of such a change may be indicated. Chemists, ac- cordingly, being guided by probabilities, are divided in opinion, and I shall, therefore, give a brief statement of both views, with the arguments in favour of each. According to one view, then, chloride of potassium and all similar com- pounds dissolve in water without undergoing any other change, and are de- posited in their original state by crystallization. When any hydracid, such as hydrochloric or hydriodic acid, is mixed with potassa or any similar 25 290 GENERAL PROPERTIES OF METALS. metallic oxide, the acid and salifiable base do not unite; but the oxygen of the oxide combines with the hydrogen of the acid, and the metal itself with the radical of the hydracid. This kind of double decomposition unquestion- ably takes place in some instances, as when hydrosulphuric acid acts upon acetate of oxide of lead, the insoluble sulphuret of lead being actually pre- cipitated ; but it is also thought to occur even when the transparency of the solution is undisturbed. It is argued, accordingly, that hydrochlorate of potassa, and the salts of the hydracids in general have no existence. Thus, when nitrate of the oxide of silver is added to a solution of chloride or cyanuret of potassium, metallic silver is said to unite with chlorine or cy- anogen, while the oxygen of the oxide of silver combines with potassium; so that nitrate of potassa and chloride or cyanuret of silver are generated. On adding sulphuric acid to a solution of chloride of potassium, hydrochlo- ric acid and potassa, which did not previously exist, are instantly formed in consequence of water being decomposed, and yielding its hydrogen to chlo- rine, and its oxygen to potassium; exactly as happens when concentrated sulphuric acid is brought into contact with solid chloride of potassium. It is further believed that the crystallized hydrochlorate of lime, baryta, and strontia, which contain water or its elements, are metallic chlorides com- bined with water of crystallization; and the same view is applied to all analogous compounds. According to the other doctrine, chloride of potassium is converted into hydrochlorate of potassa in the act of dissolving; and when the solution is evaporated, the elements existing in the salt reunite at the moment of crys- tallization, and crystals of chloride of potassium are deposited. The same explanation applies in all cases, when the salt of a hydracid crystallizes without retaining the elements of water. Of those compounds which in crystallizing retain water or its elements in combination, two opinions may be formed. Thus crystallized hydrochlorate of baryta, which consists of one equivalent of chlorine, one of barium, two of oxygen, and two of hydro- gen, may be regarded as a compound either of hydrochlorate of baryta with one equivalent of water of crystallization, or of chloride of barium with two equivalents of water. When exposed to heat, two equivalents of water are expelled, and chloride of barium is left. When nitrate of the oxide of silver is mixed in solution with hydrochlorate of potassa, the oxy- gen of the oxide of silver unites with the hydrogen of the hydrochloric acid, chloride of silver is precipitated, and nitrate of potassa remains in the liquid. On adding sulphuric acid to a hydrochlorate, hydrochloric acid is simply displaced, just as when carbonic acid in marble is separated from lime by the action of nitric acid. On comparing these opinions it is manifest that both are consistent with well-known affinities. When a metallic chloride is dissolved in water, the attraction of chlorine for the metal, and that of oxygen for hydrogen, tend to prevent chemical change; but the affinities of the metal for oxygen, of chlorine for hydrogen, and of hydrochloric acid for metallic oxides, co-ope- rate in determining the decomposition of water, and the production of a hydrochlorate. In favour of the latter view, the following considerations may be adduced :—1. The solution of some compounds, such as sulphuret and cyanuret of potassium, actually emit an odour of hydrosulphuric and hydrocyanic acid. 2. Other compounds, such as the chlorides of copper, Gobalt, and nickel, instantly acquire, when put into water, the colour pecu- liar to the salts of the oxides of those metals. 3. The solution of protochlo- ride of iron, like the protosulphate, absorbs oxygen from the atmosphere; an effect which seems to indicate the presence of the protoxide of iron in the liquid. 4. In some instances there is direct proof of decomposition of water. Thus when sulphuret of aluminium is put into that fluid, alumina is generated, and hydrosulphuric acid gas disengaged with effervescence. In like manner chloride and sulphuret of silicium are converted by water into silica, and hydrochloric and hydrosulphuric acid. In these cases the want of affinity between the new compounds causes their separation, and GENERAL PROPERTIES OF METALS. 291 thus affords direct proof that water is decomposed. But the affinities which produce this change do not appear so likely to be effective, as those which are in operation when chloride of potassium is put into water; espe- cially when it is considered that the attraction of chlorine for hydrogen, and potassium for oxygen, is aided by that of the resulting acid and oxide for each other. These arguments may be successively answered in the following man- ner:—1. That solutions of cyanuret and sulphuret of potassium smell of hydrocyanic and hydrosulphuric acids, because the carbonic acid of the at- mosphere gradually decomposes them. 2. That metals may yield with chlorine compounds of the same colour as the oxides of the same metals. Thus the terchloride and terfluoride of chromium have a red colour closely resembling that of chromic acid. 3. Protochloride of iron may attract oxygen from the air because of its known tendency to pass into the state of a sesquichloride, a portion of iron being at the same time converted into peroxide. 4. That while certain chlorides do really decompose water, it must be conceded that others dissolve directly without change. The bichlo- ride of platinum and terchloride of gold are soluble in ether, forming solu- tions which must be regarded as chlorides and not hydrochlorates, since pure ether is anhydrous; and when aqueous solutions of these chlorides are agitated with ether, ethereal solutions of platinum and gold are formed, ex- actly similar to those made with ether alone. It can scarcely be doubted, then, that these chlorides exist as such in water. In favour of the same view it may with truth be alleged, that the chlorides of potassium and sodium dissolve in and crystallize out of water without evincing the least sign of any other change than mere solution and mere crystallization. Again, crystals of the so-called hydrochlorate of baryta become chloride of barium with loss of water by mere exposure to a dry air; a cause appa- rently inadequate to determine the hydrogen of the acid to unite with the oxygen of the oxide, but sufficient to explain the phenomena if the crystals were chloride of barium with water of crystallization. On weighing these and other considerations of a like kind, it appears un- deniable that some metallic chlorides, iodides, cyanurets, and similar com- pounds dissolve as such in water; that all do so, is a position which cannot, I think, be maintained; and, therefore, the existence of such compounds as hydracids united with metallic oxides can scarcely be denied. At the same time it is necessary, to avoid a perpetually recurring two-fold explanation, to adhere consistently to one view; and the reader may have observed that I have, in this edition, uniformly gone on the supposition that chlorides, and the same class of bodies, dissolve as such in water. The considerations which have led to this preference are principally drawn from the history of the sulphur-salts. Chemists are acquainted with several metallic phosphurets; and it is pro- bable that phosphorus, like sulphur, is capable of uniting with all the metals. Little attention, however, has hitherto been devoted to these compounds; and for the greater part of our knowledge concerning them we are indebted to the researches of Pelletier and Rose. (An. de Ch. i. and xiii.; and Pog. Annalen, vi. 205.) The metallic phosphurets may be prepared in several ways. The mos direct method is by bringing phosphorus in contact with metals at a high temperature, or by igniting metals in contact with phosphoric acid and charcoal. Several of the phosphurets may be formed by transmitting a current of phosphuretted hydrogen gas over metallic oxides heated to red- ness in a porcelain tube, when water is generated, and a phosphuret of the metal remains. By similar treatment the chlorides and sulphurets of many metals may be decomposed, and phosphurets formed, provided the metal is capable of retaining phosphorus at a red heat. According to Rose, the phosphurets of copper, nickel, cobalt, and iron are the only ones which admit of being advantageously prepared by this method. When chlorides are employed, hydrochloric acid, and with sulphurets hydrosulphuric acid gas, is of course generated. 292 GENERAL PRpPERTlES OF METALS. Phosphorus is said to unite with metallic oxides. For example, phosphu- ret of lime is said to be formed by conducting the vapour of phosphorus over that earth at a low red heat; but it is probable that in this instance, as with a mixture of sulphur and an alkali, part of the metallic oxide is decomposed, and that the product contains phosphuret of calcium and phosphate of lime. The only metallic carburets of importance are those of iron, which will be described in the section on that metal. Hydrogen unites with few metals. The only metallic hydrogurets, or hvdurets known are those of zinc, potassium, arsenic, and tellurium. No definite compound of nitrogen and a metal has hitherto been discovered. The discoveries of modern chemistry have materially added to the num- ber of the metals, especially by associating with them a class of bodies which was formerly believed to be of a nature entirely different. The metallic bases of the alkalies and earths, previous to the year 1807, were altogether unknown; and before that date the list of metals, with few excep- tions, included those only which are commonly employed in the arts, and which are hence often called the common metals. In consequence of this increase in number, it is found convenient, for the purpose of description, to arrange them in separate groups; and as the alkalies and earths differ in several respects from the oxides of other metals, it will be convenient to describe them separately. I have accordingly divided the metals into the two following classes :— Class I. Metals which by oxidation yield alkalies or earths. Class II. Metals, the oxides of which are neither alkalies nor earths. Class I. This class includes thirteen metals, which may properly be arranged in three orders. Order 1. Metdlic bases of the alkalies. They are three in number; namely, Potassium, Sodium, Lithium. These metals have such a powerful attraction for oxygen, that they decompose cold water and even ice at the moment of contact, and are oxidized with disengagement of hydrogen gas. The resulting oxides are distinguished by their causticity and solubility in water, and by possessing alkaline properties in an eminent degree. They are called alkalies, and their metallic bases are sometimes termed alkaline or alkaligenous metals. Order 2. Metallic bases of the alkaline earths. These are four in number; namely, Barium, Strontium, Calcium, Magnesium. Thise metals, excepting magnesium, also decompose water rapidly at common temperatures. The resulting oxides are called alkaline earths; because, while in their appearance they resemble the earths, they are similar to the alkalies in having a strong alkaline reaction with test paper, and in neutralizing acids. The three first are strongly caustic, and baryta and strontia are soluble in water to a considerable extent. Order 3. Metallic bases of the earths. These are six in number; namely, Aluminium, Yttrium, Zirconium, Glucinium, Thorium, Silicium. The oxides of these metals are well known as the pure earths. They are white and of an earthy appearance, in their ordinary state are quite insolu- ble in water, and do not affect the colour of turmeric or litmus paper. As salifiable bases they are inferior to the alkaline earths. Silica is even con- sidered by several chemists as an acid, and its chemical relations appear to justify the opinion. For reasons to be afterwards mentioned, the propriety of placing silicium among the metals is exceedingly doubtful. GENERAL PROPERTIES OF METALS. 293 Class II. The number of the metals included in this class amounts to twenty-nine. They are all capable of uniting with oxygen, and generally in more than one proportion. Their protoxides have an earthy appearance, but with few exceptions are coloured. They are insoluble in water, and in general do not affect the colour of test paper. Most of them act as salifiable bases in uniting with acids, and forming salts; but in this respect they are much inferior to the alkdies and alkaline earths, by which they may be separated from their combinations. Several of these metals are capable of forming with oxygen compounds, which possess the character of acids. The metals in which this property has been noticed are manganese, arsenic, chromium, vanadium, molybdenum, tungsten, columbium, antimony, tita- nium, tellurium, and gdd. The metds belonging to the second class may be conveniently arranged in the three following orders:— Order 1. Metals which decompose water at a red heat. They are seven in number; namely, Manganese, Cadmium, Cobalt, Iron, Tin, Nickel, Zinc, Order 2. Metals which do not decompose water at any temperature, and the oxides of which are not reduced to the metallic state by the sole action of heat. Of these there are fourteen in number ; namely, Arsenic, Columbium, Titanium, Chromium, Antimony, Tellurium, Vanadium, Uranium, Copper, Molybdenum, Cerium, Lead. Tungsten, Bismuth, Order 3. Metals, the oxides of which are decomposed by a red heat. These are Mercury, Platinum, Osmium, Silver, Palladium, Iridium. Gold, Rhodium, 25* 294 POTASSIUM, CLASS I. METALS WHICH BY OXIDATION YIELD ALKALIES OK EARTHS. ORDER I. METALLIC BASES OF THE ALKALIES. SECTION I. POTASSIUM. Potassium, or kalium, was discovered in the year 1807 by Sir H. Davy. and the circumstances which led to the discovery have already been de- scribed. Hydrate of potassa, slightly moistened for the purpose of increas- ing its conducting power, was made to communicate with the opposite poles of a galvanic battery of 200 double plates ; when the oxygen both of the water and the potassa passed over to the positive pole, while the hydrogen of the former, and the potassium of the latter, made their appearance at the negative pole. By this process potassium is obtained in small quantity only ; but Gay-Lussac and Thenard invented a method by which a more abun- dant supply may be procured. (Recherches Physico-Chimiques, vol. i.) Their process consists in bringing fused hydrate of potassa in contact with turnings of iron heated to whiteness in a gun-barrel. The iron, under these circumstances, deprives the water and potassa of oxygen, hydrogen gas com- bined with a little potassium is evolved, and pure potassium sublimes, and may be collected in a cool part of the apparatus. Potassium may also be prepared, as first noticed by M. Curaudau, by mixing dry carbonate of potassa with half its weight of powdered charcoal, and exposing the mixture, contained in a gun-barrel or spheroidal iron bot- tle, to a strong heat. An improvement on both processes has been made by M. Brunner, who decomposes potassa by means of iron and charcoal. From eight ounces of fused carbonate of potassa, six ounces of iron filings, and two ounces of charcoal mixed intimately and heated in an iron bottle, he obtained 140 grains of potassium. (Quarterly Journal, xv. 379.) Berzelius has observed that the potassium thus made, though fit for all the usual pur- poses to which it is applied, contains a minute quantity of carbon; and, therefore, if required to be quite pure, must be rendered so by distillation in a retort of iron or green glass. A modification of this process has been since described by Wohler, who effects the decomposition of the potassa solely by means of charcoal. The material employed for the purpose is car- bonate of potassa, prepared by heating cream of tartar to redness in a co- vered crucible. (PoggcndorfT's Annalen, iv. 23.) Potassium is solid at the ordinary temperature of the atmosphere. At 70° it is somewhat fluid, though its fluidity is not perfect till it is heated to 150°. At 503 it is soft and malleable, and yields like wax to the pressure of the fingers ; but it becomes brittle when cooled to 32°. It sublimes at a low red heat, without undergoing any change, provided atmospheric air be com- POTASSIUM. 295 pletely excluded. Its texture is crystalline, as may be seen by breaking it across while brittle. In colour and lustre it is precisely similar to mercury. At 60° its density is 0.865, so that it is considerably lighter than water. It is quite opaque, and is a good conductor of heat and electricity. The most prominent chemical property of potassium is its affinity for oxygen gas. It oxidizes rapidly in the air, or by contact with fluids which contain oxygen. On this account it must be preserved either in glass tubes hermetically sealed, or under the surface of liquids, such as naphtha, of which oxygen is not an element.* If heated in the open air, it takes fire, and burns with a purple flame and great evolution of heat. It decomposes water on the instant of touching it, and so much heat is disengaged, that the potassium is inflamed, and burns vividly while swimming upon its sur- face. The hydrogen unites with a little potassium at the moment of separa- tion ; and this compound takes fire as it escapes, and thus augments the brilliancy of the combustion. When potassium is plunged under water, vio- lent reaction ensues, but without light, and pure hydrogen gas is evolved. The combining weight or equivalent of potassium is easily deducible from the composition of potassa and chloride of potassium, which are ad- mitted to consist of single equivalents of their elements. Gay-Lussac and Thenard, and Davy inferred the composition of potassa from the hydrogen gas evolved, when a known weight of potassium is oxidized under water, the volume of the oxygen which unites with the metal being equal to half the volume of the hydrogen. Berzelius analyzed chloride of potassium by means of nitrate of oxide of silver, and inferred that 39.15 is the equivalent of potasdum. Potassium. Equiv. Formulas. Proto.xide Peroxide Chloride Iodide Bromide Fluoride Hydurets Carburet Sulphuret Bisulphuret Tersulphuret 39.15 Quadrosul phuret Quintosul- phurct 1 eq.-f-Oxygen 8 1 eq.-f do. 24 1 eq.+Chlor. 35.42 1 eq.-}-Iodine 126.3 1 eq.-f-Brom. 78.4 1 eq.-f-Fluor. 18.68 Composition uncertain. 39.15 39.15 30.15 3:U5 39.15 39.15 39.15 39.15 39.15 1 eq.+ S leq.-f- leq.+ leq.+ ilphur 16.1 do. do. ■3-1:2 48.3 1 eq.: 3 eq. 1 eq. 1 eq.: 1 eq.: 1 eq.: 1 eq.r 2 eq.= 3 eq.= = 47.15 , 63.15 = 7457 = 165.45 :117.55 : 57.83 K+O or K. K-f-30 or K. K-j-Cl or KC1. K-f-I or KI. K+BrorKBr. K-f-F or KF. 55.25 K+S or KS. 71.35 K+2S or KSa. 87,15 K+3S or KSs. 103.55 K+4S or KS4. 39.15 1 eq.-f- Phosphurets > Coin ition anQ number uncertain Seleniurets $ l Cyanurets 39.15 1 eq.-f-Cyanog.26 39 leq. do. 64.4 4 eq 5 eq.= 119.65 K+5S or KS" do. 80.5 Sulphycya- nuret 39.15 1 eq.+ fC}58. ~ o J 65.54 K-f-CyorKCy. 59 1 eq.= 97.74 K+CyS*. * Mr. Durand, pharmaceutist of Philadelphia, has ascertained that the es- sential oil of copaiba is a good liquid for the preservation of potassium. I'have used it myself for this purpose, and am satisfied that it is superior to the ordinary naphtha. The brightness of the metal is but slightly impaired, while in common naphtha it becomes covered with a blackish film. Several chemists have used this oil on the recommendation of Mr. Durand, and with satisfactory results.—Ed. 296 POTASSIUM. Protoxide of Potassium.—This compound, commonly called potash or po- tassa, and by the Germans kali (an Arabic word), is always formed when potassium is put into water, or when it is exposed at common temperatures to dry air or oxygen gas. By the former method the protoxide is obtdned in combination with water; and in the latter it is anhydrous. In perform- ing the last-mentioned process, the potassium should be cut into very thin slices; for otherwise the oxidation is incomplete. The product, when par- tially oxidized, is regarded by Berzelius as a distinct oxide; but most che- mists admit it to be a mere mixture of potassa and potassium. Anhydrous potassa can only be prepared by the slow oxidation of potas- sium, as already mentioned. In its pure state it is a white solid substance, highly caustic, which fuses at a temperature somewhat above that of red- ness, and bears the strongest heat of a wind furnace without being decom- posed or volatilized. It has a powerful affinity for water, and intense heat is disengaged during the act of combination. With a certain portion of that liquid it forms a solid hydrate, the elements of which are united by an affinity so energetic, that no degree of heat hitherto employed can effect their separation. This substance was long regarded as the pure alkali, but it is in reality a hydrate of potassa. It is composed of 47.15 parts or one eq. of potassa, and 9 parts or one eq- of water; and its formula is (K-j-O) -f- (H + 0),or K-f-H. Hydrate of potassa is solid at common temperatures. It fuses at a heat rather below redness, and assumes a somewhat crystalline texture in cool- ing. It is highly deliquescent, and requires about half its weight of water for solution. It is soluble, likewise, in alcohol. It destroys all animal textures, and on this account is employed in surgery as a caustic. It was formerly called lapis causticus, but it is now termed potassa and potassa fusa by the Colleges of Edinburgh and London. This preparation is made by evapo- rating the aqueous solution of potassa in a silver or clean iron capsule to the consistence of oil, and then pouring it into moulds. In this state it is impure, containing oxide of iron, together wilh chloride of potassium, and carbonate and sulphate of potassa. It is purified from these substances by solution in alcohol, and evaporation to the same extent as before in a silver vessel. The operation should be performed expeditiously, in order to pre- vent, as far as possible, the absorption of carbonic acid. When common caustic potassa of the druggists is dissolved in water, a number of small bubbles of gas are disengaged, which is pure oxygen. Mr. Graham finds its quantity to be variable in different specimens, and to depend apparently on the impurity of the specimen. The aqueous solution of potassa, aqua potassce of the Pharmacopoeia, is prepared by decomposing carbonate of potassa by lime. To effect this object completely, it is advisable to employ equal parts ofquicklime and carbonate of potassa. The lime, as soon as it is slaked, is added to the carbonate, dis- solved in six or ten times its weight of hot water, and the mixture is boiled briskly in a clean iron vessel for about ten minutes. The liquid, after sub- siding, is filtered through a funnel, the throat of which is obstructed by a piece of clean linen. This process is founded on the fact that lime de- prives carbonate of potassa of its acid, forming an insoluble carbonate of lime, and setting the pure alkali at liberty. If the decomposition is complete, the filtered solution should not effervesce when neutralized with an acid. Liebig finds that a strong solution of caustic potassa actually deprives car- bonate of lime of its acid, and that, from this circumstance, carbonate of potassa cannot be rendered quite caustic by lime, unless diluted with about ten times its weight of water. As pure potassa absorbs carbonic acid rapidly when freely exposed to the atmosphere, it is desirable to filter its solution in vessels containing as small a quantity of air as possible. This is easily effected by means of the filter- ing apparatus devised by Mr. Donovan. It consists of two vessels a and d, POTASSIUM. 297 of equal capacity, and connected with each other as represented in the annexed wood-cut. The neck 6 of the upper vessel contains a tight cork, perforated to admit one end of the glass tube c, and the lower extremity of the same vessel terminates in a funnel pipe, which fits into one of the necks of the under vessel d by grinding, luting, or a tight cork. The vessel n is furnished with another neck e, which receives the lower end of the tube c, the junction being secured by means of a perforated cork, or luting. The throat of the funnel pipe is obstructed by a piece of coarse linen loosely rolled up, and not pressed down into the pipe itself. The solution is then poured in through the mouth at b, the cork and tube having been removed; and the first drop- pings, which are turbid, are not received in the lower vessel. The parts of the apparatus are next joined together, and the filtration may proceed at the slowest rate, without exposure to more air than was contained in the vessels at the beginning of the process. This apparatus should be made of green in preference to white glass; as the pure alkdies act on the former much less than on the latter. (Annals of Philosophy, xxvi. 115.) The mode by which this apparatus acts scarcely needs explanation. In order that the liquid should descend freely, two conditions are required: —first, that the air above the liquid should have the same elastic force, and there- fore exert the same pressure, as that below; and, secondly, as one means of securing the first condition, that the air should have free egress from the lower vessel. Both objects, it is manifest, are accomplished in the filtering apparatus of Mr. Donovan; since for every drop of liquid which descends from the upper to the lower vessel, a corresponding portion of air passes along the tube c from the lower vessel to the upper. Solution of potassa is highly caustic, and its taste intensely acrid. It possesses dkaline properties in an eminent degree, converting the vegetable blue colours to green, and neutralizing the strongest acids. It absorbs car- bonic acid gas rapidly, and is consequently employed for withdrawing that substance from gaseous mixtures. For the same reason k should be pre- served in well-closed bottles, that it may not absorb carbonic acid from the atmosphere. Potassa is employed as a reagent in detecting the presence of bodies, and in separating them from each other. The solid hydrate, owing to its strong affinity for water, is used for depriving gases of hygrometric moisture, and is admirably fitted for forming frigorific mixtures. (Page 39.) Potassa may be distinguished from all other substances by the following characters. 1. If tartaric acid be added in excess to a salt of potassa, dis- solved in cold water, and the solution be stirred with a glass rod, a white precipitate, bitartrate of potassa, soon appears, which forms peculiar white streaks upon the glass by the pressure of the rod in stirring. 2. It is pre- cipitated by perchloric acid in the cold, the perchlorate of potassa having nearly the same degree of solubility as the bitartrate. 3. A solution of chlo- ride of platinum causes a yellow precipitate, the double chloride of platinum and potassium. A drop or two of hydrochloric acid should be added at the same time as the test, the mixture be evaporated to dryness at 212°, and a little cold water be afterwards added, when the double chloride is left in the form of small shining yellow crystals. Chloride of platinum dissolved in alcohol often gives an immediate precipitate, which falls of a pale yellow colour. 4. The alcoholic solution of carbazotic acid throws down potassa in yellow crystals of carbazotate of potassa, which is very sparingly soluble. 298 POTASSIUM. 6. It yields a light gelatinous precipitate, the double fluoride of potassium and silicium, with silicated hydrofluoric acid. Of these tests carbazotic acid is the most delicate in a solution of pure potassa; but when the alkali is combined with a strong acid, the chloride of platinum is preferable. The following test has been recommended by M. Harkort for distinguish- ing between potassa and soda in minerals. Oxide of nickel, when fused by the blowpipe flame with borax, gives a brown glass; and this glass, if melt- ed with a mineral containing potassa, becomes blue, an effect which is not produced by the presence of soda. Peroxide.—When potassium burns in the open air or in oxygen gas, it is converted into an orange-coloured substance, which is peroxide of potas- sium. It may likewise be formed by conducting oxygen gas over potassa at a red heat; and it is produced in small quantity when potassa is heated in the open air. It is the residue of the decomposition of nitre by heat in metallic vessels, provided the temperature be kept up for a sufficient time.* When the peroxide is put into water, it is resolved into oxygen and potassa, the former of which escapes with effervescence, and the latter is dissolved. Chloride of Potassium.—Potassium takes fire spontaneously in an atmo- sphere of chlorine, and burns with greater brilliancy than in oxygen gas. This chloride is generated with evolution of hydrogen when potassium is heated in hydrochloric acid gas; and it is the residue after the decomposi- tion of chlorate of potassa by heat. It is formed when potassa is dissolved in a solution of hydrochloric acid, and is deposited by slow evaporation in anhydrous colourless cubic crystals. It has a saline and rather bitter taste, is insoluble in alcohol, and requires for solution 3 parts ofwater at 60°, and still less of hot water. Iodide of Potassium.—This compound is formed with evolution of heat and light, when potassium is heated in contact with iodine: it is the sole residue after decomposing iodate of potassa by heat; and by neutralizing potassa with hydriodic acid it is obtained in solution. The simplest process for preparing it in quantity is to add iodine to a hot solution of pure potassa until the alkali is neutralized, when iodide of potassium and iodate of potassa are generated, evaporate to dryness, and expose the dry mass in a platinum crucible to a gentle red heat in order to decompose the iodate. The fused mass is then dissolved out by water and crystallized. Another process is to digest iodine with zinc or iron filings in water, and then decompose the resulting iodide of zinc or iron by a quantity of potassa just sufficient to precipitate the oxide. Iodide of potassium fuses readily when heated, and rises in vapour at a heat below full redness, especially in an open vessel. It is very soluble in water, requiring only two-thirds of its weight at 60° for solution, and in a moist atmosphere deliquesces. It dissolves also in strong alcohol, even in the cold, and the solution, when evaporated, yields colourless cubic crystals of iodide of potassium. The commercial iodide is frequently impure, often containing chloride of * This fact was ascertained by Dr. Bridges of Philadelphia, in the spring of 1827, while investigating the nature of the gas given off, on the addition ofwater, from the residue of nitre, after exposure in an iron bottle to a red heat. This gas proved to consist of oxygen nearly pure, and the residue was converted into a solution of hydrate of potassa. These results evident- ly prove, that the residue in question consists of peroxide of potassium. Dr. Bridges suggests that the employment of this residue might prove conve- nient to the chemist for obtaining oxygen extemporaneously; as it would be necessary only to add water in order to obtain the gas.—North American Medical and Surgical Journal, v. 241. About the same time that Dr. Bridges made the above observations, similar ones were made by Mr. Phillips in London.—Annals of Philosophy, April 1827.—Ed. POTASSIUM. 299 potassium or sodium, and sulphate or carbonate of potassa, the last some- times in a very large quantity. It is well to purchase it in crystals, which ought not to deliquesce in a moderately dry air, but when in powder are completely soluble in the strongest alcohol. Iodine is freely soluble in water which contains iodide of potassium, a brown solution resulting, which has been thought to arise from potassium uniting with two or more equivalents of iodine. No solid compound of the kind, however, has been obtained. Bromide of Potassium.—This compound is formed by processes similar to that for preparing the iodide, and is analogous to it in most of its proper- ties. It is very soluble in water, and crystallizes by evaporation in anhy- drous cubic crystals, which fuse readily, and decrepitate when heated like sea-salt. It is but slightly soluble in alcohol. Fluoride of Potassium.—This compound is best formed by nearly satura- ting hydrofluoric acid with carbonate of potassa, evaporating to dryness in platinum, and igniting to expel any excess of acid. The resulting fluoride has a sharp saline taste, is alkaline to test paper, deliquesces in the air, and dissolves freely in water. On evaporating its solution at a temperature of 100° it may be obtained in cubes or rectangular four-sided prisms, which deliquesce rapidly. The solution acts on glass in which it is kept or evaporated. Heated with silicic acid, it forms a fusible limpid glass, which when cold is opaque and deliquescent. Water dissolves fluoride of potas- sium, and the silicic acid is left. Hydrogen and Potassium.—These substances unite in two proportions, forming in one case a solid and in the other a gaseous compound. The latter is produced when hydrate of potassa is decomposed by iron at a white heat; and it appears also to be generated when potassium burns on the sur- face of water. It inflames spontaneously in air or oxygen gas ; but on standing for some hours over mercury, the greater part, if not the whole of the potassium, is deposited. The solid hyduret of potassium was made by Gay-Lussac and Thenard, by heating potassium in hydrogen gas. It is a gray solid substance, which is readily decomposed by heat or contact with water. It does not inflame spontaneously in oxygen gas. Carburet of Potassium.—This compound has not been obtained in a pure state; but it is thought to form part of the residue in the preparation of potassium from charcoal (page 294); for on pouring that matter into water, effervescence ensues owing to the escape of carburetted hydrogen gas, and carbonate of potassa is found in solution. Sulphurets of Potassium.—Potassium unites readily with sulphur by the dd of gentle heat, emitting so much heat that the mass becomes incan- descent. The nature of the product depends on the proportions which are employed. The protosulphuret is readily prepared by decomposing sul- phate of potassa by charcoal or hydrogen gas at a red heat (page 288). It may be prepared in the moist way by a process which will be mentioned in describing the sulphur-salts. The protosulphuret of potassium fuses below a red heat, and acquires on cooling a crystalline texture. It has a red colour, its taste is at first strong- ly alkaline and then sulphurous, has an alkaline reaction with test paper, deliquesces on exposure to the air, and is soluble in water and alcohol. Most of the acids decompose it with evolution of hydrosulphuric acid gas, and without any deposite of sulphur. It takes fire when heated before the blowpipe, and quickly acquires a coating of sulphate of potassa, which stops the combustion; but when mixed in fine division with charcoal, it kindles spontaneously, forming a good pyrophorus. The bisulphuret is formed by exposing a saturated solution in alcohol of hydrosulphate of sulphuret of potassium (KS-f- HS), until a pellicle begins to form upon its surface, and then evaporating to dryness without further exposure. The first change consists in oxygen of the air uniting with hy- drogen of hydrosulphuric acid, the sulphur of which unites with potassium. 300 POTASSIUM. Then the formation of hyposulphurous acid begins; and as the hyposulphite of potassa is insoluble in alcohol, it gives a pellicle on its surface. The tersulphuret is prepared pure by transmitting the vapour of bisulphu- ret of carbon over carbonate of potassa at a red heat, as long as carbonic acid or carbonic oxide gases are disengaged. It is also formed when car- bonate of potassa is heated to low redness with half its weight of sulphur, until the mass appears in tranquil fusion: the oxygen of 3-4ths of the po- tassa unites wilh sulphur to form sulphuric acid, which exactly suffices to neutralize l-4th of the alkali, and all the carbonic acid is evolved as gas.— Thus 4 eq. potassa and 10 eq. sulphur 2 3 eq. tersulphuret & 1 eq. sulphate. 4 (K+O) 10S -2 3(K+3S) (K4-0)+(S-f-30). This is known under the name of liver of sulphur (p. 288). The quadrosulphuret is prepared by transmitting the vapour of bisulphu- ret of carbon over sulphate of potassa at a red heat, until carbonic acid gas ceases to be disengaged; or by conducting the same process with the ter- sulphuret prepared by the second method, until its sulphuric acid and potassa are decomposed. The quintosulphuret is formed by fusing carbonate of potassa with its own weight of sulphur, the residue containing sulphate of potassa as in pre- paring the tersulphuret. Each equivalent of potassium is united with five of sulphur, being the highest degree of sulphuration which can be formed by fusion. These four last sulphurets are deliquescent in the air, have a sulphurous odour, and are soluble in water; and those who consider them to decompose water in dissolving, suppose the formation of corresponding compounds of hydrogen and sulphur. On decomposing the solutions with hydrochloric or sulphuric acid, the changes ensue which have already been explained (page 260). As the solution of the quintosulphuret dissolves sulphur, a still higher degree of sulphuration must probably exist. Phosphurets of Potassium.—When potassium is heated in phosphuretted hydrogen gas, it takes fire, phosphuret of potassium is formed, and hydrogen set free; and combination is also effected by gently heating phosphorus with potassium. The number and proportion of these compounds have not yet been determined. They decompose water with formation of phosphu- retted hydrogen, potassa, and some acid of phosphorus. Seleniurets of Potassium.—These elements unite when fused together, sometimes with explosive violence, forming a crystalline fusible compound of an iron-gray colour and metallic lustre. It dissolves completely in water, yielding a deep red solution, very similar in tas«j and odour to solutions of sulphuret of potassium. On adding an acid, hydroselenic acid gas is evolved, and selenium deposited. Solution of potassa dissolves selenium, and gives rise to a seleniuret of potassium and selenite of potassa; and the same compounds are formed when selenium is heated with carbonate of potassa. Cyanuret of Potassium.—This substance is always produced when animal matters are decomposed by heat along with potassa; but the best mode of preparing it, is to heat anhydrous ferrocyanuret of potassium to redness in an iron bottle, furnished with a tube for "the escape of gas, as long as any gas is disengaged. The cyanuret of iron is thus resolved into a carburet of iron and nitrogen, and the former remains mixed with cyanuret of potas- sium : the latter should be dissolved in the smallest possible quantity of cold water, and the solution, after subsiding in a corked bottle,, be rapidly filtered, and brought to dryness in vacuo by means of sulphuric acid. Cyanuret of potassium is very fusible, bears a strong heat without de- composition, provided air and water are excluded, and may be preserved in a well-stoppercd bottle without change. It often crystallizes after fusion in colourless cubes. Exposed to the air it gradually becomes moist, and smells strongly of hydrocyanic acid. Its taste is pungent and alkaline, SODIUM. 301 followed by the flavour of hydrocyanic acid, and it has an alkaline reaction. It is poisonous, acting on animals in the same manner as hydrocyanic acid. In water it is very soluble, and also dissolves, though less readily, in dec- hol. Its solution is decomposed by acids, potassa and hydrocyanic acid being generated, a change which is caused even by the carbonic acid of the air. On heating its solution a complex change ensues, during which, from reaction between the elements ofwater and cyanogen, carbonic acid, hydro- cyanic acid, and ammonia are generated. The impurities which cyanuret of potassium is apt to contain, are ferro- cyanuret of potassium and carbonate of potassa. The former may be de- tected by its forming Prussian blue with sulphate of peroxide of iron, an effect which pure cyanuret of potassium does not occasion; and it may be got rid of by renewed exposure to heat. Carbonate of potassa is present if a solution of the cyanuret gives a precipitate of carbonate of lime in a solution of chloride of calcium ; and the cyanuret is purified by solution in boiling alcohol, out of which it separates by evaporation. Sulphocyanuret of Potassium.—To prepare this compound take anhydrous ferrocyanuret of potassium in powder, mix it with an equal weight of sul- phur, and heat the mixture in a porcelain vessel to a strong heat, somewhat short of redness. The mixture speedily fuses, takes fire, and burns briskly for a few minutes, during which it should be well stirred. In this process sulphur unites with 2-3rds of the cyanogen, forming bisulphuret of cyano- gen; l-3rd of the cyanogen is decomposed, yielding bisulphuret of carbon and nitrogen, which escape; and the iron combines with sulphur. On add- ing water and filtering, a colourless solution is obtained, and on the filter remains sulphuret of iron of a green colour. It often happens that the solution after a few hours becomes red, owing apparently to the presence of sulphocyanuret of iron, the metal of which is oxidized by the oxygen of the dr: in that case a quantity of carbonate of potassa should be added, which just suffices to precipitate the iron, and the solution be again filtered. The second process may be prevented, and the product increased, by mixing with the ferrocyanuret of potassium about l-5th of its weight of pure car- bonate of potassa before fusion with sulphur. Sdphocyanuret of potassium is very fusible, and in close vessels bears a strong heat without decomposition. It has a cool saline taste, similar to that of nitre, deliquesces in the air, is very soluble in water, and is likewise dissolved by alcohol. A hot concentrated solution deposites confused pris- matic crystals as it cools. It is quite neutral to test paper, and may be kept even in water without change. SECTION II. SODIUM. SoniUM, the natrium of the Germans, was discovered in 1807, a few days after the discovery of potassium. The first portions of it were obtained by means of galvanism; but it may be procured in much larger quantity by chemical processes, precisely similar to those described in the last section. Sodium has a strong metallic lustre, and in colour is very analogous to silver. It is so soft at common temperatures, that it may be formed into leaves by the pressure of the fingers. It fuses at 200°, and rises in vapour at a red heat. Its specific gravity is 0.972. Sodium soon tarnishes on exposure to the air, though less rapidly than potassium. Like that metal it is instantly oxidized by water, hydrogen gas in temporary union with a little sodium being disengaged. When thrown on cold water, it swims on its surface and is rapidly oxidized, though in Protoxide 23.3 Peroxide 46.6 Chloride 23.3 Iodide 23.3 Bromide 23.3 Fluoride 23.3 Sulphuret 23.3 Cyanuret 23.3 302 SODIUM. general without inflaming ; but with hot water it scintillates, or even takes fire. Dr. Ducatel finds that the heat rises high enough for inflammation with cold water, if the sodium be confined to one spot, and the water rest on a non-conducting substance, such as charcoal. (Silliman's Journal, xxv. 90.) In each case, soda is generated, and the water acquires an alkaline re- action. The Composition of soda was determined by the same methods as that of potassa, and agreeably to the observations of Berzelius 23.3 may be taken as the equivalent of sodium. The composition of the compounds of sodium described in this section is as follows:— Sodium. Equiv. Formulae. 1 cq.-f-Oxygen 8 1 eq.= 31.3 Na-f-O or Na. 2 eq.-f- do. 24 3 eq.= 70.6 2Na-f-30 or Na. 1 eq.-f-Chlorine 35.42 1 eq.= 58.72 Na-f CI or NaCI. 1 eq.-f Iodine 126.3 1 eq.= 149.6 Na-f-I or Nal. 1 eq.-f-Bromine 78.4 1 eq.= 101.7 Na-f-Br or NaBr. 1 eq.-f-Fluorine 18.68 1 eq.= 41.98 Na-f F or NaF. 1 eq.-f Sulphur 16.1 1 eq.= 39.4 Na-f SI or NaS. 1 eq.-f-Cyanogen 26.39 1 eq.= 49.69 Na-f-Cy or NaCy. Sulphocy- )„„. } Cyanogen 26.39 ( 58.59 1 eq.= ) Na-f (Cy-f 2S) anuret {'"•* l eq"+ ) Sulphur 32.2 \ 81.89 J orNa + CyS*. Chloride of soda. Composition uncertain. ' Soda.—The protoxide of sodium, commonly called soda, and by the Ger- mans natron, is formed by the oxidation of sodium in air and water, as po- tassa is from potassium. In its anhydrous state it is a gray solid, difficult of fusion, and very similar in all its characters to potassa. With water it forms a solid hydrate, easily fusible by heat, which is very caustic, soluble in wa- ter and alcohol, has powerful alkaline properties, and in all its chemical re- lations is exceedingly analogous to potassa. It is prepared from the solution of pure soda, exactly in the same manner as the corresponding preparation of potassa. The solid hydrate is composed of 31.3 parts or one equivalent of soda, and 9 parts or one equivalent ofwater. Soda is readily distinguished from other alkaline bases by the following characters. 1. It yields with sulphuric acid a salt, which by its taste and form is easiby recognized as Glauber's salt, or sulphate of soda. 2. All its salts are soluble in water, and are not precipitated by any reagent. 3. On exposing its salts by means of platinum wire to the blowpipe flame, they communicate to it a rich yellow colour. Peroxide of Sodium.—This compound is formed when sodium is heated to redness in an excess of oxygen gas. It has an orange colour, has neither acid nor alkaline properties, and is resolved by water into soda and oxygen. Chloride of Sodium.—This compound may be formed directly by burn- ing sodium in chlorine, by heating sodium in hydrochloric acid gas, and by neutralizing soda with hydrochloric acid. It exists as a mineral under the name of rock salt, is the chief ingredient of sea-water, and is contained in many saline springs. From these sources are derived the different varieties of common salt, such as rock, bay, fishery, and stovcd salt, which differ from each other only in degrees of purity and mode of preparation. The common varieties of salt, of which rock and bay salt are the purest, always contain small quantities of sulphate of magnesia and lime, and chlo- ride of magnesium. These earths may be precipitated as carbonates by boil- ing a solution of salt for a few minutes with a slight excess of carbonate of soda, filtering the liquid, and neutralizing with hydrochloric acid. On eva- porating this solution rapidly, chloride of sodium crystallizes in hollow four- sided pyramids; but it occurs in regular cubic crystals when the solution is allowed to evaporate spontaneously. These crystals contain no water of crystallization, but decrepitate remarkably when heated, owing to the expan- sion ofwater mechanically confined within them. SODIUM. 303 Pure chloride of sodium has an agreeably saline taste. It fuses at a red heat, and becomes a transparent brittle mass on cooling. It deliquesces slightly in a moist atmosphere, but undergoes no change when the air is dry. In pure alcohol it is insoluble. It requires twice and a half its weight ofwater at 60° for solution, and its solubility is not increased by heat. Hy- drous sulphuric acid decomposes it with evolution of hydrochloric acid gas, and formation of sulphate of soda. The uses of chloride of sodium are well known. Besides its employment in seasoning food, and in preserving meat from putrefaction, a property which when pure it possesses in a high degree, it is used for various pur- poses in the arts, especially in the formation of hydrochloric acid and chlo- ride of lime. Iodide of Sodiu?n.—It is obtained pure by processes similar to those for preparing iodide of potassium; but it is contained in sea-water, in many salt springs, and in the residual liquor from kelp (page 228.) It is a neutral compound, deliquescent in the air, soluble in water and alcohol, fuses rea- dily by heat, and is volatile, though in a less degree than iodide of potas- sium. Evaporated at 123° it crystallizes from its aqueous solution in cubes, which Berzelius found to contain 20.23 per cent, of water. Bromide of Sodium.—This compound is very analogous to sea-salt, and is associated with it in sea-water and most salt springs. At 86° it crystal- lizes from its aqueous solution in anhydrous cubes; but at lower tempera- tures it separates in hexagonal tables, which Mitscherlich found to contain 26.37 per cent, ofwater, or four equivalents to one eq. of the bromide. Fluoride of Sodium.—This compound is formed by neutralizing hydro- fluoric acid by soda, and by igniting the double fluoride of sodium and sili- cium, when the fluoride of silicium is expelled. When obtained by the se- cond process, it crystallizes from its aqueous solution in rhomboidal crystals, but is obtained in cubes, its proper form, by a second crystallization: when carbonate of soda is present it crystallizes in octohedrons. Fluoride of sodium in crystals is anhydrous, is almost insoluble in alcohol, and requires 25 times its weight both of hot and cold water for solution. It attacks glass vessels when evaporated in them, and by fusion unites with silicic acid, forming a glass which is more fusible than the pure fluoride; but water dissolves out the fluoride, and leaves the silicic acid. Sulphuret of Sodium.—The protosulphuret is obtained by processes simi- lar to those for protosulphuret of potassium, to which in its taste and chemi- cal relations it is very similar. A concentrated solution of it yields hydrated, square, four-sided prisms, which, when heated, fuse in their water of crys- tallization, and then leave a white anhydrous mass*. It deliquesces in the air, is very soluble in water, and is also dissolved, though in a smaller degree, by alcohol. In solution it absorbs oxygen very rapidly from the air, and passes into hyposulphate of soda. Sodium unites with sulphur in other proportions; but the resulting com- pounds have not been studied. According to Gmelin of Tubingen, sulphuret of sodium is the colouring principle of lapis lazuli, to which the colour of ultra-marine is owing ; and he has succeeded in preparing artificial ultra-marine by heating sulphuret of sodium yyith a mixture of silicic acid and alumina. (An. de Ch. et de Ph. xxxvii. 409.) Chloride of Soda.—This compound has lately acquired the attention of scientific men under the name of Labarraque's disinfecting soda liquid, which was announced by M. Labarraque as a compound of chlorine and soda, analogous to the well-known bleaching powder, chloride of lime. The nature of this liquid has been since investigated by Mr. Phillips and Mr. Faraday, especially by the latter; and it appears from the experiments of this chemist, that while chloride of soda is the active ingredient, its proper- ties are considerably modified by the presence of carbonate of soda. (Quar- terly Journal of Science, N. S. ii. 84.) Pure chloride of soda is easily prepared by transmitting to saturation a, 304 SODIUM. current of chlorine gas into a cold and rather dilute solution of caustic soda. Common carbonate of soda may be substituted for the pure alkali; but con- siderable excess of chlorine must then be employed in order to displace the whole of the carbonic acid. It may also be formed easily, cheaply, and of uniform strength, by decomposing chloride of lime with carbonate of soda, as proposed by M. Payen. (Quart. Journal of Science, N. S. i. 236.) How- ever prepared, its properties are the same. As its constituents are retained in combination by a feeble affinity, the compound is easily destroyed. It emits the odour of chlorine, and possesses the bleaching properties of that substance in a very high degree. When kept in open vessels, it is slowly de- composed by the carbonic acid of the atmosphere with evolution of chlorine ; and the change is more rapid in air charged with putrid effluvia, because the carbonic acid produced during putrefaction promotes the decomposition of the chloride. On this, as was proved by M. Gaultier de Chaubry, depends the efficacy of an alkaline chloride in purifying air loaded with putrescent exhalations. When the solution is heated to the boiling point, or concen- trated by means of heat, the chloride undergoes a change previously ex- plained (page 211), and is converted into chlorate of soda and chloride of so- dium. Chloride of soda may be employed in bleaching, and for all purposes to which chlorine gas or its solution was formerly applied. It is now much used in removing the offensive odour arising from drains, sewers, or all kinds of animal matter in a state of putrefaction. Bodies disinterred for the purpose of judicial inquiry, or parts of the body advanced in putrefaction, may by its means be rendered fit for examination; and it is employed in surgical practice for destroying the fetor of malignant ulcers. Clothes worn by persons during pestilential diseases are disinfected by being washed with this compound. It is also used in fumigating the chambers of the sick; for the disengage- ment of chlorine is so gradual, that it does not prove injurious or annoying to the patient. In all these instances chlorine appears actually to decom- pose noxious exhalations by uniting with the elements of which they consist, and especially with hydrogen. In preparing the disinfecting liquid of Labarraque, it is necessary to be exact in the proportion of the ingredients employed. The quantities used by Mr. Faraday, founded on the directions of Labarraque, are the following. He dissolved 2800 grains of crystallized carbonate of soda in 1.28 pints of water, and through the solution, contained in Woulfe's apparatus, was trans- mitted the chlorine evolved from a mixture of 967 grains of sea-salt and 750 grains of peroxide of manganese, when acted on by 967 grains of sul- phuric acid, diluted with 750 grains of water. In order to remove any ac- companying hydrochloric acid gas, the chlorine before reaching the soda was conducted through pure water, by which means nearly a third part was dissolved, but the remaining two-thirds were fully sufficient for the purpose, The gas was readily absorbed by the solution, and from the beginning to the end of the process, not a particle of carbonic acid gas was evolved j whereas, by employing an excess of chlorine, the carbonic acid may be en- tirely expelled. The solution thus prepared has all the characters of Labarraque's soda liquid. Its colour is pale yellow, and it has but a slight odour of chlorine. Its taste is at first sharp, saline, and scarcely at all alkaline; but it produces a persisting biting effect upon the tongue. It first reddens and then de- stroys the colour of turmeric paper. When boiled it does not give out chlo- rine, nor is its bleaching power perceptibly impaired; and if carefully eva- porated, it yields a mass of damp crystals, which, when re-dissolved, bleach almost as powerfully as the original liquid. When rapidly evaporated to dryness, the residue contains scaroely any chlorate of soda or chloride of sodium; but it has nevertheless lost more than half of its bleaching power, and, therefore, chlorine must have been evolved during the evaporation. The solution deteriorates gradually by keeping, chloric acid and chloride of LITHIUM. 305 sodium being generated. When allowed to evaporate spontaneously, chlo- rine gas is gradually evolved, and crystals of carbonate of soda remain. In some respects the nature of this liquid is still obscure; but from the preceding facts, drawn from the essay of Mr. Faraday, two points seem to be established. First, that the liquid contains chlorine, carbonic acid, and soda. Secondly, that the chlorine is not simply combined either with water or soda; for by boiling, the gas is neither expelled as it would be from an aqueous solution, nor does the liquid yield chloric acid and chloride of so- dium as when pure chloride of soda is heated. It may perhaps be regard- ed as a compound of chloride and bicarbonate of soda. Its production may be conceived by supposing, that when chlorine is introduced in due quantity into a solution of carbonate of soda, it combines with half of the alkali, while the remainder, with all the carbonic acid, constitutes bicarbonate of soda. Should this salt unite, though by a feeble affinity, with chloride of soda, both may thence derive a degree of permanence which neither singly possesses. During spontaneous evaporation, the tendency of the common carbonate to crystallize may occasion its reproduction, and the disengage. ment of chlorine. These remarks, however, are merely speculative. SECTION III. LITHIUM. Davy succeeded by means of galvanism in obtaining from lithia a white- coloured metal like sodium; but it was oxidized, and thus reconverted into lithia, with such rapidity that its properties could not be farther examined. Its equivalent, inferred from the composition of sulphate of lithia by Stro- meyer and Thomson, is 10; but the accuracy of this estimate is rendered doubtful by the experiments of M. Herrman, according to which 6 is a nearer estimate. The compounds of lithium described in this section are thus constituted:— Lithium. Equiv. Formulae. Lithia . 10 1 eq. +Oxygen . 8 1 eq.=18 L-f-O or L. Chloride . 10 1 eq. +Chlorine . 35.42 1 eq.=45.42 L-f-Cl or LCI. Fluoride . 10 1 eq.-f Fluorine . 18.68 1 eq.=MGS L-f F or LF. Lithia.—This, the only known oxide of lithium, was discovered in 1818 by M. Arfwedson (An. de Ch. et de Ph. x.) in a mineral called petalite ; and its presence has since been detected in spodumene, lepidolite, and in several varieties of mica. Berzelius has found it also in the waters of Carlsbad in Bohemia. From the circumstance of its having been first obtained-from an earthy mineral, Arfwedson gave it the name of lithion, (from \iQues, lapi. deus,) a term since changed in this country to lithia. It has hitherto been procured in small quantity only, because spodumene and petalite are rare, and do not contain more than 6 or 8 per cent, of the alkali. It is combined in these two minerals with silicic acid and alumina ; whereas potassa is like- wise present in lepidolite and lithion-mica, and, therefore, lithia should be prepared solely from the former. The best process for preparing lithia is that which was suggested by Berzelius. One part of petalite or spodumene, in fine powder, is mixed in- timately with two parts of fluor spar, and the mixture is heated with three or four times its weight of sulphuric acid, as long as any acid vapours are disengaged. The silicic acid of the mineral is attacked by hydrofluoric acid, and dissipated in the form of fluosilicic gas, while the alumina and lithia unite with sulphuric acid. After dissolving these salts in water, the 306 BARIUM. solution is boiled with pure ammonia to precipitate the alumina: it is then filtered and evaporated to dryness, and the dry mass heated to redness to expel the sulphate of ammonia. The residue is pure sulphate of lithia.* Lithia, in its alkalinity, in forming a hydrate with water, and in its chemical relations, is closely allied to potassa and soda. It is distinguished from them by its greater neutralizing power, by forming sparingly soluble compounds with carbonic and phosphoric acids, and by its salts, when heat- ed on platinum wire before the blowpipe, tinging the flame of a red colour. Also, when fused on platinum foil, it attacks that metal and leaves a dull yellow trace round the spot where it lay. It is distinguished from baryta, strontia, and lime, by forming soluble salts with sulphuric and oxalic acids, and from magnesia by its carbonate, though sparingly soluble in water, forming with it a solution which has an alkaline reaction. Chloride of Lithium.—It is readily obtained by dissolving lithia or its carbonate in hydrochloric acid. Like the chlorides of sodium and potas- sium, it yields by evaporation in a warm place colourless, anhydrous, cubic crystals, which differ from those chlorides in being very deliquescent, dis- solving freely in alcohol as well as water, and in its alcoholic solution burn- ing with a red flame. The iodide and bromide of lithium have not been examined. Fluoride of Lithium.—This is a fusible compound, prepared by dissolving lithia in hydrofluoric acid, and possesses about the same solubility in water as the carbonate. CLASS I. ORDER II. METALLIC BASES OF THE ALKALINE EARTHS. SECTION IV. BARIUM. Davy discovered barium, the metallic base of baryta, in the year 1808, by a process suggested by Berzelius and Pontin. It consists in forming car- bonate of baryta into a paste with water, placing a globule of mercury in a little hollow made in its surface, and laying the paste on a platinum tray which communicated with the positive pole of a galvanic battery of 100 double plates, while the negative wire was in contact with the mercury. The baryta was decomposed, and its barium combined with mercury. This amalgam was then heated in a vessel free from air, by which means the mercury was expelled, and barium obtained in a pure form. Barium, thus procured, is of a dark gray colour, with a lustre inferior to cast iron. It is far denser than water, for it sinks rapidly in strong sulphu- ric acid. It attracts oxygen with avidity from the air, and in doing so yields a white powder, which is baryta. It effervesces strongly, from the escape of hydrogen gas, when thrown into water, and a solution of baryta is produced. It has hitherto been obtained in very minute quantities, and consequently its properties have not been determined with precision. * The sulphate of lithia may be decomposed by acetate of baryta, and the acetate of lithia thus obtained, by exposure to a red heat, is converted into the carbonate. The carbonate may then be brought to the state of a caustic hydrate by the action of lime in the usual manner.—Ed. 307 The equivalent of barium, deduced from an analysis of the chloride by Berzelius and myself, is 68.7. The composition of its compounds described in this section is as follows :— Protoxide Peroxide Chloride Iodide Bromide Fluoride Sulphuret 68.7 Cyanuret 68.7 Sulphocy. 68.7 1 Barium 68.7 68.7 68.7 68.7 68.7 68.7 1 eq.-f-Oxygen 8 1 eq.: 1 eq.-f-Ditto 16 2eq.= 1 eq.-f Chlorin. 35.42 1 eq.= 1 eq.-f-Iodine 126.3 1 eq.= 1 eq.-f-Bromine 78.4 1 eq.: 1 eq.-f Flu'rine 18.68 1 eq.= 1 eq.-j-Sulphur 16.1 1 eq.= 1 eq. -f Cyanog. 26.39 1 eq.: < Peroxide . 43.7 or one eq. Protoxide . 35.7 or one eq. \ \ Protoxide . 71.4 or two eq. 115.1 115.1 It contains 27.586 per cent of oxygen, and loses 6.896 per cent, when con- verted into the green oxide. Varvicite.—This compound is known only as a natural production; having been first noticed a few years ago by Mr. Phillips among some ores of man- ganese found at Hartshill, in Warwickshire. The locality of the mineral suggested its name; but I have also detected it as the constituent of an ore of manganese from Jhlefeld, sent me by Professor Stromeyer. Varvicite was at first mistaken for peroxide of manganese, to which in the colour of its powder it bears considerable resemblance; but it is readily distinguished from that ore by its stronger lustre, greater hardness, more lamellated tex- ture, which is very similar to that of manganite, and by yielding water freely when heated to redness. Its sp. gravity is 4.531. It has not been found regularly crystallized; but my specimen from Jhlefeld is in pseudo- crystals, possessing the form of the six-sided pyramid of calcareous spar. When strongly heated it is converted into red oxide, losing 5.725 per cent. of water, and 7.385 of oxygen. It is probably, like the red oxide, a com- pound of two other oxides; and the proportions just stated justify the sup- position that it consists of two equivalents of peroxide and one of sesqui- oxide of manganese, united in the mineral with an equivalent of water. (Phil. Mag. and Annals, v. 209, vi. 281, and vii. 284.) It has been inferred from some experiments of Berzelius and John, that there are two other oxides of manganese, which contain less oxygen than 332 MANGANESE. the green or protoxide. We have no proof, however, of the existence of such compounds. Manganic Acid.—Manganese is one of those metals which is capable of forming an acid with oxygen. Manganate of potassa is generated when hydrate or carbonate of potassa is heated to redness with peroxide of man- ganese; and nitre may be used successfully, provided the heat be high enough to decompose the nitrate of potassa. The materials absorb oxygen from the air when fused in open vessels; but manganate of potassa is equal- ly well formed in close vessels, one portion of oxide of manganese then supplying oxygen to another. The product has been long known under the name of mineral chameleon, from the property of its solution to pass rapidly through several shades of colour: on the first addition of cold water, a green solution is formed which soon becomes blue, purple, and red; and ultimately a brown flocculent matter, hydrated peroxide of manganese, sub- sides, and the liquid becomes colourless. These changes, which are more rapid by dilution and with hot water, have been successively elucidated by Chevillot and Edwards, Forchammer, and Mitscherlich. (An. de Ch. et de Ph. viii, and xlix. 113, and An. of Phil, xvi.) The phenomena above mentioned are owing to the formation of manga- nate of potassa of a green colour, and to its ready conversion into the red permanganate of potassa, the blue and purple tints being due to a mixture of these compounds. Manganic acid itself cannot be obtained in an uncom- bined state, because it is then resolved into the hydrated peroxide and oxy- gen, a property which Mitscherlich availed himself of in analyzing this acid; but Mitscherlich has proved that it is anal< gous in composition to sulphuric acid, and its salts isomorphous with the sulphates. Manganate of potassa is obtained in crystals by forming a concentrated solution of mineral chame- leon in cold water, very pure and free from carbonic acid, allowing it to subside in a stoppered bottle, and evaporating the clear green solution in vacuo with the aid of sulphuric acid. All contact of paper and other organic matter must be carefully avoided, since they deoxidize the acid, and the process be conducted in a cool apartment. The crystals are anhydrous, and permanent in the dry state; but in solution the carbonic acid of the air suffices to decompose the acid, or even simple dilution with cold water. Mixed with a solution of potassa, the manganate may be crystallized a second time in vacuo without change. Permanganic Acid.—This acid is more stable than the manganic, though itself very prone to decomposition. Contact with paper or linen as in filter- ing, particles of cork, organic particles floating in the atmosphere decompose it rapidly; colouring matters are bleached by it; and in pure water its de- composition begins at 86°, and is complete at 212°. On these occasions oxygen gas is abstracted or given out, and hydrated peroxide of manganese subsides. The acid has a rich red colour, and is obtained by adding to a solution of permanganate of baryta, a quantity of dilute sulphuric acid ex- actly sufficient for precipitating the baryta. The salts of permanganic acid are more permanent than the free acid; so that most of them may be boiled in solution, especially if concentrated, Permanganate of potassa is obtained by heating a green solution of mineral chameleon however prepared; but a very good process has been indicated by Wohler. (Pog. Annalen, xxvii. 626.) It consists in fusing chlorate of potassa in a platinum crucible, introducing a few fragments of hydrate of potassa, and then adding peroxide of manganese in fine powder. Manga. nate of potassa is instantly formed ; and after decomposing any excess of chlorate of potassa by a red heat, the soluble parts are taken up in pure water, the manganate converted by boiling into permanganate of potassa, and the hot solution, decanted from the insoluble oxide of manganese, is set aside to crystallize. As the permanganate is much less soluble than the chloride of potassium in cold water, the former separates in opaque prisma. tic crystals, and the latter is left in solution. These crystals have so intense a colour that they appear black, and have a lustre like bronze; but their MANGANESE. 333 powder has a purple-red tint They are permanent at common tempera- tures; but when heated give out oxygen gas, and are reconverted into manganate of potassa. They deflagrate like nitre with burning charcoal, and detonate powerfully with phosphorus. Their colour in solution is a rich purple, and a small quantity of the salt imparts this colour to a very large quantity of water. When mixed with dilute nitric acid and boiled, oxygen gas is evolved, and hydrated peroxide of manganese subsides, from the respective quantities of which Mitscherlich ascertained the composition of the acid. In addition to the remarkable analogy which its constitution bears to perchloric acid, Mitscherlich finds that permanganate and per- chlorate of potassa are isomorphous, an observation confirmed by Mr. Miller. Protochloride of Manganese.—This compound is best prepared by evapo- rating a solution of the chloride to dryness by a gentle heat, and heating the residue to redness in a glass tube, while a current of hydrochloric acid gas is transmitted through it. The heat of a spirit-lamp is sufficient for the purpose. It fuses readily at a red heat, and forms a pink-coloured lamel- lated mass on cooling. It is deliquescent, and of course very soluble in water. Perchloride of Manganese.—Dumas discovered this compound, which is readily formed by putting a solution of permanganic into strong sulphuric acid, and then adding fused sea-salt. The hydrochloric and permanganic acids mutually decompose each other ; water and perchloride of manganese are generated, and the latter escapes in the form of vapour. The best mode of preparation is to form the green mineral chameleon, and acidulate with sulphuric acid: the solution, when evaporated, leaves a residue of sulphate and permanganate of potassa. This mixture, treated by strong sulphuric acid, yields a solution of permanganic acid, into which are added small fragments of sea-salt, as long as coloured vapour continues to be evolved. (Edin. Journ. of Science, viii. 179.) The perchloride, when first formed, appears as a vapour of a copper or greenish colour; but on traversing a glass tube cooled to 5° or —4°, it is condensed into a greenish-brown coloured liquid. When generated in a capacious tube, its vapour gradually displaces the air, and soon fills the tube. If it is then poured into a large flask, the sides of which are moist, the colour of the vapour changes instantly on coming into contact with the moisture, a dense smoke of a pretty rose-tint appears, and hydrochloric and permanga- nic acids are generated. It is hence analogous in composition to perman- ganic acid, its elements being in such a ratio that 1 eq. perchloride and 7 eq. water ~~. 1 eq. permang. acid and 7 eq. hydrochloric acid. 2Mn-f-7Cl 7(H-f O) ■* 2Mn-f 70 7(H-f-Cl). Perfluoride of Manganese.—This gaseous compound, discovered by Du- mas and Wohler (Edin. Journ. of Science, ix.), is best formed by mixing common mineral chameleon with half its weight of fluor spar, and decom- posing the mixture in a platinum vessel by fuming sulphuric acid. The fluoride is then disengaged in the form of a greenish-yellow gas or vapour, of a more intensely yellow tint than chlorine. When mixed with atmo- spheric air, it instantly acquires a beautiful purple-rcd colour; and it is freely absorbed by water, yielding a solution of the same red tint. It acts instantly on glass, with formation of fluosilicic acid gas, a brown matter being at the same time deposited, which becomes of a deep purple-red tint on the addi- tion of water. It may be inferred from the experiments of Wohler that this yellow gas is a fluoride of manganese; that when mixed with water both compounds are decomposed, and hydrofluoric and permanganic acids generated, which are dissolved; that a similar formation of the two acids ensues from the admix- ture of the yellow gas with atmospheric air, owing to the moisture contained in the latter; and that by contact with glass, fluosilicic acid gas is produced, and anhydrous permanganic acid deposited. In consequence of its acting 334 IRON. bo powerfully on glass, its other properties have not been ascertained; but from those above mentioned, its composition is obviously similar to that of the gaseous chloride of manganese. Protosulphuret of Manganese may be procured by igniting the sulphate with one-sixth of its weight of charcoal in powder. (Berthier.) It is also formed by the action of hydrosulphuric acid gas on the protosulphate at a red heat. (Arfwedson in Ann. of Phil. vol. vii. N. S.) It occurs native in Cornwall, and at Naygag in Transylvania. It dissolves completely in dilute sulphuric or hydrochloric acid, with disengagement of very pure hydrosul- phuric acid gas. SECTION XII. IRON. Iron has been known from the remotest antiquity. It has a pecdiar gray colour, and strong metallic lustre, which is susceptible of being heightened by polishing. In ductility and malleability it is inferior to several metals, but exceeds them all in tenacity. (Page 282.) At common temperatures it is very hard and unyielding, and its hardness may be increased by being heated and then suddenly cooled ; but it is at the same time rendered brittle. When heated to redness it is remarkably soft and pliable, so that it may be beaten into any form, or be intimately incorporated or welded with another piece of red-hot iron by hammering. Its texture is fibrous. Its specific gravity may be estimated at 7.7; but it varies slightly according to the de- gree with which it has been rolled, hammered, or drawn, and it is increased by fusion. In its pure state it is exceedingly infusible, requiring for fusion the highest temperature of a wind furnace. It is attracted by the magnet, and may itself be rendered permanently magnetic by several processes ;—a property of great interest and importance, and which is possessed by no other metal excepting cobdt and nickel. The occurrence of native iron, except that of meteoric origin, which al- ways contains nickel and cobalt, is exceedingly rare; and few of the speci- mens said to be such have been well attested. In combination, however, especially with oxygen and sulphur, it is abundant; being contained in ani- mals and plants, and being diffused so universally in the earth, that there are few mineral substances in which its presence may not be detected. Minerals which contain iron in such form, and in such quantity, as to be employed in the preparation of the metal, are called ores of iron; and of these the principal are the following. The red oxides of iron, included under the name of red haematite ; the brown haematite of mineralogists, con- sisting of hydrated peroxide of iron ; the black oxide, or magnetic iron ore ; and carbonate of protoxide of iron, either pure, or in the form of clay iron ore, when it is mixed with siliceous, aluminous, and other foreign sub- stances. The three former occur most abundantly in primary districts, and supply the finest kinds of iron, as those of Sweden and India; while clay- iron stone, from which most of the English iron is extracted, occurs in se- condary deposites, and chiefly in the cod formation. The extraction of iron from its ores is effected by exposing the ore, pre- viously roasted and reduced to a coarse powder, to .the action of charcoal or coke, and lime at a high temperature. The action of carbonaceous matter in depriving the ore of its oxygen is obvious; and the lime plays a part equally important. It acts as a flux by combining with all the impurities of the ore, and forming a fusible compound called a slag. The whole mass being thus in a fused state, the particles of reduced metal descend by reason of their greater density, and collect at the bottom; while the slag forms a stratum IRON. 335 above, protecting the melted metal from the action of the air. The latter, as it collects, runs out at an aperture in the side of the furnace; and the fused iron is let off by a hole in the bottom, which was previously filled with sand. The process is never successful unless the flux, together with the im- purities of the ore, are in such proportion as to constitute a fusible com- pound. The mode of accomplishing this object is learned only by expe- rience; and as different ores commonly differ in the nature or quantity of their impurities, the workman is obliged to vary his flux according to the composition of the ore with which he operates. Thus if the ore is deficient in siliceous matter, sand must be added ; and if it contain a large quantity of lime, proportionally less of that earth will be required. Much is often ac- complished by the admixture of different ores wilh each other. The slag consists of a compound of earthy salts, similar to some siliceous minerals, in which silicic acid is combined with lime, alumina, magnesia, protoxide of manganese, and sometimes oxide of iron. The most usual combination, ac- cording to Mitscherlich, is bisilicate of lime and magnesia, sometimes with a little protoxide of iron; a compound which he has obtained in crystals, having the precise form and composition of pyroxcn. Artificial minerals may in fact by such processes be procured, similar in form and composition to those which occur in the earth. We are indebted to Mitscherlich for some valuable facts on this subject. (An. de Ch. et de Ph. xxiv. 355.) The iron obtained by this process is the cast iron of commerce, and con- tains a considerable quantity of carbon, unreduced ore, and earthy sub- stances. It is converted into soft or malleable iron by exposure to a strong heat while a current of air plays upon its surface. By this means any un- decomposed ore is reduced, earthy impurities rise to the surface as slag, and carbonaceous matter is burned. The exposed iron is also more or less oxidized at its surface, and the resulting oxide, being stirred with the fused metal below, facilitates the oxidation of the carbon. As the purity of the iron increases, its fusibility diminishes, until at length, though the tempera- ture continue the same, the iron becomes solid. It is then subjected, while still hot, to the operation of rolling or hammering, by which its particles are approximated, and its tenacity greatly increased. It is then the malleable iron of commerce. It is not, however, absolutely pure; for Berzelius has de- tected in it about one-half per cent, of carbon, and it likewise contains traces of silicium. The carbonaceous matter may be removed by mixing iron filings with a quarter of its weight of black oxide of iron, and fusing the mixture, confined in a covered Hessian crucible, by means of a blast fur- nace. A little powdered green glass should be laid on the mixture, in order that the iron may be completely protected from the air by a covering of melted glass, and any unreduced oxide dissolved. But the best and readiest mode of procuring iron in a state of perfect purity, is by transmitting hy- drogen gas over the pure oxide heated to redness in a tube of porcelain. The oxygen of the oxide unites with hydrogen, and the metal is left in the form of a porous spongy mass. Magnus has observed that the reduction takes place at a heat considerably below that of redness; and that when the iron, thus reduced, is exposed to the air, it takes fire spontaneously, and the oxide is instantly reproduced. This singular property, which Magnus has also remarked in nickel and cobalt prepared in a similar manner, appears to depend on the extremely divided and expanded state of the metallic mass; for when the reduction is effected at a red heat, which enables the metal to acquire its natural degree of compactness, the phenomenon is not observed. If the oxide be mixed with a little alumina, and then reduced at a red heat, the presence of the earth prevents that contraction which would otherwise ensue : the metal is in the same mechanical condition as when it is deoxi- dized at a low temperature, and its spontaneous combustibility is preserved. But iron, in its ordinary state, has a strong affinity for oxygen. In a per- fectly dry atmosphere it undergoes no change; but when moisture is like- wise present, its oxidation, or rusting, is rapid. In the first part of the charg? carbonate of protoxide of iron is generated ; but the prototide gra- 336 IRON. dually passes into hydrated peroxide, and the carbonic acid at the same time is evolved. Rust of iron always contains ammonia, a circumstance which indicates that the oxidation is probably accompanied by decomposition of water; and M. Chevalier has observed that ammonia is also present in the native oxides of iron. Heated to redness in the open air, iron absorbs oxy- gen rapidly, and is converted into black scales, called the black oxide of iron; and in an atmosphere of oxygen gas it burns with vivid scintillations. It decomposes the vapour ofwater, by uniting with its oxygen, at all tempera- tures, from a dull red to a white heat; a singular fact, when it is considered, that, at the very same temperatures, the oxides of iron are reduced to the me- tallic state by hydrogen gas. (Gay-Lussac in An. de Ch. et de Physique, i. 36.) These opposite effects, various instances of which are known to chemists, are accounted for by a mode of reasoning similar to that explained on a for- mer occasion. (Page 131.) The equivalent of iron has not yet been determined with accuracy. From the analysis of its oxides by Berzelius, Stromeyer, and Gay-Lussac, it may be estimated at 27.16, 27.8, and 28.3. In the uncertainty as to which of these numbers is the most accurate, I shall continue to use 28, the number generally adopted in this country. The composition of the compounds of iron described in this section is as follows :— Protoxide Peroxide Black oxide Proto- chloride Perchlo- ride Protio- dide Periodide Protobro- mide Perbro- mide Protofluo- ride Perfluo- ride Tetrasul- phuret Disulphu- ret Protosul- phuret Sesquisul- phuret Bisulphu- ret Magnetic pyrites Diphos- phuret Perphos- phuret Carburets. Iron. 28 1 eq.-f Oxy. 8 1 eq. 56 2 eq.-f do. 24 3 eq. ( Protoxide ... 36 1 eq. > I Peroxide ... 80 1 eq. J i 28 1 eq.-f Chlor. 35.42 1 eq. ( 56 2 eq.-f do. 106.26 3 eq. I 28 1 eq.-f Iodine 126.3 1 eq. 56 2 eq.-f do. 378.9 3 eq. * 28 1 eq.-f Brom. 78.4 1 eq. I 56 2 eq.-f do. 235.2 3 eq. { 28 1 eq.-f Fluor. 18.68 1 eq. £ 56 2 eq.+do. 56.04 3 eq. 1112 4 eq.-f Sulph. 16.1 1 eq. i 56 2 eq.-f do. 16.1 1 eq. i 28 1 eq.-f do. 16.1 1 eq. I 56 2 eq.-f do. 48.3 3 eq. i 28 1 eq.-f do. 32.2 2 eq. ! Bisulph. of iron 60.2 1 eq. ) Protosulph. of /• = iron 220.5 5 eq. ) | 56 2 eq.-f Phosp. 15.7 1 eq. I 84 3 eq.-f do. 62.8 4 eq. Constitution not determined. Equiv. Formulae. = 36 Fe-fOorFe. = 80 2Fe-f 30 or Fe. = 116 Fe-fF^ = 63.42 Fe+Cl or FeCl. = 162.26 2Fe-f3ClorFe*C13. = 154.3 Fe-f I or Fel. =434.9 2Fe-f3IorFe2l3. = 106.4 Fe-f-Br or FeBr. =2915 2Fe-f3BrorFeaBr3. = 46.68 Fe-f F or FeF. = 112.04 2Fe-f 3F or Fe*F3. = 128.1 4Fe+S or Fe*S. = 72.1 2Fe-f-S or Fe S. = 44.1 Fe-fSorFeS. = 104.3 2Fe+3Sor Fe2S3. = 60.2 Fe-f2SorFeS2. 280.7 5FeS-fFeS2. 71.7 2Fe+P or Fe2P. 146.8 3Fe-f 4P or Fe3P*. 377 Iron, Equiv. Formulae. PlMrSa" ( 28 * e(l-+Cyan- ~6-39 1 eq-=54.39 Fe-f Cy or FeCy. phocyanu- i 28 1 eq.) gj,a™- g|39 158.59 1 eq.= 86.59 Fe-fCyS2. Sesquisul- ) I c- i l f ) phocyanu- > 56 2 eq J Blsu,Ph- ot C 175.77 3 eq.=231.77 2Fe-f3CvS?. 1 ) cyanogen ( n ' ret OXIDES OF IRON Protoxide.—This oxide is the base of the native carbonate of iron, and of the green vitriol of commerce. Its existence was inferred some years ago by Gay-Lussac; (An. de Ch. vol. lxxx.) but it is doubtful if it has ever been obtained in an insulated form. Its salts, particularly when in solution, ab- sorb oxygen from the atmosphere with such rapidity that they may even be employed in eudiometry. This protoxide is always formed with evolution of hydrogen gas when metallic iron is put into dilute sulphuric acid ; and its composition may be determined by collecting and measuring the gas which is disengaged. Protoxide of iron is precipitated from its salts as a white hydrate by pure alkdies, as a white carbonate by alkaline carbonates, and as a white ferro- cyanuret by ferrocyanuret of potassium. The two former precipitates be- come first green and then red, and the latter, green and blue by exposure to the air. The solution of gdl-nuts produces no change of colour. Hydro- sulphuric acid does not act if the protoxide is united with any of the stronger acids; but alkaline hydrosulphates cause a black precipitate, protosulphu- ret of iron. Peroxide.—The red or peroxide is a natural product, known to mineralo- gists under the name of red hematite. It sometimes occurs massive, at other times fibrous, and occasionally in the form of beautiful rhomboidal crystals. It may be made chemically by dissolving iron in nitro-hydrochloric acid, and adding an dkali. The hydrate of the red oxide of a brownish-red co- lour subsides, which is identical in composition with the mineral called brown hamatite, and consists of 80 parts or one equivdent of the peroxide, and 18 parts or two equivalents of water. Peroxide of iron is not attracted by the magnet. Fused with vitreous substances, it communicates to them a red or yellow colour. It combines with most of the acids, forming sdts, the greater number of which are red. Its presence may be detected by very decisive tests. The pure alkalies, fixed or volatile, precipitate it as the hydrate. Alkaline carbonates have a similar effect, peroxide of iron not forming a permanent salt with carbonic acid. With ferrocyanuret of potassium it forms Prussian blue. Sulphocya- nuret of potassium causes a deep blood-red, and infusion of gall-nuts, a black colour. Hydrosulphuric acid converts the peroxide into protoxide of iron, with deposition of sulphur. These reagents, and especially ferrocyanuret and sulphocyanuret of potassium, afford an unerring test of the presence of minute quantities of peroxide of iron. On this account it is customary, in testing for iron, to convert it into the peroxide, an object which is easily ac- complished by boiling the solution with a small quantity of nitric acid. Black Oxide.—This substance, the oxidum ferroso-ferricum of Berzelius, long supposed to be protoxide of iron, contains more oxygen than the pro- toxide, and less than the red oxide. It cannot be regarded as a definite compound of iron and oxygen ; but it is composed of the two real oxides united in a proportion which is by no means constant. It occurs native, frequently crystallized in the form of a regular octohedron ; and it is not only attracted by the magnet, but is itself sometimes magnetic. It is always formed when iron is heated to redness in the open air; and is likewise gene- rated by the contact of watery vapour with iron at elevated temperatures. 29 338 IRON! The composition of the product, however, varies with the duration of the process and the temperature which is employed. Thus, according to Buch- holz, Berzelius, and Thomson, 100 parts of iron, when oxidized by steam, unite with nearly 30 of oxygen ; whereas in a similar experiment performed by Gay-Lussac, 37.8 parts of oxygen were absorbed. The oxide of Gay- Lussac has the composition stated in the table; and Berzelius thinks that of magnetic iron ore to be similar. M. Mosander states, that on heating a bar of iron in the open air, the outer layer of the scales contains a greater quan- tity of peroxide than the inner layer. The former consists of one equivalent of peroxide to four of the protoxide, and in the latter are contained one equi- valent of peroxide to six equivalents of protoxide. The inner layer seems uniform in composition; but the outer is variable, its more exposed parts being richer in oxygen. The nature of the black oxide is farther elucidated by the action of acids. On digesting the black oxide in sulphuric acid, an olive-coloured solution is formed, containing two salts, sulphate of the peroxide and protoxide, which may be separated from each other by means of alcohol. (Proust and Gay- Lussac.) The solution of these mixed sdts gives green precipitates with alkalies, and a very deep blue ink with infusion of gall-nuts. The black oxide of iron is the cause of the dull green colour of bottle glass. Protochloride of Iron.—This compound is formed by transmitting dry hydrochloric acid gas over iron at a red heat, when hydrogen gas is evolved and the surface of the iron is covered with a white crystalline protochloride which at a stronger heat is sublimed. Also, on acting with hydrochloric acid on iron, which is dissolved with evolution of hydrogen gas, evaporating to dryness, and heating to redness in a tube without exposure to the air, a gray crystalline protochloride is left; but it contains some protoxide formed by an interchange of elements between the last portions of water and the chloride, hydrochloric acid being also generated. Protochloride of iron dissolves freely in water, yielding a pale green solu- tion, from which rhomboidal prisms of the same colour are obtained by evaporation. The crystals contain several equivalents of water of crystalli- zation, deliquesce by exposure to the air, owing to the formation of perchlo- ride, and are soluble in alcohol as well as water. The aqueous solution ab- sorbs oxygen from the air, and becomes yellow from the formation of perchlo- ride of iron : one portion of iron takes oxygen from the air, and yields its chlorine to another portion of iron, whereby perchloride and peroxide of iron are generated, and the latter falls as an ochreous sediment combined with some of the perchloride. A solution of the protochloride of iron dissolves binoxide of nitrogen with the same phenomena as the protosulphate (page 178), a circumstance favourable to the view entertained by many that protochloride of iron is converted by water into hydrochlorate of the pro- toxide. Perchloride of Iron.—It is formed by the combustion of iron wire in dry chlorine gas, and by transmitting that gas over iron moderately heated ; when it is obtained in small iridescent plates of a red colour, which are volatile at a heat a little above 212°, deliquesce readily, and dissolve in water, alcohol, and ether. On agitating ether with a strong aqueous solution of the per- chloride, the ether abstracts a part of it, and acquires a gold-yellow colour. The readiest mode of obtaining a solution of the perchloride is to dissolve peroxide of iron in hydrochloric acid. On concentrating to the consistence of syrup and cooling, it separates as red crystals, which by distillation yield at first water and hydrochloric acid, and then anhydrous perchloride of iron, leaving a compound of peroxide and perchloride of iron in crystalline laminae. The formation of peroxide appears due to an interchange of elements be- tween it and water. The same kind of interchange ensues between the vapours of water and the perchloride at a high temperature; and this is pro- bably the source, as Mitscherlich suggests, of the cr}stals of peroxide of iron found in volcanic products. Protiodide of Iron.—It exists as a pale green solution when iodine is di- IRON. 339 gested with water and iron wire, the latter being in excess; and on evapo- rating the solution, without exposure to the air, to dryness, and heating mode- rately, thq, protiodide is fused, and on cooling becomes an opaque crystalline mass of an iron-gray colour and metallic lustre. It is deliquescent and very soluble in water and alcohol. Its aqueous solution attracts oxygen rapidly from the air, undergoing the same kind of change as the protochloride: to preserve a solution of protiodide as such, a long piece of iron wire should be kept permanently in the liquid. This compound has been very successfully employed in medical practice by my colleague Dr. A. T. Thomson. The periodide, of a yellow or orange colour according to the strength of the solution, is obtained by freely exposing a solution of the protiodide to the air, or digesting iron wire with excess of iodine, gently evaporating, and sub- liming the periodide. It is a volatile red compound, deliquescent, and solu- ble in water and alcohol. The bromides of iron are formed under similar conditions to the chlorides and iodides, and are very analogous to them in their properties. Protofluoride of Iron is best prepared by dissolving iron in a solution of hydrofluoric acid, out of which it crystallizes as the acid becomes saturated, in small white square tables, which are sparingly soluble in water, and be- come pale yellow by the action of the air. By heat they part with their water of crystallization, and afterwards bear a red heat without decomposi- tion. (Berzelius.) The perfluoride is formed by dissolving peroxide of iron in hydrofluoric acid, and yields a colourless solution even when saturated. By evaporation it is left as a crystalline mass of a pale flesh-colour, and of a mild astringent taste. It is sparingly soluble in water. Sulphurets of Iron.—These elements have for each other a remarkably strong affinity, and unite under various circumstances and in several propor- tions. The two lowest degrees of sulphuration, the tetrasulphuret and disul- phuret, were prepared by Arfwedson by transmitting a current of hydrogen gas, at a red heat, over the anhydrous disulphate of peroxide of iron to pro- cure the tetrasulphuret, and over anhydrous sulphate of protoxide of iron for the disulphuret. In both cases sulphurous acid and water are evolved, and the resulting sulphurets are left as grayish-black powders, susceptible of a metallic lustre by friction. They both dissolve in dilute sulphuric acid with evolution of hydrogen and hydrosulphuric acid gases. Protosulphuret of Iron is prepared by heating thin laminae of iron to red- ness with sulphur in a covered Hessian crucible, and continuing the heat until any excess of sulphur is expelled. The iron is found with a crust of protosulphuret, which is brittle, of a yellowish-gray colour and metallic lustre, and is attracted by the magnet. When pure it is completely dissolved by dilute sulphuric acid, yielding pure hydrosulphuric acid. The protosulphu- ret of iron exists in nature as an ingredient in variegated copper pyrites ; and it falls on mixing hydrosulphate of ammonia with sulphate of protoxide of iron as a black precipitate, which oxidizes rapidly by absorbing oxygen from the air, as soon as the excess of hydrosulphate of ammonia is removed by washing. The sesquisulphuret is formed in the moist way by adding perchloride of iron drop by drop to hydrosulphate of ammonia or sulphuret of potassium in excess, and falls as a black precipitate, which is oxidized readily by the air. In the dry way it is slowly produced by the action of hydrosulphuric acid gas on peroxide of iron at a heat not exceeding 212°, water being also formed; and by the action of the same gas on the hydrated peroxide at common temperatures. This sulphuret, when anhydrous, has a yellowish- gray colour, is not attracted by the magnet, and dissolves in dilute sulphuric or hydrochloric acid, yielding hydrosulphuric acid and a residue of bisuh phuret of iron. (Berzelius.) Bisulphuret of Iron, iron pyrites of mineralogists, exists abundantly in the earth. It occurs in cubes or some allied form, has a yellow colour, metallic lustre, a density of 4.981, and is so hard that it strikes fire with 340 IRON. steel. Some varieties have a white colour; but these usually contain arsenic. Others occur in rounded nodules, have a radiated structure divergent from a common centre, are often found in beds of clay, and are much disposed by the influence of air and moisture to yield sulphate of oxide of iron: these are suspected by Berzelius to be compounds of proto- sulphuret and bisulphuret of iron. Bisulphuret of iron is not attacked by any of the acids except the nitric, and its best solvent is the nitro-hydrochloric acid. Heated in close vessels it gives off nearly half its sulphur, and is converted into magnetic iron pyrites. By heat and air together it yields peroxide of iron. Magnetic Iron Pyrites.—This is a natural product, termed magnetic pyrites from being attracted by the magnet, and was formerly regarded as protosulphuret of iron; but Stromeyer has shown that its elements are in such a ratio, that it may be regarded as a compound of bisulphuret and pro- tosulphuret. It is formed by heating the bisulphuret to redness in close vessels, by fusing iron filings with half their weight of sulphur, or by rub- bing sulphur upon a rod of iron heated to whiteness. It is soluble in dilute sulphuric acid, yielding hydrosulphuric acid gas and a residue of sulphur. It is much more oxidable by air and moisture than the pure bisulphuret. Diphosphuret of Iron.—It is prepared by exposing the phosphate of pro- toxide of iron to a strong heat in a covered crucible lined with charcoal, the excess of phosphorus being dissipated in vapour. It is a fused granular mass, of the colour and lustre of iron, but very brittle, and is not attacked by hydrochloric acid. It is sometimes contained in metallic iron, to the properties of which it is very injurious by rendering it brittle at common temperatures. The perphosphuret has been obtained by Rose by the action of phosphu- retted hydrogen gas on sulphuret of iron at a moderate temperature, and resembles the former in its properties. Carburets of Iron.—Carbon and iron unite in very various proportions; but there are three compounds very distinct from each other—namely, graphite, cast or pig iron, and steel. Graphite, also known under the name of plumbago and black-lead, occurs not unfrequently as a mineral production, and is found in great purity at Borrowdale in Cumberland. It may be made artificially by exposing iron with excess of charcoal to a violent and long-continued heat; and it is com- monly generated in small quantity during the preparation of cast iron. Pure specimens contain about four or five per cent, of iron, but sometimes its quantity amounts to 10 per cent. Most chemists believe the iron to be chemically united with the charcoal; but according to the researches of Dr. Karsten of Berlin, native graphite is only a mechanical mixture of charcoal and iron, while artificial graphite is a real carburet. Graphite is exceedingly unchangeable in the air, and like charcoal is at- tacked with difficulty by chemical reagents. It may be heated to any ex- tent in close vessels without change; but if exposed at the same time to the air, its carbon is entirely consumed, and oxide of iron remains. It has an iron-gray colour, metallic lustre, and granular texture; and it is soft and unctuous to the touch. Its chief use is in the manufacture of pencils and crucibles; and in burnishing iron to protect it from rust. Cast iron is the product of the process for extracting iron from its ores, and is commonly regarded as a real compound of iron and charcoal. It always contains impurities, such as charcoal, undecomposed ore, and earthy matters, which are often visible by mere inspection; and sometimes traces of chromium, manganese, sulphur, phosphorus, and arsenic are present. It fuses readily at 2786° F. (Daniell), which is a full red heat, and in cooling it acquires a crystalline granular texture. The quality of different speci- mens is by no means uniform; and two kinds, white and gray cast iron, are in particular distinguished from each other. The former is exceedingly hard and brittle, sometimes breaking like glass from sudden change of tem- perature ; while the latter is softer and much more tenacious. This differ- ZINC. 341 ence appears owing to the mode of combination, rather than to a difference in the proportion of carbon; for the white variety may be converted into the gray by exposure to a strong heat and cooling slowly, and the gray may be changed into the white by being heated and rapidly cooled. According to Karsten, the carbon of the latter is combined with the whole mass of iron, and amounts as a maximum to 5:2.r> per cent ; but in some specimens its proportion is considerably less. The former, on the contrary, contains from 3.15 to 4.0'.} per cent, of carbon, of which about three-fourths are in the state of graphite, and are left as such after the iron is dissolved by acids ; while the remaining fourth is in combination with the whole mass of metal, constituting a carburet which is very similar to steel. Gray cast iron may hence be regarded as a kind of steel, in which graphite is mechanically mixed. Steel is commonly prepared in this country by the process of cementa- tion, which consists in filling a large furnace with alternate strata of bars of the purest malleable iron and powdered charcoal, closing every aperture so as perfectly to exclude atmospheric air, and keeping the whole during seve- ral days at a red heat. By this treatment the iron gradually combines with from 1.3 to 1.75 per cent, of carbon, its texture is greatly changed, and its surface is blistered, it is subsequently hammered at a red heat into small bars, and may be welded either with other bars of steel or with malleable iron. Mr. Mackintosh of Glasgow has introduced an elegant process of forming steel by exposing heated iron to a current of coal gas; when carbu- retted hydrogen is decomposed, its carbon enters into combination with iron, and hydrogen gas is evolved. In ductility and malleability it is far inferior to iron ; but exceeds it greatly in hardness, sonorousness, and elasticity. Its texture is also more compact, and it is susceptible of a higher polish. It sustains a full red heat without fusing, and is, therefore, less fusible than cast iron; but it is much more so than malleable iron. By fusion it forms cast steel, which is more uniform in composition and texture, and possesses a closer grain than ordi- nary steel. Protocyanuret of Iron.—This compound is obtained as a yellow hydrate by mixing in solution cyanuret of potassium with sulphate of protoxide of iron ; but by the action of the air, it passes into Prussian blue. Berzelius obtains it as a yellowish-gray powder by heating in a tube the double cyanuret of iron and hydroeyanate of ammonia until the latter is ex- pelled. Protosulphocyanuret of Iron.—Hydrosulphocyanic acid dissolves iron with evolution of hydrogen gas, yielding a pale green solution, which may be obtained in a dry state by evaporation in vacuo. Atmospheric oxygen reddens it rapidly, and an ochreous sediment falls. Sesquisulphocyanuret of Iron.—This compound, of a blood-red colour, is formed by mixing in solution any salt of the peroxide of iron with sulpho- cyanuret of potassium, its intense colour affording a very delicate test of iron (page 277). By dissolving hydrated peroxide of iron in hydrosulpho- cyanic acid, and evaporating, the sesquisulphocyanuret is obtained as a red, mass, deliquescent, and soluble in water and alcohol. SECTION XIII. ZINC—CADMIUM. ZINC. This metal was first mentioned under the term zinetum in the sixteenth century by Paracelsus; but it was probably known at a much earlier period. 342 ZINC. In commerce it is often called spelter, and is obtained either from calamine, native carbonate of zinc, or from the native sulphuret, zinc blende of mine- ralogists. It is procured from the former by heat and carbonaceous mat- ters; and from the latter by a similar process after the ore has been pre- viously oxidized by roasting, that is, by exposure to the air at a low red heat. Its preparation affords an instance of what is called distillation by descent. The furnace or crucible for reducing the ore is closed above, and in its bottom is fixed an iron tube, the upper aperture of which is in the in- terior of the crucible, and its lower terminates just above a vessel of water. The vapour of zinc, together with all the gaseous products, passes through this tube, and the zinc is condensed. The first portions are commonly very impure, containing cadmium and arsenic, the period of their disengage- ment being indicated by what the workmen call the brown blaze; but when the blue blaze begins, that is, when the metallic vapour burns with a bluish- white flame, the zinc is collected. As thus obtained, it is never quite pure: it frequently contains traqes of charcoal, sulphur, cadmium, arsenic, lead, and copper; and iron is always present. It may be freed from these impu- rities by distillation,—by exposing it to a white heat in an earthen retort, to which a receiver full of water is adapted; but the first portions, as liable to contain arsenic and cadmium, should be rejected. Zinc has a strong metallic lustre, and a bluish-white colour. Its texture is lamellated, and its density about 7. It is a hard metal, being acted on by the file with difficulty. At low or high degrees of heat it is brittle; but at temperatures between 210° and 300° F. it is both malleable and ductile, a property which enables zinc to be rolled or hammered into sheets of consid- erable thinness. Its malleability is considerably diminished by the impu- rities which the zinc of commerce contains. It fuses at 773° F. (Daniell), and when slowly cooled crystallizes in four or six-sided prisms. Exposed in close vessels to a white heat, it sublimes unchanged. Zinc undergoes little change by the action of air and moisture. When fused in open vessels it absorbs oxygen, and forms the white oxide, called flowers of zinc. Heated to full redness in a covered crucible, it bursts into flame as soon as the cover is removed, and burns with a brilliant white light. The combustion ensues with such violence, that the oxide as it is formed is mechanically carried up into the air. The heat at which it begins to burn is estimated -by Daniell at 941° F. Zinc is readily oxidized by dilute sul- phuric or hydrochloric acid, and the hydrogen which is evolved contains a small quantity of metallic zinc in combination. Gay-Lussac and Berzelius found that the protoxide of zinc consists of 100 parts of metallic zinc and 24.8 of oxygen, being a ratio of 32.3 to 8. Its other combinations justify the adoption of 32.3 as the equivalent of zinc. The composition of its compounds described in this section is as follows :— Zinc. - Equiv. Formulae. Protoxide 32.3 1 eq.-f Oxygen 8 1 eq.= 40.3 Zn + OorZn. Peroxide Composition uncertain. Chloride 32.3 1 eq.-f Chlorine 35.42 1 eq.= 67.72 Zn-f CI or ZnCl. Iodide 32.3 1 eq.-f-Iodine 126.3 1 eq.= 158.6 Zn-florZnl. Bromide 32.3 1 eq.-f Bromine 78.4 1 eq.= 110.7 Zn-f Br orZnBr. Fluoride 32.3 1 eq.-f Fluorine 18.68 1 eq.= 50.98 Zn + F or ZnF. Sulphuret 32.3 1 eq.-f-Sulphur 16.1 1 eq.= 48.4 Zn-fSorZnS. Cyanuret 32.3 1 eq.-|-Cyanogen 26.39 1 eq.= 58.69 Zn-f CyorZnCy. Protoxide of Zinc.—This is the only oxide of zinc which acts as a sali- fiable base, and the only one of known composition. It is generated during the solution of zinc in dilute sulphuric acid, and may be obtained in a dry state by collecting the flakes which rise during the combustion of zinc, or by heating the carbonate to redness. At common temperatures it is white ; but when heated to low redness, it assumes a yellow colour, which gradudly disappears on cooling. It is quite fixed in the fire. It is insoluble in water, CADMIUM. 343 and, therefore, does not affect the blue colour of plants ; but it is a strong sali- fiable base, forming regular salts with acids, most of which are colourless. It combines also with some of the alkalies. The presence of zinc is easily recognized by the following characters.__ The oxide is precipitated from its solutions as a white hydrate by pure po- tassa or ammonia, and as carbonate by carbonate of ammonia, but is com- pletely redissolved by an excess of the precipitant. The fixed alkaline carbonates precipitate it permanently as white carbonate of oxide of zinc. Hydrosulphate of ammonia causes a white precipitate, a hydrated sulphuret of zinc. Hydrosulphuric acid acts in a similar manner, if the solution is quite neutrd; but it has no effect if an excess of any strong acid is present. When metallic zinc is exposed for some time to air and moisture, or is kept under water, it acquires a superficial coating of a gray matter, which Berzelius describes as a sub-oxide. It is probably a mixture of metallic zinc and the white oxide, into which it is resolved by the action of acids. The peroxide is prepared, according to Thenard, by acting on hydrated white oxide of zinc with peroxide of hydrogen diluted with water. It re- solves itself so readily into oxygen and the oxide already described, that it cannot be preserved even under the surface of water ; and its composition is quite unknown. Chloride of Zinc.—This compound is formed, with evolution of heat and light, when zinc filings are introduced into chlorine gas; and it is readily prepared bv dissolving zinc in hydrochloric acid, evaporating to dryness, and heating the residue in a tube through which dry hydrochloric acid gas is transmitted. It is colourless, fusible at a heat a little above 212°, has a soft consistence at common temperatures, hence called butter of zinc, sublimes at a red heat, and deliquesces in the air. Iodide of Zinc is prepared by digesting iodine in water with zinc filings in excess. A colourless solution results, which by evaporation yields a deli- quescent iodide. By heat in close vessels it may be sublimed, and then crystallizes in brilliant needles; but if heated in the open air, oxide of zinc is formed, and iodine expelled. If zinc is digested in water with an excess of iodine, a brown solution results, which probably contains a biniodide. Bromide of Zinc may be formed by a process similar to that for the iodide, but its properties have not been studied. Fluoride of Zinc is obtained by acting directly on oxide of zinc with hy- drofluoric acid, and is a white compound of sparing solubility. Sulphuret of Zinc.—This compound is well known to mineralogists under the name of zinc blende, and occurs in dodecahedral crystals or some allied form. Its structure is lamellated, lustre adamantine, and colour variable being sometimes yellow, red, brown, or black. It may be formed artifi- cially by igniting, in a closed crucible, a mixture of oxide of zinc and sul- phur, or sulphate of oxide of zinc and charcoal, or by drying the hydrated sulphuret of zinc. CADMIUM. Cadmium so called (from k*Sjufiu, a term applied both to calamine and to the volatile matters which rise from the furnace in preparing brass) because it is associated with zinc, was discovered in the year 1817 by Stromeyer in an oxide of zinc which had been prepared for medical use; and he has since found it in several of the ores of that metal, especially in a radiated blende from Bohemia, which contains about five per cent, of cadmium. The late Dr. Clarke detected its existence in some of the zinc ores of Derbyshire, and in the common zinc of commerce. Mr. Herapath has found it in consi- derable quantity in the zinc works near Bristol. During the reduction of calamine by coal, the cadmium, which is very volatile, flies off in vapour mixed with soot and some oxide of zinc, and collects in the roof of the vault, just above the tube leading from the crucible. Some portions of this substance yielded from 12 to 20 per cent, of cadmium. (An. of Phil. xiv. and xvii.) 341 CADMIUM. The process by which Stromeyer separates cadmium from zinc or other metals is the following. The ore of cadmium is dissolved in dilute sulphu- ric "or hydrochloric acid, and after adding a portion of free acid, a current of hydrosulphuric acid gas is transmitted through the liquid, by which means the cadmium is precipitated as sulphuret, while the zinc continues in solu- tion. The sulphuret of cadmium is then decomposed by nitric acid, and the solution evaporated to dryness. The dry nitrate is dissolved in water, and an excess of carbonate of ammonia added. The white carbonate of oxide of cadmium subsides, which, when heated to redness, yields a pure oxide. By mixing this oxide with charcoal, and exposing the mixture to a red heat, metallic cadmium is sublimed. A very elegant process for separating zinc from cadmium was proposed by Wollaston. The solution of the mixed metals is put into a platinum cap- sule, and a piece of metallic zinc is placed in it. If cadmium is present, it is reduced, and adheres so tenaciously to the capsule, that it may be washed with water without danger of being lost. It may then be dissolved either by nitric or dilute hydrochloric acid. Cadmium, in colour and lustre, has a strong resemblance to tin, but is somewhat harder and more tenacious. It is very ductile and mdleable. Its specific gravity is 8.604 before being hammered, and 8.694 afterwards. It melts at about the same temperature as tin, and is nearly as volatile as mercury, condensing like it into globules which have a metallic lustre. Its vapour has no odour. When heated in the open air, it absorbs oxygen, and is converted into an oxide. Cadmium is readily oxidized and dissolved by ni- trie acid, which is its proper solvent. Sulphuric and hydrochloric acids act upon it less easily, and the oxygen is then derived from water. The equivalent of cadmium, deduced from Stromeyer's analysis of its oxide, is 55.8. The composition of its compounds described in this section is as follows :— Cadmium. Equiv. Formulae. °Cadmf( 55-8 1 eq.-f Oxygen 8 1 eq.= 63.8 Cd-fOorCd. Chloride 55.8 1 eq.-f Chlorine 35.42 1 eq.= 91.22 Cd-f CI or CdCl. Iodide 55.8 1 eq. +Iodine 126.3 1 eq.= 182.1 Cd-f-IorCdI. Sulphuret 55.8 1 eq.-f-Sulphur 16.1 1 eq.= 71.9 Cd-fSorCdS. Oxide of Cadmium.—This, the only known oxide of cadmium, is pre- pared by igniting its carbonate, has an orange colour, is fixed in the fire, and is insoluble in water. It has no action on test paper, but is a strong alkaline base, forming neutral salts with acids. It is precipitated as a white hydrate by pure ammonia, but is redissolved by excess of that alkali. It is precipitated permanently by pure potassa or soda as a hydrate, and by all the alkaline carbonates as carbonate of oxide of cadmium. Chloride of Cadmium.—By dissolving oxide of cadmium in hydrochloric acid and concentrating duly, the chloride with water of crystallization crys- tallizes in transparent four-sided rectangular prisms, which lose their water by heat and even in a dry air, fuse at a heat short of redness, and acquire a lamellated texture in cooling. At a high temperature it is sublimed. Iodide of Cadmium may be formed in tlie same manner as iodide of zinc, is soluble in water and dcohol, and crystallizes by evaporation in large, colourless, transparent, hexagonal tables, which do not change in the air, and have a pearly lustre. By heat they lose water and then fuse. Sulphuret of Cadmium occurs in mixture or combination in some kinds of zinc blende, and is easily prepared by the action of hydrosulphuric acid on a salt of cadmium. It has a yellowish-orange colour, and is distinguish- ed from the sulphurets of arsenic by being insoluble in pure potassa, and by sustaining a white heat without subliming. (Stromeyer.) TIN. 345 SECTION XIV. TIN. Tin was known to the ancients, who obtained it principally, if not solely, from Cornwall. The tin of commerce is distinguished into two varieties, called block and grain tin, both of which are procured from the native oxide by means of heat and charcoal. In Cornwall, which has been celebrated for its tin mines during many centuries, the ore is both extracted from veins, and found in the form of rounded grains among beds of rolled materials, which have been deposited by the action of water. These grains, common- ly called stream tin, contain a very pure oxide, and yield the purest kind of grain tin. An inferior sort is prepared by heating bars of tin, extracted from the common ore, to very near their point of fusion, when the more fusible parts, which are the purest, flow out; and the less fusible portions constitute block tin. The usual impurities are iron, copper and arsenic. Tin has a white colour, and a lustre resembling that of silver. The bril- liancy of its surface is but very slowly impaired by exposure to the atmo- sphere, nor is it oxidized even by the combined agency of dr and moisture. Its malleability is very considerable; for the thickness of common tin-foil does not exceed 1-1000th of an inch. In ductility and tenacity it is inferior to several metals. It is soft and inelastic, and when bent backwards and forwards, emits a peculiar crackling noise. Its specific gravity is about 7.2. At 442° F. it fuses, and if exposed at the same time to the air, its sur- face tarnishes, and a gray powder is formed. When heated to whiteness, it takes fire and burns with a white flame, being converted into peroxide of tin. ' The equivalent of tin deduced by Berzelius from his analysis of its oxides is 57.9. The composition of the compounds of tin described in this section is as follows:— Tin. Equiv. Formulae. Protoxide . 57.9 1 eq.-f-Oxygen 8 1 eq.= 65.9 Sn + OorSn. Sesquioxide 115.8 2 eq.-f do. 24 3 eq.=139.8 2Sn + 30 or Sn. Binoxide . 57.9 1 eq.-f do. 16 2 eq.= 73.9 Sn-f20orSnT Protochloride 57.9 1 eq.-f Chlorine35.42 1 eq.= 93.32 Sn-fCl or SnCl. Bichloride . 57.9 1 eq.-f do. 70.84 2 eq.=128.74 Sn-f-2Clor SnCl3. Protiodide . 57.9 1 eq.-f Iodine 126.3 1 eq.=184.2 Sn + IorSnI. Biniodide . 57.9 1 eq.-f do. 252.6 2 eq.=3l0.5 Sn-f2I or SnR Protosulphuret57.9 1 eq.-f-Sulphur 16.1 1 eq.= 74. Sn + SorSnS. Sesquisulph. 115.8 2 eq.-f do. 48.3 3 eq.=164.1 2Sn-f 3S or Sn^S3. Bisulphuret 57.9 1 eq.-f do. 32.2 2 eq.== 90.1 Sn-f-2S or SnSa. Terphosphuret 57.9 1 eq. -f Phosph. 47.1 3 eq.=105. Sn -f 3P or SnPa. Protoxide of Tin.—When chloride of tin in solution is mixed with an alkaline carbonate, hydrated oxide of tin falls, which may be obtained as such in a dry form by washing with warm water, and drying at a heat not above 196°, with the least possible exposure to the air. This hydrate is white, burns like tinder when suddenly heated, and is decomposed by being boiled in water. The best mode of obtaining the anhydrous oxide is by heating the hydrate to redness in a tube from which air is excluded by a current of carbonic acid gas. The same oxide is formed when tin is kept for some time fused in an open vessel. Protoxide of tin has a sp. gravity of 6.666. At common temperatures it is permanent in the air, but if touched by a red hot body, it takes fire and is converted into the peroxide. It is dissolved by the sulphuric and hydro- chloric acids, as also by dilute nitric acid; and the pure fixed alkalies like- wise dissolve it. From the alkaline solution, metallic tin is gradually de- posited, and peroxide of tin remains in solution, Its salts are remarkably 346 TIN. prone to absorb oxygen, both from the air and from compounds which yield oxygen readily. Thus it converts peroxide of iron into protoxide, and throws down mercury, silver, and platinum in the metallic state from their salts. With a solution of gold it causes a purple precipitate, the purple of Cassius, which appears to be a compound of peroxide of tin and protoxide of gold. By this character protoxide of tin is recognized with certainty. It is thrown down by hydrosulphuric acid as black protosulphuret of tin. Sesquioxide of Tin.—Fuchs has lately succeeded in preparing this oxide by mixing recently precipitated and moist hydrated peroxide of iron with a solution of protochloride of tin as free as possible from hydrochloric acid; when by an interchange of elements 1 eq. perox. iron & 2 eq. chloride of tin 2 ] eq. sesquiox. tin & 2 eq. chlo. iron. 2Fe-f30 2(Sn + Cl) -2 2Sn-f-30 2(Fe + Cl). The sesquioxide falls as a slimy gray matter, and in general rather yellow from adhering oxide of iron. Berzelius obtained it purer by using a solu- tion made by saturating hydrochloric acid as far as possible with hydrated peroxide of iron. The sesquioxide of tin, while moist, is soluble in hydro- chloric acid, and the solution strikes the purple of Cassius with gold; and it is readily soluble in a solution of ammonia, which distinguishes it from the protoxide of tin, just as its action on gold does from the binoxide. (Pog. Annalen, xxviii. 443.) Binoxide of Tin is most conveniently prepared by the action of nitric acid on metallic tin. Nitric acid, in its most concentrated state, does not act easily upon tin; but when a small quantity of water is added, violent effervescence takes place owing to the evolution of nitrous acid and bin- oxide of nitrogen, and a white powder, the hydrated binoxide, is produced. On edulcorating this substance, and heating it to redness, watery vapour is expelled, and the pure binoxide, of a straw-yellow colour, remains. In this process ammonia is generated, a circumstance which proves water as well as nitric acid to be decomposed. Binoxide of tin may likewise be obtained by precipitation from a solution of perchloride of tin by potassa, ammonia, or the alkaline carbonates; but in this case it falls as a very bulky hydrate, different from the other hydrate both in appearance and several of its chemical properties. Thus the latter dissolves readily in sulphuric, nitric, and hydrochloric acid, even when diluted; while the former is completely insoluble in the same acids, even when concentrated. It unites, indeed, with hydrochloric acid, and the compound is soluble in pure water. Binoxide of tin has very little disposition in any state to unite with acids, and, when dissolved by them, is very apt to separate itself spontaneously as a gelatinous hydrate. It acts the part of a feeble acid: it reddens litmus when its hydrate moistened is laid upon it, and it unites with the pure alka- lies, forming soluble compounds which are called stannatcs. Binoxide of tin is recognized by its insolubility in acids in its anhydrous state; by separating from its solution by means of hydrochloric acid as a bulky hydrate by any of the alkalies or alkaline carbonates, which is easily and completely dissolved by pure potassa or soda in excess; and by yielding with hydrosulphuric acid the yellow bisulphuret of tin, which is also soluble in pure potassa. Binoxide of tin, when melted with glass, forms a white enamel. Protochloride of Tin.—This compound is obtained by transmitting hydro- chloric acid gas over metallic tin heated in a glass tube, when hydrogen gas is evolved; or by distilling a mixture either of granulated tin with an equal weight of bichloride of mercury, or of an amalgam of tin with calomel, urging the heat till the mercury is expelled. In this state it is a gray solid, of a resinous lustre, which fuses below redness, and at a high temperature sublimes. It is obtained by crystallization from a concentrated solution of the chloride in crystals, which are sometimes in small white needles, and at others in large transparent prisms, and consist of 93.32 parts or one eq. of protochloride of tin and 27 parts or three eq, of water. On heating these TIN. 347 crystals, they not only lose water, but reaction ensues between the elements of water and the chloride, hydrochloric acid gas is evolved, and protoxide of tin remains combined with the chloride. The same kind of compound is formed when a large quantity of the water is poured upon the crystals: the solution contains protochloride of tin and hydrochloric acid, and a white powder subsides which consists of one eq. of the protochloride, one eq. of protoxide, and two eq. of water. (Berzelius.) A solution of protochloride of tin is obtained by heating granulated tin in strong hydrochloric acid as long as hydrogen gas continues to be evolved. This solution is much employed as a deoxidizing agent, being more powerful than the sulphate or nitrate of the protoxide ; owing appa- rently to the tendency of the protochloride of tin to resolve itself into bi- chloride and metallic tin, the latter taking oxygen or chlorine from any me- tallic solutions which yield them readily. Bichloride of Tin.—When protochloride of tin is heated in chlorine gas, or on distilling a mixture of 8 parts of granulated tin with 24 of bichloride of mercury, a very volatile, colourless liquid passes over, which is bichlo- ride of tin. In an open vessel it emits dense white fumes, caused by the moisture of the air, and hence it was formerly called the fuming liquor of Libavius, who discovered it. At 248- it boils, and the sp. gravity of its va- pour was found by Dumas to be 9.1997. With one-third of its weight of water it forms a solid hydrate, and in a larger quantity ofwater dissolves. The solution of bichloride of tin, commonly called permuriate of tin, is much used in dyeing, and is prepared by dissolving tin in nitro-hydrochlo- ric acid. The process requires care ; for if the action be very rapid, as is sure to happen if strong acid be employed and much tin added at once, the peroxide will be spontaneously deposited as a bulky hydrate, and be subsequently redissolved with great difficulty. But the operation will rarely fail if the acid is made with two measures of hydrochloric acid, one of nitric acid, and one of water, and if the tin is gradually dissolved, one portion dis- appearing before another is added. The most certain mode of prepara- tion, however, is to prepare a solution of the protochloride, aid convert it into the bichloride either by chlorine, or by gentle heat and nitric acid. Iodides of Tin.—The protiodide is formed by heating granulated tin with about 2£ times its weight of iodine, and is a brownish-red, translucid sub- stance, very fusible, volatile at a high temperature, and soluble in water. The biniodide is prepared by dissolving in hydriodic acid the hydrate of the peroxide, precipitated by alkalies from the bichloride. It crystallizes in yellow crystals of a silky lustre, which are resolved by boiling water into hydriodic acid and peroxide of tin. Protosulphuret of Tin.—This compound is prepared by pouring melted tin upon its own weight of sulphur, and stirring rapidly with a stick during the action; as some tin usually escapes the sulphur from the latter being rapidly expelled, the product should be pulverized, mixed with its weight of sulphur, and projected in successive portions into a hot Hessian crucible, and then heated to redness. It is a brittle compound, of a bluish-gray, nearly black, colour and metallic lustre, which fuses at a red heat, and ac- quires a lamellated texture in cooling. It is dissolved by hydrochloric acid with evolution of hydrosulphuric acid. The same sulphuret is obtained in the moist way by adding hydrosulphuric acid to a solution of protochloride of tin. The sesquisulphuret is formed by mixing the protosulphuret in fine powder wilh a third of its weight of sulphur, and heating the mixture to low redness until sulphur ceases to escape. Its colour is of a deep grayish- yellow ; it is reconverted by a strong heat into the protosulphuret, and dis- solves in hydrochloric acid gas, yielding hydrosulphuric acid gas and a resi- due of bisulphuret of tin. Bisulphuret of Tin, formerly called mosaic gold, is prepared by heating in a glass or earthen retort a mixture of 2 parts of peroxide of tin, 2 of sul- phur, and 1 part of sal ammoniac, and maintaining a low red heat until sul- 348 COBALT. phurous acid ceases to be evolved. These materials are sometimes employed without sal ammoniac, but Berzelius says that the latter is essential for ob- taining the bisulphuret. The product when successfully prepared, is in crys- talline scales, and sometimes even in regular six-sided tables, of a golden- yellow colour and metallic lustre. It is soluble in pure potassa and in its carbonate by boiling; but its only solvent among the acids is the nitro- hydrochloric. The bisulphuret is obtained as a bulky hydrate of a dirty yellow colour by the action of hydrosulphuric acid or hydrosulphate of am- monia on bichloride of tin in solution. Terphosphuret of Tin.—Rose formed this compound by acting on a solu- tion of protochloride of tin by phosphuretted hydrogen. It is readily oxidized by the action of the air. SECTION XV. COBALT.—NICKEL. COBALT. This metal is met with in the earth chiefly in combination with arsenic, constituting an ore from which all the cobalt of commerce is derived. It is a constant ingredient of meteoric iron, though in very small quantity. (Stro- meyer.) Its name is derived from the term Kobold, an evil spirit, applied to it by the German miners at a time when they were ignorant of its value, and considered it unfavourable to the presence of valuable metals. When native arseniuret of cobalt is broken into small pieces, and exposed in a reverberatory furnace to the united action of heat and air, its elements are oxidized, most of the arsenious acid is expelled in the form of vapour, and an impure oxide of cobalt, called zaffre, remains. On heating this sub- stance with a mixture of sand and potash, a beautiful blue-coloured glass is obtained, which, when reduced to powder, is known by the name of smalt. Metallic cobalt may be obtained by dissolving zaffre in hydrochloric acid, and transmitting through the solution a current of hydrosulphuric acid gas until the arsenious acid is completely separated in the form of orpiment The filtered liquid is then boiled with a little nitric acid, in order to convert the protoxide into peroxide of iron, and an excess of carbonate of potassa is added. The precipitate, consisting of peroxide of iron and carbonate of pro- toxide of cobalt, after being well washed with water, is digested in a solution of oxalic acid, which dissolves the oxide of iron and leaves the oxide of co- balt in the form of an insoluble oxalate. (Laugier.) On heating this oxalate in a retort from which atmospheric air is excluded, a large quantity of car- bonic acid is evolved, and a black powder, metallic cobalt, is left. (Thomson in Annals of Philosophy, N. S. i.) The pure metal is easily procured also by passing a current of dry hydrogen gas over oxide of cobalt heated to redness in a tube of porcelain. In this state it is porous, and if formed at a low temperature it inflames spontaneously, as stated in the section on iron (page 335). A solution of cobalt may also be made by acting on the native arseniuret with sulphuric mixed with a fourth part of nitric acid, separating as much arsenious acid as possible by evaporation, and conducting the remainder of the process as above described. The arseniuret from Tonaberg should be preferred for this purpose, as it is in general free from nickel, which always accompanies the cobalt ores of Germany. Cobalt is a brittle metal, of a reddish-gray colour, and weak metallic lustre. Its density, according to my observation, is 7.834. It fuses at a heat rather lower than iron, and when slowly cooled it crystallizes. It is attracted by COBALT. 349 the magnet, and is susceptible of being rendered permanently magnetic. It undergoes little change in the air, but absorbs oxygen when heated in open vessels. It is attacked with difficulty by sulphuric or hydrochloric acid, but is readily oxidized by means of nitric acid. Like iron and the other metals of this order, it decomposes water at a red heat with disengagement of hy- drogen gas. (Despretz.) According to the analyses of Rothoff on the oxides of cobalt, its equiva- lent is inferred to be 29.5 (Ann. of Phil. iii. 356.) The composition of its compounds described in this section is as follows:— Cobalt. Equiv. Formulas. ^ide^ £29'5 l eq, + 0xvSen 8 I eq.= 37.5 Co-fO or Co. ^Oxide 88.5 3eq. + do. 32 4 eq.= 120.5 3Co-f 40 orCo30*. Peroxide 59 2 eq.-f do. 21 3 eq.= 83 2Co-i-30 or Co. Chloride 29.5 1 eq.-f Chlor. 35.12 1 cq.= 64.92 Co-fClorCoCl ^hme't1- I 29.5 1 eq.-f-Sulphur 16.1 1 eq.= 45.6 Co + S or CoS. Sesqui- J sulphu- >59 2eq.-f-do. 48.3 3 eq.= 107.3 2Co-f-3SorCo2S3. ^29.5 1 eq.-f do. 32.2 2 eq.= 61.7 Co-f-2S or C0S2. ret Bisul phuret Subphos- > 88.5 3 eq.-f Phosp. 31.4 2 eq.= 119.9 3Co + 2P or Co3P-\ phuret S Protoxie of Cobalt.—This oxide is of an ash-gray colour, and is the basis of the salts of cobalt, rao Sesquifluoride of Chromium is formed by dissolving the oxide in hydro- fluoric acid, and evaporating the solution to dryness, when the sesquifluoride remains as a green crystalline residue, which is soluble in water. Terfluoride of Chromium.—When a mixture of 3 parts of fluor spar and 4 of chromate of oxide of lead is distilled with 5 parts of fuming or even common sulphuric acid in a leaden or silver retort, a red-coloured gas is disengaged, which acts rapidly upon glass, with deposition of chromic acid and formation of fluosilicic acid gas. It is decomposed by water, and the solution is found to contain a mixture of hydrofluoric and chromic acids. The watery vapour of the atmosphere effects its decomposition ; so that when mixed with air, red fumes appear, owing to the separation of minute crystds VANADIUM. 363 of chromic acid. Its composition is inferred from the products which it forms with water ; since 1 eq. terfluoride & 3 eq. water 2 1 eq. chromic it 3 eq. hydrofl acid. Cr+3F; ,3(H + 0) •« Cr-f-30 3(H + F). The red colour of terfluoride of chromium naturally excites the suspicion that the gas itself may consist, not of fluoride of chromium, but of hydroflu- oric and chromic acids; and its production by means of hydrous sulphuric acid is consistent with this idea. But since' the gas may also be formed from fluor spar, chromate of oxide of lead, and anhydrous sulphuric acid, it is clear that this view is inadmissible. The changes which accompany the formation of the gas will be apparent on comparing the formulas of the ingredients used and of the products obtained. Thus, leq. chromate ox. lead 4 eq. real sulphuric acid 3 eq. fluor spar (Pb + 0)+(Cr+30), 4(S + 30), 3(Ca + F), yield by mutual reaction 1 eq. terfluoride of chromium ; 1 eq. sulphate ox. lead ; 3 eq. sulphate of lime. Cr + 3F; Pb + S; 3(Ca + S). This gas was discovered in the year 1825 by M. Unverdorben, (Ed. Journal of Science, iv. 129.) Sesquisulphuret of Chromium may be formed by transmitting the vapour of bisulphuret of carbon over oxide of chromium at a white heat; by heating in close vessels an intimate mixture of sulphur and the hydrated oxide; by fusing the oxide with a persulphuret of potassium, and dissolving the soluble parts in water; or by transmitting hydrosulphuric acid gas aided by hpat over the sesquichloride of chromium. It cannot be prepared in the moist way. It is of a dark gray colour, and acquires metdlic lustre by friction in a mortar. It is readily oxidized when heated in the open air, and is dissolved by nitric or nitro-hydrochloric acid. Protophosphuret of Chromium.—Rose prepared this compound by acting on the sesquichloride of chromium by phosphuretted hydrogen gas at a red heat. By mutual interchange of elements 1 eq. sesquichloride 2 Cr-f 3C1 2 2 eq. phosphuret 2(Cr-l-P) & 1 eq. phosph. hyd. 3H-f 2P ~ & 3 eq. hydrochloric acid 3(H +CI) This phosphuret is black, insoluble in hydrochloric acid, feebly attacked by nitric and nitro-hydrochloric acid, and burns before the blowpipe with a flame of phosphorus. Another phosphuret of a gray colour may be formed by exposing the phosphate of oxide of chromium to a strong heat in a covered crucible lined with charcoal. Its composition is unknown. VANADIUM. Vanadium, so called from Vanadis, the name of a Scandinavian Deity was discovered in the year 1830 by Professor Sefstrom, of Fahlun, in iron prepared from the iron-ore of Jaberg in Sweden. The state in which it occurs in the ore is unknown ; but Sefstrom separated it from the iron by dissolving the latter in hydrochloric acid, when a black powder came into view containing a small quantity of vanadium, together with iron, copper, cobalt, silica, alumina, and lime. He afterwards found a more abundant source in the slag or cinder formed during the conversion of the cast-iron of Jaberg into malleable iron. Soon after SefstrOm's discovery, the same metal was found by Mr. Johnston, of Durham, in a mineral from Wanlock head in Scotland, where it occurs as a vanadiate of oxide of lead. A similar mineral, found at Zimapan in Mexico, was examined in the year 1801 bv Professor del Rio, who, in the belief of having discovered a new metal gave 364 VANADIUM. it the name of Erythronium, apparently from the red colour of its acid ; but as M. Collet Descotils, on being appealed to, declared the mineral to be chromate of lead, del Rio abandoned his own opinion in deference to a higher authority. Thus have three persons noticed the existence of vana- dium, without the knowledge of each other's labours; but the merit of being the first discoverer is fairly due to Professor Sefstrom.* From the slag above mentioned vanadic acid may be obtained by the follow- ing process, contrived by Sefstrom and improved by Berzelius1. The slag in fine powder, mixed with its own weight of nitre and twice its weight of carbonate of potassa, is strongly ignited for the space of one hour. The soluble parts are then removed by boiling water, and the solution, after being filtered and neutralized with colourless nitric acid, is precipitated by chloride of barium or acetate of lead. The precipitate, which consists of vanadiate and phosphate of baryta or oxide of lead, zirconia, alumina, and silicic acid, is decomposed, while still moist, by digestion with strong sul- phuric acid; to the deep-red solution alcohol is then added, when by con- tinued digestion ether is disengaged, and all the vanadic acid Converted into the salifiable oxide, the solutions of which are blue;—a change effected in order the more completely to remove the vanadic acid from the insoluble matters. The blue liquid is then evaporated, and when it acquires a syrupy consistence, it is mixed in a platinum crucible with a little hydrofluoric acid, and sharply heated in an open fire. By this means the silicic acid, which can only be got rid of in this way, is converted into the gaseous fluoride of silicium, the sulphuric acid expelled, and the oxide reconverted into the acid of vanadium. The vanadic acid still contains phosphoric acid, alumina, and zirconia. For its further purification it is fused with nitre added in successive small portions, until, on cooling a small quantity, the red tint is found to have disappeared. In this process the acid of the nitre is displaced by the phos- phoric and vanadic acids, the object being to cause those acids to unite with potassa without employing an excess of nitre. The vanadiate and phosphate of potassa are then taken up by as small a quantity of water as will suffice, and into the filtered liquid a piece of sal ammoniac, larger than can be dis- solved by it, is introduced : as it dissolves, vanadiate of ammonia, insoluble in a saturated solution of sal ammoniac, subsides as a white powder, leaving the phosphoric acid in the liquid. The vanadiate of ammonia should be first washed with a solution of sal ammoniac, and then with alcohol of specific gravity 0.86. By heating this salt in an open platinum crucible, vanadic acid is obtain- ed; but the temperature ought to be kept below that of redness, and the mass be well stirred until it acquires a dark red colour. Heated in close vessels the vanadiate of ammonia is converted principally into the salifiable oxide ; though some of the protoxide and acid are mixed with it. With the zirconia and alumina, left by the water after fusion with nitre, some vana- dium remains: it may be extracted by fusion with sulphur and carbonate of potassa, when a double s.lphuret of vanadium and potassium is generated, which is soluble in water. On adding sulphuric acid to the solution, sul- phuret of vanadium is precipitated. The preparation of vanadium from the native vanadiate of lead is much less complicated than the process above described. It suffices to dissolve the ore, as Mr. Johnson advises, in nitric acid, and to precipitate the lead by hydrosulphuric acid, which also throws down any arsenic that may be present As vanadic acid is deoxidized by hydrosulphuric acid, a blue solu- tion is formed; but by evaporating to dryness, the acid is reproduced. The residue is then dissolved by a solution of ammonia, and the vanadiate of ammonia precipitated as before by a piece of sal ammoniac. The vanadic * Phil. Mag. and Annals, x. 321. An. de Ch. et de Ph. xlvii. 337. Brew- Btcr's Journal, v. 318. N. S. PoggendorPs Annalen, xxii. I. VANADU1M. 365 acid is thus separated from arsenic, phosphoric, and hydrochloric acids, with which in the ore of Wanlockhead it is generally associated. The attempts of Berzelius to reduce vanadic acid to the metallic state by the agency of hydrogen or charcoal at high temperatures proved unsuccess- ful, as the protoxide alone was obtained. He procured the metal, however, in the form of heavy black powder, by placing fragment of fused vanadic acid and potassium of equal size in alternate layers in a porcelain crucible, the po- tassium being in the largest proportion: a cover was then luted on, and heat applied by means of a spirit-lamp. The reduction took place suddenly and with violence; and when the mass had cooled, the potassa and redundant potassium were separated by water. But Berzelius succeeded better by a pro- cess similar to that of H. Rose for procuring metallic titanium. The liquid chloride of vanadium is introduced into a glass bulb blown in a barometer tube, and through it is transmitted dry ammoniacal gas until a white saline mass is produced, during the formation of which the gas is rapidly absorbed, and heat disengaged. A spirit-lamp flame is then applied, which expels a quantity of hydrochlorate of ammonia, and metallic vanadium is left ad- hering to the interior of the bulb. The production of hydrochloric acid is obviously owing to chlorine leaving the vanadium and uniting with the hydrogen of part of the ammonia. The pulverulent vanadium, produced by means of potassium, has but little of the tenacity and appearance of a metal; though under strong pressure it assumes a lustre like that of graphite. Heated in the open air to com- mencing redness it takes fire, and is converted into the black protoxide. It conducts electricity however, and is strongly electro-negative in relation to zinc. As procured by Rose's process, the vanadium has a strong metallic lustre and a white colour considerably resembling silver, but still more like molybdenum. It is so extremely brittle that it cannot be removed from the glass bulb without falling into powder. It is not oxidized either by air or water; although by continued exposure to the air its lustre gradually glows weaker, and it acquires a reddish tint. It is not dissolved by boiling sul- phuric, hydrochloric, or hydrofluoric acid; but by nitric and "nitro-hydro- chloric acid it is attacked, and the solution has a beautiful dark blue colour. It is not oxidized by being boiled with caustic potassa, nor by carbonated alkalies at a red heat. The equivalent of vanadium, according to the analysis of its oxides by Berzelius, is 68.5; and its compounds described in this section are thus constituted:— idium. Equiv. Formulae. Protoxide 68.5 1 eq.-f Oxygen 8 1 cq.= 76.5 V-f O or V. Binoxide 63.5 1 cq.-f-do. 16 2 eq.= 64.5 V-f 20 or V. Vanadic acid 68.5 Bichloride 68.5 Terchloride 68.5 Bibromide 68.5 Bisulphuret 6-'.5 Tersulphuret 68.5 1 eq.-f do. 24 1 eq.-f-Chlorine 70.31 1 eq.-f-do. 106.26 1 eq.-f Bromine 156.8 1 eq.-f Sulphur 32.2 1 eq.-f do. 48.3 3eq.= 92.5 2 eq.= 139.34 3 eq.= 174.76 2 eq =225.3 2eq.= 100.7 3eq.= 116.8 V-f 30 or V. V+2C1 or VCV V-f 3C1 or VCP V-f2BrorVBr3 V-f 2S or VS-. V-f-3Sor VS'. Protoxide.—This compound is readily formed from vanadic acid by the combined agency of heat and charcoal or hydrogen gas. By means of the latter Berzelius found that the reduction is effected as perfectly at a tempe- rature short of ignition, as at the strongest heat of a wind furnace. When prepared from fused vanadic acid, the protoxide retains the crystalline struc- ture of the acid, and has a black colour and a semi-metallic lustre ; but it is easily broken down into a fine black powder. When rendered coherent by compression it possesses a property very unusual in oxides, that of conduct- 31* . 366 VANADIUM. ing electricity, and in relation to zinc of being as strongly electro-negative as silver or copper. This oxide is very infusible. When heated in open vessels it takes fire and burns like tinder, being converted into the binoxide. On exposure to air and moisture it is slowly oxidized, a process which is best seen by putting it into water, when the liquid gradually acquires a green tint. In both cases the oxygen is derived from the atmosphere. A similar change occurs in acid and alkaline solutions, which, with the exception of nitric acid, do not dissolve it even at a boiling temperature. Heated in nitric acid, oxidation soon ensues with escape of nitric oxide gas, and a blue nitrate of the binoxide of vanadium is generated. The character of an alkaline base seems wholly wanting in the protoxide, and hence Berzelius considers it as a suboxide. Binoxide.—This oxide is best prepared, in the dry way, by heating to full redness an intimate mixture of 10 parts of the protoxide with 12 of vanadic acid in a vessel filled with carbonic acid, or from which combustible matter on the one hand, and oxygen gas on the other, are carefully excluded. From the salts of the binoxide, and especially the sulphate, it is precipitated as a grayish-white hydrate by means of a very slight excess of carbonate of soda. The residual solution is colourless when the process has been properly conducted : it remains blue, from undecomposed salt, if an insufficient quan- tity of alkali is used; it is brown when the alkaline carbonate is too freely em- ployed, because some of the binoxide is then dissolved by the free alkali; and if the solution contained vanadic acid, its colour after precipitation is green. The presence of the latter is avoided by transmitting hydrosulphuric acid gas into the solution, whereby vanadic acid is effectually converted into the binoxide, but the redundant gas should be expelled by gentle heat before the oxide is precipitated. As the hydrate, while moist, readily absorbs oxygen, and hence acquires a tint of brown, it must be washed and dried without exposure to the air. When thus prepared it retains its gray tint. By ex- posure to heat in a vessel from which the air is excluded, it gives out water, and acquires all the characters of the oxide prepared in the dry way. The binoxide of vanadium is a black pulverulent substance, very infusi- ble, insoluble in water, and free from any acid or alkaline reaction. When heated in the open air, it is converted into vanadic acid, and when moist it gradually suffers the same change at ordinary temperatures. It is dissolved by acids more readily as a hydrate than after being heated to redness, and forms salts, most"of which have a blue colour, and are more or less soluble in water. They may all be conveniently formed by the direct action of acids on the hydrated oxide. The nitrate may be made by acting on vanadium, or either of its oxides, by nitric acid ; the salt, when diluted with water, may be boiled without change ; but when evaporated, even spontaneously, the blue colour passes through green into red, owing to the production of vanadic acid. The sulphate is easily prepared by dissolving vanadic acid in warm sulphuric acid diluted with an equal weight of water, decomposing the vanadic acid by hydrosulphuric acid, concentrating the solution in order that the salt may be deposited, and washing away adhering sulphuric acid by means of alcohol. The dcoxidation of vanadic acid in the preceding process may also be effected by adding pure oxalic acid as long as carbonic acid gas is evolved. The salts of the binoxide of vanadium are distinguished by their blue colour, by yielding with the alkalies or their carbonates in very slight excess the hydrated binoxide, which becomes red by oxidation, and by forming with solution of gall-nuts a black compound, a tannate of the binoxide, very simi- lar to ink. The binoxide is disposed to act the part of an acid by uniting with alkaline bases, with which it forms definite and in some cases crystalline compounds. On digesting the hydrated binoxide in pure potassa or ammonia, combination is readily effected, and a dark brown solution is formed. These compounds, though soluble in water, are very sparingly so in strong and cold alkaline solutions, and may be precipitated by them. Most of the other salts formed by VANADIUM. 367 the binoxide and salifiable bases are insoluble in water, and may be form- ed from the preceding by way of double decomposition. Vanadic Acid.—When vanadiate of ammonia, prepared as already men- tioned, (page 364,) is heated in close vessels, the acid is decomposed by the hydrogen of the ammonia, and binoxide of vanadium is formed, mixed with a little protoxide and undecomposed acid. If the salt is heated in an open vessel, and well stirred, the whole mass acquires a dark red colour, and pure vanadic acid is obtained ; but a red heat should be avoided, since fusion would thereby be occasioned, and free exposure of every part to the atmo- sphere prevented. Its colour in the state of fine powder is a light rust-yellow ; but the fused acid is red with a shade of orange, and has a strong lustre. By light transmitted through thin layers it appears yellow. In the fire it is fix- ed, and is not decomposed by a very strong heat, provided combustible mat- ers are excluded. It fuses at a heat lower than that of redness, and crystal- lizes readily as it cools. In the act of becoming solid it contracts consider- ably in volume, and emits so much heat of fluidity that the acid, after ceasing to be luminous, is again rendered incandescent, and remains so until the congelation is complete. Vanadic acid is tasteless, insoluble in alcohol, and very slightly soluble in water, which takes up rather less than l-1000lh of its weight acquiring a yellow colour and an acid reaction Heated with combustible matter it is deoxidized, being converted into the protoxide or binoxide, or mixtures of these oxides. In solutions it is deprived of oxygen by all deoxidizing agents, such as alcohol, sugar, and most organic substances, including the oxalic and several vegetable acids, by hydrosulphuric acid and most of the other hydracids, not excepting the hydrochloric, by sulphurous and phosphorous acids, and even by nitrous acid. Like molybdic and tungstic acids, it is dis- posed to act as a base to such of the stronger acids as do not decompose it, and to form with them definite compounds, which are soluble in water. It unites on this principle with sulphuric and phosphoric acid; and Berzelius has remarked a compound of the phosphoric, silicic, and vanadic acids', a sort of double salt in which the latter acid is a base to the two former, and which crystallizes in scales: it is formed in ScfstrOm's process for preparing vanadic acid (page 364), and its solubility opposes a great obstacle to the separation of vanadic from silicic acid. Vanadic acid unites with salifiable bases often in two or more proportions, forming soluble salts with the alkalies, and in general sparingly soluble salts with the other metallic oxides. Those wilh excess of acid are commonly of a red or orange-red colour. Most of the neutral salts are yellow ; but it is singular that the neutral vanadiates of the alkalies, the alkaline earths, and the oxides of lead, zinc, and cadmium may be yellow at one time and colour- less at another without suffering any appreciable change in composition. Thus, on neutralizing vanadic acid with ammonia a yellow salt is obtained, the solution of which gradually becomes colourless if kept for some hours, and suffers the same change rapidly when heated. The solution, as it is coloured or colourless, gives a yellow or white residue by evaporation, and a yellow or white precipitate with a salt of baryta or oxide of lead. These changes appear to be of the same kind as those already noticed in the de- scription of phosphoric acid. Vanadic acid unites in different proportions with binoxide of vanadium, and forms compounds which are soluble in pure water but sparingly so in saline solutions, and which are purple, green, yellow, or orange, according as the acid is in a smaller or larger proportion. They are best formed by exposing the hydrated binoxide to the atmosphere, when the^e different colours successively appear, as a gradually increasing quantity of the acid is generated. Vanadic acid is distinguished from all other acids except the chromic by its colour, and from this acid by the action of deoxidizing substances, which give a blue solution with the former and a green with the latter. (Page 361.) When heated with borax in the reducing flame of the blowpipe, both of the acids yield a green glass ; but in the oxidizing flame the bead becomes yellow 368 MOLVBDENUM. if vanadium is present, while the green colour produced by chromium is permanent. Chlorides.—The bichloride is prepared by digesting a mixture of the vanadic and hydrochloric acids, deoxidizing any undecomposed vanadic acid by hydrosulphuric acid, and evaporating the solution to dryness. A brown residue is obtained, which yields a blue solution with water, part being left as an insoluble subsalt. It may also be generated by acting directly on the ignited binoxide with strong hydrochloric acid. As thus obtained its solu- tion is brown instead of blue, though in composition it seems identical with the preceding. The terchloride may be formed by transmitting a current of dry chlorine gas over a mixture of protoxide of vanadium and charcoal heated to low red- ness, when the terchloride passes over in vapour, and condenses in the form of a yellow liquid, from which free chlorine may be removed by a current of dry air. It is converted by water into hydrochloric and vanadic acid, and at- mospheric humidity produces the same change, which is indicated by the escape of red fumes. A bibromide of vanadium may be formed in the same manner as the bi- chloride, substituting the hydrobromic for hydrochloric acid. Similar com- pounds maybe procured with iodine, fluorine, and cyanogen, by dissolving binoxide of vanadium in hydriodic, hydrofluoric, and hydrocyanic acid. Sulphurets.—When the binoxide of vanadium is heated to redness in a current of hydrosulphuric acid gas, it is converted into protoxide, and both water and sulphur are obtained : on continuing the process the protoxide is decomposed, hydrogen gas and water pass over, and bisulphuret of vanadium is generated. This compound may also be procured by mixing hydrosul- phate of ammonia with a salt of the binoxide of vanadium until the precipi- tate at first formed is redissolved, and then decomposing the deep purple- coloured solution by sulphuric or hydrochloric acid. The bisulphuret of a brown colour subsides, which becomes black when it is dried. It is un- changed at common -temperatures by exposure to the air, but takes fire when heated. In the hydrated state it is dissolved by alkalies and alkaline sulphurets; but it is insoluble in acids, with the exception of the nitric and nitro-hydrochloric, by which it is converted into sulphate of the binoxide. When hydrosulphuric acid gas is transmitted through an aqueous solution of vanadic acid, a grayish-brown precipitate is formed, consisting of hydrated binoxide of vanadium mixed mechanically with sulphur. But if a solution of vanadic acid in hydrosulphate of ammonia is acidulated by hydrochloric or sulphuric acid, the hydrated tersulphuret of vanadium sub- sides. Its colour is of a much lighter brown than the bisulphuret, becomes almost black in drying, and is resolved by a red heat in close vessels into the bisulphuret, with loss of water and sulphur. It is soluble in alkalies and alkaline sulphurets, and is oxidized by nitric acid. The phosphuret of vanadium, of a leaden-gray colour, may be formed by exposing to a white heat phosphate of the binoxide of vanadium mixed with a small quantity of sugar. SECTION XVIII. MOLYBDENUM.—TUNGSTEN.—COLUMBIUM. MOLYCDENUM. When native sulphuret of molybdenum, in fine powder, is digested in nitro-hydrochloric acid until the ore is completely decomposed, and the residue is briskly heated in order to expel sulphuric acid, molybdic acid re- MOLYBDENUM. 369 mains in the form of a white heavy powder. From this acid metallic molyb- denuni may be obtained by exposing it with charcoal to the strongest heat of a smith's forge; or by conducting over it a current of hydrogen gas while strongly heated in a tube of porcelain. (Berzelius.) The sulphuret, which was long mistaken for graphite, was distinguished in 1778 by Scheele; but the metal was first obtained in a separate state by Hjelm. It likewise occurs in nature in the form of molybdate of oxide of lead. Molybdenum is a brittle metal, of a white colour, and so very infusible, that hitherto it has only been obtained in a state of semi-fusion. In this form it has a specific gravity varying between 8.615 and 8.636. When heated in open vessels it absorbs oxygen, and is converted into molybdic acid ; and the same compound is generated by the action of chlorine or nitro-hydrochloric acid. It has three degrees of oxidation, forming two oxides and one acid, from the composition of which Berzelius estimates the equivalent of molybdenum at 47.7. The composition of its compounds described in this section is as follows:— Molybdenum. Equiv. Formulae. Protoxide 47.7 1 eq.-f Oxygen 8 1 eq.= 55.7 Mo-fOorMo. Binoxide 47.7 1 eq.-f do. 16 2eq.= 63.7 Mo-f 20 or Mo. Molybdic acid 47.7 1 eq.-f do. 24 3 eq.= 71.7 Mo-f 30 or Mo. Protochloride 47.7 1 eq.-j- Chlorine 35.42 1 eq.= 83.12 Mo-f CI or MoCl. Bichloride 47.7 1 eq-f do. 70.84 2 eq.=118.54 Mo-f 2Clor M0CI2. Terchloride 47.7 1 eq.-f do. 106.26 3 eq.= 153.96 Mo-f-3Clor MoC13. Bisulphuret 47.7 1 eq.-f Sulphur 32.2 2eq.= 79.9 Mo-f-2 S or MoSs. Tersulphuret 47.7 1 eq_f do. 48.3 3 eq.= 96 Mo-f 3S or MoS3. Persulphuret 47.7 1 eq.-j.do. 64.4 4eq.= 112.1 Mo-f 4S or MoS*. Protoxide of Molybdenum. On dissolving molybdate of potassa or soda in a small quantity of water, adding hydrochloric acid until the molybdic acid at first thrown down is redissolved, and digesting with a piece of pure metallic zinc, the latter deoxidizes the molybdic acid, the liquid changes to blue, red, and black, and then contains chloride of zinc and protochloride of molybdenum. From the black solution pure potassa throws down the pro- toxide of molybdenum as a black hydrate, an excess of the alkali being used in order to hold the zinc in solution. The hydrate is washed with the least possible exposure to the air, and dried in vacuo by sulphuric acid. When heated to low redness in the open air, it takes fire and is converted into the binoxide; but if not exposed to the air, it becomes incandescent at the mo- ment of losing its water, like hydrated oxide of chromium. The anhydrous oxide is black and insoluble in acids; but in the state of hydrate acids dis- solve it. The recently precipitated hydrate is soluble in the cold by carbo- nate of ammonia, but in none of the other alkalies. Binoxide of Molybdenum.—It is obtained as a deep brown anhydrous powder by mixing molybdate of soda with half its weight of sal ammoniac in fine powder, projecting the mixture into a red-hot crucible which is to be instantly covered, and the beat continued until vapours of sal ammoniac cease to appear. In this process chloride of sodium is generated, and mo- lybdic acid is reduced by the ammonia to the state of binoxide : by adding water to the mass when cold, chloride of sodium is dissolved, and the dark brown, nearly black, binoxide left. The hydrate, of a rust-brown colour, may be formed by digesting molybdenum in powder with molybdic acid dissolved in hydrochloric acid, until the liquid acquires a deep red colour, and then adding ammonia ; or by adding ammonia to a solution of the bichloride; or digesting with metallic copper a solution of molybdic in hydrochloric acid until a deep red solution is formed, and employing an ex- cess of ammonia in order to keep oxide of copper in solution, 370 MOLYBDENUM. The anhydrous binoxide is insoluble in acids and is changed into molybdic acid by strong nitric acid. The hydrate is very like hydrated peroxide of iron, reddens litmus paper when placed on it, is dissolved by acids with which it forms red salts, is insoluble in the alkalies, but dissolves in alka- line carbonates. It is soluble, though sparingly, in pure water, so that it should be washed after precipitation by a solution of sal ammoniac, which Bait is afterwards removed by alcohol. On exposure to the air the hydrate absorbs oxygen and becomes blue at its surface: this blue compound is more soluble in water than the hydrate, and was supposed by Buchholz to be a distinct acid, which he termed molybdous acid; but Berzelius has Bhown that it is a bimolybdatc of the binoxide. (Berzelius.) Molybdic Acid —When sulphuret of molybdenum is roasted in an open crucible kept at a low red heat, and constantly stirred until sulphurous acid ceases to escape, a dirty yellow powder is left, which contains impure molybdic acid. The acid is taken up by ammonia and the filtered solution evaporated to dryness; it is again taken up by water, a little ammonia being added, and filtered; and it is then purified by crystallization. On heating gently in an open platinum crucible, taking care to prevent fusion, the am- monia is expelled, and pure acid remains. It is also obtained by oxidizing the binoxide with nitric acid. Molybdic acid, as thus formed, is a white powder, of sp. gravity 3.49, fusible by a red heat into a yellow liquid, which bears a strong red heat in closed vessels without subliming, but in an open crucible rises with the cur- rent of air, and collects on cold surfaces in colourless crystalline scales. It requires 570 times its weight of water for solution, which nevertheless has an acid reaction. It is soluble in U12 alkalies, forming colourless molyb- dates, from which molybdic acid is precipitated by the stronger acids, though an excess of the acids dissolves it; but after exposure to a red heat it is insoluble in acids. Chlorides.—Berzelius has described three chlorides of molybdenum which are analogous in composition to the oxides. The protochloride is formed by dissolving the hydrated protoxide in hydrochloric acid, forming a deep nearly black coloured solution, which leaves a black viscid mass by evapora- tion. The bichloride is obtained as above mentioned, and yields a red solution. It is obtained in the anhydrous state by gently heating molybdenum in powder in dry chlorine gas, atmospheric air being excluded. The metal takes fire at its surface, but it is soon extinguished, after which the chlorine is replaced by a red vapour of such intensity that it is completely opaque in a vessel | of an inch in diameter: this vapour condenses in the cooler parts of the apparatus in brilliant black crystals just like those of iodine, which are very fusible, and sublime at a gentle heat. Exposed to dry oxygen gas, it is transformed gradually into terchloride of molybdenum and molybdic acid; so that 3 eq. bichloride and 3 eq. oxygen 2 2 cq. terchloride and 1 eq. molybdic acid. 3010+201) 30 -p, 2(Mo + 3Cl) Mo+30. With water the bichloride acts violently from the intense heat evolved, and the whole is dissolved. • • v. j The terchloride is obtained in solution by dissolving molybdic in hydro- chloric acid ; and in the anhydrous stale by heating molybdic acid in a cur- rent of dry chlorine gas. It is white with a shade of yellow, sublimes at a heat short of redness and condenses in crystalline scales, and is soluble in water and alcohol. Sulphurets.—Molybdenum combines with sulphur in three proportions. The lowest grade is the bisulphuret, which is the most common ore of mo- lybdenum and iajisually associated with ores of tin, has a lead-gray colour and metallic lustre resembling graphite, for which it was formerly mistaken. Its density varies from 4.138 to 4.569. It bears a strong heat in close vessels TUNGSTEN. without change or fusion ; but it is oxidized by nitric acid or by the joint action of heat and air. The tersulphuret is obtained by saturating molybdate of potassa, soda, or ammonia with hydrosulphuric acid gas, and adding hydrochloric acid, when the tersulphuret falls of a deep brown colour, which becomes black on dry- ing. It is partially oxidized when dried in the air. By heat in close vessels it is changed into the bisulphuret with loss of sulphur. The persulphuret is made by boiling the sulphur-salt formed of tersulphu- ret of molybdenum and sulphuret of potassium for a long time with the bisul- phuret of molybdenum, when a precipitate collects which is to be well washed with cold water. It is a sulphur-salt composed of persulphuret of molybde- num and sulphuret of potassium, which forms with boiling water a deep red solution, from which on the addition of hydrochloric acid the persulphu- ret subsides. TUNGSTEN. It derives its name from the Swedish words Tung Sten, heavy stone, from the density of its ores; and it is cdled wolfram from the mineral of that name, which is a tungstate of the oxides of iron and manganese. This metal may be procured in the metallic state by exposing tungstic acid to the action of charcoal or dry hydrogen gas at a red heat; but though the reduction is easily effected, an exceedingly intense temperature is required for fusing the metal. Tungsten has a grayish-white colour, and considerable lustre. It is brittle, nearly as hard as steel, and less fusible than manganese. Its specific gravity is near 17.6. When heated to redness in the open air it takes fire, and is converted into tungstic acid; and it undergoes the same change by the action of nitric acid. Digested with a concentrated solution of pure po- tassa, it is dissolved with disengagement of hydrogen gas, and tungstate of potassa is generated. Chemists are acquainted with two compounds of this metal and oxygen, namely, the dark brown oxide and the yellow acid of tungsten ; and according to the analysis of Berzelius, (An. de Ch. et de Ph. x\ii.) the oxygen of the former is to that of the latter in the ratio of two to three. From the compo- sition of the latter, and assuming that it contains three atoms of oxygen, the equivalent of tungsten is 99.7. Its compounds described in this section are thus constituted :— Tungsten. Equiv. Formulae. Binoxide 99.7 1 eq. +Oxygen 16 2 eq.= 115.7 W+20orW. Tungstic acid 99.7 1 eq. +ditto 24 3 eq.= 123.7 W + 30 or W. Bichloride 99.7 1 eq. + Chlor. 70.84 2 eq.= 170.54 W+2C1 or WC1*. Terchloride 99.7 1 eq. +ditto 106.26 3 eq.=205.96 W+3C1 or WC13. Bisulphuret 99.7 1 eq. + .^dph. 32.2 2 eq. =131.9 W+2S or WS2 Tersulphuret 99.7 1 eq. +ditto 48.3 3 eq.= 483 W + 3SorWS3 Binoxide.—This oxide is formed by the action of hydrogen gas on tungstic acid at a low red heat; but the best mode of procuring it both pure and in quantity, is that recommended by Wohler. (Quarterly Journal of Scicnce,xx. 177(. This process consists in mixing wolfram in fine powder with twice its weight of carbonate of potassa, and fusing the mixture in a platinum crucible. The resulting tungstate of potassa is dissolved in hot water, mixed with about half its weight of hydrochlorate of ammonia in solution, evaporated to dryness, and exposed in a Hessian crucible to a red heat. The mass is well washed with boiling water, and the insoluble matter digested in dilute potassa to remove any tungstic acid. The residue is oxide of tungsten. It appears that in this process the tungstate of potassa and hydrochlorate of ammonia mutually deconpose each other, so that the dry mass consists of chloride of TUNGSTEN. potassium and tungstate of ammonia. The elements of the latter react on each other at a red heat, giving rise to water, nitrogen gas, and binoxide of tung- sten; and this compound is protected from oxidation by the fused chloride of potassium with which it is enveloped. This oxide is also formed by putting tungstic acid in contact with zinc in dilute hydrochloric acid. The tungstic acid first becomes blue and then assumes a copper colour; but the oxide in this state can with difficulty be preserved, as by exposure to the air, and even under the surface of water, it absorbs oxygen, and is reconverted into tung- stic acid. Binoxide of tungsten, when prepared by means of hydrogen gas, has a brown colour, and when polished acquires the colour of copper; but when procured by Wohler's process, it is nearly black. It does not unite, so far as is known, with acids; and when heated to near redness, it takes fire and yields tungstic acid. Tungstic Acid.—A convenient method of preparing tungstic acid is by digesting native tungstate of lime, very finely levigated, in nitric acid; by which means nitrate of lime is formed, and tungstic acid separated in the form of a yellow powder. Long digestion is required before all the-lime is removed; but the process is facilitated by acting upon the mineral alternately by nitric acid and ammonia. The tungstic acid is dissolved readily bv that alkali, and may be obtained in a separate state by heating the tungstate of ammonia to redness. Tungstic acid may also be prepared by the action of hydrochloric acid on wolfram. It is also obtained by heating the brown oxide to redness in open vessels. Tungstic acid is of a yellow colour, is insoluble in water, and has no action on litmus paper. With alkaline bases it forms salts called tungstates, which are decomposed by the stronger acids, the tungstic acid in general falling combined with the acid by which it is precipitated. When strongly heated in open vessels, it acquires a green colour, and becomes blue when ex- posed to the action of hydrogen gas at a temperature of 500° or 600° F. The blue compound, according to Berzelius, is a tungstate of the binoxide of tungsten; and the green colour is probably produced by an admixture oi this compound with the yellow acid. Chlorides of Tungsten—According to Wohler, tungsten and chlorine united in three proportions. When metallic tungsten is heated in chlorine gas, it takes fire, and yields the bichloride. The compound appears in the form of delicate needles, of a deep red colour, resembling wool, but more frequently as a deep red fused mass which has the brilliant fracture of cin- nabar. When heated, it fuses, boils, and yields a red vapour. By water it is changed into hydrochloric acid and binoxide of tungsten. It is entirely dissolved by solution of pure potassa and disengagement of hydrogen gas, yielding tungstate of potassa and chloride of potassium. A similar change is produced by ammonia, except that some binoxide of tungsten is left undis- solved. The terchloride is generated by heating the binoxide of tungsten in chlo- rine gas. The action is attended with the appearance of combustion, dense fumes arise, and a thick sublimate is obtained in the form of white scales, like native boracic acid. It is volatile at a low temperature without pre- vious fusion. It is converted by the action of water into tungstic and hydro- chloric acids. Another chloride has been described by Wohler. It is formed at the same time as the first; and though it is converted into hydrochloric and tungstic acids by the action of water, and would thus seem identical with the ter- chloride in the proportion of its elements, its other properties are never- theless different. It is the most beautiful of all these compounds, existing in long transparent crystals of a fine red colour. It is very fusible and volatile, and its vapour is red like that of nitrous acid. The difference betweccn this compound and the chloride first described has not yet been discovered. , Sulphurets of Tungsten.—The bisulphuret is obtained as a black powder by transmitting hydrosulphuric acid gas, or the vapour of sulphur, over COLUMBIUM. 373 tungstic acid heated to whiteness in a tube of porcelain.* The tersulphuret is prepared by dissolving tungstic acid in a solution of sulphuret of potassium or hydrosulphate of ammonia, and adding an excess of hydrosulphuric acid. It falls as a brown precipitate, which becomes black in drying. It is soluble to a certain extent in water which is free from saline matter. COLUMBIUM. This metal was discovered in 1801 by Mr. Hatchett, who detected it in a black mineral belonging to the British Museum, supposed to have come from Massachusetts in North America; and from this circumstance appli- ed to it the name of columbium. About two years after, M. Ekebcrg, a Swedish chemist, extracted the same substance from tantalite and yttro-lanta- lite; and, on the supposition of its being different from columbium, described it under the name of tantalum. The identity of these metals, however, was established in the year 1809 by Dr. Wollaston. Columbic acid is with difficulty reduced to the metallic state by the action of heat and charcoal; but Berzelius succcded in obtaining this metal by the same process which he employed in the preparation of zirconium and silicium ; namely, by heating potassium with the double fluoride of potassium and colum- bium. On washing the reduced mass with hot water, in order to remove the fluoride of potassium, columbium is left in the form of a black powder. In this state it does not conduct electricity; but in a denser state it is a perfect conductor. By pressure it acquires metallic lustre, and has an iron-gray colour. It is not fusible at the temperature at which glass is fused. When heated in the open air, it takes fire considerably below the temperature of ignition, and glows with a vivid light, yielding columbic acid. It is scarcely at all acted on by the sulphuric, hydrochloric, or nitro-hydrochloric acid; whereas it is dissolved with heat and disengagement of hydrogen gas by hydrofluoric acid, and still more easily by a mixture of nitric and hydro- fluoric acids. It is also converted into columbic acid by fusion with hydrate of potassa, the hydrogen gas of the water being evolved. From the experiments of Berzelius on the composition of the oxide and acid of columbium, its equivalent may be estimated at 185. Its compounds described in this section are thus constituted:— Columbium. Equiv. Formula?. Binoxide 185 1 eq.+Oxygen 16 2eq.= 201 Ta+20 or f a. Columbic acid 185 1 eq.+ do. 24 3 eq.= 209 Ta+30 or fa. Terchloride 185 1 eq.+Chlorine 106.26 3eq.= 291.26 Ta+3ClorTaC13. Terfluoride 185 1 eq.-f Fluorine 56.04 3 eq.=241.04 Ta+3F or TaF3. Sulphuret Composition uncertain. Binoxide of Columbium.—It is generated by placing columbic acid in a crucible lined with charcoal, luting carefully to exclude atmospheric air, and exposing it for an hour and a half to intense heat. The acid, where in direct contact with charcoal, is entirely reduced; but the film of metal is very thin. The interior portions are pure binoxide of a dark gray colour, very hard and coherent When reduced to powder its colour is dark brown. It is not attacked by any acid, even by the nitro-hydrofluoric; but it is converted into columbic acid either by fusion with hydrate of potassa, or deflagration with nitre. When heated to low redness, it takes fire and glows, yielding a light gray powder ; but in this way it is never completely oxidized. Berzelius states that this oxide, in union with protoxide of iron and a little protoxide of manganese, occurs at Kimito in Finland, and may be distinguished from the other ores of columbium by yielding a chestnut- brown powder. Columbic Acid.—Columbium exists in most of its ores as an acid, united either with the oxides of iron and manganese, as in tantalite, or with the 32 374 ANTIMONY. oarth yttria, as in the yttro-tantalite. This acid is obtained by fusing its ore With three or four times its weight of carbonate of potassa, when a soluble columbate of that alkali results, from which columbic acid is precipitated as a white hydrate by acids. Berzelius also prepares it by fusion with bisulphate of potassa. Hydrated columbic acid is tasteless and insoluble in water; but when placed on moistened litmus paper, it communicates a red tinge. It is dis- solved by the sulphuric, hydrochloric, and some vegetable acids; but it does not diminish their acidity, or appear to form definite compounds with them. With alkalies it unites readily; and though it does not neutralize their pro- perties completely, crystallized salts may be obtained by evaporation. When the hydrated acid is heated to redness, water is expelled, and the anhydrous columbic acid remains. In this state it is attacked by alkalies only. Terchloride of Columbium.—When columbium is heated in chlorine gas, it takes fire and burns actively, yielding a yellow vapour, which condenses in the cold parts of the apparatus in the form of a white powder with a tint of yellow. Its texture is not in the least crystalline. By contact with water, it is converted, with a hissing noise and increase of temperature, into columbic and hydrochloric acids. Terfluoride of Columbium.—Hydrofluoric acid takes up hydrated columbic acid, and forms with it a compound of terfluoride of columbium and hydro- fluoric acid, which, by evaporation at 76°, is deposited in crystals, which are soluble in water and effervesce in the air. By gently evaporating the solution, an uncrystalline mass, white and opaque, is left, which Berzdius considers to be the terfluoride of columbium. By water part of it is con- verted into columbic and hydrofluoric acids, the latter goluble and the former insoluble ; but both of these acids retain some terfluoride in combi- nation. Sulphuret of Columbium.—This compound, first prepared by Rose, is generated, with the phenomena of combustion, when columbium is heated to commencing redness in the vapour of sulphur; or by transmitting the vapour of bisulphuret of carbon over columbic acid in a porcelain tube at a white heat, carbonic oxide being also evolved. SECTION XIX. ANTIMONY. Antimony was first made known as a metal in the 15th century by Basil Valentine, and is said to derive its name {antimoine, anti-monk) from having proved fatal to some monks to whom it was given as a medicine. It some- times occurs native; but its only ore which is abundant, and from which the antimony of commerce is derived, is the sulphuret. This ore, the stibium of the ancients, was long regarded as the metal itself, and was called antimony, or crude antimony; while the pure metal was termed the regulus of antimony. Metallic antimony maybe obtained either by heating the native sulphuret in a covered crucible with half its weight of iron filings; or by mixing it with two-thirds of its weight of cream of tartar and one-third of nitre, and throwing the mixture, in small successive portions, into a red-hot crucible. By the first process the sulphur unites with iron, and in the second it is ex- pellcd in the form of sulphurous acid; while the fused antimony, which m both cases collects in the bottom of the crucible, may be drawn off and re- ceived in moulds. The antimony, thus obtained, is not absolutely pure; and, therefore, for chemical purposes, should be procured by heating trie ox- idc with an equal weight of cream of tartar. ANTIMONY. 375 Antimony is a brittle metal, of a white colour running into bluish-gray, and is possessed of considerable lustre. Its density is nearly 6.7. At 810° it fuses, and on cooling acquires a highly lamellated texture, and sometimes yields crystals: like arsenic, but unlike most other metals, its primary form is a rhombohedron. It has the character of being a volatile metal; but Thenard found that it bears an intense white heat without suohming, pro- vided atmospheric air be perfectly excluded, and no gaseous matters, such as carbonic acid or watery vapour, be disengaged during the process. Its surface tarnishes by exposure to the atmosphere ; and by the continued action of air and moisture, a dark matter is formed, which Berzelius regards as a definite compound. It appears, however, to be merely a mixture of the ses- quioxide and metallic antimony. Heated to a white or even full red heat in a covered crucible, and then suddenly exposed to the air, it inflames, and burns with a white light. During the combustion a white vapour rises, wliich condenses on cool surfaces, frequently in the form of small shining needles of silvery whiteness. These crystals were formerly called argentine flowers of antimony, and in chemical works are generally described as anti- monious acid; but they are correctly considered by Berzelius as the sesqui- From the experiments of Berzelius on the composition of the oxide and acids of antimony (An. de Ch. et de Ph. xvii.), the equivalent of that metal may be estimated at 64.6. The composition of its compounds described in this section is as follows :— Two eq. Antimony. Sesquioxide 129.2 +Oxygen Antimoni-j ous acid S ' Antimonic acid Sesqui- chloride Bichlo- ride Perchlo- ride Bromide Sesqui ( 129.2 +do. £ 129.2 + Chlorine | 1295 +do. 1129.2 +do. Formulae. 2Sb + 30or Sb. 2Sb + 40 or sb. 2Sb+50 or Sb". 3 eq.=235.46 2Sb + 3Cl or SbzCl 141.68 4 eq. ^270.88 2Sb + 4C1. or Sb 2 CI *. 5 eq.=306.3 2Sb+5Cl or Sb2Cls. 24 32 40 106.26 Equiv. 3 eq. =153.2 4 eq.=161.2 5eq.= 169.2 177.1 Composition uncertain. 48.3 A 129.2 + Sulphur sulphuret \ Bisulphu- h29.2 +do. ret \ Persulphu- ( 129#2+do. ret. S Oxy-sulph. ) J Sesquisulph. of antim. S \ Sesquioxide. 64.4 80.5 355 153.2 3 eq.=177.5 2Sb + 3S or Sb2S3. 4eq.= 193.6 2Sb + 4S or Sb2S*. 5 eq. =209.7 2Sb + 5Sor Sb2Ss. ^eqi =508.2 2Sb2S3+Sb. 1 eq. <, — Sesquioxide.—When sesquichloride of antimony, made by boiling the native sulphuret in hydrochloric acid (page 258), is poured into water, a white curdy precipitate subsides, formerly called powder of Algaroth, which consists of sesquioxide of antimony combined with a little hydrochloric acid, or more probably with undecomposed chloride. On decomposing the latter by digestion with carbonate of potassa and then washing with water, the sesquioxide is obtained in a state of purity. It may also be procured by adding carbonate of potassa or soda to a solution of tartar emetic, and by sublimation during the combustion of antimony. When slowly sublimed it condenses in fine needles of silvery whiteness. It occurs as a mineral, the oxide of antimony of mineralogists, the primary form of which is a right rhombic prism, isomorphous with the crystals of arsenious acid lately ob- served by Wohler. (Page 354.) 376 ANTIMONY. Sesquioxide of antimony, when prepared in the moist way, is a white powder with a somewhat dirty appearance. When heated it acquires a yellow tint, and at a dull red heat in close vessels it is fused, yielding a yellow fluid, which becomes an opaque grayish crystalline mass on cooling. Its sp. gravity is 5.566. It is very volatile, and if protected from atmospheric air may be sublimed without change. When heated in open vessels it absorbs oxygen ; and when the temperature is suddenly raised, and the oxide is porous, it takes fire and burns. In both cases antimonious acid is generated. It is the only oxide of antimony which forms regular salts wilh acids, and is the base of the medicinal preparation tartar emetic, the tartrate of antimony and potassa. Most of its salts, however, are either insoluble in water, or, like chloride of antimony, are decomposed by it, owing to the affinity of that fluid for the acid being greater than that of the acid for oxide of antimony. This oxide is, therefore, a feeble base; and, indeed, possesses the property of uniting with alkalies. To the foregoing remark, however, tartrate of antimony and potassa is an exception; for it dissolves readily in water without change. By excess of tartaric or hydrochloric acid, the insoluble salts of antimony may be rendered soluble in water. The presence of antimony in solution is easily detected by hydrosul- phuric acid. This gas occasions an orange,-coloured precipitate, hydrated sesquisulphuret of antimony, which is soluble in pure potassa, and is dis- solved wilh disengagement of hydrosulphuric acid gas by hot hydrochloric acid, forming a solution from which the white oxy-chloride (powder of Algarolh) is precipitated by water. In trying the effect of reagents on solutions of sesquioxide of antimony, it is convenient to employ tartar emetic, from its property of dissolving in pure water without decomposition. From a solution of this salt, when moderately concentrated, a little pure potassa throws down the oxide, but excess of the alkali dissolves the precipitate. The oxide is more perfectly separated by alkaline carbonates. Lime-water causes a white precipitate, a mixed tartrate of lime and sesquioxide of antimony; and earthy and metal- lic salts decompose tartar emetic by forming, like lime, sparingly soluble compounds with tartaric acid. Decomposition is also occasioned by most acids, which throw down a sparingly soluble salt of antimony and cream of tartar; and a recently made, pretty strong, infusion of gall-nuts gives a yellowish-white precipitate, which consists of tannin and sesquioxide of antimony. But these appearances are by no means to be relied on as tests of the presence of antimony: a mixture of other substances might be simi- larly influenced by the same reagents; in a moderately dilute solution of tartar emetic most of them produce no effect whatever; and the too free addition of a pure alkali or of an acid, even to a strong solution, may altogether prevent that precipitate from forming, which a smaller quantity of the same reagents would have produced. The only certain method of bringing the antimony into view, even in a very weak solution, is to acidu- late with tartaric acid, and then transmit through the liquid a current of hydrosulphuric acid gas. The hydrated sesquisulphuret of antimony, of a characteristic orange-red colour, is immediately formed. The detection of antimony in mixed fluids, as when tartar emetic is mixed with articles of food, is conducted in the following manner.—The substances arc first digested in water acidulated with about a drachm of hydrochloric and tartaric acids, which coagulate some organic matters, and give complete solubility to the sesquioxide of antimony. Through the filtered liquid, hydrosulphuric acid is then transmitted, when the orange-red sesquisulphuret of antimony subsides, which preserves its characteristic tint even when deposited from coloured solutions, and may be further recog- nized by solution in hot hydrochloric acid and precipitation by water. The metal itself may in general be obtained by placing the dry sulphuret in a glass tube, transmitting through it a current of hydrogen gas, and then, when all the atmospheric air is displaced, heating the sulphuret by the flame of a spirit-lamp. The sulphur is carried off in the form of hydrosulphuric ANTIMONY. 377 acid gas, and the metallic antimony, recognizable by its lustre, remains. The metal is principally found where the sulphuret lay; but if the current of gas during the reduction happen to be rapid, it causes mechanically a spurious sublimation of antimony, which lines part of the tube with a thin film of metal. When much organic matter is mixed with the sulphuret, the metal is sometimes indistinctly seen. In that case it should be dissolved in a few drops of nitro-hydrochloric acid with heat, and be precipitated by water: it may then be redissolved by tartaric acid, and again precipitated with its characteristic tint by hydrosulphuric acid. Orfila recommends that the metal should be obtained from the sulphuret by fusion wilh black flux; but I have elsewhere shown this process to be very precarious, and my opinion is supported by the experience of Dr. Christison. (Treatise on Poisons, 2nd Ed. 429.) Antimonious Acid.—When metallic antimony is digested in strong nitric acid, the metal is oxidized at the expense of the acid, aid hydrated antimonic acid is formed ; and on exposing this substance to a red heat, it gives out water and oxygen gas, and is converted into antimonious acid. It is also generated when the oxide is expesd to heat in open vessels. Thus, on heating sulphuret of antimony with free exposure to the air, sulphurous acid and sesquioxide of antimony are generated ; but on continuing the roasting until all the sulphur is burned, the oxide gradually absorbs oxygen and passes into antimonious acid. Hence this acid is formed in the process of pre- paring the Pulvis Antimonialis of the Pharmacopoeia. Antimonious acid is white while cold, but acquires a yellow tint when heated, is very infusible, and fixed in the fire, two characters by which it is readily distinguished from the oxide. It is insoluble in water, aid likewise in acids after being heated to redness. It combines in definite proportions with alkalies, and its salts are called antimonites. Antimonious acid is precipitated from these salts by acids as a hydrate, which reddens litmus paper, and is dissolved by hydrochloric and tartaric acids, though without appearing to form with them definite compounds. Antimonic Acid, sometimes called peroxide of antimony, is obtained as a white hydrate, either by digesting the metal in strong nitric acid, or by dis- solving it in nitro-hydrochloric acid, concentrating by heat to expel excess of acid, and throwing the solution into water. When recently precipitated it reddens litmus paper, and may then be dissolved in water by means of hy- drochloric or tartaric acid. It does not enter into definite combination with* acids, but with alkalies forms salts, which are called antimoniates. When hydrated antimonic acid is exposed to a temperature of 500° or 600° F. the water is evolved, and the anhydrous acid of a yellow colour remains. In this state it resists the action of acids. When exposed to a red heat, it parts with oxygen, and is converted into antimonious acid.. Chlorides of Antimony.—When antimony in powder is thrown into a jar of chlorine gas, combustion ensues, and the sesquichloride of antimony is generated. The same compound may be formed by distilling a mixture of antimony with about twice and a half its weight of corrosive sublimate, when the volatile sesquichloride of antimony passes over into the recipient, and metallic mercury remains in the retort. At common temperatures it is a soft solid, thence called butter of antimony, which is liquefied by gentle heat, and crystallizes on cooling. It deliquesces on exposure to the air ; and when mixed wilh water, hydrochloric acid and sesquioxide are generated, and the latter, combined with a little undecomposed chloride, subsides. The bichloride of antimony is formed by acting on hydrated antimonious by hydrochloric acid, when a solution is formed, which appears to be a com- pound of bichloride of antimony and hydrochloric acid. It possesses little permanence, and on the addition of water, antimonious acid subsides, and hydrochloric acid remains in solution. 'The perchloride is generated by passing dry chlorine gas over heated me- tallic antimony. It is a transparent volatile liquid, which emits fumes on exposure to the air. Mixed with water, it is converted into hydrochloric 32* 378 ANTIMONY. acid, and hydrated antimonic acid which subsides. (Rose, in the Annals of Philosophy, N. S. x.) Bromide of Antimony.—The union of bromine and antimony is attended with disengagement of heat and light, and the compound is readily obtained by distillation, as in the process for preparing bromide of arsenic. It is solid at common temperatures, is fused at 206°, and boils at 518U F. It is colourless and crystallizes in needles ; it attracts moisture from the air, and is decomposed by water. Sesquisulphuret of Antimony.—This is by far the most abundant ore of antimony, and is hence employed in making the preparations of antimony. Though generally compact or earthy, it sometimes occurs in acicular crys- tals and in rhombic prisms. Its sp. gravity is 4.62, colour red-gray, and its lustre metallic. When heated in close vessels, it enters into fusion without undergoing any other change. It may be formed artificially by fusing to- gether antimony and sulphur, or by transmitting a current of hydrosulphuric acid gas through a solution of tartar emetic : in this case it falls as a hydrate of an orange-red colour, and does not acquire its dark colour till its water is expelled by heat. The bisulphuret is formed, according to Rose, by transmitting hydrosul- phuric acid gas through a solution of antimonious acid in dilute hydrochloric acid. (An. of Phil. N.S. x.) Rose formed the persulphuret by the action of hydrosulphuric acid on a solution of antimonic acid. The golden sulphuret, prepared by boiling sul- phuret of antimony and sulphur in solution of potassa, a process which is not adopted by either of our colleges, is a persulphuret Oxy-sulphurtt of Antimony.—Rose has shown that this compound occurs in the mineral kingdom, being the red antimony ore (rothspiesglanzerz) of mineralogists. The pharmaceutic preparations known by ihe terms glass, liver, and crocus of antimony, are of a similar nature, though less definite in composition. They are made by roasting the native sulphuret, so as to form sulphurous acid and sesquioxide of antimony, and then vitrefying the oxide together with undecomposed ore, by means of a strong heat. The product will of course differ according as more or less of the sulphuret escapes oxidation during the process. When sulphuret of antimony is boiled in a solution of potassa or soda, a liquid is obtained, from which on cooling an orange-red matter called *kermes mineral is deposited; and on subsequently neutralizing the cold solution with an acid, an additional quantity of a similar substance, the golden sulphuret of the Pharmacopoeia, subsides. These compounds may also be obtained by igniting sulphuret of antimony with an alkali or alkaline car- bonate, and treating the product with hot water; or by boiling the mineral in a solution of carbonate of soda or potassa. The finest kermes is obtained, according to M. Ouzel, from a mixture of 4 parts of sulphuret of antimony, 90 of crystallized carbonate of soda, and 1000 of water. These materials are boiled for half or three-quarters of an hour; the hot solution is filtered into a warm vessel, in order that it may cool slowly; and after 24 hours, the deposite is collected on a filter, moderately washed with cold water, and dried at a temperature of 70° or b0° F. Very great diversity of opinion has long existed among chemists as to the nature of kermes. Berzelius and Rose gave experiments to show that it is a hydrated sesquisulphuret, differing from the native sulphuret solely in being combined with water. Subsequently Gay-Lussac and others observed that kermes contains sesquioxide of antimony, which may be removed by digestion with cream of tartar; and Gay-Lussac inferred from the quantity of water formed when kermes, previously rendered anhydrous, is reduced by hydrogen gas, that it is a hydrated oxy-sulphurct, identical, when de- prived of its water, with the red ore of antimony above referred to. Still more recently Berzelius has explained, that the ordinary process for making kermes leads to the separation of a compound of sesquioxide of antimony and potassa, wliich tenaciously adheres to kermes, but is not chemically URANIUM. 379 united with it: he rightly argues that the question is not, whether sesqui- oxide of antimony is sometimes or generally present in kermes, but whether the latter can exist without sesquioxide of antimony. This question he has answered affirmatively. He fused sulphuret of antimony with black flux, boiled the residue in water, and set aside the solution to cool: a perfect kermes was deposited, which he considers, and I apprehend with good reason, to be quite free from sesquioxide of antimony. (Pog. Annalen, xx. 364.) The theory of the preparation of kermes, as given by Berzelius, is the following.—When sesquisulphuret of antimony is fused with potassa, part of each interchanges elements with the other in such a ratio that leq. sesquisulphuretofant.2Sb+3S2 1 eq. sesquioxide of antimony 2Sb+30 and 3 eq. potassa 3(K+0)^& 3 eq.sulphuretof potassium 3(K+S). The sulphuret of potassium unites with undecomposed sesquisulphuret of antimony, forming a sulphur-salt which will be again referred to hereafter, and sesquioxide of antimony with undecomposed potassa; and on adding hot water both compounds are dissolved, and coexist independently of each other in the solution. As the solution cools, the sesquisulphuret of antimony subsides, simply because the solvent power of sulphuret of potassium is tliereby diminished; but a variable quantity of potassa and sesquioxide of antimony falls with the deposite, and cannot be entirely removed by washing with water. The cold solution still contains a double sulphuret of antimony and potassium, together with sesquioxide of antimony united with potassa : on acidulating with sulphuric acid, the sulphuret of potassium is resolved, by decomposition of water, into potassa and hydrosulphuric acid, and the sesquioxide of antimony is deprived of its potassa; and, therefore, the sesqui- sulphuret and sesquioxide of antimony, both losing at the same instant the principles which gave them solubility, are thrown down either in combina- tion or in mixture with each other. Berzelius believes the same change to occur when the ingredients are boiled instead of fused together. The golden sulphuret differs from kermes, in the absence of potassa, in contain. ing more sesquioxide of antimony, and perhaps in being or containing an oxy-sulphuret. It commonly contains free sulphur, derived apparently from the oxidizing influence of the air on the sulphuret of potassium. When alkaline carbonates are employed instead of pure alkalies, the same phenom- ena ensue, except that carbonic acid is evolved. SECTION XX. URANIUM—CERIUM. URANIUM. This metal was discovered in 1789 by the German analyst Klaproth, who named it after the new planet Uranus, the discovery of which took place in the same year. It was obtained from a mineral of Saxony, called from its black colour pitchblende, which consists of protoxide of uranium and oxide of iron. From this ore the uranium may be conveniently extracted by the following process.—After heating the mineral to redness, and reducing it to fine powder, it is digested in pure nitric acid diluted with three or four parts of water, taking the precaution to employ a larger quantity of the mineral than the nitric acid present can dissolve. By this mode of operating, the protoxide is converted into peroxide of uranium, which unites with the nitric acid almost to the total exclusion of the iron. A current of hydro- 380 URANIUM. sulphuric acid gas is then transmitted through the solution, in order to sepa- rate lead and copper, the sulphurets of which are always mixed with pitch- blende. The solution is bdlcd to expel free hydrosulphuric acid, and, after being concentrated by evaporation, is set aside to crystallize. The nitrate of peroxide of uranium is gradually deposited in flattened four-sided prisms of a beautiful lemon-yellow colour. - The properties of metallic uranium are as yet known imperfectly. It was prepared by Arfwedson by conducting hydrogen gas over the protoxide of uranium heated in a glass tube. The substance obtained by this process was crystalline, of a met.Hie lustre, and of a reddish-brown colour. It suffered no change on exposure to air at common temperatures; but when heated in open vessels it absorbed oxygen, and was reconverted into the protoxide. From its lustre it was inferred to be metallic uranium. From the experiments of Arfwedson and Perzelius on the oxides of ura- nium, the equivalent of the metal may be estimated at 217. (An. of Ph. N. S. vii. 258.) Its compounds described in this section are thus con- stituted : Uranium. Equiv. Formulae. Protoxide 217 leq. + Oxygen 8 1 eq.=225 U + O or U. Peroxide 434 2cq. + do. 24 3cq.=458 2U + 30orU. Protochloride 217 leq. + Chlorine 35.42 1 eq. =252.42 U + ClorUCl. Perchloride 434 2eq. + do. 106.26 3oq.=540.26 2U + 3ClorU2Cl3, Sulphuret Composition unknown. Protoxide.—This oxide is of a very dark green colour, and is obtained by exposing nitrate of the peroxide to a strong heat It is exceedingly infu- sible, and bears any temperature hitherto tried without change. It unites with acids, forming salts of a green colour. It is readily oxidized by nitric acid, yielding a nitrate of the peroxide. The protoxide is employed in the arts for giving a black colour to porcelain. Peroxide of Uranium is of a yellow or orange colour, and most of its salts have a similar tint. It not only combines with acids, but likewise unites with alkaline bases, a property which was first noticed by Arfwedson. It is precipitated from acids as a yellow hydrate by pure alkalies, fixed or volatile; but retains a portion of these bases in combination. It is thrown down as a carbonate by alkaline carbonates, but is redissolved by an excess of carbonate of soda or ammonia, a circumstance which affords an easy method of separating uranium from iron. It is not precipitated by hydro- sulphuric acid, but acquires a green tint from partial deoxidation. With ferrocyanuret of potassium it gives a brownish-red precipitate, very like fer- rocyanuret of copper. Peroxide of uranium is decomposed by a strong heat, and converted into the protoxide. From its affinity for alkalies, it is difficult to obtain it in a state of perfect purity. It is employed in the arts for giving an orange colour to porcelain. Chlorides.—These compounds arc obtained in solution by dissolving the corresponding oxides in hydrochloric acid. The protochloride is green, very soluble, and does not crystallize. The perchloride is yellow, deliques- cent, soluble in alcohol, ether, and water, and yields yellow solutions. Sulphuret of Uranium may be formed by transmitting the vapour of bisulphuret of carbon over protoxide of uranium strongly heated in a tube of porcelain. (Rose.) It is of a dark gray or nearly black colour, is converted into protoxide of uranium when heated in the open air, and is readily dis- solved by nitric acid. Hydrocldoric acid attacks it feebly. 381 CERIUM. Cerium, named after the planet Ceres, was discovered in the year 1803 by Hisinger and Berzelius, in a rare Swedish mineral known by the name^of cerite, and its existence was recognized about the same time by Klaproth. Dr. Thomson has since found it to the extent of thirty-four per cent, in a mineral from Greenland, called Altanite, in honour of the late Mr. Allan, who first distinguished it as a distinct species. The properties of cerium are in a great measure unknown. It appears from the experience of Vauquelin, who obtained it in minute buttons not larger than the head of a pin, that it is a white brittle metal, which resists the action of nitric, but is dissolved by nitro-hydrochloric acid. According to an experiment made by Mr. Children and Dr. Thomson, metallic cerium is volatile in very intense degrees of heat. (An. of Phil, ii.) From the experiments of Hisinger, the equivalent of cerium may be estimated at 46, and its compounds described in this section are thus constituted:— Cerium. Equiv. Formulae. Protoxide 46 1 eq.+Oxygen 8 1 eq.= 54 Ce+O or Ce. Peroxide 92 2 eq.+do. 24 3 eq.= 116 2Ce+30orCe. Chloride ( 46 1 cq-+chIorine 35-42 l e1-= 81-42 Ce+Cl or CeCl. ^ride0" i 92 2 e9-+do- 10626 3 eq.= 198.26 2Ce+3Cl or CesCls. ^uret" \ 46 * eq--f SulPhur 161 1 e1-= 621 Ce+S or CeS- Protoxide.—This oxide is a white powder, which is insoluble in water, and forms salts with acids, all of which if soluble have an acid reaction. Exposed to the air at common temperatures it suffers no change; but if heated in open vessels, it absorbs oxygen, and is converted into the peroxide. It is precipitated from its suits as a white hydrate by pure alkalies ; as a white carbonate by alkaline carbonates, but is redissolved by the precipitant in excess ; and as a white oxalate by oxalate of ammonia. Peroxide of Cerium has a fawn-red colour. It is dissolved by several of the acids, but is a weaker base than the protoxide. Digested in hydro- chloric acid, chlorine is disengaged and a protochloride results. The most convenient method of extracting peroxide of cerium from cerite is by the process of Laugier. After reducing cerite to powder, it is dis- solved in nitro-hydrochloric acid, and the solution is evaporated to perfect dryness. The soluble parts are then redissolved by water, and an excess of ammonia is added. The precipitate thus formed, consisting of the oxides of iron and cerium, is well washed and afterwards digested in a solution of oxalic acid, which dissolves the iron, and forms an insoluble oxalate with the cerium. By heating this oxalate to redness in an open fire, the acid is decomposed, and the peroxide of cerium is obtained in a pure state. Chlorides of Cerium.—The protochloride is obtained as a porous white mass by heating protosulphuret of cerium in a current of dry chlorine gas. It is fusible at a low red heat, deliquesces in the air, and is soluble in water and alcohol. Its solution is colourless; but in open vessels it becomes yellow from the formation of perchloride, undergoing the same kind of change as the protochlorides of iron and tin. The perchloride is formed by dissolving peroxide of cerium in hydro- chloric acid, and it forms an orange-yellow solution. Protosulphuret of Cerium.—Dr. Mosander has succeeded in forming this compound by two different processes. The first method is by transmitting 382 bisl._ the vapour of sulphuret of carbon over carbonate of protoxide of cerium at a red heat; and the second is by fusing oxide of cerium at a white heat with a large excess of persulphuret of potassium and afterwards removing the soluble parts by water. I'he product of the first operation is porous, light, and of a red colour like red lead ; and that of the second is in small brilliant scales, and of a yellow colour, like bisulphuret of tin. These sulphurets, though different in appearance, arc similar in point of composition, contain- ing 26 per cent of sulphur. They are insoluble in water, but are dissolved in acids with evolution of hydrosulphuric acid gas, without any residuum of sulphur. (Philos. Mug. and Annals, i. 71). SECTION XXI. BISMUTH.—TITANIUM.—TELLURIUM. BISMUTH. This metal was known to the ancients, though often confounded by'thein with lead and tin; but it appears to have derived the name of bismuth, or properly wismvth, from the German miners. It occurs in the earth both native and in combination with other substances, such as sulphur, oxygen, and arsenic. That which is employed, in the arts is derived chiefly from native bismuth, and commonly contains small quantities of sulphur, iron, and copper. It may be obtained pure for chemical purposes by heating the oxide or subnitrate to redness along with charcoal. Bismuth has a reddish-white colour and considerable lustre. Its structure is highly lamellaled, and when slowly cooled, it crystallizes in octohedrons. Its density is about 9.8. It is brittle when cold, but may be hammered into plates while warm. At 476° it fuses, and sublimes in close vessels at a red heat. It is a less perfect conductor of heat than most other metals. Bismuth undergoes little change by exposure to air at common tempera- tures. When fused in open vessels, its surface becomes covered with a gray film, which is a mixture of metallic bismuth with the oxide of the metal. Heated to its subliming point it burns with a bluish-white flame, and emits copious fumes of oxide of bismuth. The metal is attacked with diffi- culty by hydrochloric or sulphuric acid, but it is readily oxidized and dis- solved by nitric acid. The equivalent of bismuth, deduced by Lagerhjelm from the composition of its protoxide, is 71. (An. of Phil. iv. 357.) Its compounds described in this section are thus constituted :— Bismuth. Equiv. Formulae. leq.+ Oxygen 8 1 eq.= 79 Bi + OorBi. 2eq+do. 24 3 eq.= 166 2Bi + 30orBi. 1 eq.+ Chlorine 35.42 1 eq.= 106.42 Bi+ClorBiCl. 1 eq.+ Bromine 78.4 1 eq.= 149.4 Bi-f Br or BiBr. 1 eq.+ Sulphur 16.1 1 eq.= 87.1 Bi + SorBiS. Protoxide of Bismuth.—This compound is readily prepared by heating to redness the nitrate or subnitrate of oxide of bismuth. Its colour is yellow; at a full red heat it is fused into a brown liquid, which on cooling becomes a yellow transparent glass of sp. gravity 8.211. At intense temperatures it is sublimed. It unites with acids, and most of its salts are white. Protoxide 71 Peroxide 142 Chloride 71 Bromide 71 Sulphuret 71 TITANIUM. 383 When nitrate of oxide of bismuth, either in solution or in crystals, is put into water, a copious precipitate, the subnitrate, of a beautifully white colour, subsides, which was formerly called the magistery of bismuth. From its whiteness it is sometimes employed as a paint for improving the complexion; but it is an inconvenient pigment, owing to the facility with which it is blackened by hydrosulphuric acid. If the nitrate with which it is made contains no excess of acid, and a large quantity of water is em- ployed, nearly the whole of the bismuth is separated as a subnitrate.—By this character bismuth may be both distinguished and separated from other metals. Peroxide.—This oxide was first noticed by Buchholz and Brandes, but its nature and composition have been recently examined by Mr. A. Stromeyer. It is generated when hydrate of potassa is fused at a moderate heat with protoxide of bismuth; but the best mode of preparation is first to prepare the protoxide by igniting the subnitrate, and then gently heating it for some time in a solution of chloride of potassa or soda. After washing with water, any unchanged protoxide is dissolved by a solution made with 1 part of nitric acid (quite free from nitrous acid) and 9 ofwater. As thus prepared, peroxide of bismuth is a heavy powder of a brown colour, very like peroxide of lead, manifests little disposition to unite either with acids or alkalies, and is reconverted by heat with loss of oxygen into the protoxide. Heated with sulphuric or phosphoric acid, it gives off oxygen gas, and a sulphate or phosphate of the protoxide is formed; and with hydrochloric acid, chlorine is evolved, and the protochloride produced. (An. de Ch. et de Ph. Ii. 267.) Chloride of Bismuth.—When bismuth in fine powder is introduced into chlorine gas, it takes fire, burns with a pale blue light, and is converted into a chloride, formerly termed butter of bismuth. It may be prepared conveniently by heating two parts of corrosive sublimate with one of bis- muth, and afterwards expelling the excess of the former, together with the metallic mercury, by heat Chloride of bismuth is of a grayish-white colour, opaque, and of a granular texture. It fuses at a temperature a little above that at which the metal itself is liquefied, and bears a red heat in close vessels without subliming. Bromide of Bismuth is prepared by heating the metal with a large excess of bromine in a long tube; when a gray-coloured bromide results, similar in its aspect to fused iodine. At 392° it enters into fusion, and at a low red heat sublimes. With water it is converted into oxide of bismuth and hydro- bromic acid, the former of which combines with some undecomposed bromide of bismuth as an oxy-bromide. (Serullas.) Sulphuret of Bismuth.—This sulphuret is found native, and may be formed artificially by fusing bismuth with sulphur, or by the action of hydrosul- phuric acid on the salts of bismuth. It is of a lead gray colour, and metallic lustre. TITANIUM. This metal was first recognized as a new substance by Mr. Gregor of Cornwall, and its existence was afterwards established by Klaproth, who fancifully gave it the name of titanium, after the Titans of ancient fable. (Contributions, i.) But the properties of the metal were not ascertained in a satisfactory manner until the year 1822, when Dr. Wollaston was led to examine some minute crystals which were found in a slag at the bottom of a smelting furnace at the great iron works at Merthyr Tydvil in Wales, and presented to him by the Rev. Dr. Buckland. (Philosophical Transactions, 1823.) These crystals, which have since been found at other iron works, are of a cubic form, and in colour and lustre resemble burnished copper. They conduct electricity, and are attracted slightly by the magnet, a prop- erty which seems owing to the presence of a minute quantity of iron. Their Bp. gravity is 5.3 ; and their hardness is so great, that they scratch apolished surface of rock crystal. They are exceedingly infusible; but when ex- 384 TITANIUM. posed to the united action of heat and air, their surface becomes covered with a purple-coloured film, which is an oxide. They resist the action of nitric and nitro-hydrochloric acids, but are completely oxidized by being strongly heated with nitre. They are then converted into a white substance, which possesses all the properties of titanic acid. Liebig prepares metallic titanium by putting fragments of recently made chloride of titanium and ammonia into a glass tube half an inch wide and two or three feet long, transmitting through it a current of perfectly dry ammonia, and, when atmospheric air is entirely displaced, applying heat until the glass softens. Complete decomposition ensues, nitrogen gas is dis- engaged, hydrochlorate of ammonia sublimes, and metallic titanium is left in the state of a deep blue-coloured powder. If exposed to the air while warm, it is apt to take fire. The equivalent of titanium, determined by Rose from his analysis of the bichloride, is 24.3. The composition of its compounds described in this section is as follows:— 1 eq. Titanium. Equiv. Formulae. Oxide (probably) 24.3 +Oxygen 8 1 eq.=32.3 Ti + O or Ti. Titanic acid . 24.3 +do 16 2 eq.=40.3 Ti + 20 or Ti. Bichloride . . 24.3 +Chlorine 70.84 2 eq.=95.14 Ti + 2C1 or TiCl». Bisulphuret . 24.3 + Sulphur 32.2 2 eq.=56.5 Ti + 2S or TiS2. Oxide of Titanium.—This oxide is of a purple colour, and is supposed to exist pure in the mineral called anatase; but its composition and chemical properties are unknown. Titanic Acid.—This compound, called also peroxide of titanium, exists in a nearly pure state in titanite or rutile. Menaccanite, in which titanium was originally discovered by Mr. Gregor, is a titanite of the oxides of iron and manganese. It is best prepared from rutile. The mineral, after being reduced to an exceedingly fine powder, is fused in a platinum crucible with three times its weight of carbonate of potassa, and the mass afterwards washed with water to remove the excess of alkali. A gray mass remains, which consists of potassa and titanic acid. This compound is dissolved in concentrated hydrochloric acid; and on diluting with water, and boiling the solution, the greater part of the titanic acid is thrown down. It is then col- lected on a filter, and well washed with water acidulated with hydrochloric acid. In this state the oxide is not quite pure; but contains a little oxide of manganese and iron, derived from the rutile. The best mode of separating these impurities is to digest the precipitate, while still moist, with hydrosul- phate of ammonia, wThich converts the oxides of iron and manganese into sulphurets, but docs not act on the titanic acid. The two sulphurets are readily dissolved by dilute hydrochloric acid; and the titanic acid, after being collected on a filter and well washed as before, may be dried and heat- ed to redness. This method, proposed by Professor Rose of Berlin, has been thus simplified by himself. Either rutile or titaniferous iron, after being pulverized and washed, is exposed in a porcelain tube at a very strong red heat to a current of hydrosulphuric acid gas, which acts upon the oxide of iron, giving rise to water and sulphuret of iron. As soon as water ceases to appear, the process is discontinued, the mass digested in hydrochloric acid to remove the iron, and the titanic acid separated from adhering sulphur by heat. A little iron is still usually retained; but the whole may be re- moved by a repetition of the same process. (An. de Ch. et de Ph. xxiii. and xxxviii. 131.) Titanic acid, when pure, is quite white. It is exceedingly infusible and difficult of reduction ; and after being once ignited it ceases to be soluble in acids, except in the hydrofluoric. In its chemical relations it is analogous to silicic acid, being a feeble acid, insoluble in water, without action on test paper, but combining with metallic oxides. In the state of hydrate, as when TELLURIUM. 385 precipitated from hydrochloric acid by boiling, or when combined with an alkali after fusion, it has a singular tendency to pass through the pores of a filter when washed with pure water; but the presence of a little acid, alkali, or a sdt, prevents this inconvenience. . If previously ignited with carbonate of potassa, titanic acid is soluble in dilute hydrochloric acid; but it is retained in solution by so feeble an at- traction, that it is precipitated merely by boiling. It is likewise thrown down by the pure and carbonated alkalies, both fixed and volatile. A solution ot gall-nuts causes an orange-red colour, which is very characteristic of titanic acid, an effect which appears owing to tannin, and not to gallic acid. Wrjen a rod of zinc is suspended in the solution, a purple-coloured powder, probably the protoxide, is precipitated, which is gradually converted into titanic acid. Bichloride of Titanium.—This substance was discovered in the year 18^4 by Mr. George of Leeds, by transmitting dry chlorine gas over metallic tita- nium at a red heat. Rose prepared it for his analysis by heating a mixture of titanic acid and charcoal in a tube, through which dry chlorine gas was passing: the resulting bichloride was purified from adhering free chlorine by agitation either with mercury or potassium, and repeated distillation. At common temperatures it is a transparent colourless fluid of considerable specific gravity, boils violently at a temperature a little above 212°, and con- denses again without change. Dumas has shown that the density of its vapour may be estimated at 6.615. In open vessels it is attacked by the moisture of the atmosphere, and emits dense white fumes of a pungent odour similar to that of chlorine, but not so offensive. On adding a few drops ofwater to a few drops of the liquid, combination ensues with almost explosive violence, from the evolution of intense heat; and if the water is not in excess a solid hydrate is obtained. On exposure to the air it deliquesces, and on adding water the greater part is dissolved. The bichloride, when ex- posed to an atmosphere of dry ammonia, absorbs a large quantity of the gas, and becomes solid. It was from this compound Liebig prepared metallic titanium. Bisulphuret of Titanium.—This compound was discovered by Rose, who prepared it by transmitting the vapour of bisulphuret of carbon oyer titanic acid, heated to whiteness in a tube of porcelain. It occurs in thick green masses, which by the least friction acquire a dark yellow colour and metal- lic lustre. When heated in the open air, it is converted into sulphurous and titanic acids. By acids it is slowly decomposed, and is dissolved by hydro- chloric acid with disengagement of hydrosulphuric acid gas. TELLURIUM. Tellurium is a rare metal, hitherto found only in the gold mines of Tran- sylvania, and even there in very small quantity. Its existence was inferred by Mdler in the year 1782, and fully established in 1798 by Klaproth, who gave it the name of tellurium, from tellus, the earth, suggested by the source from which he drew the name of uranium. (Contributions, iii.) It occurs in the metallic state, chiefly in combination with gold and silver. Tellurium has a tin-white colour running into lead-gray, a strong metallic lustre, and lamellated texture. It is very brittle, and its density is 6.115. It fuses at a temperature below redness, and at a red heat is volatile. When heated before the blowpipe it takes fire, burns rapidly with a blue flame bordered with green, and is dissipated in gray-coloured pungent inodorous fumes. The odour of decayed horse-radish is sometimes emitted during the combustion, and was thought by Klaproth to be peculiar to tellurium; but Berzelius ascribes it solely to the presence of selenium. From some late experiments of Berzelius the equivalent of tellurium is 64.2, being nearly double the number stated in the table at page 141. Its compounds described in this section are thus constituted:— 33 86 TELLURIUM. One eq. Tellurium. Equiv. Formulae. Tellurous acid 64.2 + Oxyg. 16 2 eq.= 80.2 Te + 20orfe Telluric acid 64.2 +do. 24 3 eq.= 88.2 Te + 30orTe. Chloride 64.2 + Chlor. 35.42 1 eq.= 99.62 Te + Cl or TeCl. Bichloride 64.2 +do. 70.84 2 eq. =135.04 Te-f2Cl or TeCl*. Bisulphuret 64.2 + Sulph. 32.2 2 eq.= 96.4 Te+2SorTeS2. Persulphuret Composition uncertain. Hydrotelluric acid 64.2+Hydr. 1 1 eq.= 65.2 Te + HorTeH. Tellurous Acid.—This compound, also called oxide of tellurium, is gene- rated by the action of nitric acid on tellurium, by which acid it is dissolved ; but the solution possesses such little permanence that mere affusion ofwater precipitates part of it, and the rest is obtained by evaporating to dryness. In this state, it is a white granular anhydrous powder, which slowly reddens moist litmus paper, and is insoluble in water and acids. By pure potassa or soda in solution it is dissolved, and is rendered soluble by fusion with the alkaline carbonates, forming with those alkalies crystallizable salts. Acids, added in slight excess to the alkaline solutions, throw down tellurous acid as a white flaky hydrate, which if washed in ice-cold water, and dried at a temperature not exceeding 53°, may be preserved unchanged. In this state it is freely soluble in acids, in ammonia, in the alkaline carbonates, from which it expels carbonic acid, and even to considerable extent in pure water. Its aqueous solution reddens litmus paper : it becomes turbid at 68°, and the acid which falls is no longer soluble in acids. In these properties tellurous acid closely resembles the titanic and several other feeble acids, which have a soluble hydrated state easily convertible into an insoluble anhydrous one. Its salts are precipitated black by hydrosulphuric acid, bisulphuret of tellu- rium being formed. It is deoxidized, when metallic tellurium falls as a black powder, when a piece of zinc, tin, iron, or antimony is left in its solution. Telluric Acid.—The process which Berzelius recommends for preparing this compound is either to deflagrate tellurous acid with nitre, or to mix pure potassa freely with a solution of tellurite of potassa, and to saturate fully with chlorine. Nitric acid in slight excess and a little chloride of barium are added, in order to precipitate any traces of sulphuric and selenic acids: and after separating the precipitate by filtration, the liquid is exactly neutralized with ammonia, and chloride of barium added as long as it causes a precipi. tate. The tellurate of baryta is washed, dried by a gentle heat, and then digested with a fourth of its weight of strong sulphuric acid previously diluted with water : the filtered solution is then concentrated by a water bath, and, on cooling or subsequent spontaneous evaporation, yields hydrated telluric acid in the form of flat six-sided prisms. Adhering sulphuric acid is re- moved by alcohol. This hydrate consists of one eq. of acid and three eq. ofwater. When heated at 212° it loses two of its equivalents of water; and on heating still further all its water is expelled, and the anhydrous acid of a lemon-yellow colour re- mains. In this state it is insoluble in all fluids, whereas the hydrated acid is soluble in water; and the salts of the former differ from those which the latter forms with the same bases. Hence heat modifies the character of tel- luric acid much in the same way as that of phosphoric acid. At a heat be- yond that required to render it anhydrous, telluric acid loses oxygen and is reduced to tellurous acid. (Pog. Annalen, xxviii. 392.) Chloride.—Rose obtained it by passing a feeble current of chlorine gas over tellurium at a strong heat, when the chloride passes over as a violet vapour, which at first condenses into a black liquid, and when quite cold becomes a solid of the same colour. By the action ofwater it deposites me- tallic tellurium, and the bichloride is dissolved. Bichloride.—Rose obtained this in the same manner as the preceding chloride, except using a lower heat and a more liberal supply of chlorine. COPPER. 381 The bichloride is dso volatile, and, after being purified from free chlorine by agitation with mercury, and a second distillation, it condenses into a white crystalline solid. By a gentle heat it yields a brown liquid, but recovers its whiteness on cooling. (Tog. Annalen, xxi. 443.) Bisulphuret.—This compound falls of a dark brown, nearly black colour, when hydrosulphuric acid gas is transmitted through a solution of bichloride of tellurium, tellurous acid, or any soluble tellurite. This sulphuret is what Berzelius calls a sulphur-acid, forming a soluble sulphur-salt by uniting with sdphuret of potassium. Hence a solution of caustic potassa dissolves bisul- phuret of tellurium,'producing the same kind of change as on sulphuret of antimony. (Page 379.) Persulphuret.—This compound falls of a deep yellow colour, when a salt of telluric acid is mixed in solution with persulphuret of potassium. Its ex- istence is but transient, as it is quickly transformed into bisulphuret and be- comes black. Hydrotelluric Acid.—This gas, discovered by Davy in 1809, is formed by acting with hydrochloric acid on an alloy of tellurium with zinc or tin. It has the properties of a feeble acid, very analogous in odour, and apparently in composition, to hydrosulphuric acid ; it is absorbed by water, forming a claret-coloured solution ; and it precipitates many metallic salts, yielding an alloy of tellurium with the other metal. It is deprived of its hydrogen by chlorine, nitric acid, or oxygen of the atmosphere, tellurium being separated. SECTION XXII. COPPER. Copper is one of the most abundant of the metds, and was well known to the ancients. Native copper is by no means uncommon, being found more or less in most copper mines : it occurs in large amorphous masses in some parts of America, and is sometimes met with in octohedral crystals or in some of the forms allied to the octohedron. Stromeyer has lately discovered it in several specimens of meteoric iron, but in a quantity not exceeding 2-1000ths of the mass. The copper of commerce is extracted chiefly from the native sulphuret; especially from copper pyrites, a double sulphuret of iron and copper. The first part of the process consists in roasting the ore, so as to burn off some of the sulphu r, and leave the remainder as a subsulphate of the oxides of iron and copper. The mass is next heated with some unroasted ore and siliceous substances, by which means much of the iron unites in the state of black oxide with silicic acid, and rises as a fusible slag to the sur- face; while most of the copper returns to the state of sulphuret. It is then subjected to long-continued roasting, when the greater part of the sulphur escapes as sulphurous acid and the metal is oxidized ; after which it is re- duced by charcoal, and more of the iron separated as a silicate by the addi- tion of sand. Lastly, the metal is strongly heated while a current of air plays upon its surface: the impurities, chiefly sulphur and iron, being more oxidable than copper, combine with oxygen by preference, and the copper is at length left in a state of purity sufficient for the purposes of com- merce. Copper is distinguished from all other metals, titanium excepted, by having a red colour. It receives a considerable lustre by polishing. Its density, when fused, is 8.895, and it is increased by hammering. It is both ductile, and malleable, and in tenacity is inferior only to iron. It is hard and elastic, and consequently sonorous. Its point of fusion is 1996° F. accord- ing to Mr. Daniell, being less fusible than silver and more so than gold. Copper undergoes little change in a perfectly dry atmosphere; but is 388 COPPER. rusted in a short time by exposure to air and moisture, being converted into a green substance, carbonate of the black oxide of copper. At a red heat it absorbs oxygen, and is converted into black scales of oxide. It is attacked with difficulty by hydrochloric and sulphuric acids, and not at all by solu- tions of the vegetable acids, if atmospheric air be excluded ; but if air have frae access, the metal absorbs oxygen with rapidity, the attraction of the acid for the oxide of copper co-operating with that of the copper for oxygen. Nitric acid acts with violence on copper, forming a nitrate of the black oxide. The most trustworthy experiments for determining the equivalent of copper are those of Berzelius on the reduction of the black oxide by means of hydrogen gas at a red heat. According to the best of his analyses, 8 parts of oxygen unite with 31.6 parts of copper to constitute the black oxide ; and, therefore, if this oxide be formed of an atom of oxygen united with an atom of copper, the equivalent of this metal will be 31.6. This opinion, which I have adopted, is maintained by Thomson, Berzelius, and many Continental chemists. Others consider it as a binoxide, regarding red oxide of copper as the real protoxide ; and these take twice 31.6 or 63.2 as the equivalent of copper. The principal arguments in favour of the former view are these :—1, the red oxide has very much the character of a suboxide, a term frequently used to designate an oxide which has little or no tendency to unite with acids, and which contains less than one atom of oxygen to one atom of metal; 2, the product of the equivalent and specific heat of most metals is a constant quantity, and copper coincides with the law, provided the black oxide contain an atom of each element (page 35); 3, the salts of the black oxide are isomorphous with the salts of protoxide of iron, which gives a strong presumption that these oxides possess the same atomic con- stitution. The composition of the compounds described in this section is as follows:— Copper. Equiv. Formulae. 2eq.+ Oxygen 8 1 eq.= 71.2 2Cu + 0. 1 eq.+do." 8 1 eq.= 39.6 Cu + O. 1 eq.+do. 16 2 eq.= 47.6 Cu+20. 2 eq.+Chlorine 35.42 1 eq.= 98.62 2Cu+Cl. 1 eq.+ do. 35.42 1 eq.= 67.02 Cu+Cl. 2eq.+ Iodine 126.3 1 eq.= 189.5 2Cu+I. 2 cq.+ Sulphur 16.1 1 eq.= 79.3 2Cu+S. 1 eq.-i-do. 16.1 1 eq.= 47.7 Cu+S. 3 eq.+ Phosph. 15.7 1 eq.= 110.5 3Cu+P. 3eq.Tdo. 31.4 2eq.= 126.2 3Cu + 2P 1 eq.!j_Cyanogen 26.39 1 eq.= 57.99 Cu + Cy. ^5et0C;1«-2 »1+)£5S£? \5™ ""I-""-™ ** + <**> Red Oxide.—This compound occurs native in the form of octohedral crystals, and is found of peculiar beauty in the mines of Cornwall. It may be prepared artificially by heating, in a covered crucible, a mixture of 31.6 parts of copper filings with 39.6 of the black oxide; or still better by ar- ranging thin copper plates one above the other, with interposed strata of the black oxide, and exposing them to a red heat carefully protected from the air. Another method is by boiling a solution of acetate of protoxide of copper with sugar, when the suboxide subsides as a red powder; and another is to fuse at a low red heat the dichloride of copper with about an equal weight of carbonate or bicarbonate of soda, subsequently dissolving the sea- salt by water, and drying the red powder. In this case, by an interchange of elements, 1 eq. dichloride of copper 2Cu + C12 1 eq. red oxide . . . 2Cu + O and 1 eq. soda .... Na + ° "p.andl eq. chloride of sodium Na+ C The red oxide of copper has a density of 6.093, and in colour is very Red or dioxide 63.2 Black or protoxide 31.6 Superoxide 31.6 Dichloride 63.2 Chloride 31.6 Diniodide 63.2 Disulphuret 63.2 Sulphuret 31.6 Triphosphuret . 948 Subsesquiphosph. 94.8 Cyanuret 31.6 COPPER. 389 similar to copper. It may be preserved in a dry atmosphere; but at a red heat it absorbs oxygen, and is converted into the protoxide. Dilute acids act on it very slowly; and the resulting solution, as is indicated by its tint, does not arise from the union of the red oxide itself with the acid, but from its being resolved, like other suboxides, into metal and a protoxide. With strong nitric acid it is oxidized, binoxide of nitrogen escapes, and a nitrate of the black oxide is formed. Strong hydrochloric acid forms with it a colourless solution, from which alkalies throw it down as a hydrate of an orange tint. In this state it readily absorbs oxygen from the air. The red oxide of copper is soluble in ammonia, and the solution is quite colour- less ; but it becomes blue with surprising rapidity by free exposure to air, owing to the formation of the black oxide. Black Oxide.—This compound, the copper black of mineralogists, ia sometimes found native, being formed by the spontaneous oxidation of other ores of copper. It may be prepared artificially by calcining metallic cop- per, by precipitation from the salts of copper by means of pure potassa, and by heating nitrate of copper to redness. It varies in colour from a dark brown to a bluish-black, according to the mode of formation, and its density is 6.401. It undergoes no change by heat alone, but is readily reduced to the metallic state by heat and combustible matter. It is insoluble in water, and does not affect the vegetable blue colours. It combines with nearly all the acids, and most of its salts have a green or blue tint. It is soluble like- wise in ammonia, forming with it a deep blue solution, a property bv which protoxide of copper is distinguished from all other substances. The salts of protoxide of copper are distinguished from most substances by their colour, and are easily recognized by reagents. When pure soda or potassa is mixed with a solution of sulphate of the protoxide, a greenish-blue disdphate at first subsides; but as soon as the alkali is added in excess, a blue bulky hydrate of the oxide is formed, which is decomposed by boiling, and consequently becomes black. Pure ammonia also throws down the disdphate when carefully added; but an exces3 of the alkali instantly re- dissolves the precipitate, and forms a deep blue solution. Alkaline carbo- nates cause a bluish-green precipitate, carbonate of the protoxide, which is redissolved by an excess of carbonate of ammonia. It is precipitated as a dark brown sulphuret by hydrosulphuric acid, and as a reddish-brown ferro- cyanuret by ferrocyanuret of potassium. It is thrown down of a yellowish- white colour by albumen, and M. Orfila has proved that this compound is inert, so that albumen is an antidote to poisoning by copper. Copper is separated in the metallic state by a rod of iron or zinc. The copper thus obtained, after being digested in a dilute solution of hydrochloric acid, is almost chemically pure. The best mode of detecting copper, when supposed to be present in mixed fluids, is by hydrosulphuric acid. The sulphuret, after being collected, and heated to redness in order to char organic matter, should be placed on a piece of porcelain, and be digested in a few drops of nitric acid. Sulphate of protoxide of copper is formed, which, when evaporated to dryness, strikes the characteristic deep blue tint on the addition of ammonia. Superoxide.—This oxide was prepared by Thenard by the action of per- oxide of hydrogen diluted with water on the hydrated black oxide. It suffers spontaneous decomposition under water; but it may be dried in vacuo by means of sulphuric acid. Dichloride.—When copper filings are introduced into an atmosphere of chlorine gas, the metal takes fire spontaneously, and both the chlorides are generated. The dichloride may be conveniently prepared by heating copper filings with twice their weight of corrosive sublimate. In this way it was originally made by Boyle, who termed it resin of copper, from its resem blance to common resin. Proust, who called it white muriate of cower procured it by the action of protochloride of tin on chloride of copper- and also by decomposing the chloride by heat, air being excluded. It is slowly 33* ' 390 COPPER. deposited in crystalline grains, when the green solution of chloride of copper is kept in a corked bottle in contact with metallic copper. The dichloride of copper is fusible at a heat just below redness, and bears a red heat in close vessels without subliming. It is insoluble in water, but dissolves in hydrochloric acid, and is precipitated unchanged by water as a white powder. Its colour varies with the mode of preparation, being white, yellow, or dark brown. It is apt to absorb oxygen from the atmo- sphere, forming a green-coloured compound of oxide and chloride of cop- per ; a change to which the dichloride prepared in the moist way is pecu- liarly prone. Chloride.—The chloride of copper is obtained in solution of a green colour by dissolving protoxide of copper in hydrochloric acid, and crystal- lizes by due concentration in green needles, which are deliquescent and very soluble in alcohol. When heated, they fuse, lose water, and the anhydrous chloride in form of a yellow powder is left; but the heat must not exceed 400°, as beyond that degree the chloride loses half its chlorine, and is con- verted into the dichloride. Diniodide of Copper.—This substance is obtained by adding iodide of potassium to a solution made of the sulphates of the protoxides of copper and iron, both in crystals, in the ratio of 1 to 2J, when the protoxide of iron takes the oxygen of the oxide of copper, and the iodine the metallic copper, forming a white precipitate, the diniodide. It may be dried, and will bear a high temperature in close vessels without change; but if heated with the oxides of iron, manganese, or copper, iodine is expelled, and the copper oxidized. (Page 226.) Iodide of Copper is scarcely known. For on mixing a salt of oxide of copper with iodide of potassium, iodine is set free and the diniodide of cop- per falls. A small quantity of iodide of copper remains in solution. Sulphurets of Copper.—The disulphuret is a natural production, well known to mineralogists under the name of copper glance; and in combina- tion with protosulphuret of iron, it is a constituent of variegated copper ore. It is formed artificially by heating copper filings with a third of their weight of sulphur, the combination being attended with such free disengagement of heat, that the mass becomes vividly luminous. Sulphuret of Copper is a constituent of copper pyrites, in which it is combined with protosulphuret of iron. It may be formed artificially by the action of hydrosulphuric acid on a salt of copper. When ignited without exposure to the air, it loses half of its sulphur, and is converted into the disulphuret. Phosphurets of Copper.—Rose states that the triphosphuret is generated bj the action of phosphuretted hydrogen gas on dichloride of copper; the mutual interchange of elements being such that 3 eq. dichloride of copper 3(2Cu+CI) 2 2 eq. triphosphuret 2(Cu+P) aridleq.phosphuiettedhyd. 3H+2P -^ and 3 eq. hydrochl. acid 3(H+C1.) The suhsesquiphosphuret is formed by a similar interchange between chloride of copper and phosphuretted Ivydrogen, so that 3 eq. chloride of copper 3(C+uCl) 2 1 eq. suhsesquiphosphuret 3Cu+2P and 1 eq. phosp. hyd. 3H+2P -^ and 3 eq. hydrochloric acid3(H+Cl). Rose obtained the protophosphuret by the action of hydrogen gas on phosphate of protoxide of copper at a red heat. All these phosphurets resemble each other, being pulverulent, of a gray colour, insoluble in hydro- chloric acid, are oxidized and dissolved by nitric acid, and burn with a phosphorus flame before the blowpipe. A phosphuret of copper is also ob- tained by transmitting phosphuretted hydrogen gas through a solution of sulphate of oxide of copper; but the dark precipitate which falls seems to be a variable mixture of different phosphurets, phosphoric acid being gene- rated at the same time. (An. de Ch. et de Ph. Ii. 47.) LEAD. 391 Cyanuret of Copper.—This compound is formed by the action of hydro- cyanic acid on hydrated protoxide of copper, or by mixing the sulphate of that oxide in solution with cyanuret of potassium. It falls as a yellow pre- cipitate, insoluble in water, but soluble in hydrochloric or sulphuric acid. Disulphocyanuret of Copper.—It appears as a white precipitate, insoluble in most acids, when sulphocyanuret of potassium is added to a mixed solution of the sulphates of the protoxides of copper and iron. SECTION XXIII. LEAD. This metal was well known to the ancients. As a native production it is very rare; but in combination with sulphur it occurs in great quantity. All the lead of commerce is extracted from the native sulphuret, the galena of mineralogists. This ore, in the state of coarse powder, is heated in a revcr- beratory furnace; when part of it is oxidized, yielding sulphate of protoxide of lead, sulphuric acid which is evolved, and free oxide of lead. These oxidized portions then react on sulphuret of lead: by the reaction of two equivalents of oxide of lead and one of the sulphuret, tliree equivalents of metallic lead and one of sulphurous acid result; while one equivalent of the sulphuret and one of sulphate mutually decompose each other, giving rise to two equivalents of sulphurous acid and two of metallic lead. The slag which collects on the surface of the fused lead contains a large quantity of sulphate of protoxide of lead, and is decomposed by the addition of quicklime, the oxide so separated reacting as before on sulphuret of lead. The lead of commerce commonly contains silver, iron, and copper. Lead has a bluish-gray colour, and when recently cut, a strong metallic lustre; but it soon tarnishes by exposure to the air, acquiring a superficial coating of carbonate of protoxide of lead. (Christison.) Its density is 11.352. It is soft, flexible, and inelastic. It is both malleable and ductile, possessing the former property in particular to a considerable extent. In tenacity, it is inferior to all ductile metals. It fuses at about 612° F., and when slowly cooled forms octohedral crystals. It may be heated to whiteness in close vessels without subliming. Lead absorbs oxygen quickly at high temperatures. When fused in open vessels, a gray film is formed upon its surface, which is a mixture of metal- lic lead and protoxide; and when strongly heated it is dissipated in fumes of the protoxide. In distilled water, previously boiled and preserved in close vessels, it undergoes no change; but in open vessels it is oxidized with con- siderable rapidity, yielding minute, shining, brilliantly white, crystalline scales of carbonate of the protoxide, the oxygen and the carbonic acid being derived from the air. The presence of saline matter in water retards the oxidation of the lead; and some salts, even in very minute quantity, prevent it altogether. The protecting influence, exerted by certain substances, was first noticed by Guyton-Morveau; but it has been minutely investigated by Dr. Christison of Edinburgh, who has discussed the subject in his excellent Treatise on Poisons. He finds that the preservative power of neutral salts is materially connected with the insolubility of the compound which their acid is capable of forming with lead. Thus, phosphates and sulphates, as well as chlorides and iodides, are highly preservative; so small a quantity as l-30,000th part of phosphate of soda or iodide of potassium in distilled water preventing the corrosion of lead. In a preservative solution the metal gains weight during some weeks, in consequence of its surface gradually acqui- ring a superficial coating of carbonate, which is slowly decomposed by the 392 LEAD. saline matter of the solution. The metallic surface being thus covered with an insoluble film, which adheres tenaciously, all further change ceases. Many kinds of spring water, owing to the salts which they contain, do not corrode lead; and hence, though intended for drinking, it may be safely collected in leaden cisterns. Of this, the water of Edinburgh is a remark- able instance. Lead is not attacked by the hydrochloric or the vegetable acids, though their presence, at least in some instances, accelerates the absorption of oxy- gen from the atmosphere in the same manner as with copper. Cold sulphu- ric acid does not act upon it; but when boiled in that liquid, the lead is slowly oxidized at the expense of the acid. The only proper solvent for lead is nitric acid. This reagent oxidizes it rapidly, and forms with its oxide a salt which crystallizes in opaque octohedrons by evaporation. From late experiments on the composition of the protoxide of lead, and of the nitrate and sulphate of that oxide, I have deduced 103.6 as the equivalent, a number which agrees very closely with the researches of Berzelius on the same subject. (Phil. Trans. 1833, part ii.) The composition of its compounds described in this section is as follows :— Lead. Equiv. Formulae. Dinoxide 207.2 2 eq. +Oxygen 8 1 eq.=215.2 2Pb + 0. Protoxide 103.6 1 eq. + do. 8 1 eq.=111.6 Pb + O. Peroxide 103.6 1 eq. + do. 16 2 eq.=119.6 Pb + 20. Red oxide J 310-813 %±d9°- 3 32 *«& . } =342.83Pb+40 { or protox. 223.2 or 2eq. + perox. 119.6 or leq. \ 2Ph4-Pb. Chloride 103.6 1 eq. + Chlorine 35.42 1 eq.=139.02 Pb + Cl. Iodide 103.6 1 eq. +Iodine 126.3 1 eq.=229.9 Pb + I. Bromide 103.6 1 eq. +Bromine 78.4 1 eq.=182 Pb + Br Fluoride 103.6 1 eq. +Fluorine 18.68 1 eq.=122.28 Pb + F. Sulphuret 103.6 1 eq. +Sulphur 16.1 1 eq.=119.7 Pb+S. Phosphuret > ^ ... . . C rh t ( Composition uncertain. Cyanuret 103.6 1 eq. + Cyanogen 26.39 1 eq=129.99 Pb + Cy. Dinoxide of Lead.—Dulong observed that on heating dry oxdate of prot- oxide of lead in a glass tube to low redness, air being excluded, a mixture of carbonic acid and carbonic oxide gases is evolved, and a suboxide remains of a dark gray, nearly, black, colour. Boussingault has lately proved that it is a dinoxide. It does not unite with acids, but is resolved by them into a salt of the protoxide with separation of metallic lead. (An. de Ch. et de Ph. liv. 263.) Protoxide.—This oxide is prepared on a large scale by collecting the gray film which forms on the surface of melted lead, and exposing it to heat and air until it acquires a uniform yellow colour. In this state it is the massicot of commerce; and when partially fused by heat, the term litharge is applied to it. As thus procured it is always mixed with the red oxide. It may be obtained pure by adding ammonia to a cold solution of nitrate of protoxide of lead until it is faintly alkaline, washing the precipitated subnitrate with cold water, and when dry, heating it to moderate redness for an hour in a platinum crucible. An open fire should be used, and great care taken to prevent combustible matter in any form from contact with the oxide. Protoxide of lead is red while hot, but has a rich lemon-yellow colour when cold, is insoluble in water, fuses at a bright red heat, and is fixed and unchangeable in the fire. Its density is 9.4214. The fused protoxide has a highly foliated texture, and is very tough, so as to be pulverized with diffi- culty. By transmitted light it is yellow; but by reflected light it appears green in some parts and yellow in others. Heated with combustible matters, tho protoxide parts with oxygen and is reduced. From its insolubility it does not change the vegetable colours under common circumstances; but when rendered soluble by a small quantity of acetic acid, it has a distinct LEAD. 393 alkaline reaction. It unites with acids, and is the base of all the salts of lead, most of which are of a white colour. Protoxide of lead is precipitated from its solutions by pure alkalies as a white hydrate, which is redissolved by potassa in excess; as a white carbonate, which is the well-known pigment white lead, by alkaline carbonates; as a white sulphate by soluble sulphates; as a dark brown sulphuret by hydro- sulphuric acid ; and as yellow iodide of lead by hydriodic acid or iodide of potassium. With regard to the poisonous property of the salts of lead, a remarkable fact has been observed by my colleague Dr. A. T. Thomson, who has proved that of all the ordinary preparations of lead, the carbonate is by far the most virulent poison. Any sdt of lead which is easily convertible into the carbonate, as for instance the subacetate, is also poisonous; but he has given large doses of the nitrate of the protoxide and chloride of lead to rabbits without producing perceptible inconvenience. He finds that acetate of prot- oxide of lead, mixed with vinegar to prevent the formation of any carbonate, may be freely and safely administered in medical practice. The best method of detecting the presence of lead in wine or other suspect- ed mixed fluids is by means of hydrosulphuric acid. The sulphuret of lead, after being collected on a filter and washed, is to be digested in nitric acid diluted with twice its weight of water, until the dark colour of the sulphuret disappears. The solution of the nitrate shodd then be brought to perfect dryness on a watch-glass, in order to expel the excess of nitric acid, and the residue be redissolved in a smdl quantity of cold water. On dropping a particle of iodide of potassium into a portion of this liquid, yellow iodide of lead will instantly appear. Protoxide of lead unites readily with earthy substances, forming with them a transparent colourless glass. Owing to this property it is much em- ployed for glazing earthenware and porcelain. It enters in large quantity into the composition of flint glass, which it renders more fusible, transparent, and uniform. Lead is separated from its salts in the metallic state by iron or zinc. The. best way of demonstrating this fact is by dissolving 1 part of acetate of pro toxide of lead in 24 ofwater, and suspending a piece of zinc in the solution by means of a thread. The lead is deposited upon the zinc in a peculiar ar- borescent form, giving rise to the appearance called arbor Saturni. Red Oxide.—This compound, the minium of commerce, is employed as a pigment, and in the manufacture of flint glass. It is formed by oxidizing lead by heat and air without allowing it to fuse, and then exposing it in open vessels to a temperature of 600° or 700°, while a current of air plays upon its surface. It slowly absorbs oxygen and is converted into minium. This oxide does not unite with acids. When heated to redness it gives off pure oxygen gas, and is reconverted into the protoxide. When digested in nitric acid it is resolved into protoxide and peroxide of lead, the former of which unites with the acid, while the latter remains as an insoluble powder. From the facility with which this change is effected even by acetic acid, most chemists consider red lead, not so much as a definite compound of lead and oxygen, but as a salt composed of the protoxide and peroxide as stated at page 392. This oxide has been long considered as a sesquioxide, an error first corrected by Dalton (New System of Chemistry, ii. 41.), whose obser- vation has been confirmed by Dumas and Phillips. (An. de Ch. et de Ph. xlix. 398, and Phil. Mag. N. S. iii. 125.) Dumas shows that the minium is not uniform in composition, but consists of variable mixtures of the protoxide with real red lead. The former may be oxidized by continued exposure to air and heat, and may be dissolved by acetic acid very much diluted with cold water. Peroxide.—This oxide may be obtained by the action of nitric acid on minium, as just mentioned; by fusing protoxide of lead with chlorate of potassa, at a temperature short of redness, and removing the chloride of potassium by solution in water; and by transmitting a current of chlorine 394 LEAD. gas through a solution of acetate of the protoxide of lead. In the last the reaction is such, that 1 eq. chlorine & 2 eq. protox. lead 2 1 eq. perox. lead & 1 eq. chloride lead CI. 2(Pb + 0) -S Pb + 20 Pb+Cl. The chloride is removed by washing with warm water. Peroxide of lead is of a puce colour, is insoluble in water, and is resolved by strong ox-acids, such as the sulphuric and nitric, into a salt of the pro- toxide and oxygen gas. With hydrochloric acid it yields chlorine gas and chloride of lead. At a red heat it emits oxygen gas and is converted into the protoxide. Chloride of Lead.—This compound, sometimes called horn lead, is slowly formed by the action of chlorine gas on thin plates of lead, and may be ob- tained more easily by adding hydrochloric acid- or a solution of sea-salt to acetate or nitrate of oxide of lead dissolved in water. This chloride dissolves to a considerable extent in hot water, especially when acidulated with hydro- chloric acid, and separates on cooling in small acicular anhydrous crystals of a white colour. It fuses at a temperature below 'redness, and forms as it cools a semi-transparent mass, which has a density of 5.133. It bears a full red heat in close vessels without subliming; but in open vessels it smokes from spurious evaporation, loses some of its chlorine and absorbs oxygen, yielding an oxy-chloride of a yellow colour. Iodide of Lead is easily formed by mixing a solution of hydriodic acid in excess with the nitrate of" protoxide of lead dissolved in water; and it is of a rich yellow colour. It is dissolved by boiling water, forming a colourless solution, and is deposited on cooling in yellow crystalline scales of a bril- liant lustre. Bromide of Lead.—It falls as a white crystalline powder, of sparing solu- bility in water, when a soluble salt of lead is mixed with bromide of potas- sium in solution. Exposed to heat it fuses into a red liquid which becomes yellow when cold. Fluoride of Lead is formed by mixing hydrofluoric acid with acetate of protoxide of lead, and falls as an uncrystdline white powder of very sparing solubility. It is soluble in nitric and hydrochloric acids, but is decomposed when the solution is evaporated. Sulphurets of Lead.—It is probable that lead unites with sulphur in several different proportions ; but the Only one of these compounds well known to chemists is the native sulphuret, galena, which occurs in cubic crystals, or in forms allied to the cube. It may be formed artificially by fusing lead with sulphur, or by the action of hydrosulphuric acid on a salt of lead. Phosphuret of Lead has been little examined. It may be formed by heating phosphate of oxide of lead with charcoal, by mixing a solution of phosphorus in alcohol or ether with the solution of a salt of lead, or by the action of phosphuretted hydrogen on a similar solution. Carburet of Lead may be obtained by reducing oxide of lead in a state of fine division and intimate admixture with charcoal. It is also generated when salts of lead, which contain a vegetable acid, are decomposed by heat in close vessels. (Berzelius.) Cyanuret of Lead falls as a heavy white powder when cyanuret of potas- sium is mixed with a solution of nitrate of" oxide of lead, or hydrocyanic acid with the acetate. It is soluble in nitric acid, and to a considerable extent in hot water, yielding colourless solutions. Heated to gentle red- ness, it gives out nitrogen gas, and pyrophoric carburet of lead remains. MERCURY. CLASS II. ORDER III. METALS, THE OXIDES OF WHICH ARE REDUCED TO THE METALLIC STATE BY A RED HEAT. ^ SECTION XXIV. MERCURY OR QUICKSILVER. This metal was well known to the ancients. The principal mines from which it is obtained are those of Idria in Carniola, and Almaden in Spain, where it is found both in the native state and combined with sulphur as cin- nabar, the latter being the most abundant. From this ore the metal is ex- tracted by heating it with lime or iron filings, by which means the mercury is volatilized and the sulphur retained. As prepared on a large scale it is usually mixed in small quantity with other metals, from which it may be purified by cautious distillation. Mercury is distinguished from all other metals by being fluid at common temperatures. It has a tin-white colour and strong metallic lustre. It be- comes solid at a temperature which is 39 or 40 degrees below zero; and in congeding, it evinces a strong tendency to crystallize in octohedrons. It contracts greatly at the moment of congelation ; for while its density at 47° is 13.568, that of frozen mercury is 15.612. When solid it is malleable, and may be cut with a knife. At 662° or near that degree, it enters into ebulli- tion, and condenses again on cool surfaces into metallic globules. Mercury, if quite pure, is not tarnished in the cold by exposure to air and moisture; but if it contain other metals, the amalgam of those metals oxi- dizes readily, and collects a film upon its surface. Mercury is said to be oxidized by long agitation in a bottle half full of air, and the oxide so formed was calld by Boerhaave ethiops perse ; but is very probable that the oxida- tion of mercury observed under these circumstances was solely owing to the presence of other metals. When mercury is exposed to air or oxygen gas, while in the form of vapour, it slowly absorbs oxygen, and is converted into peroxide of mercury. The only acids that act on mercury are the sulphuric and nitric acids. The former has no action whatever in the cold; but on the application of heat, the mercury is oxidized at the expense of the acid, pure sulphurous acid gas is disengaged, and a sulphate of mercury is generated. Nitric acid acts energetically upon mercury both with and without the aid of heat, oxi- dizing and dissolving it with evolution of binoxide of nitrogen. From some late analyses of the peroxide and chlorides of mercury, I have inferred that its equivalent is 202. (Phil. Trans. 1833, part ii.) The composition of its compounds described in this section is as follows :— Mercury. Equiv. Formulae. Protoxide 202 1 eq.+Oxygen 8 1 eq.=210 Hg + O or Hg. Peroxide 202 1 eq.+ do 16 2 eq.=218 Hg + 20orHg. Protochloride 202 1 eq+Chlorine 35.42 1 eq.= 237.42 Hg + Cl or HgCl. Bichloride 202 1 eq.+do 70.84 2 eq.=272.84 Hg + 2C1 or HgCR Protiodide 202 1 eq.+Iodine 126.3 1 eq,= 328.3 Hg + I or Hgl. Sesquiodide 404 2 eq.-f do. 378.9 3 eq.= 782.9 2Hg + 3I or Hg2l3. Biniodide 202 1 eq.+ do. 252.6 2 eq.=454.6 Hg+2IorHgl2. Protobromide 202 1 eq.+Bromine 78.4 1 eq.=280.4 Hg + BrorHgBr. 396 MERCURY. Mercury. Equiv. Formulae. Bibromide 202 1 eq.+Bromine 156.8 2 eq.= 358.8 Hg + 2Br or HgBr3. phuret111' ) 202 1 eq.+Sulphur 16.1 1 eq.=218.1 Hg + S or HgS. Bisulphuret 202 1 eq.+ do. 32.2 2 eq.= 234.2 Hg + 2S or HgS2. Bicyanuret 202 1 eq.+Cyano. 52.78 2 eq.=228.39 Hg + 2Cy or HgCy. Protoxide.—This oxide, which is a black powder, insoluble in water, is best prepared by the process recommended by Donovan. (An. of Phil, xiv.) This consists in mixing calomel briskly in a mortar with pure potassa in excess, so as to effect its decomposition as rapidly as possible: the protoxide is then washed with cold water, and dried spontaneously in a dark place. These precautions are rendered necessary by the tendency of the protoxide to resolve itself into the peroxide and metallic mercury, a change which is easily effected by heat, by the direct solar rays, and even by daylight. It is on this account very difficult to procure protoxide of mercury in a state of absolute purity. This oxide is precipitated from its salts, of which the nitrate is the most interesting, as the black protoxide by pure alkalies ; as a white carbonate, which soon becomes dark from the loss of carbonic acid, by alkdine car- bonates ; as calomel by hydrochloric acid or any soluble chloride; and as the black protosulphuret by hydrosulphuric acid. Of these tests, the action of hydrochloric acid is the most characteristic. The oxide is reduced to the metallic state by copper, phosphorous acid, or protochloride of tin. Peroxide.—This oxide may be formed either by the combined agency of heat and air, as already mentioned, or by dissolving mercury in nitric acid, and exposing the nitrate so formed to a temperature just sufficient for expelling the whole of the nitric acid. It is commonly known by the name of red precipitate. The peroxide prepared from the nitrate almost always contains a trace of nitric acid, which may be detected by heating it in a clean glass tube by means of a spirit-lamp: a yellow ring, formed of subnitrate of oxide of mercury, collects within the tube just above the part which is heated. (Dr. Clarke.) Peroxide of mercury, thus prepared, is commonly in the form of shining crystalline scales of a nearly black colour while hot, but red when cold : when very finely levigated, the peroxide has an orange colour. It is soluble to a small extent in water, forming a solution which has an acrid metdlic taste, and communicates a green colour to the blue infusion of violets. When heated to redness, it is converted into metallic mercury and oxygen. Long exposure to light has a similar effect. (Guibourt) Some of the neutral salts of this oxide, such as the nitrate and sulphate, are converted by water, especially at a boiling temperature, into insoluble yellow subsalts, leaving a strongly acid solution, in which a little of the original salt is dissolved. The oxide is separated from all acids as a red, or when hydratic as a yellow precipitate, by the pure and carbonated fixed alkalies. Ammonia and its carbonate cause a white precipitate, which is a double salt, consisting of one equivalent of the acid, one equivalent of the peroxide, and one equivalent of ammonia. The oxide is readily reduced to the metallic state by metallic copper. Hydrosulphuric acid, phosphorous acid, and protochloride of tin, reduce the peroxide into the protoxide; and when added in larger quantity, the first throws down a black sulphuret, and the two latter metallic mercury. The action of hydrosulphuric acid on a solution of corrosive sublimate is, however, peculiar; for at first it occasions a white precipitate which, according to Rose, is a compound of two equiva- lents of bisulphuret to one of bichloride of mercury. This gas acts on bibromide and biniodide of mercury in a similar manner. (An. de Ch. et de Ph. xl. 46.) Protochloride.—Protochloride of mercury, or calomel, is always generated when chlorine comes in contact with mercury at common temperatures; and also by the contact of metallic mercury and the bichloride. It may be made by precipitation, by mixing nitrate of protoxide of mercury in solution MERCURY. 397 witli hydrochloric acid or any soluble chloride. It is more commonly prepared by sublimation. This is conveniently done by mixing 272.84 parts or one equivalent of the bichloride with 202 parts or one equivalent of mercury, until the metallic globules entirely disappear, and then subliming. When first prepared it is dways mixed with some corrosive sublimate, and, there- fore, should be reduced to powder and well washed, before being employed for chemicd or medical purposes. Protochloride of mercury is a rare mineral production, called horn quick- silver, which occurs crystallized in quadrangular prisms, terminated by pyramids. When obtained by sublimation it is in semi-transparent crystal- line cakes; but as formed by precipitation, it is a white powder. Its density is 7.2. At a heat short of redness, but higher than the subliming point of the bichloride, it rises in vapour without previous fusion; but during the sublimation a portion is always resolved into mercury and the bichloride. It is yellow while warm, but recovers its whiteness on cooling. It is distinguished from the bichloride by not being poisonous, by having no taste, and by being exceedingly insoluble in water. Acids have little effect upon it; but pure alkalies decompose it, separating the black protoxide of mercury. When calomel is boiled in a solution of hydrochlorate of am- monia, it is converted into corrosive sublimate and metallic mercury. Chlo- ride of sodium has a similar effect, though in a less degree. Bichloride.—When mercury is heated in chlorine gas, it takes fire, and burns with a pale red flame, forming the well-known medicinal preparation and virulent poison corrosive sublimate, or bichloride of mercury. It is pre- pared for medical purposes by subliming a mixture of bisulphate of the peroxide of mercury with chloride of sodium or sea-salt. The exact quanti- ties required for mutual decomposition are 298.2 parts or one equivalent of the bisulphate, to 117.44 parts or two equivdents of the chloride. Thus, Bisulphate of Mercury 1 eq. Sulphuric acid 80.2 or 2 eq. 2S. Chloride of Sodium 2 eq. Chlorine . 70.84 or 2 eq. 2C1. Peroxide of mer. 218 or 1 eq. Hg. j Sodium . . 46.6 or 2 eq. 2Na. 298.2 Hg + 2S.| 117.44 2(Na + Cl). And by mutual interchange of elements they produce Bichloride of Mercury 1 eq. Mercury . 202 or 1 eq. Hg. Chlorine . 70.84 or 2 eq. 2C1. 272.84 Hg + 2C1. Sulphate of Soda 2 eq. Soda . . . 62.6 or 2 eq. . 2Na. Sulphuric ac. 80.2 or 2 eq. . 2S. 142.8 2(Na + S). The products have exactly the same weight (272.84 + 142.8=415.64) as the compounds (298.2 + 117.44=415.64) from which they were prepared. Bichloride of mercury, when obtained by sublimation, is a semi-trans- parent colourless substance, of a crystalline texture. It has an acrid, burning taste, and leaves a nauseous metallic flavour on the tongue. Its specific gravity is 5.2. When exposed to a heat short of incandescence, it is fused, enters into ebullition from the rapid formation of vapour, and is deposited without further change on cool surfaces as a white crystalline sublimate. It requires twenty times its weight of cold, and only twice its weight of boiling water for solution, and is deposited from the latter, as it cools, in the form of prismatic crystals. Strong alcohol and ether dissolve it in the 34 398 MERCURY. same proportion as boiling water; and it is soluble in half its weight of con- centrated hydrochloric acid at the temperature of 70°. With the chlorides of potassium and sodium, hydrochlorate of ammonia, and several other bases, it enters into combination, forming double salts, which are more soluble than the chloride itself. When its solution in water is agitated with ether, the latter abstracts the bichloride, and rises with it to the surface of the former, thus affording strong evidence of the bichloride having existed as such in the water. Its aqueous solution is gradually decomposed by light, calomel being deposited. The presence of mercury in a fluid, supposed to contain corrosive subli- mate, may be detected by concentrating and digesting it with an excess of pure potassa. Oxide of mercury, which subsides, is then sublimed in a small glass tube by means of a spirit-lamp, and obtained in the form of metallic globules. But in cases of poisoning, when the bichloride is mixed with organic substances, Dr. Christison recommends that the liquid, without previous filtration, be agitated with a fourth of its volume of ether, which separates the poison from the aqueous part, and rises to the surface. The ethereal solution is then evaporated on a watch-glass, the residue dissolved in water, and the mercury precipitated in the metallic state by protochloride of tin at a boiling temperature- If, as is probable, most of the poison is already converted into calomel, and thereby rendered insoluble, as many vegetable fibres should be picked out as possible, and the whole at once digested with protochloride of tin. The organic substances are then dis- solved in a hot solution of caustic potassa, and the insoluble parts washed and sublimed to separate the mercury. (Christison on Poisons.) A very elegant method of detecting the presence of mercury is to place a drop of the suspected liquid on polished gold, and to touch the moistened surface with a piece of iron wire or the point of a penknife, when the part touched instantly becomes white, owing to the formation of an amalgam of gold. This process was originally suggested by Mr. Sylvester, and has since been simplified by Dr. Paris. (Medical Jurisprudence, by Paris and Fonblanque.) Many animal and vegetable solutions convert bichloride of mercury into calomel, a portion of hydrochloric acid being set free at the same time. Some substances effect this change slowly; while others, and especially al- bumen, produce it in an instant. Thus, when a solution of corrosive subli- mate is mixed with albumen, a white flocculent precipitate subsides, which Orfila has shown to be a compound of calomel and albumen, and which he has proved experimentally to be inert. (Toxicologie, vol. i.) Consequently, a solution of the white of eggs is an antidote to poisoning by corrosive sublimate. The muscular and membranous parts, even of a living animal, produce a similar effect; and the causticity of corrosive sublimate seems owing to the destruction of the animal fibre by which the decomposition of the bichloride is accompanied, and which constitutes an essential part of the chemical change. Protiodide of Mercury.—This compound is obtained by mixing nitrate of protoxide of mercury in solution with iodide of potassium. It is a green powder, insoluble in water, and disposed to resolve itself under the influence of heat or solar light into mercury and the biniodide. However, when the heat is quickly supplied, it is fused and sublimed without material change. Sesquiodide.—This compound falls as a yellow powder when iodide of potassium is added in solution to the mixed nitrates of the protoxide and peroxide of mercury, the latter being in excess. The precipitate is digested with a solution of sea-salt which takes up any biniodide which may have fallen. Biniodide.—This compound is formed by mixing nitrate of the peroxide or bichloride of mercury with iodide of potassium in solution, and falls as a rich red-coloured powder of a tint which vies in beauty with that of vermilion, though, unfortunately, the colour is less permanent. Though insoluble in water, it dissolves freely in an excess of either of its precipitants. If taken MERCURY. 399 up in a hot solution of nitrate of peroxide of mercury, the biniodide crystal- lizes out on cooling in scales of a beautiful red tint. The same crystals separate from a solution in iodide of potassium; but if the liquid be con- centrated, a double iodide of mercury and potassium subsides. The biniodide, when exposed to a moderate heat, gradually becomes yel- low ; and the particles, though previously in powder, acquire a crystalline appearance. At about 400° it forms a yellow liquid which slowly sublimes in small transparent scales, or in large rhombic tables, when a considerable quantity is sublimed. The crystals retain their yellow colour at 60° if kept very tranquil; but if the temperature be below a certain point, or they are rubbed or touched, they quickly become red. This phenomenon is entirely due to a change in molecular arrangement: the different colours so often witnessed in the same substances at different temperatures, as in peroxide of mercury and the protoxides of lead and zinc, appear to be phenomena of the same nature. Protobromide of Mercury.—It is precipitated as a white insoluble powder by mixing nitrate of protoxide of mercury with bromide of potassium. The bibromide is a white crystallizable compound, soluble in water and dcohol, fusible and volatile, and in many respects analogous to the bichlo- ride. It is formed by acting on peroxide of mercury with hydrobromic acid, or digesting the preceding compound with bromine. Sulphurets of Mercury.—The protosulphuret may be prepared by trans- mitting a current of hydrosulphuric acid gas through a dilute solution of nitrate of protoxide of mercury, or through water in which calomel is sus- pended. It is a black-coloured substance, which is oxidized by digestion in strong nitric acid. When exposed to heat it is resolved into the bisulphuret and metallic mercury. The bisulphuret is formed by fusing sulphur with about six times its weight of mercury, and subliming in close vessels. When procured by this process it has a red colour, and is known by the name of factitious cinnabar. Its tint is greatly improved by being reduced to powder, in which state it forms the beautiful pigment vermilion. It may be obtained in the moist way by pouring a solution of corrosive sublimde into an excess of hydrosulphate of ammonia. A black precipitate subsides, which acquires the usual red colour of cinnabar when sublimed. The black precipitate formed by the action of hydrosulphuric acid on bicyanuret of mercury, is likewise a bisul- phuret. Cinnabar, as already mentioned, occurs native. When equal parts of sulphur and mercury are triturated together until metallic globules cease to be visible, the dark coloured mass called ethiops mineral results, which Mr. Brande has proved to be a mixture of sulphur and bisulphuret of mercury. (Journal of Science, vol. xviii. p. 294.) Cinnabar is not attacked by alkalies, or any simple acid; but it is dis- solved by the nitro-hydrochloric, with formation of sulphuric acid and per- oxide of mercury. Bicyanuret of Mercury.—This compound is best prepared by boiling, in any convenient quantity of water, finely levigated Prussian blue, quite pure and well dried on a sand-bath, with an equal weight of peroxide of mercury in powder, until the blue colour of the pigment entirely disappears. A colourless solution is formed, which, when filtered and concentrated by eva- poration, yields crystals of bicyanuret of mercury in the form of quadran- gular prisms. The essential part of this process consists in the peroxide of mercury and the Prussian blue exchanging elements, whereby bicyanuret of mercury and peroxide of iron result; but the entire change is very complex and not well understood. Much cyanogen remains behind in the insoluble part; for on digesting it in hydrochloric acid, so as to remove the peroxide of iron, a considerable quantity of Prussian blue is left. This process is so very uneconomical, that I suspect it would be better, as Winckler proposes, to prepare hydrocyanic acid from ferrocyanuret of potassium and sulphurio acid (page 269), and to agitate with peroxide of mercury until the odour of hydrocyanic acid ceases. 400 SILVER. Bicyanuret of mercury is colourless and inodorous, has a disagreeable metallic taste, and is highly poisonous. It dissolves very freely in hot water, and crystallizes readily as it cools. By alcohol it is very sparingly dis- solved. When heated it is resolved into mercury and cyanogen. (Page 265.) SECTION XXV. SILVER. This metal was known to the ancients. It frequently occurs native in silver mines, both massive and in octohedral or cubic crystals. It is also found in combination with gold, tellurium, antimony, copper, arsenic, and sulphur. In the state of sulphuret it so frequently accompanies galena, that the lead of commerce is rarely quite free from traces of silver. Silver is extracted from its ores by two processes which are essentially distinct; one of them being contrived toseparate.it from lead, the other, the process by amalgamation, being especially adapted to those ores which are free from lead. The principle of its separation from lead is founded on the different oxidability of lead and silver, and on the ready fusibility of litharge. The lead obtained from those kinds of galena which are rich in sulphuret of silver is kept at a red heat in a flat furnace, with a draught of air constantly playing on its surface: the lead is thus rapidly oxidized ; and as the oxide, at the moment of its formation, is fused, and runs off through an aperture in the side of the furnace, the production of litharge goes on uninterruptedly until all the lead is removed. The button of silver is again fused in a smaller furnace, resting on a porous earthen dish, made with lixiviated wood-ashes, called a test, the porosity of which is so great, that it absorbs any remaining portions of litharge which may be formed on the silver. The ores commonly employed in the process of amalgamation, which has been long used at Freyberg in Saxony, and is extensively practised in the silver and gold mines of South America, are native silver and its sulphuret. At Freyberg the ore in fine powder is mixed with sea-salt, and carefully roasted in a reverberatory furnace. The production of sulphuric acid leads to the formation of sulphate of soda, while the chlorine of the sea-salt com- bines with silver. The roasted mass is ground to a fine powder, and, to- gether with mercury, water, and fragments of iron, is put into barrels, which are made to revolve by machinery. In this operation, intended to insure perfect contact between the materials, chloride of silver is decomposed by the iron, the silver unites with the mercury, and the chloride of iron is dissolved by the water. The mercury is then squeezed through leathern bags, the pores of which permit the pure mercury to pass, but retain the amalgam of silver. The combined mercury is then distilled off in close vessels, and the metals obtained in a separate state. Goldsmith's silver commonly contains copper and traces of gold, the latter appearing in dark flocks when the metal is dissolved in nitric acid. It may be obtained pure for chemical uses by placing a clean piece of copper in a solution of nitrate of oxide of silver, washing the precipitate with pure water, and then digesting it in ammonia, in"order to remove any adhering copper. A better process is to decompose chloride of silver by means of carbonate of potassa. For this purpose precipitate a solution of nitrate of oxide of silver with chloride of sodium, wash the precipitate with water, and dry it. Then put twice its weight of carbonate of potassa into a clean Hessian or black-lead crucible, heat it to redness, and throw the chloride by successive portions into the fused alkali. Effervescence takes place from the evolution of carbonic acid and oxygen gases, chloride of potassium is SILVER. generated, and metallic silver subsides to the bottom. The pure metal may be granulated by pouring it while fused from a height of seven or eight feet into a vessel of water. Silver has the clearest white colour of all the metals, and is susceptible of receiving a lustre surpassed only by polished steel. In malleability and ductility it is inferior only to gold, and its tenacity is considerable. It is very soft when pure, so that it may be cut with a knife. Its density after being hammered is 10.51. At a full red heat, corresponding to 1873° F., according to Mr. Daniell, it enters into fusion. Pure silver does not rust by exposure to air and moisture. When fused in open vessels it absorbs oxygen in considerable quantity, amounting some- times to 22 times its volume ; but it parts with the whole of it in the act of becoming solid. This fact, first noticed by M. Lucas, has been studied by Gay-Lussac, who attributes to it the peculiarly beautiful aspect of granulated silver : he observed the absorption and subsequent evolution of oxygen to be most abundant in the purest silver, and is entirely prevented by a very smdl per-centage of copper. If silver is heated to redness, without fusing, in contact with glass or porcelain, it readily absorbs oxygen, and the oxide fuses with the earthy matters, forming a yellow enamel. When silver in the form of leaves or fine wire is intensely heated by means of electricity, galva- nism, or the ox-hydrogen blowpipe, it burns with vivid scintillations of a greenish-white colour. The only pure acids that act on silver are the sulphuric and vnitric acids, by both of which it is oxidized, forming with the first a sulphate, and with the second a nitrate of oxide of silver. It is not attacked by sulphuric acid unless by the aid of heat. Nitric acid is its proper solvent, and forms with its oxide a salt, which, after fusion, is known by the name of lunar caustic. From recent experiments on the composition of the chloride and nitrate of the oxide of silver, I have deduced 108 as the equivalent of silver, an estimate closely corresponding with the previous researches of Berzelius. (Phil. Trans. 1833, part ii.) The compounds of silver described in this section are thus constituted :— Silver. Equiv. Formulae. Oxide 108 1 eq. + Oxygen 8 1 eq.= 116 Ag + O or Ag. Chloride 108 1 eq. + Chlorine 35.42 1 eq.= 143.42 Ag + Cl or AgCI. Iodide 108 1 eq. +Iodine 126.3 1 eq.=234.3 Ag + IorAgl. Sulphuret 108 1 eq. +Sulphur 16.1 1 eq.= 124.1 Ag + SorAgS. Cyanuret 108 1 eq. +Cyanogen 26.39 1 eq.= 134.39 Ag+CyorAgCy, Oxide of Silver.—This oxide is best procured by mixing a solution of pure baryta with nitrate of oxide of silver dissolved in water. It is of a brown colour, insoluble in water, and is completely reduced by a red heat. Silver is separated from its solution in nitric acid by pure alkalies and al. kaline earths as the brown oxide, which is redissolved by ammonia in excess; by alkaline carbonates as a white carbonate, which is soluble in an excess of carbonate of ammonia ; aa a dark brown sulphuret by hydrosul. phuric acid; and as a white curdy chloride of silver, which is turned violet by light and is very soluble in ammonia, by hydrochloric acid or any soluble chloride. By the last character, silver may be both distinguished and sepa- rated from other metallic bodies. Silver is precipitated in the metallic state by most other metals. When mercury is employed for this purpose, the silver assumes a beautiful arbores- cent appearance, called arbor Diana. A very good proportion for the expe- riment is twenty grains of lunar caustic to six drachms or an ounce ofwater. The silver thus deposited always contains mercury. When oxide of silver, recently precipitated by baryta or lime-water, and separated from adhering moisture by bibulous paper, is left in contact for ten or twelve hours with a strong solution of ammonia, the greater part of it is, 402 GOLD. dissolved; but a black powder remains which detonates violently from heat or percussion. This substance, which was discovered by Berthollet, (An. de Chimie, i.) appears to be a compound of ammonia and oxide of silver; for the products of its detonation are metallic silver, water, and nitrogen gas. It should be made in very small quantity at a time, and dried spontaneously in the air. On exposing a solution of oxide of silver in ammonia to the air, its surface becomes covered with a pellicle, which Faraday considers to be an oxide containing a smaller proportion of oxygen than that just described. This opinion he has made highly probable; but further experiments are requisite before the existence of this oxide can be regarded as certain. Chloride of Silver.—This compound, which sometimes occurs in silver mines, is always generated when silver is heated in chlorine gas, and may be prepared conveniently by mixing hydrochloric acid, or any soluble chlo- ride, with a solution of nitrate of oxide of silver. As formed by precipitation it is quite white; but by exposure to the direct solar rays it becomes violet, and almost black, in the course of a few minutes; and a similar effect is slowly produced by diffused day-light Hydrochloric acid is set free during this change, and, according to Berthollet, the dark colour is owing to separa- tion of oxide of silver. (Statique Chimique, vol. i. p. 195.) Chloride of silver, sometimes called horn silver, is insoluble in water, and is dissolved very sparingly by the strongest acids; but it is soluble in am- monia. Hyposulphurous acid likewise dissolves it. At a temperature of about 500° it fuses, and forms a semi-transparent horny mass on cooling, which has a density of 5.524. It bears any degree of heat, or even the com- bined action of pure charcoal and heat, without decomposition ; but hydrogen gas decomposes it readily with formation of hydrochloric acid. Iodide of Silver.—This compound is formed when iodide of potassium is mixed with a solution of nitrate of oxide of silver. It is of a greenish-yel- low colour, and is insoluble in water and ammonia. Cyanuret of Silver is formed by mixing hydrocyanic acid with nitrate of oxide of silver. It is a white curdy substance, similar in appearance to chloride of silver, insoluble in water, but soluble in a solution of ammonia and in hot nitric acid. It is decomposed by hydrochloric acid with forma- tion of hydrocyanic acid and chloride of silver. Sulphuret of Silver.—Silver has a strong affinity for sulphur. This metal tarnishes rapidly when exposed to an atmosphere containing hydrosulphuric acid gas, owing to the formation of a sulphuret. On transmitting a current of this gas through a solution of lunar caustic, a dark brown precipitate sub- sides, which is a sulphuret of silver. The silver glance of mineralogists is a similar compound, and the. same sulphuret may be prepared by heating thin plates of silver with alternate layers of sulphur. This sulphuret is remarkable for being soft and even malleable. Silver unites also by the aid of heat with phosphorus, forming a soft, brittle, crystalline compound. SECTION XXVI. GOLD. Gold appears to have been known to the earliest races of man, and to have been esteemed by them as much as by the moderns. It has hitherto been found only in the metallic state, either pure or in combination with other metals. It occurs massive, capillary, in grains, and crystallizes in octohedrons and cubes, or their allied forms. It is sometimes found in GOLD. 403 primary mountains; but more frequently in alluvial depositions, especially among sand in the beds of rivers, having been washed by water out of dis- integrated rocks in which it originally existed. There are few countries in which gold washings have not formerly existed; but the principd supply of gold is from South America, from the gold mines of Hungary, and from the Uralian mountains of Siberia, especially on the Asiatic side of the chain, where separate masses in sand have been found weighing 18 or 20 pounds. Rich deposites of gold appear also to exist in some of the southern provinces of North America. Gold is generally separated from accompanying impu- rities by the process of amalgamation, similar to that described in the last section; by which means it is freed from iron and all associated metals, excepting silver. In Hungary the gold is purified by cupellation. The silver, which in variable quantity is present in native gold, may be brought into view by dissolving the gold in nitro-hydrochloric acid. The best mode of separation consists in fusing the gold wilh so much silver that the former may constitute one-fourth of the mass: nitric acid will then dissolve all the silver, and leave the gold. The silver may also be removed by digestion in sulphuric acid. Gold is the only metal which has a yellow colour, a character by which it is distinguished from all other simple metallic bodies. It is capable of receiving a high lustre by polishing, but is inferior in brilliancy to steel, silver, and mercury. In ductility and malleability it exceeds all other metals; but it is surpassed by several in tenacity. Its density is 19.257; when pure it is exceedingly soft and flexible; and it fuses according to Mr. Daniell at 2016° F Gold may be exposed for ages to air and moisture without change, nor is it oxidized by being kept in a state of fusion in open vessels. When in- tensely ignited by means of electricity or the oxy-hydrogen blowpipe, it burns with a greenish-blue flame, and is dissipated in the form of a purplo powder, which is supposed to be an oxide. Gold is not oxidized or dissolved by any of the pure acids; for it may be boiled even in nitric acid without undergoing any change. Its best solvents are chlorine and nitro-hydrochloric acid; and it appears from the observa- tions of Davy that chlorine is the agent in both cases, since nitro-hydro- chloric acid does not dissolve gold, except when it gives rise to the formation of chlorine. (Page 215.) It is to be inferred, therefore, that the chlorine unites directly with the gold. The most convenient method of dissolving it, is to digest fragments of the metal in a mixture composed of two measures of hydrochloric and one of nitric acid, until the acid is saturated. The excess of acid is then expelled by evaporating the orange-coloured solution until a ruby-red liquid remains, which is the neutral terchloride of gold. On adding water, the chloride is dissolved, forming a solution of a gold-yellow colour. The equivalent of gold, estimated from the analysis of the terchloride by Berzelius, is 199 2. The composition of its compounds described in this sec- tion is as follows :— 1 eq. Gold. Eqdv. Formulae. Protoxide 199.2+ Oxygen 8 1 eq.= 207.2 Au + O or Au. Binoxide 199.2 + do. 16 2 eq.=215.2 Au + 20orAu. Peroxide 199.2 + do. 24 3 eq.=223.2 Au + 30 or Au. Protochloride 199.2-j-Chlorine 35.42 1 eq.=234.62 Au+Clor AuCl. Terchloride 199.2 + do. 106.26 3 eq. =305.46 Au + 3C1 or AuCl3. Tersulphuret 199.2 + Sulphur 48.3 3 eq. =247.5 Au+3SorAuS3. Protoxide of Gold.—It is obtained by the action of a cold solution of potassa on the protochloride of gold, and is separated as a green precipitate, 404 GOLD. which is partially soluble in the alkaline solution. It spontaneously changes soon after its preparation into metallic gold and the peroxide. The binoxide is supposed to be the purple oxide which is formed by the combustion of gold: but its composition has not been demonstrated by ana- lysis. Peroxide.—This, the only well-known oxide of gold, is prepared by the ac tion of alkalies on the terchloride, but is obtained quite pure with difficulty. Pelletier recommends that it should be formed by digesting a solution of the terchloride with pure magnesia, washing the precipitate with water, and re- moving the excess of magnesia by dilute nitric acid. It is apt, however, to retain magnesia, and I am informed by Dr. Wagner, of Pesth in Hungary, that the most certain mode of procuring the peroxide is the following. Dis- solve 1 part of gold in the usual way, render it quite neutral by evaporation, and redissolve in 12 parts of water: to the solution add 1 part of carbonate of potassa dissolved in twice its weight of water, and digest at about 170°. Carbonic acid gradually escapes, and the hydrated peroxide of a brownish- red colour subsides. After being well washed it is dissolved in colourless nitric acid of specific gravity 1.4, and the solution decomposed by admixture with water. The hydrated peroxide is thus obtained quite pure, and is ren- dered anhydrous by a temperature of 212° F. Peroxide of gold is yellow in the state of hydrate, and nearly black when anhydrous, is insoluble in water, and completely decomposed by solar light or a red heat. Hydrochloric acid dissolves it readily, yielding the common solutions of gold; but it forms no definite compound with any acid which contains oxygen. It may indeed be dissolved by nitric and sulphuric acids; but the affinity is so slight that the oxide is precipitated by the addition of water. It combines, on the contrary, with alkaline bases, such as potassa and baryta, apparently forming regular salts, in which it acts the part of a weak acid. This property, which constitutes the difficulty of procuring per- oxide of gold quite pure, induced Pelletier to deny that the peroxide of gold is a salifiable base, and to propose for it the name of auric acid, its com- pounds with alkalies being called aurates. (An. de Ch. et de Ph. xv.) When recently precipitated peroxide of gold is kept in strong ammonia for about a day, a detonating compound of a deep olive colour is generated, analogous to the fulminating silver described in the last section. According to the analysis of Dumas, its elements are in the ratio of one equivalent of gold, two of nitrogen, six of hydrogen, and three of oxygen, as expressed by the symbols Au + 2N + 6H+30. With regard to the mode in which these elements are arranged, different opinions may be formed. Dumas thinks the real combination is indicated by the formula (Au+\)+(3H+N)+3H, being a hydrated nituret of gold united with ammonia; but it appears more simple to consider it as a diaurate of ammonia, expressed by the formula 2(3H+N)+Au. Its detonation should give rise to metallic gold, water, nitrogen, and ammonia. A similar compound is obtained, and this is the ordinary mode of procuring fulminating gold, by digesting terchloride of gold with an excess of ammonia: a yellow precipitate subsides, the fulminat- ing ingredient of which appears identical with that above described; but a subchloride of gold and ammonia falls at the same time, and adheres sq obstinately that it cannot be wholly removed by boiling water. Fulminat- ing gold may be dried at 212"; but friction, or a heat suddenly raised to about 290° or upwards, produces a violent detonation. It is best to make it in small quantities at a time, and to dry it in the open air. (An. de Ch. et dePh. xliv. 167.) Chlorides of Gold. On concentrating the solution of gold to a sufficient extent by evaporation, the terchloride may be obtained in ruby-red prismatic crystals, which arc very fusible. It deliquesces on exposure to the air, and is dissolved readily by water without residue. It is also soluble in alcohol and ether; and the latter withdraws it from the aqueous solution. It begins GOLD. to lose chlorine at a temperature of about 400°, being changed into a brown dry mass, which is a mixture of the protochloride and terchloride, soluble in water. At about 600° the terchloride is completely resolved into the yellow insoluble protochloride, which by boiling in water is changed into metallic gold and the soluble terchloride. At a red heat the protochloride loses its chlorine altogether, and metallic gold remains. The terchloride of gold, is the usual and most convenient form of obtain- ing a solution of gold, and examining its properties in that state. On adding to the solution sulphate of protoxide of iron, a brown precipitate ensues, which is gold in very fine division, and the solution contains sesquisulpiiate of peroxide, and perchloride of iron. The action is such that 6 eq. sulphate of protoxide of iron 6(Fc+S) and 1 eq. terchloride of gold Au+3C1 yield 2 eq. tersulphate of peroxide of iron 2(Fe+3S) 1 eq. perchloride of iron 2Fe+3Cl. and 1 eq. gold Au. The precipitate, when duly washed with dilute hydrochloric acid in order to separate adhering iron, is gold in a state of prefect purity. A similar reduction is effected by most of the metals, and by sulphurous and phospho- rous acids, and by oxalic add with escape of carbonic acid gas. When a piece of charcoal is immersed in a solution of gold, and exposed to the direct solar ray?:, its surface acquires a coating of metallic gold, and ribands may be gilded by moistening them with a dilute solution of gold, and ex- posing them to a current of hydrogen or phosphuretted hydrogen gas. When a strong aqueous solution of gold is shaken in a phial with an equal volume of pure ether, two fluids result, the lighter of which is an ethereal solution of gold. From this liquid flakes of metal are deposited on standing, especially by exposure to light, and substances moistened with it receive a coating of metallic gold* The reduction in most of these instances is owing to the chlorine quitting the gold in obedience to some stronger attraction: metals deprive it directly of its chlorine; and deoxidizing agents do so indirectly by combining with the oxygen of water, while the hydro- gen acts on the chlorine. When protochloride of tin is added to a dilute aqueous solution of gold, a purple-coloured precipitate, called the purple of Cassius, is thrown down; and the same substance may be prepared by fusing together 150 parts of silver, 20 of gold, and 35.1 of tin, and acting on the alloy with nitric acid, which dissolves out the silver and leaves a purple residue, containing the tin and gold which were employed. To prevent the oxidation of the tin during fusion, the three metals should be projected into a red-hot black-lead crucible, which contains a little melted borax. When the powder of Cassius is fused with vitreous substances, such as flint-glass, or a mixture of sand and borax, it forms with them a purple enamel, which is employed in giving pink colours to porcelain. The essential cause of the colour is probably a compound of the purple or supposed binoxide of gold with earthy matters, similar to the enamel formed by glass and oxide of silver: the oxide of tin is not essential, since finely divided metallic gold alone will give the same tint of purple. The chemical nature of the purple of Cassius is very obscure. From its formation by protochloride of tin, it is inferred to contain peroxide of tin, and gold either in the metallic state or oxidized to a degree inferior to the peroxide. According to Berzelius, its sole loss when heated to redness is * With respect to the revival of gold from its solutions, the reader may consult an Essay on Combustion by Mrs. Fulhame, and a paper by Count Rumford in the Philosophical Transactions for 1798. 406 PLATINUM. 7.65 per cent, of water, and the residue has a brick-red colour arising front a mechanical mixture of metallic gold and peroxide of tin, a statement which is confirmed by Gay-Lussac. (An. de Ch. et de Ph. xlix. 396.) The proportion of these products corresponds to six equivalents of peroxide of tin, one of gold, and six of water. Nevertheless, the purple of Cassius, as is indicated both by its colour and its solubility in ammonia, is not a mechanical mixture of these ingredients; nor can it well be regarded as a chemical compound of gold and peroxide of tin, since no definite compound of the kind is known to chemists. The more probable supposition is, that it is a hydrated double salt, composed of peroxide of tin as the acid, united with protoxide of tin and binoxide of gold as bases, in such proportion that the oxygen of the gold exactly suffices to convert the protoxide into peroxide of tin. A compound of this nature is expressed by the formula 2(Sn + Sn) + (Au + Sn) + 6H. Tersulphuret of Gold.—On transmitting a current of hydrosulphuric acid gas through a solution of gold, a black precipitate is formed, which is the tersulphuret. It is resolved by a red heat into gold and sulphur. The compounds of gold with the other non-metallic bodies have been little examined. SECTION XXVII. PLATINUM. This valuable metal occurs only in the metallic state, associated or com- bined with various other metals, such as copper, iron, lead, titanium, chro- mium, gold, silver, palladium, rhodium, osmium, and iridium. It has hitherto been found chiefly in Brazil, Peru, and other parts of South America, in the form of rounded or flattened grains of a metallic lustre and white colour, mixed with sand and other alluvial depositions. The particles rarely occur so large as a pea; but they are sometimes larger, and a sped- men brought from South America by Humboldt was rather larger than a pigeon's egg, and weighed 1088.6 grains. In the year 1826, however, Boussingault discovered it in a syenitic rock in the province of Antioquia in South America, where it occurs in veins associated with gold. Rich mines of gold and platinum have also been discovered in the Uralian Mountdns. (Edinburgh Journal of Science, v. 323.) Pure platinum has a white colour very much like silver, but of inferior lustre. It is the heaviest of known metals, its density after forging being about 21.25, and 21.5 in the state of wire. Its malleability is considerable, though far less than that of gold and silver. It may be drawn into wires, the diameter of which docs not exceed the 2000th part of an inch. It is a soft metal, and like iron admits of being welded at a high temperature. Dr. Wollaston* observed that it is a less perfect conductor of heat than several other metals. Platinum undergoes no change from the combined agency of air and moisture; and it may be exposed to the strongest heat of a smith's forge * The reader will find, in the Philosophical Transactions"for 1829, some important directions by Dr. Wollaston, both as to the mode of extracting platinum from its ores, and of communicating to the pure metal its highest degree of malleability. The essay receives additional interest, from being one of those which were composed during the last illness of this truly illus- trious philosopher. 'LATINUM. 407 without suffering either oxidation or fusion. On heating a small wire of it by means of galvanism or the oxy-hydrogen blowpipe, it is fused, and after- wards burns with the emission of sparks. Smithson Tennant showed that it is oxidized when ignited with nitre (Philos, Trans. 1797); and a similar effect is occasioned by pure potassa and lithia. It is not attacked by any of the pure acids. Its solvents are chlorine, or solutions, such as nitro-hy- drochloric acid, which supply chlorine; and it is dissolved with greater difficulty than gold. The remarkable property observed by Dobereiner in spongy platinum of causing the union of oxygen and hydrogen gases, was mentioned at page 159, a property which Dulong and Thenard showed to be also possessed, though in a lower degree, by platinum in its compact form of wire or foil, and by several other metals. (An. de Ch. et de Ph. xxiii. and xxiv.) Faraday (Phil. Trans. 1834, part i.) has lately discussed, with his wonted ability and success, both the conditions required for the effective action of platinum, and the cause of the phenomenon. The sole conditions are purity of the gases, and perfect cleanliness of the platinum. By cleanliness is meant perfect absence of foreign matter, pure water excepted, and this condition is easily secured by fusing pure potassa on its surface, washing off the alkdi by pure water, then dipping the platinum in hot oil of vitriol, and again washing-with water. In this state platinum foil acts so rapidly at common temperatures on oxygen and hydrogen gases mixed in the ratio of 1 to 2, that it often becomes red-hot and kindles the mixture. Handling the pla- tinum, wiping it with a towel, or exposing it to the atmosphere for a few days, suffices to soil the surface of the metal, and thereby diminish or pre- vent its action. These phenomena are supposed to result from the concur- ring influence of two forces, the self-repulsive energy of similar gaseous particles, and the adhesive attraction exerted between them and the platinum. Each gas, repulsive to itself and not repelled by the platinum, comes into the most intimate contact with that metal, and both gases are so condensed upon its surface, that they are brought within the sphere of their mutual attraction and combine. Faraday has given several instances, similar to those which I had occasion to describe some years ago (Jameson's Journal, xi. 99 and 311), where the action of platinum is retarded or altogether pre- vented by small quantities of certain gases, such as hydrosulphuric acid, carbonic oxide, and olefiant gases. One would be tempted to suppose that these gases act by soiling the metallic surface, though in some respects this explanation is not satisfactory. The equivalent of platinum, deduced by Berzelius from the analysis of the bichloride, is 98.8. The composition of its compounds described in this section is as follows:— Platinum. Equiv. Formulae. Protoxide 98.8 1 eq.+Oxygen 8 1 eq.= 106.8 Pl+O or PI. Binoxide 98.8 1 eq.+do. 16 2 eq.= 114.8 Pl+20 or PI. Sesquioxide ? 197.6 2 eq.+do. 24 3 e4._921.6 2P1+30 or PI. Protochloride 98.8 1 eq.-f Chlorine 35.42 1 eq. =134.22 Pi +C1 or P1CI. Bichloride 98-8 1 eq.+do. 70-84 2 eq.=169.64 P1+2C1 or PICR Protiodide 98.8 1 eq.+Iodine 126.3 1 eq. =225.1 Pl+I or PH. Biniodide 98-8 1 eq.+do. 252-6 2 eq.= 351.4 P1+21 or P1I». Protosulphuret 98.8 1 eq.+Sulphur 16.1 1 eq.= 114.9 Pl+S or PIS. Bisulphuret 98.81 eq.+do. 32.2 2 eq.=131 P1+2S or PIS*. Protoxide of Platinum.—This oxide is prepared by digesting protochloride of platinum in a solution of pure potassa, avoiding a large excess of the alkali, since it dissolves a portion of the oxide, and thereby acquires a green colour. In this state it. i* ?», hydrate which loses first its water and then 408 PLATINUM. oxygen when heated, and dissolves slowly in acids, yielding solutions of a brownish-green tint. Binoxide.—This oxide is prepared with difficulty, owing to its disposition, like peroxide of gold, to act rather as an acid than an alkaline base, and either to fall in combination with any alkali by which it is precipitated, or to remain with it altogether in solution. Berzelius recommends that it should be prepared by exactly decomposing sulphate of binoxide of platinum with nitrate of baryta, and adding pure soda to the filtered solution, so as to precipitate about half of the oxide ; since otherwise, a sub-salt would subside. The oxide falls in the form of a bulky hydrate, of a yellowish-brown colour: it resembles rust of iron when dry, and is nearly black when rendered an- hydrous. Sesquioxide.—This oxide, of a gray colour, is prepared, according to its discoverer Mr E. Davy, by heating fulminating platinum with nitrous acid; but the nature of the compound so formed has not yet been decisively deter- mined. (Phil. Trans. 1820.) Protochloride.—When the bichloride is heated to 450°, half of its chlorine is expelled, and the protochloride of a greenish-gray colour remains. It is insoluble in water, sulphuric acid, and nitric acid; but hydrochloric acid partially dissolves it, yielding a red solution. At a red heat its chlorine is driven off, and metallic platinum is left. It is dissolved by a solution of the bichloride. Bichloride of Platinum.—This chloride is obtained by evaporating the solution of platinum in nitro-hydrochloric acid to dryness at a very gentle heat, when it remains as a red hydrate, which becomes brown when its water is expelled. It is deliquescent, and very soluble in water, alcohol, and ether; its solution, if free from the chlorides of palladium and iridium, being of a pure yellow coldur. Its ethereal solution is decomposed by light, metallic platinum being deposited. A solution of platinum is recognized by the following characters. When to an alcoholic or concentrated aqueous solution of the bichloride, a solution of chloride of potassium is added, a crystalline double chloride of a pale yellow colour subsides, which is insoluble in alcohol, and sparingly soluble in water: at a red heat it yields chlorine gas, and the residue consists of metallic platinum and chloride of potassium. With a solution of hydro- chlorate of ammonia a similar yellow salt falls, which when ignited leaves pure platinum in the form of a delicate spongy mass, the power of which in kindling an explosive mixture of oxygen and hydrogen gases has dready been mentioned. Protiodide of Platinum.—Lassaigne prepared this compound by digesting the protochloride of platinum in a rather strong solution of iodide of potas- sium, when the protiodide gradually appeared in the form of a black powder, which is insoluble in water and alcohol. It is unchanged by the sulphuric, nitric, and hydrochloric acids, decomposed by the alkalies, and at a red heat gives off its iodine. Biniodide of Platinum.—Lassaigne prepares this compound by the action of iodide of potassium on a rather dilute solution of bichloride of platinum. At first the liquid acquires an orange-red and then a claret colour, without any precipitation ; hut when the solution is boiled, a black predpitate sub- sides, which sliauia dc washed with hot water and dried at a heat not ex- ceeding 212°. This biniodide is a black powder, sometimes crystalline, is tasteless and inodorous, insoluble in water, and may be boiled in water without change. By alcohol it is sparingly dissolved, especially when heat- ed. Acids act feebly upon it; but it is decomposed by alkalies, and begins to lose iodine at 270°. (An. de Ch. et de Ph. Ii. 113.) Protosulphuret of Platinum.—It is formed by heating in a retort the yellow ammoniacal chloride of platinum with half its weight of sulphur until all the salammoniac and excess of sulphur are expelled. The proto- sulphuret is then left as a gray powder of a metallic lustre. It may also be formed by the action of hydrosulphuric acid on protochloride of platinum. PALLADIUM. Persulphuret.—It is formed as a brown precipitate, which becomes black when dried, by letting fall a solution of bichloride of platinum drop by drop into a solution of sulphuret of potassium, or by transmitting hydrosulphuric acid gas into a solution of the double chloride of platinum and sodium. (Berzelius.) It should be dried in vacuo by aid of sulphuric acid; since, by exposure to the air in a moist state, sulphuric acid is generated. Fdminating platinum may be prepared by the action of ammonia in slight excess on a solution of sulphate of oxide of platinum. (E. Davy.) It is analogous to the detonating compounds which ammonia forms with the oxides of gold and silver. SECTION XXVIII. PALLADIUM, RHODIUM, OSMIUM, AND IRIDIUM. The four metals to be described in this section are all contained in the ore of platinum, and have hitherto been procured in very small quantity. When the ore is digested in nitro-hydrochloric acid, the platinum, together with palladium, rhodium, iron, copper, and lead, is dissolved; while a black powder is left consisting of osmium and iridium. PALLADIUM. This metal was discovered in 1803 by Wollaston (Phil. Trans. 1804 and 1805). On adding bicyanuret of mercury dissolved in water to a neutrd so- lution of the ore of platinum, either before or after the separation of that metal by hydrochlorate of ammonia, a yellowish-white flocculent precipitate is gradually deposited, which is cyanuret of palladium. When this com- pound is heated to redness, the cyanogen is expelled, and pure palladium remains. In order to obtain it in a malleable state, the metal should be heated with sulphur, and the resulting sulphuret purified by cupellation in an open crucible with borax and a little nitre. It is then roasted at a low red heat on a flat brick, and when reduced to a pasty consistence, it is pressed into a square or oblong perfectly flat cake. It is again to be roasted very patiently, at a low red heat, until it becomes spongy on the surface; and when quite cold, it is condensed by frequent tappings with a light hammer. By alternate roastings and tappings the sulphur is burned off, and the metd rendered sufficiently dense to be laminated. Thus prepared it is rather brittle while hot, which Wollaston supposed to arise from a small remnant of sulphur. (Phil. Trans. 1829, p. 7.) Palladium resembles platinum in colour and lustre. It is ductile as well as malleable, and is considerably harder than platinum. Its specific gravity varies from 11.3 to 11.8. In fusibility it is intermediate between gold and platinum, and is dissipated in sparks when intensely heated by the oxy- hydrogen blowpipe. At a red heat in oxygen gas, its surface acquires a fine blue colour, owing to superficial oxidation ; but the increase of weight is so slight as not to be appreciated. Palladium is oxidized and dissolved by nitric acid, and even the sulphuric and hydrochloric acids act upon it by the aid of heat; but its proper solvent is nitro-hydrochloric acid. Its oxide forms beautiful red-coloured salts, from which metallic palladium is precipitated by sulphate of protoxide of iron, and by all the metals described in the foregoing sections, excepting silver, gold, and platinum. From the analysis by Berzelius of the double chloride of palladium and 410 RHODIUM. potassium the equivalent of palladium is inferred to be 53.3. The composi- tion of its compounds described in this section is as follows:— Palladium. Equiv. Formulae. Protoxide 53.3 1 eq. + Oxyg. 8 1 eq.= 61.3 Pd+O or Pd. Binoxide 53.3 1 eq.+do. 16 2 eq.= 69.3 Pd+20 or Pd. Protochloride 53.3 1 eq. + Chlor. 35.42 1 eq.= 88.72 Pd + Cl or PdCl. Bichloride 53.3 1 eq.+do. 70.84 2 eq.=124.14 Pd+2ClorPdCl2. Protosulphuret 53.3 1 eq. + Sulph. 16.1 1 eq.= 69.4 Pd + S or PdS. Protoxide of Palladium.—This oxide is obtained as a hydrate of a deep brown colour by decomposing its salts with an excess of carbonate of potassa or soda; and, by washing and heating to low redness, the anhydrous protoxide of a black colour is left. It is also obtained by heating the nitrate at a low red heat. In the anhydrous state it is dissolved with difficdty by acids. When strongly heated it parts with its oxygen. Berzelius says it falls from its salts on the addition of the alkalies as a subsalt, which is dis- solved by the alkali in excess. Binoxide.—To prepare this oxide Berzelius recommends that a solution of potassa or its carbonate in excess should be poured by little and little on the solid bichloride of palladium and potassium, and the materials be well inter- mixed : water is not first added, because it decomposes the double chloride; and the alkali is not added all at once, because the binoxide would then be dissolved at first, and afterwards separate out as a gelatinous hydrate, which could not be purified by washing. When prepared with the foregoing di- rections, the binoxide is obtained as a hydrate of a deep yellowish-brown colour, which retains a little potassa in combination; but on heating the solution to 212°, the alkali is dissolved and the anhydrous black, oxide left. Protochloride of Palladium.—It is obtained by evaporating to dryness a solution of palladium in nitro-hydrochloric acid, being left as a brown crys- talline hydrate, which becomes black when its water is expelled. It loses its chlorine when strongly heated, and is soluble in water. The bichloride is formed by digesting the protochloride in nitro-hydro- chloric acid, and exists only in solution, the colour of which is of so deep a brown as to appear nearly black. It is readily distinguished from the proto- chloride by yielding with chloride of potassium a double chloride of a red colour; whereas that formed with the protochloride is yellow. Protosulphuret of Palladium.—It is readily formed by heating the metal with sulphur, and is a fusible brittle compound of a gray colour. RHODIUM. This metal was discovered by Wollaston at the time he was occupied with the discovery of palladium. On immersing a thin plate of clean iron into the solution from which palladium and the greater part of the platinum have been precipitated, the rhodium, together with small quantities of platinum, copper, and lead, is thrown down in the metallic state; and on digesting the precipitate in dilute nitric acid, the two last metals are removed. The rhodium and platinum are then dissolved by means of nitro-hydrochloric acid, and the solution, after being mixed with some chloride of sodium, is evaporated to dryness. Two double chlorides result, that of platinum and sodium, and of rhodium and sodium, the former of which is soluble, and the latter insoluble in alcohol; and they may, therefore, be separated from each other by this menstruum. The double chloride of rhodium is then dissolved in water, and metallic rhodium precipitated by insertion of a rod of zinc. Rhodium, thus procured, is in the form of a black powder, which requires the strongest heat that can be produced in a wind furnace for fusion, and when fused has a white colour and metallic lustre. It is brittle, is extremely hard, and has a specific gravity of about 11. It attracts oxygen at a red heat, a mixture of peroxide and protoxide being formed. It is not attacked OSMIUM AND IRIDIUM. 411 by any of the acids when in its pure state; but if alloyed with other metals, such as copper or lead, it is dissolved by nitro-hydrochloric acid, a circum- stance which accounts for its presence in the solution of crude platinum. It is oxidized by being ignited either with nitre, or bisulphate of potassa. When heated with the latter, sulphurous acid gas is evolved, and a double sulphate of peroxide of rhodium and potassa is generated, which dissolves readily in hot water, and yields a yellow solution. The presence of rhodium in platinum, iridium, and osmium may thus be detected, and by repeated fusion a perfect separation be accomplished. (Berzelius.) Chemists are acquainted with two oxides of rhodium. The protoxide is black, and the peroxide, which is the base of the salts of rhodium, is of a yellow colour. Most of its salts arc either red or yellow. From the composition of the double chloride of rhodium and potassium Berzelius considers 52.2 as the equivalent of rhodium; and its compounds described in this section are thus constituted:— Rhodium. Equiv. Formulae. Protoxide 52.2 1 eq.+Oxygen 8 1 eq.= 60.2 R + O or R. Peroxide 104.4 2 eq.+ do. 24 3 eq.= 128.4 2R + 30orIL_ Protochloride 52.2 1 eq. + Chlor. 35.42 1 eq.= 87.62 R+ClorRCl. Perchloride 104.4 2 eq.+ do. 106.26 3 eq.=210.66 2R + 3ClorR; equal, and juogyt} form) is applied: and those substances which assume the same figure are said to be isomorphous. Of these isomorphous bodies, several distinct groups have been described by Mitscherlich. One of the most instructive of these includes the salts of arsenic and phosphoric acid. Thus, the neutral phosphate and biphosphate of soda have exactly the same form as the arseniate and binarseniate of soda; phosphate and biphosphate of ammonia correspond to arseniate and bi- narseniate of ammonia, and the biphosphate and binarseniate of potassa have the same form. Each arseniate has a corresponding phosphate, possessed of * Annales de Ch. et de Physique, vol. xiv. 172, xix. 350, and xxiv. 264 and 355. 430 CRYSTALLIZATION, the same form possessing the same number of equivalents of acid, alkali, and water of crystallization, and differing in fact in nothing, except that one series contains arsenic and the other an equivalent quantity of phosphorus. A second remarkable group contains the salts of sulphuric, selenic, chromic, and man- ganic acids. The salts of baryta, strontia, and oxide of lead constitute a third group; and a fourth consists of lime, magnesia, and the protoxides of manganese, iron, cobalt, nickel, zinc, and copper. A fifth includes alumina, peroxide of iron, and the green oxide of chromium ; and a sixth group in- cludes the salts of permanganic and perchloric acids. In comparing together isomorphous bodies of the same group, identity of form is not to be expected unless there is similarity of composition. A neutral phosphate does not correspond to a binarseniate, nor a biphosphate to a neutral arseniate : an an- hydrous sulphate is not comparable to a hydrated seleniate of the same base; nor is sulphate of protoxide of iron, with six equivalents of water, isomor- phous with sulphate of protoxide of manganese with five equivalents. In all such instances, if chemical composition differ, crystalline form is also different. The following table contains the principal groups of isomorphous sub- stances at present observed by chemists : a more extended one, partly theo- retical, has been drawn up by Professor Johnston, of Durham, in his Report on Chemistry to the British Association :— Silver Gold Au 2. Arsenious acid ... As in its unusual form (page 354.) Sb Sesquioxide of antimony 3. Alumina Sesquioxide of iron . Salts of Phosphoric acid Arsenic acid i Salts of Sulphuric acid . Selenic acid Chromic acid Manganic acid Salts of Perchloric acid . Permanganic acid Salts of Potassa. 4. Al. Fe Se. Cr Mn CI. Mn. K. Salts of , Soda .... . Na. Oxide of silver ■ Ag. 9. Salts of Baryta .... . Ba. Strontia Sr. Lime (in Arragonite) . Ca. Protoxide of lead Pb. 10. Salts of Lime .... . Ca. Magnesia • Mg. Protoxide of iron . Fe- manganese Mn. zinc . Zn. nickel . Ni. cobalt Co. copper . Cu. Salts of Alumina lead (inplumbo- ) calcite) . j 11. Sesquioxide of iron Sesquioxide of chromium Sesquioxide of manganese Pb. Al. Fe. Cr. Mn. Ammonia with 1 eq. of waterH3N-f-H. The facts above mentioned afford indubitable proof that the form of CRYSTALLIZATION. crystals is materially dependent on their atomic constitution; and they at first induced Mitscherlich to suspect that crystalline form is determined solely by the number and arrangement of atoms, quite independently of their nature. Subsequent observation, however, induced him to abandon this view ; and his opinion now appears to be, that certain elements, which are themselves isomorphous, when combined in the same manner with the same substance, communicate the same form. Similarly constituted salts of arsenic and phosphoric acids yield crystals of the same figure, because the acids, it is thought, are themselves isomorphous ; and as the atomic constitution of these acids is similar, each containing the same number of atoms of oxy- gen united with the same number of atoms of the other ingredient, it is inferred that phosphorus is isomorphous with arsenic. In like manner it is believed that selenic acid must be isomorphous with sulphuric acid, and selenium with sulphur; and the same identity of form is ascribed to all those oxides above enumerated, the salts of which are isomorphous. The accuracy of this ingenious view has not yet been put to the test of extensive observation, because the crystalline forms of the substances in question are for the most part unknown. But our knowledge, so far as it goes, is favourable ; for sesquioxide of iron and dumina, the salts of which possess the same form, are themselves isomorphous. It may hence be in- ferred as probable, that isomorphous compounds in general arise from iso- morphous elements uniting in the same manner with the same substance. Isomorphous substances have often very close points of resemblance, quite independently of form. Thus, arsenic and phosphorus have the same odour; they both form gaseous compounds with hydrogen : they differ from nearly all other bodies in their mode of combining wilh oxygen, and yet agree with one another, and their salts are disposed to combine with the same quantity of water of crystallization. A similar analogy subsists be- tween selenium and sulphur, both being fusible, volatile, and combustible in nearly the same degree, forming with hydrogen colourless gases which are similar in odour and in their chemical relations, and giving rise to analogous compounds with oxygen. The characters of sulphuric and selenic acids in particular are very similar ; and the salts of these acids are equally allied. Sulphate of soda, for example, has the unusual property of being less soluble in water at 212° than at 100°, and the very same peculia- rity is observable in seleniate of soda. The same intimacy of relation'exists between baryta and strontia, between lime and magnesia, and between cobalt and nickel. Isomorphous substances, owing doubtless to the various points of resem- blance which have just been traced, crystallize together with great readiness, and are separated from each other with difficulty. Dr. Daubeny has re- marked that a weak solution of lime, which in pure water would be instantly indicated by oxalate of ammonia, is very sluggishly affected by that test when much sulphate of magnesia is present; and I find that chloride of manganese cannot be purified from lime by oxalate of ammonia. A mixture of the sulphates of the protoxides of copper and iron yields crystals which have the same quantity of water of crystallization (six equivalents), and the same form as green *vitriol, though they may contain a large quantity of copper. The sulphates of the protoxides of zinc and copper, of copper and mag- nesium, of copper and nickel, of zinc and manganese, and of magnesium and manganese, crystallize together, contain six equivalents of water, and have the same form as green vitriol, without containing a particle of iron. These mixed salts may be crystallized over and over again without the ingredients being separated from each other, just as it is extremely difficult to purify alum from sesquioxide of iron, with which alumina is isomorphous. In these in- stances the isomorphous salts do not occur in definite proportions: they are not chemically united as double salts, but merely crystallize together. The same intermixture of isomorphous substances which takes place in artificial salts, is found to occur in minerals, and affords a luminous expla- nation of the great variety both in the kind and proportion of substances which may coexist in a mineral species, without its externaf character being 432 CRYSTALLIZATION. thereby essentidly affected. Thus, garnet 'is a double silicate of dumina and lime, expressed by the formula (A1 + 3Si) + 3(Ca + Si); but in garnet, as in alum, the alumina may be replaced by sesquioxide of iron, yielding the compound (Fe-f3Si) + 3(Ca4-Si) or they may be both present in any pro- portion, provided that their sum is equivalent to either singly. So, while sesquioxide of iron displaces the alumina, the lime may be exchanged for protoxide of iron; and a mineral would result, (Fe_|_3Si)-r.3(Fe-r-Si), which contains neither alumina nor lime, though it has still the form of gar- net. Instead of protoxide of iron, the lime may be replaced by magnesia, protoxide of manganese, or any other isomorphous base; or any equivalent quantity of some or all of these may take the place of the lime, without the crystallographic character being destroyed. In like manner epidote is a double silicate of alumina and lime, expressed by (Al-f-3Si)-f-(Ca-f-3Si). and here again varieties of epidote are to be expected, in which alumina and lime are replaced partially or wholly by an equivalent quantity of iso- morphous bases. The discovery of Mitscherlich, while it accounts for difference of com- position in the same mineral, and serves as a caution to mineralogists against too exclusive reliance on crystallographic character, is in several other respects of deep interest to the chemist It tends to lay open new paths of research by unfolding analogies which would not otherwise have been perceived. The tendency of isomorphous bodies to crystallize together accounts for the difficulty of purifying mixtures of isomorphous salts by crystallization. The same property sets the chemist on his guard against the occurrence of isomorphous substances in crystallized minerals. The native phosphates, for example, frequently contain arsenic acid, and conversely the native arseniates phosphoric acid, without the form of the crystals being thereby affected in the slightest degree. It is a useful guide in discovering the atomic constitution of compounds. All chemists are agreed, from the composition of the oxides of iron, and from the compounds which this metal forms with other bodies, that the oxide consists of two atoms of iron and three atoms of oxygen; and, therefore, it is inferred that alumina, which is isomorphous with peroxide of iron, has a similar constitution. The green oxide and acid of chromium, the oxygen of which is as 1 to 2, affords a still better illustration. As the chromates and sulphates are isomorphous, it was inferred that chromic, like sulphuric, acid was composed of one atom of the combustible to three atoms of oxygen. On this presumption it follows that the green oxide, containing half as much oxygen as the acid, must con- tain two atoms of chromium to three atoms of oxygen; and agreeably to this inference, it is found that the green oxide is isomorphous with alumina and peroxide of iron. The phenomena presented by isomorphous bodies afford a powerful argument in favour of the atomic theory. The only mode of satisfactorily accounting for the striking identity" of crystalline form observable, first, between two substances, and, secondly, between all their compounds which have an exactly similar composition, is by supposing them to consist of ultimate particles possessed of the same figure, and arranged in precisely the same order. Hence it appears, that, in accounting for the con- nexion between form and composition, it is necessary to employ the very same theory, by which alone the laws of chemical union can be adequately explained. It has been objected to some of the facts adduced in favour of isomorphism, that the forms of substances considered isomorphous are sometimes approxi- mate rather than identical. The primary form of sulphate of strontia is a rhombic prism very similar to that of sulphate of baryta; but on measuring the inclination of corresponding sides in each prism, the difference is found to exceed two degrees; and similar differences are observable in the rhom- '6.W-SALTS. 433 bohedron of the carbonates of lime and protoxide of iron. This has in- duced Professor Miller of Cambridge to indicate this approximation by the termplesiomorphism (frxno-iot, near); and it has been brought forward in a clever essay by Mr. Brooke as an argument against the whole doctrine of isomorphism, an essay which has received an able reply from the pen of Mr. Whewell. (Phil. Mag. and An. N. S. x. 161 and 401.) It is an important matter for inquiry to ascertain the distinction between isomorphous and plesiomorphous bodies, and to discover if two substances may be plesiomor- phous in one state of combination and isomorphous in another. Such an inquiry will in dl likelihood modify our present views; but in the mean time the facts dready known leave no doubt, that some doctrine of the kind has a real foundation in nature. In one of the essays above referred to, Professor Mitscherlich observed that biphosphate of soda is capable of yielding two distinct kinds of crystals, which, though different in form, in composition appear to be identical. The more uncommon of the two forms resembled binarseniate of soda; but the more usual form is quite dissimilar. He has since discovered that sulphur is capable of yielding two distinct kinds of crystals. The crystals of carbo- nate of lime in calcareous spar and in Arragonite belong to different systems of crystallization, the former being rhombohedral, and the latter derived from a rhombic prism. Arsenious acid, and probably metallic arsenic also, affords an instance of the same kind (page 354). It would thus seem that elementary and compound bodies are capable of assuming tjvo distinct crystal- line forms. In the case of biphosphate of soda, an explanation may be de- rived from the late experiments of Mr. Graham on metaphosphoric acid; but the fact that an elementary substance is susceptible of assuming different forms is wholly unexplained. Mitscherlich has also noticed that the form of salts is sometimes changed by heat, without their losing the solid state. This change was first noticed in sulphate of magnesia, and also in the sulphates of the protoxides of zinc and iron. It appears, in these instances at least, to be owing to decompo- sition of the hydrous salt effected by increased temperature; a change of composition which is accompanied with a new arrangement in the mole- cules of the compound. SECTION I. CLASS OF SALTS. ORDER I. OXY-SALTS. This order of salts includes no compound the acid or base of which does not contain oxygen. With the exception of the ammoniacal salts, both the acid and base of the salts described in this section are oxidized bodies. As each acid, with few exceptions, is capable of uniting with every alkaline base, and frequently in two or more proportions, it is manifest that the salts must constitute a very numerous class of bodies. It is necessary, on this account, to facilitate the study of them as much as possible by classification. They may be conveniently arranged by placing together those salts which contain either the same salifiable base or the same acid. It is not very material which principle of arrangement is adopted; but I give the preference to the latter, because, in describing the individual oxides, I have already mention- ed the characteristic features of their salts, and have thus anticipated the chief advantage that arises from the former mode of classification. I shall, there- fore, divide the salts into families, placing together those saline combinations which consist of the same acid united with different salifiable bases. The salts of each family, in consequence of containing the same acid, possef-s 434 SULPHATES, certain characters in common by which they may all be distinguished; and, indeed, the description of many salts, to which no particular interest is at- tached, is sufficiently comprehended in that of its family, and may, there- fore, be omitted. All the powerful alkaline bases, excepting ammonia, are the protoxides of an electro-positive metd, such as potassium, barium, or iron; so that if M represent an equivalent of any one of those metals, M + O or M is the strong- est alkaline base, and often the only one, which that metal can form. A sin- gle equivalent of an acid neutralizes M, forming with it a neutral salt. Thus, indicating an equivalent of sulphuric and nitric acid by the signs S and N, all the neutral sulphates and nitrates of protoxides are indicated by M-|-S and M -f- N. There is, therefore, in the neutral protosalts of each family, a constant ratio in the oxygen of the base and acid, resulting from the compo- sition of each acid; that ratio for the sulphates being as 1 to 3, and for ni- trates as 1 to 5. If the metal M of a neutral sulphate pass into a higher grade of oxidation, becoming a binoxide M, then will that binoxide be disposed to unite with two equivalents of acid, and form a bisalt, M-J-2S, in which the oxygen of base and acid is still as 1 to 3; and if the metal yield a sesquioxide, M, then if sufficient acid be supplied, the resulting salt will consist of M-f- 3S, the ratio of 1 to 3 being preserved. This curious law relative to oxy-salts, which is very general, was first noticed by Gay-Lussac (Memoires d'Arcueil, ii.); and Berzelius has found it to hold in earthy minerds, and employed it as a guide in studying their composition. The combination of salts with one another gives rise to compounds which were formerly called triple salts; but as the term double salt, proposed by Ber- zelius, gives a more correct idea of their nature and constitution, it will d- ways be employed by preference. These salts may be composed of one acid and two bases, of two acids and one base, and of two different acids and two different bases. Most of the double salts hitherto examined consist of the same acid and two different bases. SULPHATES. The salts of sulphuric acid in solution may be detected by chloride of barium. A white precipitate, sulphate of baryta, invariably subsides, which is insoluble in acids and alkalies, a character by which the presence of sul- phuric acid, whether free or combined, may always be recognized. An in- soluble sulphate, such as sulphate of baryta or strontia, may be detected by mixing it, in fine powder, with three times its weight of carbonate of potassa or soda, and exposing the mixture in a platinum crucible for half an hour to a red heat.- Double decomposition ensues; and on digesting the residue in water, filtering the solution, neutralizing the free alkali by pure hydrochloric, nitric, or acetic acid, and adding chloride of barium, the insoluble sulphate of that base is precipitated. Several sulphates exist in nature, but the only ones which are abundant are the sulphates of lime and baryta. All of them may be formed by the ac- tion of sulphuric acid on the metals themselves, on the metallic oxides or their carbonates, or by way of double decomposition. The solubility of the sulphates is very variable. There are six only which may be regarded as really insoluble; namely, the sulphates of baryta, and of the oxides of tin, antimony, bismuth, lead, and mercury. The sparingly SULPHATES. 435 soluble sulphates are those of strontia, lime, zirconia, yttria, and of the oxides of cerium and silver. All the others are soluble in water. All the sulphates, those of potassa, soda, lithia, baryta, strontia, and lime excepted, are decomposed by a white heat. One part of the sulphuric acid of the decomposed sulphate escapes unchanged, and another portion is re- solved into sulphurous acid and oxygen. Those which are easily decom- posed by heat such as sulphate of oxide of iron, yield the largest quantity of undecomposed sulphuric acid. When a sulphate, mixed with carbonaceous matter, is ignited, the oxygen both of the acid and of the oxide unites with carbon, carbonic acid is disen- gaged, and a metdlic sulphuret remains. A similar change is produced by hydrogen gas at a red heat, with formation of water, and frequently of some hydrosulphuric acid. In some instances the hydrogen entirely deprives the metal of its sulphur. The composition of neutrd protosulphates is expressed, as above stated, by the formula M-f S. Consequently the acid contains three times as much oxygen as the base; and if both were deprived of their oxygen, a metallic protosulphuret wodd result, as indicated by the formula M -f- S. The following table represents the composition of the principal sulphates, both anhydrous, and with water of crystallization when they crystallize with water. Base. Acid. Names. Sulphate of potassa Sesquisulph. potassa . Do. in crystals with Bisulph. potassa Do. in crystals with Sulph. soda 47.15 1 eq.-f 40.1 94.3 2 eq.-f. 120.3 9 or 1 eq. ofwater 47.15 1 eq.-f 80.2 18 or 2 eq. ofwater 31.3 1 eq.-f 40.1 Do. in crystals with 90 or 10 eq. of water Bisdph. soda 31.3 1 eq.-f 80.2 Do. in crystals with 36 or 4 eq. of water Sulph. lithia Do. in crystals with Sulph. ammonia . Do. in crystals with Sulph. baryta Sulph. strontia . Sulph. lime 18 1 eq.-f 40.1 9 or 1 eq. of water 17.15 1 eq.-f 40.1 18 or 2 eq. of water 76.7 1 eq.-f 40.1 1 eq.-f 40.1 1 eq.-f 40.1 2 eq. of water 51.8 28.5 Do. as gypsum with 18 or Sulph. magnesia . . 20.7 1 eq.-f 40.1 Do. in crystals with 63 or 7 eq. of water Sulph. alumina . . 51.4 1 eq.-f 40.1 Da. in crystals with 81 or 9 eq. of water Equiv. 1 eq.= 87.25 3 eq.=214.6 . =223.6 2 eq.=127.35 . =145.35 1 eq— 71.4 . =161.4 2 eq.=111.5 . =147.5 1 eq.= 58.1 . = 67.1 1 eq.= 57.25 . = 75.25 1 eq.=H6.8 1 eq.= 91.9 I eq.= 68.6 . = 86.6 1 eq. 60.8 . =123.8 1 eq.= 91.5 . =172 J Formdae. K-fS. 2K+3S" K-f2S. Na-fS. Na-f2S. L-fS. H3N-fS Ba+S. Sr-fS. Ca-fS. Mg+S. AI+S. 436 Names. Tersulph. alumina SULPHATES. Base. Acid. Equiv. 51.4 1 eq.+120.3 3 eq.=171.7 Do. in crystals with 162 or 18 eq. of water . =333.7 Sulph. protox. manganese 35.7 1 eq.-f 40.1 1 eq.= 75.8 Do. in crystals with 45 or 5 eq. of water . =120.8 Sulph. protox. iron . 36 1 eq.-f 40.1 1 eq.= 76.1 Do. in crystals with 54 or 6 eq. of water . =130.1 Tersulph. sesquiox. iron 80 1 eq.-f 120.3 3 eq.=200.3 Disulph. sesquiox. iron 160 2 eq.-f 40.1 1 eq.=200.1 Do. as a hydrate with 54 or 6 eq. of water . =254.1 Sulph. protox. zinc . 40.3 1 eq.-f 40.1 1 eq.= 80.4 Do. in crystals with 63 or 7 eq. of water . =143.4 Sulph. protox. nickel . 37.5 1 eq.-f 40.1 1 eq.= 77.6 Do. in crystals with 63 or 7 eq. of water . =140.6 Sulph. protox. cobalt . 37.5 1 eq.-f 40.1 1 eq.= 77.6 Do. in crystals with 54 or 6 eq. of water . =131.6 Tersulph. sesquiox. chrom. 80 1 eq.+120.3 3 eq.=200.3 Sulph. protox. copper 39.6 1 eq.-f 40.1 1 eq.= 79.7 Disulph. protox. copper 79.2 2 eq.-f 40.1 1 eq.= 119.3 Sulph. protox. mercury 210 1 eq.-f 40.1 1 eq.= 250.1 Subsulp. perox. mercury 872 4 eq.-f 120.3 3 eq.=992.3 Bisulph. perox. mercury 218 1 eq.-f 80.2 2 eq.=298.2 Sulph. ox. silver . . 116 1 eq.-f 40.1 1 eq.= 156.1 DOUBLE SULPHATES. Names. Equiv. Sulphate of soda S Sulph. soda 71.4 1 eq. ) _j4Q and lime ( do. lime 68.6 1 eq. ( Sulph. of potassa S Sulph. potassa 87.25 1 eq. ) _i48.05 and magnesia ) do. magnesia 60.8 1 eq. \ Do. with 54 or 6 eq. ofwater . . =202.05 Sulph. of ammonia 5 Sulph. ammonia 57.25 1 eq. ) _U8 05 and magnesia ) do. magnesia 60.8 1 eq. \ — with 63 or 7 eq. of water Do. = 181.05 Sulph. of soda S Sulph. soda 71.4 1 eq. ) 1320 1 eq. 5 and magnesia ) do. magnesia 60.8 Do. with 54 or 6 eq. of water Formulee. A1+3S. Mn+S. Fe-fS. Fe-f3S. 2F_e-fs" Zn-fS. Ni-fS. Co+S. Cr-f3S. Cu-fS. 2Cu-fS. Hg+S 4Hg-f3S. Hg+2S. Ag+S. Formulae. NaS-fCaS.' KS+MgS. H'NS-fMgS. NaS+MgS. = 186.2 SULPHATES. 437 Names. Equiv. Formulae. Sulph. of potassa \ Sulph. potassa 87.25 1 eq. ?25895 KS-fAlSa and alumina ( Tersulph. alum. 171.7 1 eq. \ Da with 216 or 24 eq. ofwater . =474.95 Sulph. of soda ( Sulph. soda 71.4 1 etl-1 243.1 NaS-fAlSa and alumina ( Tersulph. alum. 171.7 1 eq. $ — Do. with 234 or 26 eq. ofwater . =477.1 Sulph. of ammonia S Sulph. ammonia 57.25 1 eq. ^ _228.95 H3NS-f AISs and alumina ( Tersulph. alum. 171.7 1 eq. £ Sulph. of potassa S Sulph. potassa 87.25 1 eq. > = 153.95 KS-f MnS. and proUx. mang. ( do. protox. mang. 75.8 1 eq. ( Do. with 54 or 6 eq. of water • . =217.05 Sulph. of ammonia \ Sulph. ammonia 5755 1 eq. > ^3395 H3NS-f MnS. and protox. mang. ) do. protox. mang. 75.8 1 eq. ^ Do. with 63 or 7 eq. ofwater . =196.05 The protoxides of iron, zinc, nickel, and cobalt yield with potassa and ammonia double salts exactly agreeing in form and composition with the preceding double salts of magnesia and oxide of manganese. Sulphate of Potassa.—This salt is easily prepared artificially by neutral- izing carbonate of potassa with sulphuric acid; arid it is procured abundant- ly by neutralizing with carbonate of potassa the residue of the operation for preparing nitric acid. (Page 181.) Its taste is saline and bitter. It gene- rally crystallizes in six-sided prisms, bounded by pyramids with six sides; the'size of which is said to be much increased by the presence of a little car- bonate of potassa. Its primary form, according to Mitscherlich, is a rhombic octohedron, and it is isomorphous with chromate and seleniate of potassa. (Pog. Annalen, xviii. 168.) The crystals contain no water of crystallization, and suffer no change by exposure to the air. They decrepitate when heated, and enter into fusion at a red heat. They require 16 times their weight of water at 60°, and 5 of boiling water for solution. Bisulphate of potassa is easily formed by exposing the neutral sulphate with half its weight of strong sulphuric acid to a heat just below redness, in a platinum crucible, until acid fumes cease to escape. The primary form of its crystals is a right rhombic prism, but which is in general so flattened as to be tabular. It has a strong sour taste, and reddens litmus paper. It is much more soluble than the neutral sulphate, requiring for solution only twice its weight of water at 60°, and less than an equal weight at 212° F. It is resolved by heat into sulphuric acid and the neutral sulphate. Mr. Phillips has described a sesquisulphate, obtained in the form of acicular crystals like asbestus, from the residue of the process for making nitric acid. The conditions for ensuring its production have not been determined. (Phil. Mag. and Annals, ii. 421.) Sulphate of Soda.—This compound, commonly called Glauber's salt, is occasionally met with on the surface of the earth, and is frequently con- tained in mineral springs. It may be made by the direct action of sulphuric acid on carbonate of soda, and it is procured in large quantity as a residue in the process for forming hydrochloric acid and chlorine. (Pages 210 and 212.) Sulphate of soda has a cooling, saline, and bitter taste. It commonly yields four and six-sided prismatic crystals, but its primary form is a rhom- bic octohedron. Its crystals effloresce rapidly when exposed to the air, losing the whole of their water. When heated they readily undergo the 438 SULPHATES. watery fusion. At 32° 100 parts of water dissolve 12 parts of the crystals, 48 parts at 64.5°, 100 parts at 77°, 270 at 89.5°, and 322 at 91.5°. On in- creasing the heat beyond this point, a portion of the salt is deposited, being less soluble than at 91.5°. (Gay-Lussac.) If a solution saturated at 91.5° is evaporated at a higher temperature, the salt is deposited in opaque anhy- drous prisms, the primary form of which is a rhombic octohedron. Its specific gravity in this state is 2.462. (Haidinger.) Bisulphate of soda may be formed in the same manner as the analogous salt of potassa. Sulphate of Lithia.—This salt is very soluble in water, fuses by heat more readily than the sulphates of the other alkalies, and crystallizes in flat prisms, which resemble sulphate of soda in appearance, but do not effloresce on exposure to the air. Its taste is saline without being bitter. Sulphate of Ammonia.—This salt is easily prepared by neutralizing car- bonate of ammonia with dilute sulphuric acid; and it is contained in con- siderable quantity in the soot from coal. It crystallizes in long flattened six- sided prisms. It dissolves in two parts of water at 60°, and in an equal weight of boiling water. In a warm dry air it effloresces and loses half of its water. When sharply heated it fuses, gives off its water of crystalliza- tion along with some ammonia, and is then decomposed, yielding nitrogen gas, water, and sulphate of ammonia. Sulphate of Baryta.—Native sulphate of baryta, commonly called heavy spar, occurs abundantly, chiefly massive, but sometimes in anhydrous crys- tals, the form of which is variable, being sometimes prismatic and some- times tabular, deducible from a right rhombic prism. Its density is about 4.4. It is easily formed artificially by double decomposition. This salt bears an intense heat without fusing or undergoing any other change, and is one of the most insoluble substances with which chemists are acquainted. It is sparingly dissolved by hot and concentrated sulphuric acid, but is pre- cipitated by the addition ofwater. Sulphate of Strontia.—This salt, the celestine of mineralogists, is less abundant than heavy spar. It occurs in anhydrous prismatic crystals of peculiar beauty in Sicily, and its primary form is a right rhombic prism. Its density is 3.858. As obtained by the way of double decomposition, it is a white heavy powder, very similar to sulphate of baryta, and requires about 3840 times its weight of boiling water for solution. Sulphate of Lime.—This salt is easily formed by mixing in solution a salt of lime with any soluble sulphate. It occurs abundantly as a natural pro- duction. The mineral called anhydrite is anhydrous sulphate of lime; and all the varieties of gypsum are composed of the same salt, united with water. The pure crystallized specimens of gypsum are sometimes called selenite; and the white compact variety is employed in statuary under the name of alabaster. The crystals are generally flattened prisms, the primary form of which is a rhombic prism. The hydrous salt is deprived of its water by a low red heat, and in this state forms plaster of Paris. Its property of be- coming hard, when made into a thin paste with water, is owing to the anhydrous sulphate combining chemically with that liquid, and thus de- priving it of its fluidity. Sulphate of lime has hardly any taste. It is considerably more soluble than the sulphate of baryta or strontia, requiring for solution about 500 parts of cold, and 450 of boiling water. Owing to this circumstance, and to its existing so abundantly in the earth, it is frequently contained in spring water, to which it communicates the property called hardness. When freshly precipitated, it may be dissolved completely by dilute nitric acid. It is commonly believed to sustain a white heat without decomposition; but Dr. Thomson states, that it parts with some of its acid when heated to red- ess Sulphate of Magnesia.—This sulphate, generally known by the name of Epsom salt, is frequently contained in mineral springs. It may be made directly, by neutralizing dilute sulphuric acid with carbonate of magnesia ; suIPhai 439 but it is procured for the purposes of commerce by the action of dilute sulphuric acid on magnesian limestone, native carbonate of lime and magnesia. Sulphate of magnesia has a saline, bitter, and nauseous taste. It crys- tdlizes readily in small quadrangular prisms, which effloresce slightly in a dry air. It is obtained also in larger crystals, which are irregular six-sided prisms, terminated by six-sided summits. Its primary form is a right rhombic prism, the angles of wliich are 90° 30' and 89° 30'. (Brooke.) Its crystals are soluble in an equal weight of water at 60°, and in three-fourths of their weight of boiling water. They undergo the watery fusion when heated; and the anhydrous salt is deprived of a portion of its acid at a white heat. Sulphates of Alumina.—The tersulphate is prepared by saturating dilute sulphuric acid with hydrated alumina, and evaporating. It crystallizes with difficulty in thin flexible plates of a pearly lustre, which contain eighteen eq. ofwater, and require twice their weight ofwater for solution. Berzelius says it occurs native at Milo, in the Grecian Archipelago. It has an acid reaction. The hydrated disulphale is known to mineralogists under the name of aluminite, which occurs at Halle on the river Saal, and at Newhaven in Sussex; and Berzelius says the same compound falls when ammonia is added to a solution of the tersulphate. It is insoluble in water, and by heat is first rendered anhydrous, and then its acid is expelled, leaving pure alumina. The composition given in the table is from an analysis of alumi- nite from both its localities by Stromeyer. Sulphate of Protoxide of Manganese.—This salt is best obtained by dis- solving pure carbonate of manganese in moderately dilute sulphuric acid, and setting the solution aside to crystallize by spontaneous evaporation. The crystals are transparent, and of a slight rose tint, in taste resembling Glauber's salt, and occur in flat rhombic prisms. It is insoluble in alcohol, but dissolves in twice and a half its weight of cold water. If the heat is gradually applied, it may be increased to redness without expelling any of the acid. Sulphates of the Oxides of Iron—Sulphate of the protoxide, commonly called green vitriol, is formed by the action of dilute sulphuric acid on metallic iron (page 162), or by exposing protosulphuret of iron in fragments to the combined agency of air and moisture. The salt has a strong styptic, inky taste. Though neutral in composition, being composed of one equiva- lent of each element, it reddens the vegetable blue colours. It is insoluble in alcohol, but soluble in two parts of cold, and in three-fourths of its weight of boiling water. It occurs in right rhombic prisms, which are trans- parent, and of a pale green tint; but when its water of crystallization is expelled, it is of a dirty white colour. This salt is employed in the manu- facture of fuming sulphuric acid. (Page 194.) The tersulphate of the sesquioxide is formed by mixing with a solution of the protosulphate exactly half as much sulphuric acid as that salt contains, and adding to the mixture in a boiling state successive portions of nitric acid until nitrous acid fumes cease to appear. The solution is then evapora- ted to dryness to expel the excess of nitric acid, and the tersulphate remains as a white salt. After being strongly heated it dissolves slowly in water; but if evaporated at a gentle heat, it is deliquescent, and very soluble in water and alcohol, but insoluble in strong sulphuric acid. At a red heat it gives out all its acid, and sesquioxide of iron is left. Its solution in water has an orange colour, which is yellow when much diluted. The disulphate of the sesquioxide falls as a hydrate of an ochreous colour, when a solution of the protosulphate is kept in an open vessel. Sulphate of Protoxide of Zinc—This salt, frequently called white vitriol, is the residue of the process for forming hydrogen gas by the action of dilute sulphuric acid on metallic zinc; but it is also made, for the purposes of com- merce, by roasting native sulphuret of zinc. It crystallizes by spontaneous evaporation in transparent flattened four-sided prisms, referable to a right rhombic prism, and isomorphous with Epsom salt. The crystals dissolve in 440 SUPLHATES. two parts and a half of cold, and are still more soluble in boiling water. The taste of this salt is strongly styptic. It reddens vegetable blue colours, though in composition it is a strictly neutral salt. Sulphate of Protoxide of Nickel.—This salt, like the salts of nickel in general, is of a green colour, and crystallizes from its solution in pure water in right rhombic prisms exactly similar to the primary form of the sulphates of zinc and magnesia. If an excess of sulphuric acid is present, the crystals are square prisms, which, according to Messrs. R. Phillips and Cooper, con- tain rather less water and more acid than the preceding; though the differ- ence is not so great as to indicate a different atomic constitution. (Annals of Philosophy, xxii. 439. Dr. Thomson says he analyzed both kinds, and found their composition identical. It is soluble in about three times its weight of water at 60° F. Sulphate of Protoxide of Cobalt.—When protoxide of cobalt is digested in dilute sulphuric acid, a red solution is formed, which by evaporation de- posites crystals of the same colour. Mitscherlich has shown that the crys- tals are identical in composition with sulphate of protoxide of iron; and Mr. Brooke's measurements prove these salts to be isomorphous. (An. of Phil. N. S. vi. 120.) They are insoluble in alcohol, and dissolve in about 24 parts of cold water. Tersulphate of Sesquioxide of Chromium.—This salt may be formed by saturating dilute sulphuric acid with hydrated sesquioxide of chromium; but it has not been obtained in crystals. Sulphates of the Oxides of Copper.—Sulphate of the red oxide of copper has not been obtained in a separate state. The sulphate of the black, or protoxide, blue vitriol, employed by surgeons as an escharotic and astringent, may be prepared by roasting the native suphuret; but it is more generally made by directly dissolving the protoxide in dilute sulphuric acid, and crys- tallizing by evaporation. This salt forms regular crystals of a blue colour, reddens litmus paper, and is soluble in about four of cold, and in two parts of boiling water. It is isomorphous with sulphate of protoxide of man- ganese. When pure potassa is added to a solution of the sulphate of protoxide of copper in a quantity insufficient for separating the whole of the acid, a pale bluish-green precipitate, the disulphate, is thrown down. Sulphate of protoxide of copper and ammonia is generated by dropping pure ammonia into a solution of the sulphate, until the sub-salt at first thrown down is nearly all dissolved. It forms a dark blue solution, from which, when concentrated, crystals are deposited by the addition of dcohol. It may be formed also by rubbing briskly in a mortar two parts of crystal- lized sulphate of protoxide of copper with three parts of carbonate of am- monia, until the mixture acquires a uniform deep blue colour. Carbonic acid gas is disengaged with effervescence during the operation, and the mass becomes moist, owing to the water of the blue vitriol being set free. This compound, which is the ammoniaret of copper of the Pharmacopoeia, contains sulphuric acid, protoxide of copper, and ammonia; but its precise nature has not been determined in a satisfactory manner. It parts gradual- ly with ammonia by exposure to the air. Sulphates of the Oxides of Mercury.—When two parts of mercury are gently heated in three parts of strong sulphuric acid, so as to cause slow effervescence, a sulphate of the protoxide of mercury is generated. But if a strong heat is employed in such a manner as to excite brisk effervescence, and the mixture is brought to dryness, a bisulphate of the peroxide results, both being anhydrous. (Donovan in An. of Phil, xiv.) When this bisul- phate, which is the salt employed in making corrosive sublimate, is thrown into hot water, decomposition ensues, and a yellow sub-salt, formerly called turpeth mineral, subsides. This salt is said by Phillips to consist of three equivalents of acid and four of the peroxide. The1 hot water retains some of the bisulphate in solution, together with free sulphuric acid. Sulphate of Oxide of Silver.—As this salt is rather sparingly soluble in DOUBLE SULPHATES. water, it mav be formed by double decomposition from concentrated solu- tions of nitrate of oxide of silver and sulphate of soda. It may also be pro- cured by dissolving silver in sulphuric acid which contains about a tenth part of nitric acid, or by boiling silver in an equal weight of concentrated sulphuric acid. It requires about 80 times its weight of hot water for solu- tion, and the greater part is deposited in small needles on cooling. By slow evaporation from a solution containing a little nitric acid, Mitscherlich ob- tained it in the form of a rhombic octohedron, the angles of which are al- most identical with those of anhydrous sulphate of soda. Seleniate of oxide of silver is isomorphous with the sulphate. Sulphate of oxide of silver forms with ammonia a double salt, which crys- tdlizes in rectangular prisms, the solid angles and laterd edges of which are commonly replaced by tangent planes. It consists of one equivalent of oxide of silver, one of acid, and two of ammonia ; and it is formed by dis- solving sulphate of oxide of silver in a hot concentrated solution of ammonia, from which on cooling the crystals are deposited. This salt is isomorphous with a double chromate and seleniate of oxide of silver and ammonia, which have a similar constitution, and are formed in the same manner. (Mitscher- lich in An. de Ch. et de Ph. xxxviii. 62.) DOUBLE SULPHATES. Sulphate of Soda and Lime.—This compound, the Glauberite of minera- logists, occurs in very flat oblique rhombic prisms. Berthier prepared it by fusing together sulphate of soda with sulphate of lime in the ratio of their equivalents. Sulphate of soda, fused in similar proportions with the sul- phates of magnesia, baryta, and oxide of lead, gives analogous compounds. In these instances, however, the affinity is so feeble, that it is overcome by the mere action of water. (An. de Ch. et de Ph. xxxviii. 255.) Sulphate of Potassa and Magnesia.—On mixing solutions of these salts in atomic proportion, the double salt is formed either by spontaneous evapora- tion or on cooling from a hot rather concentrated solution. The crystals are prismatic, but of a complicated form, derived from an oblique rhombic prism. (Brooke.) A similar double salt, isomorphous with the preceding, is formed by substituting ammonia for potassa. Their composition is given in the table (page 436). Similar pairs of double salts may be formed with the protoxides of man- ganese, iron, zinc, cobalt ar*d nickel. These salts have the same form and composition as the corresponding salt of magnesia. Alum.—This well known substance is a double sulphate of potassa and alumina, which crystallizes with great facility from a solution containing its elements. It is prepared in this country from alum-slate, an argillaceous slaty rock highly charged with pyrites: on roasting this rock the sulphuret of iron is oxidized, the resulting sulphuric acid unites with alumina and potassa present in the slate, and the alum is dissolved out by water. By frequent crystallization it is purified from the oxide of iron, which obstinate- ly adheres to it. In Italy it is prepared from alum-stone, which occurs at Tolfa near Rome, and in most volcanic districts, being formed apparently by the action of sulphurous acid vapours on felspatic rocks. The materials of the alum exist in the stone ready formed; and they are extracted by gently heating the rock, exposing it for a time to the air, and lixiviation. The alum from this source has been long prized, in consequence of being quite free from iron. In both of these processes the alkali contained in the alum- rock is inadequate for uniting with the sulphate of alumina which is obtain- ed, and hence a salt of potassa must be added. Alum has a sweetish astringent taste, and reddens litmus paper. It is soluble in five parts of water at 60° * and in little more than its own weight * The solubility of alum in cold water is probably not so great as is here mentioned. Berzelius states it to be soluble in about eighteen parts of cold water, and Thenard in between fourteen and fifteen parts.—Ed. **-* DOUBLE SULPHATES. of boiling water. It crystallizes readily in octohedrons, or in segments of the octohedron, and the crystals contain almost 50 percent ofwater of crys- tallization. On being exposed to heat, they froth up remarkably, and part with all the water, forming anhydrous alum, the Alumen Ustum of the Pharmacopoeia. At a full red heat the alumina is deprived of its acid. Alum is employed in the formation of a spontaneously inflammable mix- ture long known under the name of Homberg's pyrophorus. It is made by mixing equal weights of alum and brown sugar, and stirring the mass over the fire in an iron or other convenient vessel till quite dry: it is then put into a glass tube or bottle, and heated to moderate redness without exposure to the air, until inflammable gas ceases to be evolved. A more convenient mix- ture is made with three parts of lampblack, four of burned alum, and eight of carbonate of potassa. When the pyrophorus is well made, it speedily be- comes hot on exposure to the air, takes fire, and burns like tinder; but the experiment frequently fails from the difficulty of regulating the temperature. From some recent experiments by Gay-Lussac, it appears that the essential ingredient of Homberg's pyrophorus is sulphuret of potassium in a state of minute division. The charcoal and alumina act only by being mechanically interposed between its particles; but when the mass once kindles, the char- coal takes fire and continues the combustion. He finds that an excellent pyrophorus is made by mixing 27 parts of sulphate of potassa with 15 parts of calcined lampblack, and heating the mixture to redness in a common Hessian crucible, of course excluding the air at the same time. (An. de Ch. et de Ph. xxxvii. 415.) Alum, having exactly the same form, composition, appearance, and taste as the salt just described, may be made with ammonia, the sulphate of which replaces sulphate of potassa. It is met with occasionally as a natural pro- duct, and may be prepared by evaporating a solution of sulphate of ammonia with tersulphate of alumina. A soda alum may also be prepared, similar in form and composition to the preceding alums, except that it contains twenty-six equivalents of water. (Berzelius.) This salt is disposed to einoresee in the air. Iron Alum.—By mixing sulphate of potassa with tersulphate of sesqui- oxide of iron, and crystallizing by spontaneous evaporation, crystals are ob- tained similar to common alum in form, colour, taste, and composition. This salt has often a pink tint, but is sometimes quite colourless. A similar double salt, quite colourless, may be made with ammonia instead of potassa. In both these alums the alumina is simply replaced by an equivalent quan- tity of peroxide of iron. Chrome Alums.—The tersulphate of sesquioxide of chromium forms with the sulphates of potassa and ammonia double salts which are exactly similar in form and composition to the preceding varieties of alum. They appear black by reflected, but ruby-red by transmitted light. Manganese Alum.—Mitscherlich obtained this salt by mixing a solution of tersulphate of sesquioxide of manganese with sulphate of potassa, and eva- porating to the consistence of syrup by a very gentle heat. On cooling, octohedral crystals of a brownish-violet colour were deposited, which were similar in composition to common alum. The tersulphate used for the pur- pose is prepared by macerating sesquioxide of manganese in very fine powder with strong sulphuric acid: it is made with difficulty, owing to the indisposition of that oxide to unite with acids, and to its ready conversion by heat into sulphate of the protoxide. Anhydrous Sulphates with Ammonia.—Rose has observed that some sul- phates possess the property of absorbing ammonia, and of forming with it definite compounds, which differ from sulphates of ammonia prepared in the moist way, both by containing no water of crystallization, and by the facility with which the alkali is again given out. They are formed by placing the anhydrous sulphate in a glass tube, and transmitting over it at common tem- peratures ammoniacal gas, well dried by fused potassa, as long as any increase of weight is observed: some sulphates absorb the gas very rapidly at first, SULPHITES. 443 and with disengagement of heat; but the absorption afterwards becomes slow, and requires a day or two in order to be complete. The salts most remark- able for this property are those which, in solution, are disposed to unite with ammonia.—Sulphate of protoxide of copper greedily absorbs ammonia, and acquires a deep blue colour similar to the ammoniaret of copper, prepared with moisture; but the former compound consists of two eq. of sulphate of protoxide of copper and five eq. of ammonia, while the latter contains one eq. of sulphate of copper, two of ammonia, and one equivalent ofwater. Sul- phate of protoxide of cobalt, as well as that of nickel, unites with three equiv- alents of ammonia; that of zinc with 2.5, and that of manganese with two equivalents. The latter when heated loses all its ammonia, and returns to its oridnal condition ; whereas most of the other ammoniaco-sulphates suffer partial decomposition at the same time. Sulphate of oxide of silver unites with one equivalent of ammonia; and a similar compound was prepared by C. G. Mitscherlich, but with two equivalents of ammonia. With most of the other anhydrous sulphates ammonia refuses to unite. On considering the nature of these compounds, one is at first disposed to associate them wilh double salts, supposing the add to be divided between the two bases. But this opinion is rendered unlikely by the large quantity of combined ammonia, by the facility with which the alkali is given off, and by the absence of water, so constantly present in other ammoniacal sulphates. Rose, with much plausibility, compares these compounds to hydrates: water acts as a feeble base to saline compounds, combining with some in one or more proportions, and not at all with others, differing greatly in the ratio in which it combines with different salts, and being abandoned with great fa- cility, often by mere exposure to the air. The same features characterize the combinations of ammonia with the anhydrous sulphates. (Pog. Annalen, xx. 149.) The sulphates are not the only salts which absorb ammonia. Rose found that the nitrate of oxide of silver unites with three equivalents of ammonia, and the gas, if freely supplied, is at first absorbed with such rapidity, and the corresponding increase of temperature is so great, that the salt enters into fusion. Heat expels the ammonia before the nitrate of oxide of silver is de- composed. A similar compound, but with less ammonia, was formed by C. Mitscherlich. SULPHITES. The salts of sulphurous acid have not hitherto been minutely examined. The sulphites of potassa, soda, and ammonia, which are made by neutralizing those alkalies with sulphurous acid, are soluble in water, but most of the other sulphites, so far as is known, are of sparing solubility. The sulphites of baryta, strontia, and lime are very insoluble; and consequently the solu- ble salts of these earths decompose the alkaline sulphites. The stronger acids, such as the sulphuric, hydrochloric, phosphoric, and arsenic acids, decompose all the sulphites with effervescence, owing to the escape of sulphurous acid, which may easily be recognized by its odour. Nitric acid, by yielding oxygen, converts the sulphites into sulphates. When the sulphites of the fixed alkalies and alkaline earths are strongly heated in close vessels, a sulphate is generated, and a portion of sulphur su- blimed. In open vessels at a high temperature they absorb oxygen, and are converted into sulphates; and a similar change takes place even in the cold, especially when they are in solution. Gay-Lussac has remarked, that a neutral sulphite always forms a neutral sulphate when its acid is oxidized; a fact from which it may be inferred, that neutral sulphites consist of one equivalent of the acid and one equivalent of the base. The hyposulphates and hyposulphites are of such little practical impor- tance, that it is unnecessary to describe individual salts: their general char- acter has been already given. (Page 197.) For a particular description of 444 NITRATES. the hyposulphates, the reader is referred to an essay by Dr. Heeren. (An. de Ch. et de Ph. xl. 30.) NITRATES. The nitrates may be prepared by the action of nitric acid on metals, on the salifiable bases themselves, or on carbonates. As nitric acid forms soluble salts with all alkaline bases, the acid of the nitrates cannot be precipitated by any reagent. They are readily distinguished from other salts, however, by the characters already described. (Page 184.) All the nitrates are decomposed without exception by a high temperature ; but the changes which ensue are modified by the nature of the oxide. Ni- trate of oxide of palladium is decomposed at such a moderate temperature, that a great part of the acid passes off unchanged. Nitrate of oxide of lead requires a red heat, by which it is resolved, as already mentioned (page 180), into oxygen and nitrous acid. In some instances the changes are more complicated. With nitre, for example, nitrite of potassa is at first genera- ted, with escape of oxygen gas: as the heat increases, the nitrous acid is resolved into binoxide of nitrogen and oxygen, the former of which remains in combination with potassa; the binoxide is then resolved into protoxide of nitrogen and oxygen, the former being retained by the alkali; and, lastly, nitrogen gas is disengaged, and peroxide of potassium remains. If the operation is performed in an earthen vessel, the peroxide will be more or less decomposed, in consequence of the affinity of the earthy substances for potassa. The preceding facts have been chiefly collected from the obser- vations of Phillips and Berzelius. The tendency of potassa and soda to unite with protoxide of nitrogen was first observed by Davy; and M. Hess has lately remarked that similar compounds are obtained with soda, baryta, and lime, as well as potassa, when their nitrates are heated, until the disen- gaged gas is found to extinguish a light. As the nitrates are easily decomposed by heat alone, they must necessari- ly suffer decomposition by the united agency of heat and combustible matter. The nitrates on this account are much employed as oxidizing agents, and frequently act with greater efficacy eyen than nitro-hydrochloric acid. Thus metallic titanium, which resists the action of these acids, combines with oxygen when heated with nitre. The efficiency of this salt, which is the nitrate usually employed for the purpose, depends not only on the affinity of the combustible for oxygen, but likewise on that of the oxidized body for potassa. The process for oxidizing substances by means of nitre is called deflagration, and is generally performed by mixing the inflammable body with an equal weight of the nitrate, and projecting the mixture in small portions at a time into a red-hot crucible. All the neutral nitrates of the fixed dkalies and alkaline earths, together with most of the neutral nitrates of the common metals, are composed of one equivalent of nitric acid, and one equivalent of a protoxide. Consequent- ly, the oxygen of the oxide and acid in all such sdts must be in the ratio of 1 to 5, the general formula being M-f N The only nitrates found native are those of potassa, soda, lime, and mag- nesia. The composition of the principal nitrates is exhibited in the following table.— Names. Base. Acid. Equiv. Formulae. Nitrate of potassa . . 47.15 1 eq.-f 54.15 1 eq.=101.3 K-fN. ________soda . . 31.3 1 eq.-f 54.15 1 eq.= 85.45 Na-fN. --------ammonia . 17.15 1 eq.-f 54.15 1 eq.= 71.3 H^N-fN. Do in prisms with 9 or 1 eq. of water = 80.3 Names. Nitrate of baryta . --------- strontia . NITRATES. Base. Acid. 76.7 1 eq.-f 54.15 1 eq 51.8 1 eq.-f 54-15 1 eq Do. in prisms with 45 or 5 eq. of water . . Nitrate of lime . . 28.5 1 eq.-f 54.15 1 eq. ----------magnesia . 20.7 1 eq.-f 54.15 1 eq.: ----------protox. copper 39.6 1 eq.-f 51.15 1 eq.; Do. in prisms with 63 or 7 eq. of water . — Nitrate of protox. lead . 111.6 1 eq.-f 54.15 1 eq.= Dinitrate of ditto . . 223.2 2 eq.-f 54.15 1 eq.= Nitrate of protox. mercury 210 1 eq.-f 54.15 1 eq.= Do. in crystals with 18 or 2 eq. of water . = Nitrate of perox. mercury 218 1 eq.-f 54.15 1 eq.= Dinitrate of ditto . 436 2 eq.-f 54.15 1 eq.= Nitrate of ox. silver . 116 1 eq.-f 54.15 1 eq.= Equiv. = 130.85 =105.95 =150-95 = 82.65 = 74.85 = 93.75 156.75 : 165.75 :277.35 264.15 282.15 272.15 490.15 170.15 FormukB. Ba -f N. Sr-fN. Ca-fN"! Mg-fN. Cu-fN. Pb-fN.' 2Pb-f*N! Hg-fN! Hg-fN." 2Hg-fN! Ag+N' Nitrate of Potassa.—This salt is generated spontaneously in the soil, and crystallizes upon its surface, in several parts of the world, and especially in the East Indies, whence the greater part of the nitre used in Britain is de- rived. In some parts of the continent, it is prepared artificially from a mix- ture of common mould or porous calcareous earth with animal and vegeta- ble remains containing nitrogen. When a heap of these materials, preserved moist and in a shady situation, is moderately exposed to the air, nitric acid is gradually generated, and unites with the potassa, lime, and magnesia, which are co'nmonly present in the mixture. On dissolving these salts in water, and precipitating the two earths by carbonate of potassa, a solution is formed, which yields crystals of nitre by evaporation. The nitric acid is probably generated under these circumstances by the nitrogen of the organic matters combining during putrefaction with oxygen of the atmosphere, a change which must be attributed to the affinity of oxygen for nitrogen, aided by that of nitric add for alkaline bases. The nitre made in France is often said to be formed by this process; but the greater part is certainly obtained by lixiviation from certain kinds of plaster of old houses, where nitrate of lime is gradually generated. Nitrate of potassa is a colourless salt, which crystallizes readily in six-sided prisms. Its taste is saline, accompanied with an impression of coolness. It requires for solution seven parts of water at 60°, and its own weight of boiling water. It contains no water of crystallization, but its crystals are never quite free from water lodged mechanically within them. At 616° It undergoes the igneous fusion, and like all the nitrates is decomposed by a red heat. Nitre is chiefly employed in chemistry as an oxidizing agent, and in the formation of nitric acid. Its chief use in the arts is in making gunpowder, which is a mixture of nitre, charcoal, and sulphur. In the East Indies it is 38 446 NITRATES. employed for the preparation of cooling mixtures :—an ounce of powdered nitre dissolved in five ounces of water reduces its temperature by fifteen de- grees. It possesses powerful antiseptic properties, and is, therefore, much employed in the preservation of meat and animal matters in general. Nitrate of Soda.—This salt is analogous in its chemical properties to the preceding compound. It sometimes crystallizes in oblique rhombic prisms; but it more commonly occurs as an obtuse rhombohedron, which is its prima- ry form. (Mr. Brooke.) It is plentifully found in the soil in some parts of India; and at Atacama in Peru it covers large districts, and occurs in im- mense quantity.. With charcoal and sulphur it forms a mixture which burns much slower than common gunpowder, and, therefore, cannot be substituted for nitre; but it may be advantageously used in the manufacture both of sulphuric and nitric acid. It is disposed to deliquesce in the air, and is soluble in twice its weight of cold water, and still more freely by the aid of heat. Nitrate of Ammonia.—Nitrate of ammonia may be formed by neutraliz- ing dilute nitric acid by carbonate of ammonia, and evaporating the solu- tion. This salt may be procured in three different states, which have been described by Sir H. Davy. (Researches concerning the Nitrous Oxide.) If the evaporation is conducted at a temperature not exceeding 100°, the salt is obtained in prismatic crystals which contain one equivalent of water. If the solution is evaporated at 212°, fibrous crystals are procured; and if the heat be gradually increased to 300°, it forms a brittle compact mass on cool- ing. The fibrous and compact varieties still contain water, the former 8.2 per cent., and the latter 5.7. All these varieties deliquesce in a moist air, and are very soluble in water. The change which nitrate of ammonia undergoes at a temperature vary- ing between 400° and 500° has already been explained. (Page 174.) When heated to 600°, it explodes with violence, being resolved into water, nitrous acid, binoxide of nitrogen, and nitrogen. The fibrous variety was found by Davy to yield the largest quantity of protoxide of nitrogen. From one pound of this salt he procured nearly three cubic feet of the gas. Nitrate of Baryta,—This salt is sometimes used as a reagent and for pre- paring pure baryta. It is easily prepared by digesting the native carbonate, reduced to powder, in nitric acid diluted with 8 or 10 times its weight of water. The salt crystallizes readily by evaporation in transparent anhy- drous octohedrons, and is very apt to decrepitate by heat unless previously reduced to powder. It requires 12 parts of water at 60° and 3 or 4 of boil- ing water for solution, but is insoluble in alcohol. It undergoes the igneous fusion in the fire before being decomposed. Nitrate of Strontia.—This salt may be made from strontianite in the same manner as the foregoing compound, to which it is exceedingly analogous. It commonly crystallizes in anhydrous octohedrons which undergo no change in a moderately dry atmosphere, and are insoluble in alcohol; but sometimes it contains 30 per cent ofwater of crystallization, and then assumes the form of a prism with ten sides and two summits. Nitrates of Lime and Magnesia.—These salts crystallize in hydrated prisms when their solutions are concentrated to the consistence of syrup, but the quantity of water which they contain is not ascertained. They deliquesce rapidly in the air, are very soluble in water, and are dissolved by alcohol, the nitrate of lime more freely than nitrate of magnesia. Nitrate of Protoxide of Copper.—This salt is prepared by the action of nitric acid on copper, ^age 176.) It crystallizes, though with some difficul- ty, in prisms of a deep blue colour, which are very soluble in water and al- cohol, and deliquesce on exposure to the air. The green insoluble subsalt, procured by exposing the neutral nitrate to a heat of 400°, or by dropping an alkali into a solution of that salt, the latter being in excess, is a trinitrate, consisting of three eq. of oxide of copper, one eq. of acid, and one eq. of water. When heated to redness it yields pure oxide of copper. Nitarte of Protoxide of Lead.—This salt is formed by digesting litharge NITRATES. in dilute nitric acid, and crystallizes readily in octohedrons, which are an- hydrous and almost always opaque. It has an acid reaction, but is neutral in composition. A dinitrate was formed by Berzelius by adding to a solution of the neutral nitrate, a quantity of pure ammonia insufficient for separating the whole of the acid. Nitrates of the Oxides of Mercury.—-The prolonitrate is conveniently form- ed by digesting mercury in nitric acid diluted with three or four parts of water, until the acid is saturated, and then allowing the solution to evaporate spontaneously in an open vessel. The solution always contains at first some nitrate of the peroxide, but if metallic mercury is left in the liquid, a pure protonitrate is gradually deposited. The salt thus formed has hitherto been regarded as the neutral protonitrate; but according to the analysis of M. C. Mitscherlich (Pog. Annalen, ix. 387), it is a subsalt, in which the protoxide and acid are in the ratio of 20S to 36. This result, however, requires con- firmation. The neutral protonitrate is said by C. Mitscherlich to be obtain- ed in crystals, by dissolving the former salt in pure water acidulated with nitric acid, and evaporating spontaneously without the contact of metallic mercury or uncombined oxide. These salts dissolve completely in water slightly acidulated with nitric acid, but in pure water a small quantity of a yellow subsalt is generated. When mercury is heated in an excess of strong nitric acid, it is dissolved witli brisk effervescence, owing to the escape of binoxide of nitrogen, and transparent prismatic crystds of the pernitrate are deposited as the solution cools. When put into hot water it is resolved into a soluble salt, the compo- sition of which is unknown, and into a yellow dinitrate of the peroxide. (An. de Ch. et de Phys. xix.) Nitrate of Oxide of Silver.—Silver is readily oxidized and dissolved by nitric acid diluted with two or three times its weight of water, forming a so- lution which yields transparent tabular crystals by evaporation. These crys- tals, which are anhydrous, undergo the igneous fusion at 426°, and yield a crystalline mass in cooling; but when the temperature reaches 600° or 700°, complete decomposition ensues, the acid being resolved into oxygen and nitrous acid, while metallic silver is left. When liquefied by heat, and re- ceived in small cylindrical moulds, it forms the lapis infemails or lunar caus- tic, employed by surgeons as a cautery. The nitric acid appears to be the agent which destroys the animal texture, and the black stain is owing to the separation of oxide of silver. It is sometimes employed for giving a black colour to the hair, and is the basis of the indelible ink for marking linen. The pure nitrate, whether fused or in crystals, is colourless and transpa- rent, and does not deliquesce by exposure to the air; but common lunar caustic is dark and opaque, and dissolves imperfectly in water, owing to some of the nitrate being decomposed during its preparation. It is impure also, always containing nitrate of protoxide of copper, and frequently traces of gold. The pure salt is soluble in its own weight of cold, and in half its weight of hot water. It dissolves also in four times its weight of alcohol. Its aqueous solution, if preserved in clean glass vessels, undergoes little or no change even in the direct solar rays; but when exposed to light, especial- ly to sunshine, in contact with paper, the skin, or any organic substance, a black stain is quickly produced, owing to decomposition of the salt and re- duction of its oxide to the metallic state. This change is so constant, that nitrate of oxide of silver constitutes an extremely delicate test of the presence of organic matter, and has been properly recommended as such by Dr. Davy. Its solution is always kept in the laboratory as a test for chlorine and hydrochloric acid. Nitrate of oxide of silver, even after fusion, reddens vegetable colouring matters; but it is quite neutral in composition. 448 NITRITES.--CHLORATES. NITRITES. Little is known with certainty concerning the compounds of nitrous acid with alkaline bases. Nitrite of potassa is formed by heating nitre to redness, and removing it from the fire before the decomposition is complete. On add- ing a strong acid to the product, red fumes of nitrous acid are disengaged, a character which is common to all the nitrites. The nitrite of soda, baryta, and strontia, may be obtained in the same manner, and doubtless several others. Two nitrites of oxide of lead have been described in the Annales de Chimie, lxxxiii. by Chevreul and Berzelius. It is possible, however, that these compounds arc hyponitrites. CHLORATES. The salts of chloric acid are very analogous to the nitrates. As the chlo- rates of the alkalies, alkaline earths, and most of the common metals are composed of one equivalent of chloric acid and one equivalent of a protoxide, M-f CI, it follows that the oxygen of the former to that of the latter is in the ratio of 1 to 5. The chlorates are decomposed by a red heat, nearly all of them being converted into metallic chlorides, with evolution of pure oxygen gas. They deflagrate with inflammable substances with greater violence than nitrates, yielding oxygen with such facility that an explosion is pro- duced by slight causes. Thus, a mixture of sulphur with three times its weight of chlorate of potassa explodes when struck between two hard surfaces, With charcoal and the sulphurets of arsenic and antimony, this salt forms similar explosive mixtures; and with phosphorus it detonates violently by percussion. One of the mixtures, employed in the percussion locks for guns, consists of sulphur and chlorate of potassa, with which a little charcoal or gunpowder is mixed; but as the use of these materials is found corrosive to the lock, fulminating mercury is now generally preferred. All the chlorates hitherto examined are soluble in water, excepting the chlorate of protoxide of mercury, wliich is of sparing solubility. These salts are distinguished by the action of strong hydrochloric and sulphuric acids, the former of which occasions the disengagement of chlorine and protoxide of chlorine, and the latter of peroxide of chlorine. None of the chlorates are found native, and the only ones that require par- ticular description are those of potassa and baryta. Chlorate of Potassa.—This salt, formerly called oxymuriate or hyperoxy- muriate of potash, is colourless, and crystallizes in four and six-sided scales of a pearly lustre. Its primary form is stated by Mr. Brooke to be an ob- lique rhombic prism. It is soluble in sixteen times its weight of water at 60°, and in two and a half of boiling water. It is quite anhydrous, and when exposed to a temperature of 400° or 500° undergoes the igneous fusion. On increasing the heat almost to redness, effervescence ensues, and pure oxygen gas is disengaged, phenomena which have been explained in the section on oxygen. It can bear a heat of 660° without decomposition. Chlorate of potassa is made by transmitting chlorine gas through a con- centrated solution of pure potassa, until the dkali is completely neutralized. The solution, which, after being boiled for a few minutes, contains nothing but chloride of potassium and chlorate of potassa (page 211), is gently evapo- rated till a pellicle forms upon its surface, and is then allowed to cool. The greater part of the chlorate crystallizes, while the chloride remains in solu- tion. The crystals, after being washed with cold water, may be purified by a second crystallization. Chlorate of Baryta is of interest, as being the compound employed in the formation of chloric acid; and the readiest mode of preparing it is by the process of Mr. Wheeler. On digesting for a few minutes a concentrated solution of chlorate of potassa with a slight excess of silicated hydrofluoric IODATES. 449 acid, the alkali is precipitated in the form of an insoluble double fluoride of silicium and potassium, while chloric acid remains in solution. The liquid after filtration is neutralized by carbonate of baryta, which throws down the excess of silicated hydrofluoric acid, and chlorate of baryta is left in solution. By evaporation it yields prismatic crystals, which require for solution 4 times their weight of cold, and a still smaller quantity of hot water. They are composed of 76.7 parts or one eq. of baryta, 75.42 or one eq. of chloric acid, and 9 or one eq. of water. Perchlorates.—The neutral protosalts of perchloric acid consist of ono equivalent of acid and base, as is expressed by the formula M-f CI. Most of these salts are deliquescent, very soluble in water, and soluble in alcohol; four only were found by Serullas to be not deliquescent,—the perchlorates of potassa, ammonia, protoxide of lead, and protoxide of mercury. When heat- ed to redness they yield oxygen gas and metallic chlorides; and they are distinguished from the chlorates by not acquiring a yellow tint on the addi- tion of hydrochloric acid. The perchlorate of potassa is prepared from the chlorate by the action of heat and sulphuric acid, as already mentioned. (Page 21S.) It is the most insoluble of the perchlorates, and on this account per- chloric acid precipitates potassa from its salts, being a test of about the same delicacy as tartaric acid. The other perchlorates are made by neutraliz- ing the base with perchloric acid. The solubility in alcohol of the per- chlorates of baryta, soda, and oxide of silver, is a property which the analy- tical chemist may avail himself of in andysis for the separation of potassa and soda from each other. IODATES. From the close analogy in the composition of chloric and iodic acids, it follows that the general character of the iodates must be similar to that of the chlorates. Thus in all neutral protiodates the oxygen contained in the oxide and acid is in the ratio of 1 to 5. They form deflagrating mix- tures with combustible matters; and on being heated to low redness, oxygen gas is disengaged and a metallic iodide remains. As the affinity of ir-'.ine for metals is less energetic than that of chlorine, many of the iodates part with iodine as well as oxygen when heated, especially if a high temperature is employed. The iodates are easily recognized by the facility with which their acid is decomposed by deoxidizing agents. Thus, the sulphurous, phosphorous, hydrochloric, and hydriodic acids, deprive iodic acid of its oxygen, and set iodine at liberty. Hydrosulphuric acid also decomposes the acid of these salts; and hence an iodate of potassa may be converted into the iodide of potassium by transmitting a current of hydrosulphuric acid gas through its solution. None of the iodates have been found native. They are all of very sparing solubility, or actually insoluble in water, excepting the iodates of the alkalies. Iodate of Potassa.—This salt may be procured by adding iodine to a con- centrated hot solution of pure potassa, until the alkali is completely neutral- ized. The liquid, which contains an iodate and iodide (page 227), is evaporat- ed to dryness by a gentle heat, and the residue, when cold, is treated by repeated portions of boiling alcohol. The iodate, which is insoluble in thai menstruum, is left, while the iodide of potassium is dissolved. A better process has been recommended by M. Henry, jun., founded on the property which iodide of potassium possesses, of absorbing oxygen while in the act of escape from decomposing chlorate of potassa. For this purpose iodide of potassium is fused in a capacious Hessian crucible, and when, after removal from the fire, it is yet semi-fluid, successive portions of pulverized chlorate of potassa are projected into it, stirring well after each addition. The materials froth up considerably, and when the action is over, a white, opaque, cellular mass remains, easily separable from the crucible: tepid water dissolves out 450 PHOSPHATES. the chloride of potassium, and leaves the iodate. Convenient proportions are one part of iodide of potassium and rather more than one and a half of chlorate of potassa. (Journ. de Pharmacie, July, 1832.) All the insoluble iodates may be procured from this salt by double decom- position. Thus iodate of baryta may be formed by mixing chloride of barium with a solution of iodate of potassa. A biniodate of potassa has been described by Serullas. It is formed by incompletely neutralizing a hot solution of chloride of iodine with potassa or its carbonate, and setting it aside to cool. A peculiar compound of chloride of potassium and biniodate of potassa falls; but on dissolving this substance, filtering and exposing the solution to a temperature of 77°, the biniodate is gradually deposited in right rhombic prisms terminated by dihedral summits, It is soluble in 75 times its weight of water at 59°. A teriodate may be formed by mixing a large excess of sulphuric acid with a moderately dilute solution of iodate of potassa. On evaporating at 77°, the teriodate is deposited in regular rhomboidal crystals, which require 25 times their weight of water at 60° for solution. Serullas states that the compound of chloride of potassium and biniodate of potassa, above mentioned, may be formed by the action of hydrochloric acid on Iodate of potassa. By spontaneous evaporation it is obtained, some- times in brilliant, transparent, elongated prisms, and at other times in hexa- gonal laminae; but generally it crystallizes in right quadrangular prisms with their lateral edges truncated, and terminated by four-sided summits. (An. de Ch. etdePh.xliii.113.) Bromates.—These compounds have may characters common with the chlorates and iodates; but hitherto they have been but partially examined. PHOSPHATES. In studying these salts, the reader must bear in mind that there are three isomeric modifications of the same acid, wliich have been described under the names of phosphoric, pyrophosphoric, and metaphosphoric acid (page 202); and, therefore, it will be necessary to have three corresponding families of salts, the phosphates, pyrophosphates, and metaphosphates. This distinction, and the other facts lately recorded by Mr. Graham, render it necessary either to change the nanies of the phosphates or to retain their old names in opposition to the principles of nomenclature. The most consistent con- duct will be to describe each salt under its scientific name, and add at the same time its ordinary one. An equivalent of each of the three acids, is a compound of 31.4 parts or two eq. of phosphorus-f 40 parts or five eq. of oxygen = 71.4, expressed by the formula P. To form a salt neutral in com- position, one equivalent of an alkaline base is requisite; and in the case of any protoxide, indicated by M, the general formula will be M -f P. If two equivalents of a protoxide are united with one of the acid, we have a disalt, 2M-f P; and if three eq. of a base combine with one eq. of the acid, it is a trisalt, 3M-f P. It seems also that water plays the part of an alkaline base towards each of the three acids, cither alone or conjointly with another base: the salts with such compound bases can scarcely be viewed in the light of double salts (page 434); since the two bases act together as one electro-posi- tive element. All the protophosphates which are neutral in composition, are soluble in water, and redden litmus paper; whence they are commonly called super- phosphates. The triphosphates, except those of the pure alkalies, are either sparingly soluble or insoluble in water; but they are all dissolved by dilute nitric or phosphoric acid, being converted into the soluble phosphates. All the triphosphates with fixed and strong bases bear a red heat without change; PHOSPHATES. 451 but the phosphates and diphosphates, to judge from experiments on the soda sdts, are converted into metaphosphates and pyrophosphates. Most of the phosphates of the second class of metals are resolved into phosphurets by the conioint agency of heat and combustible matter. The phosphates of the alkalies are only partially decomposed under these circumstances, and the phosphates of baryta, strontia, and lime undergo no change. The presence of a soluble phosphate may be distinguished by the tests already mentioned (page 203) for phosphoric acid. The insolub e phosphates are decomposed when boiled with a strong solution of carbonate of potassa or soda; the acid uniting with the alkali so as to form a soluble phosphate: the earthy phosphates, indeed, are decomposed with difficulty, requiring con- tinued ebullition, and should preferably be fused with an alkaline carbonate, like an insoluble sulphate. . Several phosphates are met with in nature, such as those ot lime, alumina, and the oxides of manganese, iron, uranium, copper, and lead. The composition of the principal phosphates is given in the following table:— Names. Base. Acid. Equiv. Formula;. Triphosphate of soda . 93.9 3eq. _f 71.4 1 eq.= 165.3 3Na + P. Do. in crystals with 216 or 24 eq. of water . =381.3 Triphosph.sodaand? Soda 62.6 2 e9-£_f 71 4 i eq =143 Na3H + P. basic water . 5 Water 9 1 eq. 5 ~ - Do. in crystals with 216 or 24 eq. of water . =359 Do. 126 or 14eq. ofwater . =269 Acid triphos. soda ) Soda 31.3 * ecl-( 171 4 1 eq =120.7 NaH2+P. and basic water (Water 18 2 eq. { "*" " H — Do. in crystals with 18 or 2 eq. of water . =138.7 Triphosphate of potassa 141.45 3 eq. +71.4 leq.=212.85 3K+R Diphosphate do 94.3 2eq. + 71.4 1 eq. =165.7 2K+P. Phosphate do 47.15 leq. -f 71.4 1 eq. =118.55 K+_P. Do. in crystals with 18 or 2 eq. of water . . =136.55 Phosphate of soda 1 Soda 31.3 1 eq.? n 4 j eq.=1i9.85NaH3N+P. and ammonia $ Ammon. 17.15 leq. 3 ' * — Do. in crystals wilh 90 or 10 eq. of water . =209.85 Diphosphate of ammonia 34.3 2eq. +71.4 1 eq.= 105.7 2H*N + P. Do. in crystals with 27 or 3 eq. ofwater . =132.7 Phosphate of ammonia 17.15 leq. +71.4 1 eq.= 88.55 H^N+P. Do. in crystals with 27 or 3 eq. ofwater . =115.55 Bone phosphate of lime 228 8eq. +214.2 3 eq. =442.2 8Ca + 3J\ Triphosphate do. . 85.5 3eq. + 71.4 1 eq.= 156.9 3Ca + P. Diphosphate do. . 57 2 eq. + 71.4 1 eq=128.4 2Ca + R Phosphate do. . 28.5 leq. + 71.4 1 eq.= 99.9 Ca + P. 452 PHOSPHATES. The triphosphate of baryta, strontia, protoxides of manganese, iron, copper, lead, silver, &c. precisely correspond to the triphosphate of lime, simply sub- stituting three equivalents of those oxides. These oxides in like manner form soluble phosphates analogous in composition to that of lime. Triphosphate of Soda.—This salt, lately described by Mr. Graham as the subsesquiphosphate, is made by adding pure soda to a solution of the suc- ceeding compound until the liquid feels soapy to the fingers, an excess of soda not being injurious. The liquid is then evaporated until a pelicle ap- pears, and the crystals which form on cooling are quickly redissolved in water and recrystallized. Though the crystals do not change in the air, the solution absorbs carbonic acid, and the resulting carbonate of soda ad- heres to the triphosphate. This salt crystallizes in colourless six-sided slender prisms, which have a strong alkaline taste and reaction, require 5 times their weight of water at 60°, and still less of hot water, for solution, and at 170° fuse in their water of crystallization. They may be exposed to a red heat without losing the characters of a phosphate. The feeblest acids deprive the salt of one-third of its soda. When this salt is mixed in solution with nitrate of oxide of silver in ex- cess, there is an exact interchange of elements, such that 1 eq. triphosphate of soda 3Na + P ^ 1 eq. triphosphate of silver 3Ag+P and 3 eq. nitrate of silver 3(Ag+N) ^ and 3 eq. nitrate of soda 3(Na+N). The resulting solution is, therefore, quite neutral. The triphosphate of oxide of lead, and other insoluble triphosphates, may be prepared in like manner. Triphosphate of Soda and Basic Water.—This salt is the most common of the phosphates, being manufactured on a large scale by neutralizing with carbonate of soda the acid phosphate of hme procured by the action of sul- phuric acid on burned bones (page 198). It is generally described as the neutral phosphate of soda, and for distinction's sake is sometimes termed rhombic phosphate, from its crystals having the form of oblique rhombic prisms. This salt crystallizes best out of an alkaline solution; but however pre- pared, it is always alkaline to test paper, and requires a considerable quan- tity of acid before losing its alkalinity. The crystals effloresce on ex- posure to the air, and require four times their weight of cold, and twice their weight of hot water for solution. It often contains traces of sulphuric acid, from which it may be purified by repeated solution and crystallization. When mixed with nitrate of oxide of silver, the interchange of elements is such that 1 eq. rhombic phosphate Na2H + P 1 eq. triphosp. of silver 3Ag+P. and 3 eq. nitrate of silver 3( Ag + N.) ^ and 2 eq. nitrate of soda 2(Na + N). The yellow triphosphate of oxide of silver falls exactly as with the former salt, but one equivalent of nitric acid is left free in the solution. When a solution of the rhombic phosphate is evaporated at a temperature of 90°, it crystallizes with 14 instead of 24 equivalents of water, and the crystals differ, as might be expected, from the other salt in figure, and are permanent in the air. Both salts lose their basic water at a red heat, and are converted into a pyrophosphate. Acid Triphosphate of Soda and Water.—This salt, commonly called biphos- phate of soda from its acid reaction, may be formed by adding phosphoric acid to a solution of carbonate of soda, or to either of the preceding phosphates, until itceascs to give a precipitate with chloride of barium. Being very soluble rttWWTHATES. 453 in water, the solution must be concentrated in order that it may crystallize. This salt is capable of yielding two different kinds of crystals without vary- ing its composition. The more unusual form, isomorphous with binarseniate of soda, is a right rhombic prism, the smaller lateral edge of which is 78° 30', terminated by pyramidal planes. The primary form of its ordinary crys- tals is a right rhombic prism, the larger angle of which is 93° 54'. The crystds of this salt consist, as stated at page 451, of NaH-T+2H, When heated to 212° the water of crystallization is expelled, and the anhy- drous salt remains, still yielding a yellow precipitate with silver when neutral- ized by ammonia; but if exposed to a heat of 400°, it loses half its basic water, being reduced to NaHP, and has the character of dipyrophosphateof soda. At a red heat it is converted into metaphosphate of soda. Triphosphate of Potassa.—Mr. Graham formed this salt by adding caustic potassa in excess to a solution of phosphoric acid, as well as by fusing phos- phoric acid with a slight excess of carbonate of potassa. He obtained it in acicular crystals, which were very soluble in water but not deliquescent Diphosphate of Potassa.—This salt may be prepared by neutralizing the superphosphate of lime from bones with carbonate of potassa. It is deli- quescent, and has not been obtained in regular crystals. Phosphate of Potassa may be formed by adding phosphoric acid to carbo- nate of potassa until the liquid ceases to give a precipitate with chloride of barium, and setting it aside to crystallize. The primary form of the crys- tals is an octohedron with a square base; but they usually occur in square prisms terminated by the planes of their primary form. They are acid to test paper. When this compound is neutralized by carbonate of soda, and the solution set to crystallize, a phosphate of soda and potassa is deposited in crystals, the primary form of which is an oblique rhombic prism, which frequently occurs without any modification. Phosphate of Soda and Ammonia.—This salt is easily prepared by mixing together one equivalent of hydrochlorate of ammonia and two equivalents of the rhombic phosphate of soda, each being previously dissolved in a small quantity of boiling water. As the liquid cools, prismatic crystals of the double phosphate are deposited, while chloride of sodium remains in solution. Their primary form is an oblique rhombic prism. This salt has been long known by the name of microcosmic salt, and is much employed as a flux in experiments with the blowpipe. When heated it parts with its water and ammonia, and a very fusible metaphosphate of soda remains. Diphosphate of Ammonia.—This salt is formed by adding ammonia to con- centrated phosphoric acid until a precipitate appears. On applying heat, the precipitate is dissolved, and on abandoning the solution to itself, the neu- tral salt crystallizes. The primary form of the crystals is an oblique rhom- bic prism, the smaller angle of which is 84° 30'. They often occur in rhom- bic prisms with dihedral summits. (Mitscherlich.) The phosphate is made in the same manner as the phosphate of potassa. The crystals are less soluble than the diphosphate, and undergo no change on exposure to the air. Their primary form is an octohedron with a 6quare base; but the right square prism, terminated by the faces of the primary form, is the most frequent. Phosphates of Lime.—The peculiar compound called the bone phosphate, exists in bones after calcination, and falls as a gelatinous precipitate on pour- ing chloride of calcium into a solution of the rhombic phosphate of soda, or on adding ammonia to a solution of any phosphate of lime in acids. Triphosphate of Lime cannot be formed by precipitation, but occurs in hexagonal prisms in the mineral called apatite. Diphosphate of Lime, commonly called neutral phosphate, falls as a granu- lar precipitate, consisting of fine crystalline particles, when the rhombic 454 PHOSPHATES. phosphate of soda is added in solution drop by drop to chloride of calcium in excess. The residual liquid reddens litmus, owing to a small quantity of triphosphate of lime being generated. Phosphate of Lime, called the biphosphate from its acid reaction, is formed by dissolving either of the preceding salts in a slight excess of phosphoric acid. The compound is deliquescent, very soluble, and crystallizes with great difficulty. It exists in the urine. The solution formed by the action of sulphuric acid on bones is probably a compound of lime with two or more equivalents of phosphoric acid, being really a superphosphate. Phosphates of Magnesia.—The diphosphate, 2Mg+P, is formed by mix- ing hot saturated solutions of the rhombic phosphate of soda and sulphate of magnesia, and separates on cooling in small crystals which contain four- teen equivalents ofwater to one of the anhydrous salt. The triphosphate is principally formed when the solutions are intermixed in the cold. These salts have been but little examined. The phosphate of ammonia and magnesia subsides as a pulverulent granu- lar precipitate from neutral or alkaline solutions containing phosphoric acid, ammonia, and magnesia. It is readily dissolved by acids, and is sparingly soluble in pure water, especially when carbonic acid is present; but it is in- soluble in a solution of most neutral salts, such as hydrochlorate of ammo- nia. It constitutes one variety of urinary concretions. According to Ber- zelius it consists of Phosphoric acid . . 71.4 1 eq. P. Magnesia . . . 41.4 2 eq. 2Mg. Ammonia . . . 34.3 2 eq. 2(3H+N). Water .... 90 10 eq. 10H. The mode in which these elements are arranged is unknown. When heat- ed to redness it loses its water and ammonia, and the residue is diphosphate of magnesia, which contains 36.67 per cent, of pure magnesia. At a strong red heat it fuses, and appears when cold as a white enamel. When the materials for forming the preceding salt are mixed while hot, small acicular crystals subside on cooling, which are said by Berzelius to contain less of the two bases than the other salt. Phosphates of Protoxide of Lead.—The triphosphate is precipitated when acetate of oxide of lead is mixed with a solution of the rhombic phosphate of soda, acetic acid being set free. The diphosphate is best formed by adding the rhombic phosphate of soda gradually to a hot solution of chloride of lead. The nitrate should not be used for the purpose, as it combines with the precipitate. Both these phosphates are white, and are frequently formed at the same time. The diphosphate fuses readily into a yellow bead, which in cooling acquires crystalline facettes. Triphosphate of Oxide of Silver.—This compound subsides, of a character- istic yellow colour (page 203), when the rhombic phosphate of soda is mixed in solution with nitrate of oxide of silver, nitric acid being set free at the same time. It is apt to retain some of the nitrate in combination. This salt is very soluble in nitric and phosphoric acid, forming the soluble phosphate, and in ammonia. By exposure to light it is speedily blackened; but when protected from this agent, it yields on drying an anhydrous yellow powder, which has a specific gravity of 7.321 (Stromeyer). Its colour changes on the application of heat to a reddish-brown, but its original tint returns on cooling. It bears a red heat without fusion: at a white heat it fuses, and if kept for some time in a fused state a portion of pyrophosphate is generated. PYROPHOSPHATES. PYROPHOSPHATES. The discovery of these salts by Dr. Clark has also been mentioned (page 203). That modification of phosphoric acid, termed pyrophosphoric acid, is procured by forcing with the aid of heat phosphoric acid to combine with two equivalents either of water or some fixed base. The only pyrophos- phates which have as yet been studied are those of soda and oxide of silver. These salts are thus constituted:— Names. Base. Acid. Equiv. Formulae. Dipyrophosphate of soda . . 62.6 2eq. + 71.4 1 eq.=134 2Na+P. Do. in crystals with 90 or 10 eq. of water . =224 Acid dipyrophos. soda ) Soda 31.3 1 eq. -j 4 j _m7 NaH+P. and basic water \ V\ ater 9 leq" M ■ — Pyrophosphate of soda . 31.3 1 eq. 4 71.4 1 eq.= 102.7 Na + P. Dipyrophos. oxide of silver . 232 2 eq. + 71.4 1 eq.=303.4 2Ag+P. Dipyrophosphate of Soda.—This is the compound first prepared by Dr. Clark from the rhombic phosphate (page 203), by expelling its basic water. When the residual mass is dissolved in water and set to evaporate, crystals are ob- tained, having the outline of an irregdar six-sided prism, derived from a rhombic prism. These crystals are permanent in the air, much less soluble in water than the original rhombic phosphate, and quite neutral to test paper. Ignited with carbonate of soda, a phosphate is reproduced, because the acid is forced to unite with three equivalents of a base. Dipyrophosphate of soda is permanent both in crystals and in solution in the cold; but by long boiling, or quickly when boiled with an acid, a phos- phate is reproduced. With a salt of lead it yields a white dipyrophosphate of oxide of lead; and on washing the precipitate and decomposing by hydro- sulphuric acid gas, a solution of pyrophosphoric acid is obtained, which again forms dipyrophosphate of soda when neutralized with soda. The oxides of most metals of the second class yield with pyrophosphoric acid insoluble or sparingly soluble salts, which may be prepared by double decomposition with dipyrophosphate of soda. It should be held in view, how- ever, as Stromeyer has remarked, that most of these salts are more or less soluble in an excess of dipyrophosphate of soda; and that some of them, such as the dipyrophosphate of the oxides of lead, copper, nickel, cobalt, uranium, bismuth, manganese, and mercury, are dissolved by it with great facility. Acid Dipyrophosphate of Soda and Water.—This salt is formed by expo- sing, as stated at page 453, the acid triphosphate to a heat of 400°, when it loses one half of its basic water, and acquires the character of a pyrophosphate. This salt dissolves readily in water, has an acid reaction, and has not been obtained in crystals. Pyrophosphate of Soda.—When the preceding salt, NaH+P, is heated to 600° or a little higher, it loses its basic water, and yet the acid does not lose the character of pyrophosphoric acid. It is left, therefore, as a simple pyrophosphate of soda, Na+P. On adding water, part of it dissolves, and part is left as an insoluble white powder. The solution is quite neutral to test paper, but on adding nitrate of oxide of silver, the dipyrophosphate of that oxide falls, and free nitric acid remains in solution. The soluble and insduble pyrophosphate of soda appear identical in composition; and the 456 METAPHOSPHATES__ARSENIATES. former at a heat just short of redness may be wholly converted into the latter. Dipyrophosphate of Oxide of Silver.—This salt is readily formed by double decomposition with dipyrophosphate" of soda and nitrate of oxide of silver, the residual liquid being quite neutral to test paper. It falls as a snow-white granular precipitate, which fuses readily at a heat short of incandescence into a dark brown liquid, which becomes a crystalline enamel on cooling. METAPHOSPHATES. The only metaphosphates which have yet been examined, are those of soda, baryta, and oxide of silver, which are thus constituted:— Names. Base. Acid. Equiv. Formula? phosphate of soda . . 31.3 leq.+ 71.4 leq.=102.7 Na+P.' Do. baryta . . 76.7 leq.+71.4 1 eq. =148.1 Ba+P.' Do. ox. silver 116 leq+71.4 1 eq. =187.4 Ag+P.' Submetaphos. do. 348 3eq.+142.8 2 eq =490.8 3Ag+2P. Metaphosphate of Soda.—When the pyrophosphate or acid dipyrophosphate of soda is heated to low redness, it fuses, and on cooling becomes a trans- parent glass, which deliquesces in a damp air, and is very soluble. The solution has a feeble acid reaction. When mixed with nitrate of oxide of silver, the metaphosphate of that oxide falls in gelatinous flakes, wholly un- like the pyrophosphate, and aggregates together as a soft solid when heated to near 212°. The metaphosphate of soda does not change by keeping, and has not hitherto been made to crystallize. When its solution is evaporated, and kept for some time at 400°, it is reconverted into the acid dipyrophos- phate of soda and basic water. All the preceding facts are drawn from Mr. Graham's essay. (Phil. Trans. 1833, Part ii.) Metaphosphate of Baryta falls in gelatinous flakes on adding metaphos- phate of soda to a solution of chloride of barium, the latter being in excess as the soda salt dissolves the precipitate. By long continued boiling, meta- phosphate of baryta is at length dissolved, and at the same time converted into a phosphate. The metaphosphate of silver is obtained by precipitation, as above stated. When put, while moist, into boiling water, part of its acid is removed, and the submetaphosphate is generated. ARSENIATES. Arsenic acid resembles the phosphoric in composition and in many of its properties, and all the protarseniates of neutral composition are represented by the formula M+As, the oxygen of the oxide and acid being as 1 to 5. These salts, like the phosphates, are soluble in water and redden litmus, whence they are commonly considered as bisalts. The diarseniates, 2M + As, in which the oxygen of the base and acid is as 2 to 5, are usually termed neutral arseniates. Arsenic acid has a strong tendency to the formation of triarseniatcs. Both these series of subsalts, except those with the alkalies, are of sparing solubility in water; but they are dissolved by phosphoric or nitric acid, as well as most acids which do not precipitate the base of the salt. ARSENIATES. Many of the arseniates bear a red heat without decomposition, or being otherwise modified in their characters; but they are all decomposed when heated to redness along with charcoal, metallic arsenic being set a hberty. The arseniates of the fixed dkalies and alkaline earths W""°*'**™S temperature for reduction; while the arseniates of the second class ofmeta s, as of lead and copper, are easily reduced in a glass tube by means of a sp. nt- Lmp wfthout dangVof melting the glass. Of all the arseniates that of oxide of lead is the most insoluble. ■„:\x~a ;« th<> The soluble arseniates are easily recognized by the tests describedin the section on arsenic (page 357); and the insoluble plates, when bo led in a strong solution of the fixed alkdine carbonates, a« deprived of t'«» ™£ which may then be detected in the usual manner. The free alkali, how ever, should first be exactly neutralized by pure nitric acid. The arseniates of lime, and of the oxides of nickel, cobalt, copper, and lead, are natural productions. . . -,,,•,.,. The composition of the principal arseniates is contained in the following tables- Names. Base. Acid. Equiv. Formulae. Triarseniate of soda 93.9 3 eq. + 115.4 1 eq.=209.3 3Na+ As. Do. in crystals with 216 or 24 eq. of water =42o.3 Triarsen. soda ) soda and basic water ^ water Do. in crystals with Do. in crystals with ?ecl-+115.4 1 eq.=187 NaSH+As. 1 eq. ' — 62.6 9 leq. 216 or 24 eq. of water =40 126 or 14 eq. ofwater =313 Acid triarsen. soda ? soda and basic water y water 18 2eq. Do. in crystds with 18 or 2 eq. of water =183.7 Triarseniate of potassa 3P ie1- + 115.41eq.= 164.7 NaH3+As. 18 2eq.~ ^ — 18 or 2 eq. of water =183.7 141.45 3eq. +115.4 leq. =256.85 3K+As. Diarseniate do. 94.3 2eq. +115.4leq. =209.7 2K+As. Arseniate of potassa . 47.15 1 eq+115.4 1 eq.= 162.55 K+ As. Do. in crystals with 18 or 2 eq. ofwater =180.55 Diarseniate of ammonia 34.3 2 eq. +115.4 leq.= 149.7 2H3N+As. Do. in crystals with 27 or 3 eq. ofwater =176.7 Arseniate of ammonia . 17.15 1 eq. + l 15.4 leq.= 132.55 H^N + As^ Do. in crystals with 27 or 3eq. ofwater =159.55 Triarseniate of baryta . 230.1 3eq. + 115.4 1 eq.=345.5 3Ba + As. Diarseniate do. 153.4 2eq. +115.4 leq. =268.8 2Ba + As. Arseniate do. . 76.7 1 eq. +115.4 leq. =192.1 Ba + As. Triarseniate of lime . 85.5 3 eq.+ 115.4 1 eq.=200.9 3Ca + As. Diarseniate do. . . 57 2 eq+115.4 leq.= 172 4 2Ca+As. Arseniate do. . . 28.5 1 eq. +115.41 eq. =143.9 Ca + As. 39 458 ARSENIATES. Names. Base. Acid. Equiv. Formula. Triarseniate of protox. lead 334.8 3 eq. +115.4 1 eq. =450.2 SPb+Ajs. Diarseniate do. . 223.2 2eq.+115.4 leq.=338.6 2Pb+As. Triarseniate of ox. silver . 348 3eq.+ 115.4 leq.= 463.4 3Ag+As. Arseniates of Soda.—The triarseniate is made in the same manner as tri- phosphate of soda, with which it is isomorphous. At 60°, 100 parts ofwater dissolve 28 of the crystals, and still more by the aid of heat At 186° they fuse in their water of crystallization. The triarseniate of soda and basic water corresponds precisely in form and constitution with the corresponding phosphate, and like it parts with its last equivalent of water at a red heat; but does not, on losing it, receive any change in its characters. It is efflorescent and alkaline to test paper, and crystallizes best out of an alkaline solution. It is prepared by adding soda or its carbonate in slight excess to a solution of arsenic acid. The salt with fourteen equivalents of water coincides with the corresponding phosphate. The acid triarseniate of soda and basic water is prepared like the corre- sponding phosphate. The same observation applies to the arseniates of potassa and ammonia, each having its isomorphous phosphate. The triarseniate of potassa crys- tallizes in needles and with difficulty, like the corresponding triphosphate. The arseniate of potassa may be formed by heating nitre to redness mixed with an equal weight of arsenious acid. The double arseniate of potassa and soda agrees in form and compo^ sition with the phosphate of those bases. Arseniates of Baryta.—The triarseniate is best prepared by gradually adding in solution triarseniate of soda to chloride of barium in excess, and falls as a pulverulent heavy precipitate, which is apt to contain a little diar- seniate of baryta as well as the soda salt, and should, therefore, be well wash- ed with boiling water. On adding chloride of barium to an excess of triarse- niate of soda, the latter salt always falls with the precipitate. To prepare the diarseniate a solution of the rhombic triarseniate of soda is added drop by drop to chloride of barium in solution, when the diarseniate of baryta soon appears in white crystalline scales, which contain four equiva- lents of water. On reversing the process by adding chloride of barium to the arseniate, the precipitate is a mixture of the triarseniate and diarseniate of baryta. By the continued action of hot water on the diarseniate, it is partly changed into the soluble arseniate and insoluble triarseniate. The soluble arseniate is obtained by dissolving either of the two former salts, in a moist state, by dilute arsenic acid. Arseniates of Lime.—The three salts analogous to those of baryta are ob- tained by precisely similar processes. The diarseniate occurs in silky acicu- lar crystals as a rare mineral named pharmacolite, which contains six equiva- lents of water. Arseniates of Protoxide of Lead.—The triarseniate is formed by adding in solution acetate of oxide of lead gradually to an excess of triarseniate of soda. The same salt falls when acetate of oxide of lead and the rhombic triarse- niate of soda are intermixed, acetic acid being set free. It is a white very insoluble powder, which at a low red heat acquires a yellow tint, which it loses again on cooling. The diarseniate may be made by a similar process as for forming the di- phosphate, and is a white insoluble, easily fusible powder. Triarseniate of Oxide of Silver.—This salt falls as a briek-red powder when nitrate of oxide of silver is mixed in solution with triarseniate of soda or the rhombic triarseniate, in the latter case nitric acid being set free. It ARSENITES—CHROMATES. 459 is apt to retain some of the nitrate, which cannot be removed by washing; a property which the yellow phosphate of oxide of silver also possesses. ARSENITES. These salts have as yet been but little examined. The arsenites of potassa, soda, and ammonia may be prepared by acting with those alkalies on arsenious acid: they are very soluble in water, have an alkaline reaction, and have not been obtained in regular crystals. Most of the other arsenites are insoluble, or sparingly soluble, in pure water; but they are dissolved by an excess of their own acid, with great facility by nitric acid, and by most other acids with which their bases do not form insoluble compounds. The insoluble arsenites are easily formed by double decomposition. All the arsenites are decomposed when heated in close vessels, the arseni- ous acid being either dissipated in vapour, or converted, with disengage- ment of some metallic arsenic, into arseniates. Heated with charcoal or black flux, the acid is reduced with facility. (Page 357.) The soluble arsenites, if quite neutrd, are characterized by forming a yel- low arsenite of oxide of silver when mixed with the nitrate of that base, and a green arsenite of protoxide of copper, Scheele's green, with sulphate of that oxide. When acidulated with acetic or hydrochloric acid, hydrosulphuric acid causes the formation of orpiment The insoluble arsenites are all de- composed when boiled in a solution of carbonate of potassa or soda. The arsenite of potassa is the active principle of Fowler's arsenical solu- tion. CHROMATES. The salts of chromic acid are mostly either of a yellow or red colour, the latter tint predominating whenever the acid is in excess. The chromates of oxides of the second class of metals are decomposed by a strong red heat, by which the acid is resolved into the green oxide of chromium and oxygen gas; but the chromates of the fixed alkalies sustain a very high temperature without decomposition. They are all decomposed without exception by the united agency of heat and combustible matter. The neutral chromates of protoxides are similar in constitution to the sulphates, being formed of one equivalent of the base and one of chromic acid, the formula being M + Cr. The chromates are in general sufficiently distinguished by their colour. They may be known chemically by the following character:—On boiling a chromate in hydrochloric acid mixed with alcohol, the chromic acid is at first set free, and is then decomposed, a green solution of the chloride of chromium being generated. The only native chromate hitherto discovered is the red dichromate of protoxide of lead from Siberia, in the examination of which Vauquelin made the discovery of chromium. Chromates of Potassa.—The neutral chromate from which all the com- pounds of chromium are directly or indirectly prepared, is made by heating to redness the native oxide of chromium and iron, commonly called chromate of iron, with nitrate of potassa, when chromic acid is generated, and unites with the alkali of the nitre. The object to be held in view is to employ so small a proportion of nitre, that the whole of the alkali may combine with chromic acid, and constitute a neutral chromate, which is easily obtained pure by solution in water and crystallization. For this purpose the chromate of iron is mixed with about a fifth of its weight of nitre, and exposed to a strong heat for a considerable time ; and the process is repeated with those portions of the ore which are not attacked in the first operation. It is de- posited from its solution in small prismatic anhydrous crystals of a lemon- yellow colour, the primary form of which, according to Mr. Brooke, is a right rhombic prism. Chromate of potassa has a cool, bitter, and disagreeable taste. It is so- luble to great extent in boiling water, and in twice its weight of that liquid 460 BORATES. at 60°; but it is insoluble in alcohol. It has an alkaline reaction, and on this account M. Tassaert* regards it as a subsalt; but Dr. Thomson has proved that it is neutral in composition, consisting of 52 parts or one equivalent of chromic acid, and 47.15 parts or one equivalent of potassa. + Bichromate of potassa, which is made in large quantity at Glasgow for dyeing, is prepared by acidulating the neutral chromate with sulphuric or still better with acetic acid, and allowing the solution to crystallize by sponta- neous evaporation. When slowly formed it is deposited in four-sided tabular crystals, the primary form of which is an oblique rhombic prism. They have an exceedingly rich red colour, are anhydrous, and consist of one equivalent of the alkali, and two equivalents of chromic acid. (Thomson.) They are soluble in about ten times their weight of water at 60°, and the solution reddens litmus paper. Tho insoluble salts of chromic acid, such as the chromates of baryta and oxides of zinc, lead, mercury, and silver, are prepared by mixing the soluble salts of those bases with a solution of chromate of potassa. The three former are yellow, the fourth orange-red, and the fifth deep red or purple. The yellow chromate of lead, which consists of one equivalent of acid and one equivalent of oxide, is now extensively used as a pigment, and the chro- mate of oxide of zinc may be used for the same purpose. A dichromate, composed of one equivalent of chromic acid and two equi- valents of protoxide of lead, may be formed by boiling the carbonate of that oxide with excess of chromate of potassa. It is of a beautiful red colour, and has been recommended by Mr. Badams as a pigment. (An. of. Phil. xxv. 303.) It may be also made by boiling the neutral chromate with am- monia or lime-water. Liebig and Wohler prepare it by fusing nitre at a low red heat, and adding chromate of oxide of lead by degrees until the nitre is nearly exhausted. The chromate of potassa and nitre are then re- moved by water, and the dichromate is left crystalline in texture, and of so beautiful a tint, that it vies with cinnabar. (Pog. An. xxi. 580.) Bichromate of Chloride of Potassium.—M. Peligot has lately described a crystalline compound in which chloride of potassium acts the part of an alka- line base in relation to chromic acid. It is prepared from bichromate of potassa and concentrated hydrochloric acid in the ratio by weight of about 3 to 4, which are to be boiled together for some time in a rather small quan- tity of water; and it is deposited in flat quadrangular prisms of the same colour as bichromate of potassa. In this process there is a mutual interchange between the elements of potassa and hydrochloric acid; such that 2 eq. chromic acid . . 2(Cr + 30) ^ 2 eq. chromic acid 2(Cr + 30). 1 eq. potassa . . . K + O [3 1 eq. chlo. of potassium K +CI. 1 eq. hydrochloric acid . H + Cl ^ 1 eq. water . H + 0. For this change to ensue there ought to be a certain excess of hydrochloric acid, and yet not so much as to decompose the chromic acid. This salt should be dried on bibulous paper. It is permanent in the air. In pure water it is decomposed, the materials from which it was formed, bichromate of potassa and hydrochloric acid, being reproduced ; but it may be dissolved without such change in water acidulated by hydrochloric acid. M. Peligot has made similar bichromates with the chlorides of sodium, calcium, and magnesium, and with hydrochlorate of ammonia; this last salt being exactly similar in appearance to the bichromate of chloride of potas- sium. (An. de Ch. et de Ph. Iii. 267.) BORATES. As the boracic is a feeble acid, it neutralizes alkalies imperfectly; and hence the borates of soda, potassa, and ammonia have always an alkaline reaction. For the same reason, when the borates are digested in any of the more power- *An. de Ch. et de Ph. vol. xxii. t Annals of Philosophy, vol. xvi. CARBONATES. 461 ful acids, such as the sulphuric,nitric, or hydrochloric, the boracic acid is sepa- rated from its base. This docs not happen, however, at high temperatures; for boracic acid, owing to its fixed nature, decomposes at a red heat all sdts, not excepting sulphates, the acid of which is volatile. The borates of the alkalies are soluble in water, but most of the other sdts of this acid are of sparing solubility. They are not decomposed by heat, and the alkaline and earthy borates resist the action of heat and combustible matter. They are remarkably fusible in the fire, a property obviously ow- ing to the great fusibility of boracic acid itself. The borates are distinguished by the following character:—By digesling any borate in a slight excess of strong sulphuric acid, evaporating to dryness, and boiling the residue in strong alcohol, a solution is formed, which has the property of burning with a green flame. (Page 205.) Biborate of Soda.—This salt, the only borate of importance, occurs native in some of the lakes of Thibet and Persia, and is extracted from this source by evaporation. It is imported from India in a crude state, under the name of tincal, which, after being purified, constitutes the refined borax of com- merce. It is frequently called subborate of soda, a name suggested by the in- consistent and unphilosophical practice, now quite inadmissible, of regulating the nomenclature of salts merely by their action on vegetable colouring mat- ter. It crystdlizes in hexahedral prisms, which effloresce on exposure to the air, require twenty parts of cold, and six of boiling water for solution. When exposed to heat the crystals are first deprived of their water of crys- tallization, and then fused, forming a vitreous transparent substance called glass of borax. The crystals are composed of 69.8 parts or two eq. of boracic acid, 31.3 or one eq. of soda, and 90 or ten eq. of water. The chief use of borax is as a flux, and for the preparation of boracic acid. Biborate of magnesia is a rare natural production, which is known to mineralogists by the name of boracite. A new biborate of soda, which contains half as much water of crystalli- zation as the preceding, has been lately described by M. Buran.. It is harder and denser than borax, is not efflorescent, and crystallizes in regular octohe- drons. It is made by dissolving borax in boiling water until the specific gravity of the solution is at 30° or 32° of Baume's hydrometer: the solution is then very slowly cooled ; and when the temperature descends to about 133°, the new salt is deposited. It is found to be more convenient for the use of jewdlers than common borax. (An. de Ch. et de Ph. xxxvii. 419.) « CARBONATES. The carbonates are distinguished from other salts by being decomposed with effervescence, owing to the escape of carbonic acid gas, by nearly all the acids; and all of them, except the carbonates of potassa, soda, and lithia, may be deprived of their acid by heat. The carbonates of baryta and strontia, especially the former, require an intense white heat for decomposi- tion ; those of lime and magnesia are reduced to the caustic state by a full red heat; and the other carbonates part with their carbonic acid when heat- ed to dull redness. All the carbonates, except those of potassa, soda, and ammonia, are of sparing solubility in pure water; but all of them are more or less soluble in an excess of carbonic acid, owing doubtless to the formation of supersalts. The former nomenclature of the salts is peculiarly exceptionable as ap- plied to the carbonates. The two well-known carbonates of potassa, for ex- ample, are distinguished by the prepositions sub and super, as if the one had an alkaline, and the other an acid reaction; whereas, in fact, according to their action on test paper, they are both subsalts. I shall adopt the nomen- clature which has been employed with other salts, applying the generic name of carbonate to those salts which contain one equivalent of carbonic acid and one equivalent of the base,—compounds which may be regarded as 426 CARBONATES. neutral in composition, however they may act on the colouring matter of plants. The formula for the neutral protocarbonates is M + C, the acid con- taining twice as much oxygen as the base. Several of the carbonates occur native, among which may be enumerated the carbonates of soda, baryta, strontia, lime, magnesia, and the protoxides of manganese, iron, copper, and lead; together with some double carbonates, such as Dolomite, or the double carbonate of lime and magnesia, and baryto- calcite, or the double carbonate of baryta and lime. The composition of the principal carbonates is stated in the following table: Names. Base. Acid. Equiv. Formulae. Carbonate of potassa . 47.15 1 eq.+22.12 1 eq.= 69.27 K+C. Bicarbonate of potassa . 47.15 1 eq.+44.24 2 eq.= 91.39 K+2C. Do. in crystals with 9 or 1 eq. of water =100.39 Carbonate of soda . . 31.3 1 eq. +22.12 1 eq.= 53.42 Na + C. Do. in crystals with 90 or 10 eq. of water =143.42 Do. in crystals with 63 or 7 eq. of water =116.42 Bicarbonate of soda . 31.3 1 eq. +44.24 2 eq.= 75.54 Na + 2C. Do. in crystals with 9 or 1 eq. of water = 84.54 Carbonate of ammonia . 17.15 1 eq. +22.12 1 eq.= 39.27 H3N+C. Bicarbonate of ammonia 17.15 1 eq+44.24 2 eq.= 61.39 H3N+2C. Carbonate of baryta . 76.7 1 eq. +22.12 1 eq.= 98.82 Ba + C. Carbonate of strontia 51.8 1 eq. +22.12 1 eq.= 73.92 Sr + C. Carbonate of lime(marble)28.5 1 eq.+22.12 1 eq.= 50.62 Ca+C. Carbonate of magnesia 20.7 1 eq+22.12 1 eq.= 42.82 Mg+C. Do. in crystals with 27 or 3 eq. ofwater = 69.82 Carbonate protox. of iron 36 1 eq.+22.12 1 eq.= 58.12 Fe+C. Dicarbonate protox. copper 79.2 2 cq-+22.12 1 eq.= 101.32 2Cu+C. Do. in malachite with 9 or 1 eq. ofwater =110.32 Carbonate protox. lead 111.6. 1 eq. +22.12 1 eq.= 133.72 Dicarb. perox. mercury 436 2eq.+22.12 1 eq.=458.12 2Hg + C. Double Carbonates. Carbonate of lime S Carb. lime . . 50.62 1 eq.) _ 9344 Mgc+CaC. and magnesia } Carb. magnesia 42.02 1 eq. S Carb. of baryta. S Carb. baryta 98.82 leq. J =149.44 CaC + BaC. and lime fCarb. lime . 50.62 1 eq. $ Carbonate of Potassa.-This salt is procured in an impure form by burn- ing knd plants, lixiviating their ashes, and evaporating the solu ion, to dryness a process which is performed on a large scale in Russia and America. The carbonate, thus obtained, is known in commerce by the names of potash and pearlash, and is much employed in the arts especially in the CARBONATES. formation of soap and the manufacture of glass. When derived from this source it always contains other compounds, such as sulphate of potassa and chloride of potassium; and, therefore, for chemical purposes, it should be prepared from cream of tartar. On heating this salt to redness, the tartaric acid is decomposed, and a pure carbonate of potassa mixed with charcoal remains. The carbonate is then dissolved in water, and, after filtration, is evaporated to dryness in a capsule of platinum or silver. Pure carbonate of potassa has a taste strongly alkaline, is slightly caustic, and communicates a green tint to the blue colour of the violet It dissolves in less than an equal weight ofwater at 60°, deliquesces rapidly on exposure to the air, and crystdlizes with much difficulty from its solution. In pure dcohol it is insoluble. It fuses at a full red heat, but undergoes no other change. , „ It is often necessary, for commercial purposes, to ascertain the value ot different samples of pearlash; that is, to determine the quantity of real car- bonate of potassa contained in a given weight of impure carbonate. A con- venient mode of effecting this object is described by Mr. Faraday in this ex- cellent work on Chemical Manipulation. Into a tube sealed at one end, 9$ inches long, |ths of an inch in diameter, and as cylindrical as possible in its whole length, pour 1000 grains of water, and with a file or diamond mark the place where its surface reaches, and divide the space occupied by the water into 100 equal parts, as is shown in the annexed wood-cut Opposite to the numbers 23.44, 48.96, 54.63, and 65, draw a line, and at the first write soda, at the second potassa, at the third carbonate of soda, and at the fourth carbonate of potassa. Then prepare a dilute acid having the specific gravity of 1.127 at 60°, which may be made by mixing one measure of concentrat- ed sulphuric acid with four measures of distilled water. This is the stand- ard acid to be used in all the experiments, being of such strength that when poured into the tube till it reaches either of the four marks just mentioned, we shall obtain the exact quantity which is necessary for neutralizing 100 grains of the alkali written opposite to it. If, when the acid reaches Kie word carb. potassa, and when, consequently, we have the exact quantity which will neutra- jqqq lize 100 grains of that carbonate, pure water be added until it reaches 0, or the beginning of the scde, each division of this mixture will neutralize one grain of carbo- nate of potassa. All that is now required, in order to ascertain the quantity of real g . carbonate in any specimen of pearlash, is to dissolve 100 grains of the sample in warm water, filter to remove all the in- soluble parts, and add the dilute acid in successive small quantities, until by the test of litmus paper, the solution is exactly Potassa . . . neutralized. Each division of the mix- ^ , gQ^a ture indicates a grain of pure carbonate. It is convenient, in conducting this process, to set aside a portion of the alkaline liquid, in Carb. Potassa order to neutralize the acid, in case it should at first be added too freely. To this instru- ment the term alkalimeter is given, a name obviously derived from the use to which it is applied. Bicarbonate of Potassa is made by trans- mitting a current of carbonic acid gas through a solution of the carbonate; or by evaporat- ing a mixture of the carbonates of ammonia 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 464 CARBONATES. and potassa, the ammonia being dissipated in a pure state. By slow evapo- ration, the bicarbonate is deposited from the liquid in hydrated prisms with eight sides, terminated with dihedral summits, the primary form of wliich is a right rhomboidal prism. Bicarbonate of potassa, though far milder than the carbonate, is alkaline both to the taste and to test paper. It does not deliquesce on exposure to the air. It requires four times its weight of water at 60° for solution, and is much more soluble at 212°; but it parts with some of its acid at th*. tem- perature. At a low red heat it is converted into the carbonate. Dr. Thomson, in his " First Principles," has described a sesquicarbonate which was discovered by Dr. Nimmo of Glasgow. Its crystals contain twelve equivalents of water, as denoted by the formula K2C3 _f 12H. Carbonate of Soda.—The carbonate of commerce is obtained by lixiviating the ashes of sea-weeds. The best variety is known by the name of barilla and is derived chiefly from the salsola soda and salicornia herbacea. A very inferior kind, known by the name of kelp, is prepared from sea-weeds on the northern shores of Scotland. The purest barilla, however, though well fitted for making soap and glass, and for other purposes in the arts, always contains the sulphates of potassa and soda, and the chlorides of po- tassium and sodium. A purer carbonate is prepared by heating a mixture of sulphate of soda, saw-dust, and lime in a reverberatory furnace. By the action of carbonaceous matter, the sulphuric acid is decomposed; its sdphur partly uniting with calcium and partly being dissipated in the form of sul- phurous acid, while the carbonic acid, which is generated during the pro- cess, unites with soda. The carbonate of soda is then obtained by lixivia- tion and crystallization. It is difficult to obtain this salt quite free from sul- phuric acid. Carbonate of soda crystallizes in octohedrons with a rhombic base, the acute angles of which are generally truncated. The crystals effloresce on exposure to the air, and when heated dissolve in their water of crystallization. By continued heat they are rendered anhydrous without loss of carbonic acid. They dissolve in about two parts of cold, and in rather less than their weight of boiling water, and the solution has a strong alkaline taste and re- action. The crystals commonly found in commerce contain ten equivalents of water; but when formed at a temperature of about 80°, they retain only seven equivalents. The purity of different specimens of barilla, or other carbonates of soda, may be ascertained by means of the alkalimeter above described. Bicarbonate of Soda.—This salt is made by the same processes as bicar- bonate of potassa, and is deposited in hydrated crystalline grains by evapora- tion. Though still alkaline, it is much milder than the carbonate, and far less soluble, requiring about ten times its weight of water at 60° for solu- tion. It is decomposed partially at 212°, and is converted into the carbonate by a red heat Sesquicarbonate.—This compound occurs native on the banks of the lakes of soda in the province of Sukena in Africa, whence it is~exported under the name of trona. It was first distinguished from the two other carbonates by Mr. rhillips, (Journal of Science, vii.) whose analysis corresponds with that of Klaproth. Its formula is Na2C3-f4H. Carbonate of Ammonia.—The only method of procuring this salt is by mixing dry carbonic acid over mercury with twice its volume of ammoniacal gas. It is a dry white volatile powder of an ammoniacal odour, and dkaline reaction. Bicarbonate of Ammonia.—This salt was formed by Berthollet by trans- mitting a current of carbonic acid gas through a solution of the common carbonate of ammonia of the shops. On evaporating the liquid by a gentle heat, the bicarbonate is deposited in small six-sided prisms, which have no smell, und very little taste: their primary form, according to Mr. Miller of CARBONATES. 465 Cambridge, is a right rhombic prism. Berthollet ascertained that it contains twice as much acid as the carbonate. Sesquicarbonate of Ammonia.—The common carbonate of ammonia of the shops, Sub-carbonas Ammonia of the Pharmacopoeia, is different from botn these compounds. It is prepared by heating a mixture of one part of hydro- chlorate of ammonia with one part and a half of carbonate of lime, carefully dried. Double decomposition ensues during the process: chloride of calcium remains in the retort, and hydrated sesquicarbonate of ammonia is sublimed. The carbonic acid and ammonia are, indeed, in proper proportion in the mixture for forming the real carbonate; but from the heat employed in the sublimation, part of the ammonia is disengaged in a free state. The salt thus formed consists, according to the analysis of Phillips, Ure, and Thomson, of 34.3 parts or two eq. of ammonia, 66.36 parts or three eq. of carbonic acid, and 18 parts or two eq. of water. When recently prepared, it is hard, compact, translucent, of a crystalline texture, and pungent am- moniacal odour; but if exposed to the air, it loses weight rapidly from the escape of pure ammonia, and becomes an opaque brittle mass, which is the bicarbonate. . - , , e Carbonate of Baryta occurs abundantly in the lead mines of the north of England, where it was discovered by Dr. Withering, and has hence received the name of Wiiherite. It may be prepared by way of double decomposition by mixing a soluble salt of baryta with any of the alkdine carbonates or bicarbonates. It is anhydrous, exceedingly insoluble in distilled water, re- quiring 4300 times its weight ofwater at 60°, and 2300 of boiling water for solution; but when recently precipitated, it is dissolved much more freely by a solution of carbonic acid. It is highly poisonous. Carbonate of Strontia, which occurs native at Strontian in Argyllshire, and is known by the name of Strontianite, may be prepared in the same manner as carbonate of baryta. It is anhydrous, and very insoluble in pure water, but is dissolved by an excess of carbonic acid. Carbonate of Lime.—This salt is a very abundant natural production, and occurs under a great variety of forms, such as common limestone, chdk, marble, and Iceland spar, and in regular anhydrous crystals, the density of which is 2.7. It may also be formed by precipitation. Though sparingly soluble in pure water, it is dissolved by carbonic acid in excess; and hence the spring-water of limestone districts always contains carbonate of lime, which is deposited when the water is boiled. Daniell noticed that an aqueous solution of sugar and lime deposited crys- tdlized carbonate of lime by exposure to the air. Gay-Lussac has proved that the sugar merely acts as a solvent, presenting lime in a favourable state for combining with the carbonic acid of the atmosphere; and that all the lime is deposited in acute rhombohedrons, which contain five eq. ofwater to one eq. of carbonate of lime. These crystals are insoluble and remain un- changed in cold water ; but in water at 86°, or in air, they lose their com- bined water, and fall to powder. When boiled in alcohol they retain their form, but lose two eq. of water and retain three eq. in combination. (An. de Ch. et de Ph. xlviii. 301.) . Carbonate of Magnesia.—It is met with occasionally in small acicular crystals, and in a pulverulent earthy state, but more commonly as a com- pact mineral of an earthy fracture called magnesite. A specimen of mag- nesite from the East Indies, where I am informed it is abundant, has been analyzed by Dr. Henry, who found it to be nearly pure anhydrous carbonate of magnesia: it is of a snow-white colour, of density 2.56, and so hard that it strikes fire with steel. (An. of Phil. xvii. 252.) It is obtained in minute transparent hexagonal prisms with three eq. of water, when a solution of bicarbonate of magnesia evaporates spontaneously in an open vessel. The crystals lose their water and become opaque by a very gentle heat, and even in a dry air at 60°. By cold water they are decomposed, yielding a soluble bicarbonate and an insoluble white compound of hydrate and carbonate of 466 CARBONATES. magnesia; and hot water produces the same change with disengagement of carbonic acid, without dissolving any magnesia. (Berzelius.) When carbonate of potassa is added in excess to a hot solution of sulphate of magnesia, a white precipitate falls, which after being well washed has been long considered as pure carbonate of magnesia; but Berzelius has shown that it consists of the following ingredients: Magnesia . 44.75 82.8 or4eq.1 Probable formula is Carbonic acid . 35.77 66.36 or 3 eq. ] . . ... Water . . 19.48 36 or 4 eq. j. MgH4+3MgC. 100.00 185.16 or leq. J This compound is said to require 2493 parts of cold, and 9000 of hot water for solution. It is freely dissolved by a solution of carbonic acid, bicarbo- nate of magnesia being generated; but on allowing the solution to evapo- rate spontaneously, carbonic acid is given off, and crystals of the hydrated carbonate above mentioned are obtained. Carbonate of Protoxide of Iron.—Carbonic acid does not form a definite compound with peroxide of iron, but with the protoxide it constitutes a sdt which is an abundant natural production^ occurring sometimes massive, and at other times crystallized in rhombohedrons or hexagonal prisms. This protocarbonate is contained also in most of the chalybeate mineral waters, being held in solution by free carbonic acid; and it may be formed by mix- _ ing an alkaline carbonate with the sulphate of protoxide of iron. When prepared by precipitation, it attracts oxygen rapidly from the atmosphere, and the protoxide of iron, passing into the state of peroxide, parts with car. bonic acid. For this reason, the carbonate of iron of the Pharmacopoeia is of a red colour, and consists chiefly of the peroxide. Dicarbonate of Protoxide of Copper.—It occurs as a hydrate in the beauti- ful green mineral called malachite; and the same compound, as a green powder, the mineral green of painters, may be obtdned by precipitation from a hot solution of sulphate of protoxide of copper by carbonate of soda or po- tassa. When obtained from a cold solution, it falls as a bulky hydrate of greenish-blue colour, which contains more water than the green precipitate. By careful drying its water may be expelled. When the hydrate is boiled for a long time in water it loses both carbonic acid and combined water, and the colour changes to brown. The rust of copper, prepared by exposing metallic copper to air and moisture, is a hydrated dicarbonate. The blue-coloured mineral, called blue copper ore, appears to be a hydrate and carbonate of the protoxide of copper, and consists, according to the ana- lysis of Mr. Phillips, of (Quarterly Journal of Science, iv.) Protoxide of copper 69.08 118.8 3 eq. ) Probable formula is Carbonic acid . 25.46 44.24 2 eq.} ..... Water . . 5.46 9 leq.) CuH + 2CuC. 100.00 171.24 leq. The blue pigment called verditer, prepared by decomposing nitrate of oxide of copper with chalk, has a similar composition. (Phillips.) Carbonate of Protoxide of Lead.—This salt, which is the white lead or ceruse of painters, occurs native in white prismatic crystals derived from a right rhombic prism, the sp. gravity of which is 6.72. It is obtained as a white pulverulent precipitate by mixing solutions of an alkaline carbonate with acetate of protoxide of lead; and it is prepared as an article of com- merce from the subacetate by a current of carbonic acid, by exposing metal- lie lead in minute division to air and moisture, and by the action oni thin sheets of lead of the vapour of vinegar, by which the metal is both oxidized and converted into a carbonate. Dicarbonate of Peroxide, of Mercury.—When a solution of the nitrate of ' —11I1J!IU"UALTS. 467 peroxide of mercury is decomposed by carbonate of soda, an ochre-yellow precipitate falls, which Mr. Phillips finds to be a dicarbonate. The protoxide appears to form no compound with carbonic acid; for when a nitrate of that oxide is decomposed by any alkaline carbonate, the precipitate is either black at first or speedily becomes so, and after being washed is quite free from carbonic acid. Double Carbonates.—One of the most remarkable of these is the double carbonate of lime and magnesia, which constitutes the minerals called bitter- spar, pearl-spar, and Dolomite. The two former occur in rhombohedrons of nearly the same dimensions as carbonate of lime. The latter is met with in great perfection in the Alps, and there usually occurs in white masses of a gran- ular texture: the grains often cohere loosely, but other specimens are hard and compact, and when broken present the crystalline aspect of marble. Its density is 2.884. Some specimens consist of the two constituent carbonates in the ratio of their equivalents, as stated in the table; but the ratio of the ingredients, as may be expected, is very variable, since isomorphous sub- stances crystallize together in all proportions. Carbonate of protoxide of manganese is often associated with them. The rock called magnesian lime- stone may be viewed as an impure earthy variety of Dolomite. The double carbonate of baryta and lime constitutes the mineral called barito-calcite, which Mr. Children found to contain the two carbonates in atomic proportion. Berthier has made some interesting experiments on the production of double carbonates by fusion. Carbonate of soda, when fused with carbonate of baryta, strontia, or lime, in the ratio of their equivalents, yields uniform crystalline compounds, which have all the appearance of being definite. An equivalent of Dolomite fuses in like manner with four equivalents of carbo- nate of soda. Five parts of carbonate of potassa and four of carbonate of soda, corresponding to an equivalent of each, fuse with remarkable facility; and this mixture, by reason of its fusibility, may be advantageously employ- ed in the analysis of earthy minerals. Compounds similar to the foregoing may be generated by heating sulphate of soda with carbonate of baryta, strontia, or lime, in the ratio of their equiv- alents: or by employing the sulphate of these bases and carbonate of soda. In like manner carbonate of soda fuses with chloride of barium or calcium; and chloride of sodium with carbonate of baryta or lime. (An. de Ch, et de Ph. xxxviii. 246 SECTION II. CLASS OF SALTS. ORDER II. HYDRO-SALTS. In this section are included those salts only, the acid or base of which is a compound containing hydrogen as one of its elements. For reasons al- ready assigned (page 291) I have already described all those salts which were formerly called muriates or hydrochlorates of metallic oxides as chlorides of metals, considering that in general the neutralizing power of hydrochloric acid is not due to its direct combination with an oxide, but to chlorine uniting with the metal itself. The same remark applies to the hydriodic and other hydracids, the.salts of which are consequently reduced to a small number. The only salts, indeed, which are included in this section, are compounds of the hydracids with ammonia and phosphuretted hydrogen. Some of the compounds which might, as containing an hydracid, be compre- 468 hended in this section, may with greater propriety be placed in the fourth, seeing that in them the hydracid acts rather as a base or electro-positive in- gredient than as an acid or electro-negative substance. This double function, which chemists have long recognized in certain metallic oxides, such as alumina and oxide of zinc, appears to be performed even by so powerful an acid as the hydrochloric. Some judicious observations on this subject have been made by Professor Kane of Dublin. (Dublin Journal of Science, i. 265.) Ammonia unites with fluoride of boron, bisulphuret of carbon, and some other bi-elementary compounds, wliich contain neither oxygen nor hydrogen, constituting saline combinations, which are included in this section, and to which, considering the distinct alkaline character of ammonia, the ordinary nomenclature of salts is applicable. AMMONIACAL SALTS. These compounds, like the oxy-salts of ammonia, are readily recognized by the addition of pure potassa or lime, when the odour of ammonia may be perceived. Those which contain a volatile acid may in general be sublimed without decomposition; but the ammonia is expelled by heat from those acids which are much more fixed than itself. The most important of these salts are thus constituted:— Names. Base. Acid. Equiv. Formulae. Hydrochlorate of ammonia 17.15 leq.+ 36.42 1 eq.= 53.57 H^N+HCl. Hydriodate do. . . 17.15 1 eq.-f 127.3 1 eq.= 144.45 H3N+HI. Hydrobromate do. . 17.15 1 eq.-f 79.4 1 eq.= 96.55 HaN + HBr. Hydrofluate do. . . 17.15 1 eq.-f 19.68 1 eq.= 36.83 HsN + HF. Hydrosulphate do. . 17.15 1 eq.-f 17.1 1 eq.= 34.25 HsN + HS. Hydroeyanate do. . . 17.15 leq.+ 27.39 1 eq.= 44.54 HaN + HC^N. Hydrosulphocyanate do. 17.15 1 eq.-f 59.59 leq. = 76.74 HsN+HCyS5*. Trifluoborate do. . . 51.45 3 eq.+ 66.94 1 eq.= 118.39 3H^N + BF3. Difluoborate do. . . 34.3 2 eq.-f 66.94 1 eq.=l01.24 2H*N+BF3. Fluoborate do. . . . 17.15 1 eq.-f 66.94 1 eq.= 84.09 H*N+BFs. Fluosilicate do. . . . 17.15 1 eq.+ 26.18 1 eq.= 43.33 HsN+SiF. Carbosulphate do. . . 17.15 1 eq.-f 38.32 1 eq.= 55.47 HsN + CS3. Hydrochlorate of Ammonia.—This sdt, sal ammoniac of commerce, was formerly imported from Egypt, where it is procured by sublimation from the soot of camel's dung; but it is now manufactured in Europe by several pro- cesses. The most usual is to decompose sulphate of ammonia by the chloride dther of sodium or magnesium, when double decomposition ensues, giving rise in both cases to hydrochlorate of ammonia, and to sulphate of soda when chloride of sodium is used, and to sulphate of magnesia when chloride of magnesium is employed. The sal ammoniac is afterwards obtdned in a pure state by sublimation. The method now generally used in this country for obtaining sulphate of ammonia is to decompose with sulphuric acid the hydrosulphate and hydroeyanate of ammonia which is collected in the manu- facture of coal-gas; but it may also be procured either by lixiviating the soot of coal, which contains sulphate of ammonia in considerable quantity, or by digesting with gypsum impure carbonate of ammonia, procured from the destructive distillation of bones and other animal substances, so as to form an insoluble carbonate of lime and a soluble sulphate of ammonia. Hydrochlorate of ammonia has a pungent saline taste, has a density of 1.45, and is tough and difficult to be pulverized. It is soluble in alcohol and water, requiring for solution three times its weight of water at 60°, and an equal weight at 212°. It usually crystallizes from its solution in feathe- ry crystals, but sometimes in cubes or octohedrons. At a temperature below that of ignition it sublimes without fusion or decomposition, and condenses on cool surfaces as an anhydrous salt, which absorbs humidity in a damp atmo- AMMONIACAL SALTS. sphere, but is not deliquescent. It is generated by the direct union of hy- drochloric acid and ammoniacal gases, which unite in equal volumes. Hydriodate of Ammonia.—it is formed as a white powder by the direct union in equal measures of hydriodic acid and ammoniacal gases, or by neu- tralizing a solution of hydriodic acid with ammonia, and evaporating. It crystallizes with difficulty in anhydrous cubes, is very soluble in water, and deliquesces in a moist atmosphere. In close vessels it may be sublimed with- out change; but suffers partial decomposition when heated in the open air. When a concentrated solution of this salt is digested with iodine, a brown solution is obtained, the nature of which is not understood. Hydrobromate of Ammonia is a white anhydrous salt which may be formed by similar processes as the hydriodate. It is soluble in water, and crystal- lizes by evaporation in quadrilateral prisms. Hydrofluate of Ammonia.—It is prepared by mixing 1 part of sal am- moniac with 2J of fluoride of sodium, both dry and in fine powder, gently heating the mixture in a platinum vessel, and receiving the sublimed salt in a second platinum vessel, the temperature of which is not ullowed to exceed 212°. Chloride of sodium is generated, and hydrofluate of ammonia isob- tdned in small anhydrous prismatic crystals, which may be preserved unchanged in the air, is partly soluble in alcohol, and dissolves readily in water. At an elevated temperature it fuses before subliming.. It acts power- fully on glass even in its dry state. When this salt is introduced in a dry state into ammoniacal gas, absorp- tion ensues, and the resulting salt appears to be a dihydrofluate of ammonia. By sublimation it loses ammonia and becomes neutral. An acid salt, appa- rently a bihydrofluate, is obtained by evaporating the aqueous solution oT the neutral hydrofluate, ammonia being disengaged. If the evaporation take place at 100°, it separates in crystalline grains, which redden litmus, and deliquesce rapidly at common temperatures. Hydrosulphate of Ammonia.—This salt, also called hydrosulphuret of am- monia, and formerly the fuming liquor of Boyle, is prepared by heating a mixture of one part of sulphur, two of sal ammoniac, and two of unslaked lime. The changes which ensue have been explained by Gay-Lussac. The volatile products are ammonia and hydrosulphate of ammonia; and the fixed residue consists of sulphate of lime with chloride and sulphuret of calcium. The hydrosulphuric acid is formed from the hydrogen of hydrochloric acid uniting with sulphur, and the oxygen of the sulphuric acid is derived from decomposed lime, the calcium of which is divided between the chlorine of the hydrochloric acid and sulphur. Hydrosul- phate of ammonia may also be formed by the direct union of its con- stituent gases, and if they are mixed in a glass globe kept cool by ice, the salt is deposited in crystals. It is much used as a reagent, and for this pur- pose is usually prepared by saturating a solution of ammonia with hydro- sulphuric acid gas. Hydroeyanate of Ammonia.—This salt is a constant product of the de- structive distillation of animal substances, and may be obtained in a pure state by saturating hydrocyanic acid with ammonia. It is extremely volatile, and crystallizes when condensed in cubic crystals. It is very liable to spon- taneous decomposition, when it becomes black, from the separation of a carbonaceous mass which retains nitrogen. Sulphocyanate of Ammonia is prepared by neutralizing hydrosulphocyanic acid with ammonia, or heating a mixture of dry sulphocyanuret of potassium and sal ammoniac. It is very soluble, deliquesces in the air, and crystallizes with difficulty. J Fluoborates of Ammonia.—Fluoboric acid combines with three times and with twice its volume of ammoniacal gas, forming a trifluoborate and difluo- borate, which are liquid at common temperatures. The neutral fluoborate is formed of equal volumes of its constituent gases, and is a white volatile salt, soluble m water, but which cannot be recovered from the solution • for on evaporation, a subfluoborate of ammonia is expelled, and boracic acid is 40 470 SULPHUR-SALTS. left in solution. The neutral fluoborate is formed by heating gently either of the subfluoborates. Fluosilicate of Ammonia.—Fluosilicic acid and ammoniacal gases unite by volume in the ratio of 1 to 2, forming a white volatile salt which is de- composed by water. Carbosulphate of Ammonia.—When dry ammoniacal gas is brought into contact with bisulphuret of carbon, direct combination ensues, and there re- sults an uncrystalline solid mass of a straw-yellow colour, which may be sublimed without decomposition. By contact with water, or exposure to a moist air, an interchange ensues between the elements of water and bi- sulphuret of carhon, giving rise to hydrosulphuric and carbonic acids ; and a sulphur-salt of an orange-yellow colour, the hydrocarbosulphuret of am- monia,* is generated. Arsenio-persulphale of Ammonia.—Berzelius states that when dry persul- phuret of arsenic is exposed to ammoniacal gas, absorption ensues, and a yellowish-white compound results; but the elements are united by a feeble attraction, and on mere exposure to the air, the ammonia escapes. SALTS OF PHOSPHURETTED HYDROGEN. Rose has lately called the attention of chemists to the close analogy which exists in the composition of ammonia and phosphuretted hydrogen, and in some of their properties. The latter is a feeble alkaline base, which com- bines with some of the hydracids. The salt best known is the hydriodate of phosphuretted hydrogen, first noticed by Gay-Lussac, which is formed of 127.3 parts or one eq. of acid and 34.4 parts or one eq. of base, and crys- tallizes in cubes. The crystals are permanent while quite dry; but with water, or the moisture of the air, they yield a solution of hydriodic acid, and phosphuretted hydrogen gas escapes. These salts are all decomposed by water, and exist only in the anhydrous state. SECTION III. CLASS OF SALTS. ORDER III. SULPHUR-SALTS. The compounds described in this section are double sulphurets, just aa the oxy-salts in general are double oxides. Their resemblance in composi- tion to salts is perfect. The principal sulphur-bases are the protosulphurets of potassium, sodium, lithium, barium, strontium, calcium, and magne- sium, and hydrosulphate of ammonia; and the principal sulphur-acids are the sulphurets of arsenic, antimony, tungsten, molybdenum, tellurium, tin, and gold, together with hydrosulphuric acid, hydrosulphocyanic acid, bisd- phuret of carbon, and sulphuret of selenium. The sulphur-salts with two metals are so constituted, that if the sulphur in each were replaced by an equivalent quantity of oxygen, an oxy-salt would result. The analogy be- tween oxy-salts and sulphur-salts is rendered still closer by the circumstance that hydrosulphuric and hydrosulphocyanic acids have the characteristic properties of acidity, and unite both with ammonia and with sulphur-bases. * It is not very clear what combination Dr. Turner intends by the name, hydro-carbosulphuret of ammonia; but it is probable that he means to desig- nate the sulphur-salt called further on, carbo-sulphuret of hydrosulphate of ammonia. See page 473.—Ed. The sulphur-salts may be divided into families, characterized by contain- ing the same sulphur-acid. For the purpose of indicating that such salts are double sulphurets, as well as to distinguish them readily from other kinds of salts, I shall construct the generic name of each family from the sulphur-acid terminated with sulphuret Thus the salts which contain persulphuret of arsenic or hydrosulphuric acid as the sulphur-acid are term- ed arsenio-sulphurets and hydro-sulphurets ; and a salt composed of each ol these sulphur-acids with sulphuret of potassium is termed arsenio-sulphuret and hydro-sulphuret of sulphuret of potassium. For the sake of brevity the metal of the base may alone be expressed, it being understood that the positive metd in a sulphur-salt enters as a protosulphuret into the com- pound.* HYDRO-SULPHURETS. The sulphur-salts contained in this group have hydro-sulphuric acid for their electro-negative ingredient. Most of them which have been studied are soluble in water, and may be obtained in crystals by evaporation. They are decomposed by exposure to the air, yielding at first bisulphurcts of the metal, and then a hyposulphite. By acids the hydrosulphuric acid is ex- pelled with effervescence. They are thus constituted :— Name. Sulphur-base. Sulphur- acid. Equiv. Formula;. Hydrosulphuret of tassium . P°" \ 55.25 39.4 1 eq. + 17.1 1 eq.= 72.35 KS + HS. Ditto sodium leq.+ 17.1 1 eq.= 56.5 NaS + HS. Ditto lithium 26.1 leq.+ 17.1 1 eq.= 43.2 LS+HS. Ditto barium 8-1.8 1 eq. + 17.1 1 eq.= 101.9 BaS + HS. Ditto strontium . 59.9 leq. + 17.1 1 eq.= 77. SrS + HS. Ditto calcium 36.6 1 eq. + 17.1 1 eq.= 53.7 CaS + HS. Ditto magnesium 28.8 leq.+ 17.1 ] eq.= 45.9 MgS + HS. Hydro-sulphuret of Potassium.—This salt is obtained in the anhydrous state by introducing anhydrous carbonate of potassa into a tubulated retort, transmitting through it a current of hydrosulphuric acid gas, and heating the salt to low redness. The mass becomes black, fuses, and boils from the escape of carbonic acid gas and aqueons vapour; and after the ebullition has ceased, the gas is continued to be transmitted, until the retort is quite cold. The resulting anhydrous hydro-sulphuret of potassium, though black while in fusion, is white when cold, and of a crystalline texture; but if air had not been perfectly excluded, it has a yellow tint, owing to the presence of some bisulphuret of potassium. The same salt is prepared in the moist way by introducing a solution of pure potassa, free from carbonic acid, into a tubulated retort, expelling at- mospheric air by a current of hydrogen gas, and then saturating the" solu- tion with hydrosulphuric acid. At first the potassa, as in the former pro- cess, interchanges elements with the gas, yielding water and protosulphuret * Dr. Hare has adopted an ingenious plan for naming the sulphur-salts founded on the nomenclature of the oxy-salts. Considering the electro- negative sulphuret in the sulphur-salts as performing the part of an acid he calls it an acid, and forms its name by changing the termination of the ele- ment with which the sulphur is combined into ic, and prefixing sulph or sulpho. Thus, taking the examples given by Dr. Turner, he calls persul- phuret of arsenic, sulpharsenic acid, and its sulphur-salts, sulpharscniates; while he denominates hydrosulphuric acid, sulphydric acid, and its salts sulphydrates. In indicating the sulphur-base, Dr. Hare has adopted the same plan which Dr. Turner recommends in the text, of expressing the metal only of the sulphur-base, the metal being understood to be in the state of protosulphuret.—Ed, HYDRO-SULPHOCYANURETS. of potassium; after which the protosulphuret unites with hydrosulphuric acid. The solution should be evaporated in the retort to the consistence of syrup, a current of hydrogen gas being transmitted through the apparatus the whole time; and on cooling the salt crystallizes in large four or six- sided prisms, which are colourless if air was perfectly excluded. The crys- tals contain water of crystallization, have an acrid, alkaline, and bitter taste, deliquesce in open vessels, and dissolve freely in water and alcohol. On exposure to the air it acquires a yellow colour, from the formation of bisul- phuret of potassium. Hydro-sulphuret of Sodium.—It is prepared on the same principles as the former salt, and yields by evaporation colourless crystals. When a hot con- centrated solution is mixed with a solution of hydrate of soda also concen- trated, the mixture on cooling deposites four-sided prisms, which are proto- sulphuret of sodium with water of crystallization. The interchange of elements is such that 1 eq. hydro-sulph'iret and 1 eq. soda 2 2 eq sulphuret and 1 eq. water. (Na + S) + (H + S) Na + O ~ 2(Na + S) H + O. Hydro-sulphuret of Lithium may be prepared in the same way as the two former salts, and is left by evaporation as a crystalline solid. When heated in close vessels it parts with its water of crystallization, and like the two former salts retains its acid even at a red heat. Hydro-sulphuret of Barium.—It is prepared by the action of hydrosul- phuric acid on a solution of baryta with the precautions already mentioned for excluding atmospheric air, and crystallizes by evaporation in four-sided prisms, which are very soluble in water, but dissolve sparingly in alcohol. The crystals part with their water of crystallization when heated, and at a commencing red heat give out hydrosulphuric acid, leaving pure sulphuret of barium. Hydro-sulphuret of Strontium is prepared like the former salt, and crys- tallizes in large radiated prisms, which when quite dry may be kept several days exposed to the air without change. When heated it loses its water and acid, and protosulphuret of strontium as a white powder is left. Hydro-sulphuret of Calcium is formed in the same manner as the pre- ceding salts; but it exists only in solution; for on attempting to crystallize by evaporation, hydrosulphuric acid is driven off, and the sulphuret of cal- cium in prisms of a silky lustre is deposited. The hydro-sulphuret of mag- nesium likewise exists only in solution. HYDRO-SULPHOCYANURETS. The acid of these salts is hydrosulphocyanic acid. The only ones yet known are those of sulphuret of potassium and hydrosulphate of ammonia, which are thus constituted:— Names. Sulphur-base. Sulphur-acid. Equiv. Formula;. Hydro-sulpho- ) cyan, of po- £ 55.25 1 eq. + 59.59 1 eq.= 114.S4 KS + HCySs. tassium j Hydro-sulpho- ^ dro"iil°natc* C 34-25 1 e1- + 59,59 X c1~ 93"84 (H3N + HS) + HCySs. of ammonia j Hi/dro-sulphocyanuret of Potassium.—Zeise prepares this salt by decom- posing with pure potassa the corresponding ammoniacal salt, and evapo- rating in vacuo along with sulphuric acid, which unites both with its water and free ammonia. The residue is a white crystalline salt, soluble both in water and alcohol. Hydro-sulphocyanuret of Hydrosulphate of Ammonia.—It is prepared, ac- cording to Zeise, by saturating with ammoniacal gas 100 measures of dco- CARBO-SULPHURETS. 473 hol at a temperature of 50°, diluting with 50 measures of alcohol, and add- ing 16 measures of bisulphuret of carbon. In about half an hour a deposite- of carbo-sulphuret of hydrosulphate of ammonia ensues, which is separated by filtration through linen, and the clear liquor is received in a flask, tightly corked, and exposed for 10 hours to a temperature of 60°. It is then ex- posed for 24 hours to a cold of 32°, during which the hydro-sulphocyanuret separates in long brilliant crystals of a lemon-yellow colour. The crystals are first washed with cold alcohol and then with ether, and dried on bibu- lous paper, and in vacuo. They may be preserved in a dry state, but de- compose readily when moist, and are very soluble in water. CARBO-SULPHURETS. The acid of Ihese sulphur-salts is bisulphuret of carbon; and the salts themselves are thus constituted:— Names. Sulphur-base. Sulphur-acid. Equiv. Formulse. Carbo-sulphuret i -.q- j , 3832 l eq<= 93.57 KS + CS2. of potassium < leq.+ 38.32 1 eq.= 77.72 NaS-fCS-'. leq. + 38.32 1 eq.= 64.4-2 LS + CS'a, Ditto sodium 39.4 Ditto lithium 26.1 Ditto hydrosul- i phate of am-V34.25 1 eq. +38.32 1 eq.= 72.57 (H3N + HS)+CS2. monia Ditto barium 84.8 1 eq. +38.32 1 eq.= 123.12 BaS + CS2. Ditto strontium 59.9 1 eq.-f-38.32 1 eq.= 98.22 SrS + CS2. Ditto calcium 36.6 1 eq. + 3*32 1 eq.= 74.92 CaS + SC2." Ditto magnesium 28.8 1 eq. +38.32 1 eq.= 67.12 MgS+CSs. Carbo-sulphuret of Potassium.—On agitating bisulphuret of carbon with a strong alcoholic solution of protosulphuret of potassium, the liquid when set at rest separates into three layers, the lowest of which is carbo-sulphuret of potassium, and is of the consistence of sj'rup. Another process is to digest bisulphuret of carbon at 86° in a corked bottle full of a strong aqueous solu- tion of protosulphuret of potassium, until the latter is saturated. A concen- trated solution of this salt is of a deep orange, almost red, colour; and when evaporated at 86° to the consistence of syrup, a deliquescent yellow crystal- line salt is deposited, which is sparingly soluble in alcohol. On heating it to 150° it gives off water of crystallization; and when more strongly heated it is resolved into tersulphuret of potassium and charcoal. Carbo-sulphuret of Sodium.—It is prepared like the former salt, and sepa- rates in yellow crystals from a very concentrated solution. It is deliques- cent, and dissolves readily in alcohol as well as water. Carbo-sulphuret of Lithium resembles the preceding salt, and is very so- luble in water and alcohol. Carbo-sulphuret of Hydrosulphate of Ammonia.—Zeise prepares this salt by filling a bottle with ten measures of nearly absolute alcohol saturated with ammoniacal gas and one measure of bisulphuret of carbon, and insert- ing a tight cork. As soon as the liquid has acquired a yellowish-brown co- lour, the bottle is plunged into ice-cold water, when the carbo-sulphuret is deposited either in yellow penniform crystals or as a crystalline powder. The whole is thrown upon a linen filter, and the salt after being washed first with absolute alcohol and then with ether, is dried by pressure within folds of bibulous paper. This salt is very volatile, passing off entirely at common temperatures, and can only be preserved in well-corked bottles. Exposed to the air it absorbs humidity and acquires a red colour. Its solu- tion may be kept unchanged in bottles filled with it and tightly corked. 1 he carbo-sulphurcts of barium, strontium, and calcium, may be obtained by acting on bisulphuret of carbon with a solution of the protosulphurets of those metals. The resulting solutions are of an orange or brown colour and the salts deposited by evaporation are of a citron-yellow when quite dry,' 40* 474 ARSENIO-SULPHURETS. The carbo-sulphuret of barium is of sparing solubility. The carbo-sulphuret of magnesium is best prepared by adding sulphate of magnesia to a .solu- tion of carbo-sulphuret of barium. Berzelius has also prepared several car- bo-sulphurets of the metals of the second class. ARSENIO-SULPHURETS. Berzelius finds that each of the three sulphurets of arsenic (page 358) is capable of acting as a sulphur-acid, giving rise to three distinct families of sulphur-salts, distinguishable by the terms arsenio-persulphurets, arsenio- sesquisulphurets, and arsenio-protosulphurets. Persulphuret of arsenic is a very powerful sulphur-acid, violently dis- placing hydrosulphuric acid from its combinations with sulphur-bases, even at common temperatures ; and when digested with earthy or alkaline carbo- nates, it expels carbonic acid. The salts of this sulphur-acid may be pre- pared by several methods:— 1. By digesting the persulphuret of arsenic in a solution of a sulphur- basei such as a sulphuret of potassium or sodium, until it is saturated. The resulting soluble arsenio-persulphuret may be employed to prepare insoluble salts of the same sulphur-acid by means of double decomposition. If a per- sulphuret of potassium is used, sulphur is deposited. 2. By decomposing a hydrosulphuret of a sulphur-base with persulphuret of arsenic, in which case hydrosulphuric gas is disengaged with efferves- cence. 3. By decomposing a solution of an arseniate by means of hydrosulphuric acid or hydrosulphate of ammonia. 4. By dissolving persulphuret of arsenic in a solution of caustic dkali, such as potassa; when an interchange of elements between portions of the alkali and persulphuret ensues, whereby arsenic acid and protosulphuret of potassium are generated. In this case 1 eq. persulphuret it 5 eq. potassa 3 1 eq. arsenic acid & 5 eq. protosulphuret. 2As + 5S 5(K + 0) ~ 2As + 50 5(K + S). Two salts are thus generated and co-exist in the solution, namely, arseniate of potassa and arsenio-persulphuret of potassium. Similar changes invaria- bly occur when sesquisdphuret of arsenic, sesquisulphuret of antimony, and other sulphur-acids are boiled with alkaline solutions : an oxy-salt, the acid of which is formed of oxygen and the electro-negative metal, is always gene- rated; and this salt, if soluble in water, remains together with the sulphur- salt in solution. An alkaline carbonate may be substituted for a pure alkali, but then carbonic acid is expelled. These principles are concerned in the production of kermes, as already explained (page 378). 5. The last method which requires mention, is by exposing a mixture of persulphuret of arsenic and an alkaline carbonate to a red heat in a covered vessel. Carbonic acid gas is disengaged; and an interchange of elements. similar to that just explained, takes plaee between a portion of the alkali and the sulphuret The fused mass, accordingly, contdns an arseniate of the alkali, as well as sulphur-salt This tendency to the formation of a double sulphuret is the reason why, in decomposing orpiment by black flux, the whole of the arsenic is never sublimed ; a part is uniformly retained in the form of a sulphur-salt, the arsenio-sesquisulphuret of sulphuret of potassium. Most of the arsenio-persulphurets of the second class of metals are insolu- ble ; but those of the metals of the alkalies and alkaline earths are very so- luble in water, have a lemon-yellow colour in the anhydrous state, and are colourless when combined with water of crystallization or in solution. When exposed to heat in close vessels they give off" sulphur, and an arsenio- sesquisulphuret is generated. In the solid state they are very permanent in the air, and even in solution oxidation takes place with great slowness. When decomposed by an acid, persulphuret of arsenic subsides, hydrosul- ;i!iuric acid gas escapes, and a salt of the alkali is generated. Some cho- ARSENIO-SULPHURETS. 475 mists may doubt the possibility of the arsenio-persulphurets dissolving as such in water: they may consider the arsenic, and the metal of the sul- phur-base to be united with oxygen, and all the sulphur with hydrogen; but this supposition, if followed out, leads into such complex and improbable modes of combination, that I see no alternative but implicitly to admit the views here adopted. . . . „ ... The following table exhibits the composition of the principal arsenio- persulphurets :— Names. Sulph.-base. Sulph.-acid. Equiv. Formula?. Triarsenio-persulph. ^ 165 ?5 3 eq+l55.9 1 eq.= 321.65 3KS+As*S\ Dilrn£;eruiPi,dliio'.5 ^+155.9 icq.-^ *™+^°. Arsenio-persulpll. do. 55.25 1 eq.+155.9 1 eq.=211.15 hS+As'S*. Triarsenio-persulph. > llg2 3 eq+155.9 1 eq.=274.1 3NaS+AssS«. of sodium . ( „, „ ,.,, Do. in crystals with 270 or 30 eq. of water =544 1 Diarsenio.persuIph.do. 78.8 2 eq+155.9 1 eq.= 234 7 2NaS4-As»S*. Arsenio-persulph. do. 39.4 1 eq.+ lD5.9 1 eq.= 195.3 AaS+As-^. Triarsenio-persulph. i , lt-„ , ora rz 5 3(HaN+HS) of hydrosulphate V 102.75 3 eq.l+5a.9 1 eq.=2o?.65 j V+As Se/ °famm°nia ^ , ,«n , 00,, )2(H3N+HS) Diarsenio-persdph.do. 68.5 2 eq+155.9 1 eq.=2244 j + a8sS«. Arsenio-persulph. do. 34.25 1 eq+155.9 1 eq.= 190.15 j ( +AsSg6> } Arsenio-persulphurets of Potassium.—The diarsenio-persulphuret is best obtained by the action of hydrosulphuric acid gas on the diarseniate of po- tassa, and yields a colourless solution. By evaporation in vacuo it is reduced to a yellowish viscid mass which dries imperfectly, but, when exposed for some time to the open air, at length becomes a crystalline mass of a lemon- yellow colour, in which rhomboidal tables are perceptible. When this salt is mixed with alcohol, it is resolved into the triarsenio-persulphuret, which is insoluble in the alcohol, and the arsenio-persulphuret, which remains in solution. The latter has not been obtained in the solid state. The former is deliquescent and very soluble in water: but when its solution is gently evaporated, the residue has a radiated crystalline texture. Arsenio-persulphurets of Sodium.—The diarsenio-persulphuret is formed like the corresponding salt of potassium, is very soluble in water, and by evaporation yields a lemon-yellow mass, which attracts humidity from the air. On mixing its solution with alcohol it is resolved into the arsenio- persulphuret and triarsenio-persulphuret of sodium, and the latter falls in scaly crystals of snowy whiteness, which may be collected on a filter, wash- ed with alcohol, and dried without change. This salt by solution in water and evaporation may be obtained in rhomboidal tables or prisms derived from a rhombic prism. The crystals undergo no change in the air, and contain thirty equivalents of water. The arsenio-persulphuret has been obtained only in solution. The arsenio-persulphurets of lithium are very analogous to those of sodium. Arsenio-persulphurets of hydrosulphate of Ammonia.—The diarsenio- persulphuret is obtained as a colourless solution by decomposing with hydrosulphuric acid gas a solution of diarseniate of ammonia. By sponta- neous evaporation it becomes a viscid mass of a reddish-yellow colour, and which cannot be fully dried without decomposition. When its solution is mixed with hydrosulphate of ammonia and agitated with hot alcohol, the triarsenio-sulphuret is deposited in cdourless prisms, which, after being well washed with alcohol and dried on bibulous paper, undergo no change by exposure to the air. The arsenio-persulphuret remains in the alcoholic solution. Analogous salts may be similarly prepared with barium, strontium, cal- 476 ANTIMONIO-SULPHURETS. cium, and magnesium; and insoluble compounds of the same nature may be formed by way of double decomposition by mixing soluble arsenio-per- sulphurets with oxy-salts of the second class of metals. The salts in which sesquisulphuret of arsenic acts as an acid, resemble those of the persulphuret both in their general characters and mode of for- mation. Those formed with the protosulphuret of arsenic cannot be made in the moist way by direct union of their ingredients; but when solutions of the arsenio-sesquisulphurets are evaporated, spontaneous decomposition takes place, the salts of protosulphuret of arsenic of a reddish-brown colour subside, while arsenio-persulphurets remain in solution. MOLYBDO-SULPHURETS. The electro-negative ingredient of these salts is the tersulphuret of molybdenum, and the most remarkable of them is the molybdo-sulphuret of potassium, which is readily formed by decomposing with hydrosulphuric acid gas a rather strong solution of molybdate of potassa. ' If no iron is present, the liquid acquires a beautiful red colour like the solution of bi- chromate of potassa, and on evaporation prismatic crystals with four and eight sides are deposited. Berzelius describes this compound as one of the most beautiful which chemistry can produce: the crystals, by transmitted light, are ruby-red, and their surfaces, while moist with the solution which yielded them, shine like the wings of certain insects with a metallic lustre of a rich green tint. The crystals are anhydrous, dissolve readily in water, but are insoluble in alcohol. On the addition of sulphuric or any of the stronger acids, a salt of potassa is generated with escape of hydrosulphuric acid, and precipitation of tersulphuret of molybdenum. Soluble molybdo-sulphurets of sodium, lithium, and hydrosulphate of ammonia of a red colour, may be obtained by a process similar to that for preparing the preceding compound. The composition of these salts is as follows:— Names. Sulphur-base. Sulphur-acid. Equiv. Molybdo-sulphuret £ ernr of potassium Moly bdo-sul phuret of sodium Molybdo-sulphuret of lithium Molybdo-sulphuret of hydrosulphate of ammonia 39.4 26.1 leq.+ 96 1 eq. + 96 1 eq. + 96 1 eq.=151.25 1 eq.= 135.4 1 eq.= 122.1 34.25 leq.+ 96 1 eq.= 130.25 Formulas. KS + 3IoSa NaS + MoS^ LS + MoSs WH3N + HS) ) +MoS» Similarly constituted soluble salts of a red or orange colour may be ob- tained by boiling solutions of sulphuret of barium, strontium, and calcium with an excess of tersulphuret of molybdenum. The insoluble molybdo- sulphurets may be prepared from the former by way of double decompo- sition. ANTIMONIO-SULPHURETS. When two parts of carbonate of potassa are intimately mixed with four of sesquisulphuret of antimony and one part of sulphur, and the mixture is fused, an antimonio-persulphuret of potassium is generated. On digesting in water, a subantimonio-persulphuret is dissolved, and is deposited by gen- tle evaporation in large colourless tetrahedrons, which become yellow on exposure to the air. The salts which this sulphur-acid forms with other bases have not been examined. A sulphur-salt of potassium, in which sesquisulphuret of antimony is the acid, remains in solution after the kermes is deposited (379), and may be obtained by evaporation in vacuo In colourless irregular crystals which deli- quesce rapidly in the air, 63 mm" "~~—— -ffiftrtrriALTS. 477 TUNGSTO-SULPHURETS. The best known of these salts is that of potassium, in wliich tersulphuret of tungsten is combined with protosulphuret of potassium. It is formed when a solution of tungstate of potassa is decomposed by hydrosulphuric acid, and crystallizes by evaporation in flat quadrilateral prisms, which are anhydrous, and are of a pale red colour. It dissolves sparingly in alcohol, but is freely soluble in water, yielding an orange-coloured solution. When mixed with a quantity of acid insufficient for entire decomposition, it forms a bitungsto-sulphuret of a brown colour. The tungsto-sulphuret of potassium unites with tungstate of potassa as a double salt, which yields a yellow solution, and crystallizes in rectangular tables of a lemon-yellow colour. It combines also with nitrate of potassa, and the resulting double salt crystallizes in large transparent crystals of a ruby-red tint, and when heated detonates like gunpowder. The tungsto-sulphuret of sodium is prepared from tungstate of soda by hydrosulphuric acid, and crystallizes with difficulty in irregular crystds of a red colour. It deliquesces in the air, and is soluble in water and alcohol. SECTION IV. CLASS OF SALTS. ORDER IV. HALOID SALTS. In this section are included substances composed like the preceding salts of two bi-elementary compounds, one or both of which are analogous in composition to sea-salt. The principal groups consist of double chlorides, double iodides, double fluorides, and double cyanurets. In these the haloid- bases belong usually to the electro-positive metals, and the haloid-acids to the metals which are electro-negative. I shall apply to them the same prin- ciple of nomenclature as to the sulphur-salts.* * Dr. Hare has adopted for these compounds the same plan of nomencla- ture as for the sulphur-salts.—?ee note, page 471. Viewing, as Dr. Turner has done, the double haloid salts of Berzelius as simple salts, consisting of two bi-elementary compounds, the more electro-negative compound acting the part of an acid, the other of a base, he has applied to the former the generic appellation of acid, and named the salts themselves on the same plan as the oxy-salts. Accordingly, he has given the usual acid termination to the name of the more electro-positive element of the compound acting as the acid, considering this element as the radical, and prefixes to it syllables indicating the other element. Where oxygen is the electro-negative ele- ment of an acid, it is understood to be present, without any syllables being prefixed to indicate its presence; but where other elements replace oxygen in compounds having the nature of ordinary acids, it is obviously necessary to indicate the replacing element. Adopting these principles of nomencla- ture, Dr. Hare has chlorohydrargyrates, chloroaurates, iodoplatinates, bro- mohydrargyrates, fluohydrates, fluoborates, fluosilicates, cyanoferrates, &c., corresponding with Dr. Turner's hydrargo-chlorides, auro-chlorides, platino- iodides, hydrargo-bromides, hydro-fluorides, boro-fluorides, silico-fluorides, and ferro-cyanurets, &c.—Ed, 478 AURO-CHLORIDES. HYDRARGO-CHLORIDES. The haloid-acid of this family is bichloride of mercury, which reddens litmus paper, and loses the property when a haloid-base is present, thus bearing a close analogy to ordinary acids. Its principal salts which have been examined are thus constituted:— Names. Basic chloride. Bichlor. Merc. Equiv. Formulae. Dihydrargo-chlo- J ride of potas-V 149.14 2 eq. +272.84' 1 eq.= 421.98 2KCl + HgCl2. sium S Do. in rhombic prisms with 18 or 2 eq. of water= 439.98 Hydrargo-chlo- i ride of potas- V 74.57 1 eq. +272.84 1 eq.= 347.41 KCl + HgCh. sium } Do. in acicular crystals with 18 or 2 eq. of water=365.41 Bihydrargo-chlo- J ride of potas-V 74.57 1 eq. +545.68 2 eq.= 620.25 KCl-f2HgCl2. sium \ Do. in acicular crystals with 36 or 4 eq. of water= 656.25 ^dTSShlm} 58-72 W+272.84 ieq..331.56 NaCl + HgCl2. Do. in crystals with 36 or 4 eq. of water =367.56 Dili3'drargo-chlo- *) ItatS:['»'•" >*+««« i •*-«»*\m$£$a) monia. j Do. in flat rhombic prisms with 18 or 2 eq. of water=397.98. The preceding salts, except the last, were first prepared and examined by Bonsdorff (An. de Ch. et de Ph. xliv. 189); and they are obtained by mixing the ingredients in the ratio for combining, and setting aside the solution to crystallize. The ammoniacal salt has long been known under the name of salt of alembroth. Bonsdorff obtained similar compounds with the chlorides of lithium, barium, strontium, calcium, magnesium, manganese, iron, cobalt, nickel, and copper. Those of lithium, calcium, magnesium, and zinc are de- liquescent. The hydrargo-chlorides of iron and manganese are isomorphous, and crystallizes in rhombic prisms. Hydrochloric acid combines with b chloride of mercury, and yields a very soluble salt, which may be obtained in crystal's: the electro-positive ingredient is here probably hydrochloric acid, and as such will be considered as chloride of hydrogen, with proper- ties analogous to the chlorides of electro-positive metals. AURO-CHLORIDES. These salts, the electro-negative ingredient of which is the terchloride of gold, have been studied by Berzelius, Johnston, and Bonsdorff. They are prepared by mixing the chlorides in atomic proportions, and setting aside the solution to crystallize. Most of them have an orange or yellow colour, and consist of single equi- valents of their constituent chlorides, as is exemplified by the composition of the three following salts :— Names. Basic chloride. Terch. Gold. Equiv. Formula?. Auro-chloride of j 71#5? j eq , 305.46 1 eq.=380.03 KCl + AuCR potassium S Do. in prisms with 45 or 5 eq. ofwater =425.03 ASiCum°ride °f | 58,72 1 e1- + 305-46 1 eq.=364.18 NaCl + AuCl3. Do. in 4-sided pr. with 36 or 4 eq. of water=400.18 >*«■■«- —-~ii-ii. i inHtiio-f rtiLORiDES. 479 Names. Basic chloride. Terch. Gold. Equiv. Formulae. Auro-chloride of) „._,,. ,,QM ( (H^N+HCl) hydrochlorate [ 53.57 1 eq.+305.46 1 cq.= 359.03 J ^^ of ammonia \ . one oo Do. in acicular crystals with 36 or 4 eq. of water=395.03 Auro-chloride of Potassium.-This salt crystallizes either in striated prisms or thin hexagonal tables, which effloresce in a dry air, and lose all their water at 212°. At a red heat the terchloride of gold is decomposed, leaving chloride of potassium and metallic gold. This salt is soluble both in water and dcohol. . ... Auro-chloride of Sodium crystallizes in long quadrilateral prisms, which may be exposed to the air without change, and fuse readily m their water of crystallization. The auro-chloride of lithium is deliquescent. Auro-chloride of Hydrochlorate of Ammonia.—It crystallizes m transpa- rent needes or small prisms, which become opaque by exposure to the air, and are soluble in water and alcohol. Auro-chloride of Hydrogen.—-In this compound hydrochloric acid is pro- bably the positive chloride. It crystallizes readily in long acicular crystds of a light yellow colour when an acid solution of gold is cautiously evapo- rated. The crystals undergo no change in dry air, but in a moist atmo- sphere deliquesce into a yellow liquid. Bornsdorff has prepared the auro-chlorides of barium, strontium, calcium, magnesium, manganese, zinc, cadmium, cobalt, and nickel. Most of them crystallize in prisms and contain water of crystallization. PLATINO-CHLORIDES. Both the protochloride and bichloride of platinum act as haloid-acids. Magnus prepared the platino-protochloride of potassium by mixing chloride of potassium with a solution of protochloride of platinum in hydrochloric acid. It crystallizes by evaporation in red, anhydrous, 4-sided prisms, . which are insoluble in alcohol, but dissolve readily in water. It consists of single equivalents of its constituent chlorides. The platino-protochloride of sodium may also be prepared, is soluble in water and alcohol, and crystallizes with difficulty. A similar salt may be formed with hydrochlorate of ammonia, and is isomorphous with that of potassium, which it dso resembles in its properties, composition, and mode of preparation. The solution of protochloride of platinum in hydrochloric acid, which has a deep red tint, is doubtless a double chloride, but it has not been obtained in crystals. The principal salts of bichloride of platinum are those of potassium, so- dium, and hydrochlorate of ammonia, which are thus constituted:— Names. Basic chloride. Bichl. of Plat. Equiv. Formulae. Platino-bichloride >745? 1 , 169 64 i eq.=244.21 KC1+P1C12. of potassium \ Platino-bichloride ) 5g_72 ! eq . 169.64 i eq.=228.36 NaCl+PlCR of sodium ) Do. in prisms with 54 or 6 eq. ofwater =282.36 Platino-bichloride ) c m ■\r_i_wr,l'\ of hydrochlorate V 53.57 1 eq.+ 169.64 1 eq.=223.21 V T-p^T; of ammonia ) Platino-bichloride of Potassium.—The production of this salt by mixing its constituents in solution, constitutes one of the best tests for potassa (page 297.) It is commonly obtained as a powder of a pale lemon-yellow colour; but by slow evaporation it yields small octohedrons of a brilliant lustre. It is anhydrous, insoluble in alcohol, and is sparingly dissolved by cold, but 480 RHODIO-CHLORIDES. more freely by hot water. Heated to redness it yields chlorine, and the re- sidue consists of platinum and chloride of potassium. Platino-bichloride of Sodium.—This salt crystallizes in fine transparent prisms of a deep yellow colour, which are soluble in water and alcohol. When gently heated it loses its water of crystallization, and becomes a pde yellow powder. Platino-bichloride of Hydrochlorate of Ammonia falls as a lemon-yellow powder when sal ammoniac is mixed with a strong solution of bichloride of platinum. It resembles the double salt of potassium in its properties and form, crystallizing in small anhydrous octohedrons when its aqueous solu- tion is slowly evaporated. This salt is employed in the preparation of plati- num, and when heated to redness leaves that metal in a spongy state. Bonsdorff has prepared the platino-bichlorides of barium, strontium, cal- cium, and several other metals. Most of them crystdlize with water of crystallization, and have a yellow or orange colour. PALLADIO-CHLORIDES. Both of the chlorides of palladium act as haloid-acids, combining with many of the metallic chlorides, when their respective solutions are mixed and evaporated. The principal palladio-cdorides which have been examined are those with the chlorides of potassium and sodium, and with hydrochlo- rate of ammonia, which consist of single equivalents of their ingredients. The palladio-protochloride of potassium crystallizes in four-sided prisms of a dirty yellow colour, which are anhydrous, insoluble in alcohol, and freely soluble in water. The corresponding salt of sodium is deliquescent and soluble both in water and alcohol. That of hydrochlorate of ammonia is isomorphous with the salt of potassium, which it resembles in its other properties. The palladio-bichloride of potassium is obtained by evaporating the pal- ladio-protochloride with nitro-hydrochloric acid, when microscopic crystals of a cinnabar-red colour are deposited, which by a glass are found to be re- gular octohedrons. It is anhydrous, insoluble in alcohol, and nearly so in water. When heated, or by continued ebullition, it is reconverted into the palladio-protochloride of potassium. The corresponding salt of hydrochlo- rate of ammonia is obtained in a similar manner, and resembles the former in form and other properties. RHODIO-CHLORIDES. The sesquichloride of rhodium combines with the chlorides of potassium and sodium, and the resulting salts are thus constituted:— Names. Basic Chlor. Sesquichl. Rhod. Equiv. Formulae. "rf^S^ (149-142e1- + 210-66 leq.=39.58 2KC1 + R2C13. Do. in four-sided prisms with 18 or 2 eq. of water=377.8 Trirhodio-chloride J n6 16 3 21Q 66 j 38G82 3NaCl + R2CR of sodium > Do. in prisms with 162 or 18 eq ofwater =548.82 Dirhodio-chloride of Potassium.—It is obtained by mixing the respective chlorides in the ratio above assigned, and crystallizes in four-sided rectan- gular prisms, which are of a deep red colour, insoluble in alcohol, and con- tain 18 parts or two eq. of water combined with 359.8 parts or one eq. of the salt. Hydrochlorate of ammonia yields a similar double salt, analogous In its properties to the preceding. Trirhodio-chloride of Sodium.—This salt crystallizes in large prismatic crystals of a deep red colour, which lose part of their water in a dry air, and become covered with a red powder. They are insoluble in alcohol. 481 IRIDIO-CHLORIDES. The chlorides of iridium act as haloid-acids. The most remarkable of its salts is the iridio-bichloride of potassium, which in form and properties re- sembles the platino-bichloride of potassium, crystallizing in brilhant octohe- drons, but of a black colour, which are sparingly soluble in water. Hydro- chlorate of ammonia forms with it a similar salt, which is of a deep cherry- red colour. OSMIO-CHLORIDES. Berzelius has described the osmio-bichloride of potassium, which resem- bles in form, composition, and most of its properties the corresponding salts of platinum and iridium. It is insoluble ,in alcohol, and but sparingly dis- solved in water; but its aqueous solution, when gently evaporated, yields oc- tohedron crystals of a deep brown colour. OXY-CHLORIDES. Chemists are acquainted with a considerable number of compounds in which a metallic oxide is united with a chloride either of the same metal, which is the most frequent, or of some other metal. These compounds are commonly termed sub-muriates, on the supposition that they consist of hy- drochloric acid combined with two or more equivalents of an oxide. Oxy-chlorides of Iron.—When the crystallized protochloride of iron is heated without exposure to the air, the last portions of its water exchange elements with part of the chloride of iron, yielding hydrochloric acid, which is evolved, and protoxide of iron. On raising the heat so as to expel the pure chloride of iron, a deep green oxy-chloride in scaly crystals remains. (Berzelius.) _ The ochreous matter which falls when a solution of the protochloride of iron is exposed to the air, is hydrated peroxide of iron combined with some perchloride. A similar hydrate is obtained by mixing with a solution of the perchloride of iron a quantity of alkali insufficient for complete decomposi- tion. When a solution of the perchloride is evaporated to dryness without exposure to the air, the last portions of water exchange elements with the perchloride, hydrochloric acid is disengaged, and after subliming the pure anhydrous perchloride, a compound in large, brown, shining laminae is left, which consists of peroxide and perchloride of iron. (Berzelius.) Mr. Phillips has described a soluble oxy-chloride, which appears to consist of one eq. of perchloride of iron with nine eq. of the peroxide. It is prepared by digesting hydrochloric acid with the required proportion of the moist hy- drated peroxide. The solution is of a brownish-red colour, and a precipitate is occasioned either by a little more of the peroxide or a little acid, indi- cating the formation of other oxy-chlorides which are insoluble. (Phil. Mag. and An. viii. 406.) Oxy-chlorides of Tin.—When a large quantity ofwater is poured on crys- tallized protochloride of tin, a portion of water and protochloride exchange elements, an acid solution is formed, containing the double chloride of tin and hydrogen, and a white powder subsides, which is a compound of the protoxide and protochloride of tin. Oxy-chloride of Antimony.—It falls as a white curdy precipitate when sesquichloride of antimony is thrown into water (page 377); and according to an analysis by Phillips contains about 7.8 per cent of chlorine. Oxy-chloride of Cerium.—This compound is generated by heating the hy- drated protochloride, just as when the protochloride of iron is distilled. Oxy-chloride of Bismuth.—It is prepared by pouring a neutral solution of nitrate of oxide of bismuth into a concentrated solution of sea-salt; and a similar compound, but with more oxide, is formed when a dilute solution of sea- salt is used. They are both heavy insoluble powders of a very white colour. 41 482 CHLORIDES WITH PHOSPHURETTED HYDROGEN. Oxy-chloride of Copper.—This compound falls as a green hydrate when potassa is added to a solution of chloride of copper insufficient for its com- plete decomposition. When its water is expelled it becomes of a liver-brown colour. Berzelius states it to consist of one cq. of the chloride and three eq. of oxide of copper. It is used as a pigment under the name of Brunswick green, being prepared for that purpose by exposing metallic copper to hydro- chloric acid or a solution of sal ammoniac. The same compound is gene- rated during the corrosion of copper in sea-water. Oxy-chlorides of Lead.—A compound of one eq. protochloride to two eq. of protoxide of lead has been found as a colourless mineral. Another oxy- chloride with three eq. of the protoxide is prepared by adding pure ammonia to a hot solution of chloride of lead. It falls as a heavy white hydrate ; but on expelling its water by heat, it acquires a pale yellow colour. A third oxy-chloride with a still larger proportion of oxide is used as a pigment under the name of mineral or patent yellow; and it is prepared by the action of moist sea-salt on litharge, by which means portions of the protoxide and sea-salt exchange elements, yielding soda and chloride of lead. After wash- ing away the alkali, the mixed oxide and chloride are dried and fused. Oxy-chloride of Mercury.—This compound is obtained as a shining crys- talline powder, of a bi ownish-black colour, when peroxide of mercury is boiled with a solution of the bichloride. It is anhydrous, and consists of sin- gle equivalents of the oxide and chloride. CHLORIDES WITH AMMONIA. Several interesting compounds of chlorides with ammonia have been stu- died by Persoz and Rose. (An. de Ch. et de Ph. xliv. 315, and li. 5, and Pog. Annalen, xx. 149.) The perchlorides of chromium, tin, titanium, antimony, and iron absorb ammonia at common temperatures; and most of the other chlorides absorb it when gently warmed. The chlorides of potassium, so- dium, and barium do not absorb ammonia, while those of strontium and cal- cium combine with four equivalents of the alkdi. Chloride of copper absorbs three equivalents, and acquires the same deep blue tint as the ammoniaco- sulphate of copper. Chloride of nickel unites with three, and chloride of co- balt with two equivalents of ammonia, Chloride of silver takes up slowly one and a half equivalents. Calomel absorbs half an equivalent and forms a black compound; but on exposure to the air, the ammonia flies off, and pure white calomel remains. Corrosive sublimate, by the aid of heat, rapidly ab- sorbs half an equivalent of ammonia, and forms a white compound, wliich is insoluble in water, and bears a considerable temperature without decomposi- tion : the white precipitate of pharmacy is probably analogous in nature, though the ratio of its ingredients is different. Perchloride of titanium com- bines with two equivalents, and that of tin with one. The bromides and iodides, as well as the bicyanuret of mercury, absorb ammonia in the same manner as the chlorides. Nearly all of these compounds depend on very feeble affinities. Most of them lose their ammonia by mere exposure to the air, and it is expelled from nearly all by a very moderate heat; in some, as with perchloride of titanium, heat occasions reactions between the chlorine and ammonia, and the metal is insulated ; but in general the alkali is simply expelled, and the chloride returns to its former condition. Though these ammoniacal chlorides may be viewed as salts in which a metallic chloride acts as an acid, they appear to be more closely allied to those singular com- pounds of ammonia with the oxy-salts which have already been noticed (page 442). To this remark, some of them, of which the ammoniacal chlo- ride of mercury is an instance, are probably exceptions. CHLORIDES WITH PHOSPHURETTED HYDROGEN. The analogy which Rose has traced between ammonia and phosphuretted hydrogen is especially remarkable in the compounds which they both form with metallic chlorides. He has examined the compounds of phosphuretted hydrogen with the perchlorides of titanium, tin antimony, iron and alum - mum, all of which correspond to ammoniacal chlorides of similar composi- tion. The phosphuretted hydrogen is in all readily displaced by water or a solution of ammonia. Rose observed that the resulting chloride is the same in character and composition, whichever of the two kinds of phosphuretted hydrogen were used in its preparation. He also found that the gas, when dis- placed from perchloride of titanium by water, does not inflame spontaneously, whereas, if displaced by a solution of potassa or its carbonate, by carbonate of ammonia or hydrochloric acid, the gas is spontaneously imflammable. He was thus able to disengage at will either variety of phosphuretted hydro gen from the same compound, without reference to the kind which had been used in its preparation. These facts first led Rose to the opinion that the two gases of phosphorus and hydrogen must be similar in composition. (Page 264.) DOUBLE IODIDES. These compounds have not yet been closely studed; but there is no doubt that the iodides are capable of forming with each other an extensive series of compounds. Bonsdorff obtained the hydrargo-biniodide of potassium by saturating a strong solution of iodide of potassium with biniodide of mer- cury • it may also be formed by dissolving corrosive sublimate in a solution of iodide of potassium, evaporating to dryness, and digesting in alcohol, when the double iodide is dissolved, and chloride of potassium is left. A va- riety of double iodides have been described by Boullay, and among them a compound of biniodide of mercury and hydriodic acid. (An. de Ch. et de Ph. xxxiv.) In general the double hydrargo-biniodides contain single equi- vdents of the respective iodides. Liebig obtained a compound of the bichlo- ride and biniodide of mercury, consisting of two eq. of the former to one eq. of the latter, as indicated by the formula Hgl2-f2HgCls. Several compounds of biniodide of platinum with other iodides have been studied by Mr. Kane of Dublin, and Lassaigne. (Dublin Journal of Science, i. 304, and An. de Ch. et de Ph. Ii. 125.) The compounds at present known are thus constituted:— Names. Basic Iodide. Biniod. Plat. Equiv. Formulae. Platino-biniodide of )1GSA5 i eq 1351.4 1 eq.=516.85 KI+P1R potassium ( Do. of sodium . 149.6 1 eq+351.4 1 eq.=501 Nal+PIR Do. of hydriodate ) W4 45 Y +351.4 j eo.=495.85 \ (H,* p^HI) of ammonia J f -r-rii3. Do. of barium . 195 1 eq+351.4 1 eq.=546.4 Bal+PIR Do. of zinc . 158.6 1 eq.+35l.4 1 eq.=510 Znl+PlP. Do. of hydrogen 127.3 1 eq+351.4 1 eq.=478.7 Hl+Plla. The platino-biniodide of potassium is prepared by digesting an excess of biniodide of platinum in a rather concentrated solution of iodide of potas- sium. By spontaneous evaporation it crystallizes in small rectangular plates surmounted sometimes with a four-sided pyramid, which are anhydrous, unchanged in the air, and insoluble in alcohol. The colour of the crystals is black with a metallic lustre, and they yield a deep claret-coloured solu- tion with water. The biniodide of platinum appears to combine also with the iodide of platinum; but the compound has only been obtained in solution. The platino-biniodides of sodium, barium, and zinc, are obtained in the same manner as that of potassium, crystallize with difficulty, are deliques- cent in the air, and dissolve in water and alcohol. The ammoniacal salt is analogous in its properties to that of potassium, with which it appears also to be isomorphous. Platino-biniodide of Hydrogen,—This compound consists of hydriodic 484 BORO-FLORIDES. acid and biniodide of platinum, in which the former is regarded as the electro-positive element. It is prepared by acting on biniodide of platinum with a cold dilute solution of hydriodic acid, which gradually acquires a deep claret, colour, and by evaporation under a bell-jar with quicklime, de- posites black acicular crystals. The crystals become moist by exposure to the air. Oxy-iodides.—The principal oxy-iodides at present known to chemists are those formed by the oxide and iodide of lead. When iodide of potassium is mixed with acetate of oxide of lead in excess, the yellow iodide at first formed combines with oxide of lead and acquires a white colour; and the same compound is obtained directly by employing a subacetate. M. Denot finds that there are three oxy-iodides, in which one eq. of iodide of lead is united with one, two, and five equivalents of oxide of lead. DOUBLE BROMIDES. These compounds have not yet been studied; but Bonsdorff has proved the possibility of forming compounds similar in composition and properties to the double chlorides. He obtained the hydrargo-bibromide of potassium in crystals, consisting of an equivalent of each bromide united with two eq. of water. DOUBLE FLUORIDES. The researches of Berzelius have led to the formation of several exten- sive families of double fluorides, in which the fluorides of boron, silicium, titanium, and of other electro-negative metals are the acids, and the fluorides of electro-positive metals are bases. In some instances hydrofluoric acid is a haloid-acid; but more commonly it acts the part of a base. Hydro-fluorides.—In this family hydrofluoric acid is combined with the fluorides of electro-positive metals. If an equivalent of any electro-positive metal be indicated by M, then the general formda for this family is MF+HF. The hydro-fluoride of potassium is made by mixing hydrofluoric acid with a solution of fluoride of potassium, and evaporating by a gentle heat in a platinum capsule. It commonly crystallizes in confused laminae; but, by slow evaporation, in square tables or cubes, which are anhydrous and dissolve freely in pure water. It fuses readily when heated, and loses all its hydrofluoric acid at a low red heat. The hydro-fluoride of sodium is prepared as the preceding salt, and by spontaneous evaporation yields anhydrous rhombohedral crystals. It is sparingly soluble in cold, but much more freely in hot water. The hydro- fluoride of lithium is also of sparing solubility. The fluorides of barium, strontium, calcium, and magnesium, do not combine with hydrofluoric acid. BORO-FLUORIDES. When the terfluoride of boron (fluoboric acid gas) is acted upon by water, one out of every four equivalents of the gas interchanges elements with water, giving rise to hydrofluoric and boracic acids, the former of which combines as a haloid-base with undecomposed terfluoride of boron, consti- tuting the boro-hydrofluoric acid (page 241), but which may be viewed as the boro-fluoride of hydrogen. This change is such that 4 eq. terfluoride of boron 4(B+3F) ^ 3 eq. terfluoride of boron 3(B+3F) [o 3 eq. hydrofluoric acid 3(H+F) and 3 eq. of water 3(11 + 0) ^ and 1 eq. boracic acid B+30. By careful concentration and cooling, the boracic acid separates as a crys- talline powder, and the boro-fluoride of hydrogen remains in solution. It is strongly acid to test paper, and its composition is indicated by the formda SILIC0-FLU0RIDES. HF+BFs, being an equivalent of each fluoride. On adding potassa to this compound, it interchanges elements with hydrofluoric acid, and there re- suits the boro-fluoride of potassium, KF+BFs, the hydrogen being simply displaced by potassium. The protoxides of most other metals act precisely like potassa, and, therefore, the general formula of these compounds is MF + BF«. When exposed to a strong heat, they all give off terfluoride of boron, and a metallic fluoride is left. Boro-fluoride of Potassium.—It is prepared by dropping boro-fluoride of hydrogen drop by drop into a solution of a salt of potassa, and falls as a gelatinous transparent hydrate, which is a white very fine powder when dried. It has a slightly bitter taste, and is quite neutral to test paper, is very sparingly soluble in alcohol and cold water, but is dissolved freely by hot water, and subsides on cooling in small brilliant anhydrous crystals. At a strong red heat it gives off the terfluoride of boron and fluoride of potas- sium remains. The boro-fluoride of sodium is very soluble in water, and is, therefore, best obtained pure by the direct action of boro-fluoride of hydrogen on fluoride of sodium. It crystallizes by slow evaporation in large rectangular prisms, which redden litmus paper strongly. The boro-fluoride of lithium also crystallizes in large prisms, is very soluble in water, and deliquesces in the air. The boro-fluoride of barium is prepared by adding carbonate of baryta to boro-fluoride of hydrogen till it ceases to be dissolved, avoiding any further addition. On evaporating to the consistence of a syrup, long acicular crystals form, and by keeping the solution in a warm place, it yields flat, four-sided, rectangular prisms. It is acid to test paper, and deliquescent. The boro-fluorides of calcium and magnesium may be prepared in a similar manner, and are soluble in water. Lead forms a soluble boro-fluoride, which crystallizes in the same manner as the boro-fluoride of barium. SILICO-FLUORIDES. The acid solution, called silico-hydrofluoric acid (page 327,) may be viewed as the silico-fluoride of hydrogen, a compound of 52.36 parts or two eq. of fluoride of silicium and 19.68 or one eq. of fluoride of hydrogen (hydrofluoric acid), as indicated by the formula HF+2SiF. When the solution is neutralized with potassa, the alkali interchanges elements with fluoride of hydrogen, water and fluoride of potassium are generated, and the latter combines with the fluoride of silicium. This double fluoride consists, therefore, of 52.36 parts or two eq. of fluoride of silicium, and 57.83 or one eq. of fluoride of potassium, the formula of which is KF+ 2SiF. A similar change ensues with the protoxides of most other metals, and hence the general formula of the silico-fluorides is MF+2SiF. On ex- posing these compounds to a red heat, fluoride of silicium is disengaged. Silico-fluoride of PotassiuniM—This salt falls as a very transparent jelly, which has the property of reflecting the colours of the rainbow ; but when collected on a filter and dried, it becomes a white powder. By evaporating a saturated aqueous solution, it separates in minute anhydrous crystals. It is sparingly soluble in hot water, and still less so in cold water. The silico-fluoride of sodium resembles the former salt, but is much more soluble in hot water. By evaporation it is obtained in minute anhydrous hexagonal prisms. The silico-fluoride of lithium forms similar crystals, but is more soluble in water. The silico-fluoride of barium gradually falls in microscopic crystals, which through a glass appear as elongated prisms, when chloride of barium is mixed with the silico-fluoride of hydrogen, hydrochloric acid remaining in solution. This salt is very sparingly soluble in water whether hot or cold. The silieo-fluorides of strontium, calcium, magnesium, and lead are best prepared by dissolving their respective carbonates in silico-fluoride of hy- drogen. The salt of strontium crystallizes in short quadrilateral prisms, 486 DOUBLE CYANURETS.--FERRO-CYANURETS. which lose their water of crystallization at a gentle heat and become opaque. For complete solubility in water, they require a slight excess of hydrofluoric acid to be present, and then they dissolve freely. The salt of calcium crys- tallizes in regular quadrilateral prisms. It dissolves readily in water acidu- lated with hydrofluoric or hydrochloric acid, but is decomposed by pure water, yielding an acid soluble salt, and an insoluble subsalt. The salts of magnesium and lead are very soluble, and leave a gummy mass by evapo- ration. The silico-fluorides of manganese, iron, zinc, cobalt, nickel, and copper are soluble in water, and crystallize in similar hexagonal prisms, probably i-somorphous, which contain respectively one eq. of fluoride of silicium and seven eq. of water of crystallization. TITANO-FLUORIDES. Hydrofluoric acid dissolves titanic acid, and forms with it an acid solution which may be viewed as the titano-fluoride of hydrogen, consisting of 61.66 parts or one eq. of bifluoride of titanium, and 19.68 or one eq. of fluoride of hydrogen, expressed by the formula HF+TiF2. When mixed with po- tassa, water and fluoride of potassium are generated, and the titano-fluoride of potassium results, the formula of which is KF+TiFa. By substituting most other protoxides for potassa, similar salts may be prepared, the general formula being MF+TiF2. Few of the titano-fluorides have as yet been studied. That of potassium crystallizes by evaporation in scales like boracic acid, which are anhydrous, and but sparingly soluble in cold water. The titano-fluoride of sodium is very soluble, and crystdlizes with difficulty. Similar double fluorides may be formed, in which the fluorides of molyb- denum, tellurium, and platinum, act as the electro-negative ingredients. Few of them, however, have as yet been studied. Berzelius has prepared the alu- mino-fluorides of potassium and sodium, and the zircono-fluoride of potas- sium. He employed the latter [in the preparation of metallic zirconium. The alumino-fluoride of sodium is found in nature as a rare mineral called cryolite. OXY-FLUORIDES. Several fluorides combine with oxides in the same manner as chlorides and iodides. An oxy-fluoride of aluminium is prepared as an insoluble gela- tinous hydrate by digesting hydrate of alumina in a solution of the sesqui- fluoride of aluminium. This oxy-fluoride, combined with silicate of alumina, constitutes the topaz. The neutral fluorides of cobalt, nickel, and copper are decomposed by hot water, being resolved into soluble hydro-fluorides, and in- soluble oxy-fluorides. Several other fluorides doubtless undergo a similar change. The oxy-fluoride of lead is generated either by digesting fluoride of lead in solution of ammonia, or by fusing together the fluoride and oxide of lead. It is more soluble than the fluoride, and the solution by exposure to the air gives a precipitate of carbonate of oxide of lead. The fluoride of lead also combines by fusion with chloride of lead. Fluoride of calcium forms a very fusible compound with sulphate of lime. DOUBLE CYANURETS.—FERRO-CYANURETS. The double cyanurets constitute an extensive and important group of salts, of which a few species only have as yet been studied. Those at pre- sent'observed by chemists are the ferro-cyanurets, ferro-sesquicyanurets, zinco-cyanurcts, cobalto-cyanurets, nicco-cyanurets, and cupro-cyanurets, in which the proto-cvanurct of iron, sesqui-cyanuret of iron, cyanuret of zinc, cyanuret of cobalt, cyanuret of nickel, and cyanuret of copper, may be con- sidered as the electro-negative cyanurets. FERRO-CYANURETS. Most of the ferro-cyanurets are disalts, in which one equivalent of proto- cyanuret of iron, FeCy, is combined with two equivalents of some other cyanuret. The general formula of such compounds is 2MCy+FeCy, in which M represents any electro-positive metal. The elements are in such a ratio, that if the metals were converted into protoxides at the expense of water, the hydrogen would just suffice to form hydrocyanic acid with the cyanogen :—thus would 1 eq. ferro-cyan. 2MCy+FeCy 2 2 eq. hydrocy. oxide of M. 2(M+HCy) and 3 eq. water 3(H+0) -^ 1 eq. hydrocy. oxide of iron Fc+HCy. The ferro-cyanurets in solution may hence be regarded as hydrocyanates of protoxides. But it is better to abstain from such views; for if adopted with one family, they may with equal reason be applied to others, and be extend- ed to the haloid and sulphur-salts generally, which would create much con- fusion, and a necessity for conceiving very improbable, not to say unchemi- cal, modes of combination. The ferro-cyanurets of the metals of the alkalies and alkaline earths are soluble in water: those of the metals of the earths have not been prepared at all; and those of the second class of metds are insoluble. The latter may readily be obtained from the former by means of double decomposition. When the soluble ferro-cyanurets are dried and heated to redness in close vessels, the electro-positive cyanuret escapes decomposition, while the nega- tive cyanuret is resolved into carburet of iron and nitrogen, the latter escap- ing as gas. The insoluble ferro-cyanurets undergo complete decomposition by heat, either nitrogen gas, or nitrogen mixed with cyanogen gas being evolved. The iron in solutions of the soluble ferro-cyanurets is not detected by alkalies, or hydrosulphuric acid. When acted on by the stronger acids, such as the sulphuric, the electro-positive cyanuret is resolved by decompo- sition of water into hydrocyanic acid and an oxide of the metal; while a white precipitate falls, principally containing cyanuret of iron, which yields Prussian blue by exposure to the air. Most of the insoluble ferro-cyanurets are soluble without decomposition in strong sulphuric acid, and subside witli their former characters on dilution with water. The principal ferro-cyanurets are thus constituted :— Names. Basic Cyan. Cyan. Iron. Equiv. Formulae. F7o?asIiunm°fS13L03 2eq. + 54.39 1 eq.= 185.47 2KCy + FeCy. Do. in crystals with 27 or 3 eq. ofwater =212.47 ^sodium"' °f£ 99,38 2eq-+54-39 leq.= 153.77 2NaCy+FeCy. Do. in crystals with 108 or 12 eq. of water=261.77 ^barium"' °fyl9(U8 2 eq-+54-39 1 eq.=244.57 2BaCy+FeCy. Do. in crystds with 54 or 6 eq. ofwater =298.57 Feste0onTium0fJ140-38 2e> 2 eq. potassa 2(K+0). With a salt of manganese, copper, silver, and most other metals, similar decompositions ensue. With a salt of protoxide of iron a white precipitate fdls, which is perferro-cyanuret of potassium, that is, contains cyanuret of potassium united with two or more equivalents of protocyanuret of iron, and which on exposure to the air yields Prussian blue. With a persalt of iron Prussian blue is instantly generated. Ferro-cyanuret of Sodium is prepared like the preceding salt, and crystal- lizes by evaporation in four-sided prisms of a yellow colour, which require 4j| times their weight of cold water for solution, and much less of boiling water. They contain 39 per cent, of water, corresponding to twelve equivalents, some of which they lose in a dry air and fall into powder. Ferro-cyanuret of Barium is conveniently prepared by mixing hot, rather dilute, solutions made with 2 parts of ferro-cyanuret of potassium and 1 part of chloride of barium; when on cooling, and also by evaporation, ferro- cyanuret of barium separates in small rhomboidal crystals of a pale yellow colour, which require for solution 100 times their weight of boiling, and 1920 of cold, water. Of six eq. of water contained in the crystals, five and a half eq. are given off at 104°,' and the remainder is retained until the heat is so high as to decompose the salt itselfl Ferro-cyanuret of Strontium is best prepared by digesting pure Prussian blue in a solution of strontia. This salt is very soluble, and crystallizes less readily than the preceding in yellow crystals. The ferro-cyanurets of cal- cium and magnesium are made in a similar manner, and are very soluble in water, the latter being very deliquescent. The former, when its solution is evaporated to the consistence of thin syrup, and kept in a warm place, crys- tallizes in oblique four-sided prisms of a pale yellow colour. The crystals lose eleven and a half eq. of water at 104° without bss of form, but retain the residual half equivalent until the heat is sufficient m decompose the salt itself. . , , . When pure Prussian blue is heated with a solution of ammonia, or moist ferro-cyanuret of lead with carbonate of ammonia, a solution is obtained which by spontaneous evaporation yields small octohedral crystals of a straw-yellow colour. These crystals apparently consist of 89.08 parts or two eq. of hydroeyanate of ammonia, and 54.39 parts or one eq. of cyanuret ets. 489 of iron combined with one of water. When heated in a glass tube they lose their water and hydroeyanate of ammonia, leaving cyanuret of iron. On heating the solution in the open air, hydiocyanate of ammonia vola- tilizes, and Prussian blue is formed, oxygen gas being absorbed from the air. . Ferro-cyanuret of Hydrogen.—This compound was discovered by Mr. Porrett, and described by him under the name of ferruretted chyazic acid (Phil. Trans. 1814-15), the term chyazic being composed of the initials of carbon, hydrogen, and azote. It is now better known under the name of ferrocyanic or hydroferrocyanic acid, being regarded as a hydracid with a compound radical analogous to hydrosulphocyanic acid, a view suggested by Gay-Lussac as a deduction from the experiments of Robiquet and Ber- zelius. (An. de Ch. et de Ph. xv, xvii, and xxii.) In fact, on referring to its composition (page 487), it will be obvious that its formula 2HCy+FeCy admits of being placed in the order 2H+FeCy3, which represents a com- pound of tercyanuret of iron with two eq. of hydrogen. The supposed radical, however, has not been obtained in a separate state, nor is its com- position exactly analogous to other hydracids, wliich contain one eq. of hydrogen combined with the radical; whereas, if regarded as ferro-cyanuret of hydrogen, it is precisely similar to other ferro-cyanurets, hydrogen being substituted for an electro-positive metal. The existence of similar double cyanurets of hydrogen goes far to establish the propriety of this view. Two processes were recommended by Porrett for preparing this com- pound. The first consists in dissolving 58 grains of crystallized tartaric acid in alcohol, and mixing it wilh 50 grains of ferro-cyanuret of potassium dissolved in the smallest possible quantity of hot water, when bitartrate of potassa subsides, and the clear solution, by spontaneous evaporation, gradu- ally deposites ferro-cyanuret of hydrogen in small cubic crystals of a yel- low colour, which contain an undetermined quantity of water of crystalliza- tion. In the second process a solution of ferro-cyanuret of barium is de- composed by a quantity of sulphuric acid exactly sufficient for separating the barium as sulphate of baryta. Berzelius recommends that moist ferro- cyanuret of lead or copper should be suspended in water and decomposed by hydrosulphuric acid, the excess of the gas be removed by a little fresh ferro- cyanuret of lead, and the liquid after filtration be evaporated to dryness in vacuo along with sulphuric acid. An uncrystalline white residue is obtained, which contains one eq. of water, of which it cannot be deprived by heat without decomposition. In the process of Berzelius the hydrogen is derived from hydrosulphuric acid, the sulphur of which unites with lead; while in those of Porrett decomposed water supplies the hydrogen, and its oxygen combines with the potassium or barium. Ferro-cyanuret of hydrogen is neither volatile nor poisonous in small quantities, is inodorous, and has an agreeably acid taste followed by slight astringency. It reddens litmus paper, neutralizes alkalies, and dissolves the alkaline carbonates with effervescence, yielding ferro-cyanurets with the metals of the alkalies. With a persalt of iron it yields Prussian blue. Its solution in open vessels is gradually decomposed by the oxygen of the air, and Prussian blue is generated, changes which are hastened by the influ- ence of light. When its solution is boiled, hydrocyanic acid is gradually disengaged, and a white precipitate falls, which is rendered blue by the oxygen of the air. When the dry ferro-cyanuret of hydrogen is distilled, it first yields hydrocyanic acid, and then hydroeyanate and carbonate of ammonia, carburet of iron being left FERRO-SESQUICYANURETS. Sesquicyanuret of iron, which has not been obtained in a solid state, acts the same part towards the basic cyanurets, as protocyanurct of iron, in the ferro-cyanurets, and yields definite saline compounds, which may be termed ferro-sesquicyanurets. The principal compounds of this family are the 490 FERRO-SESQUICYANURETS. ferro-sesquicyanurets of potassium, sodium, barium, calcium, and hydrogen, all of which yield crystals of a red colour, are soluble in water, and form Prussian blue with salts of the protoxide of iron, for which they afford a test of great delicacy. To the same family belongs Prussian blue. These compounds are thus constituted:— Names. Basic Cyan. Sesquicy. Iron. Equiv. Formula?. ^ToTplSml196-62 3eq. + 13517 leq.=331.7D 3KCy+FecCy, Ditto of sodium 149.07 3 eq. +135.17 1 eq.=284.24 3NaCy+Fe2Cy3. Ditto of barium 285.27 3 eq. +135.17 1 eq.=420.44 3BaCy+Fe<€y3. Ditto of calcium 140.67 3 eq. +135.17 1 eq.=275.84 3CaCy+Fe2Cy3. Ditto of hydrogen 82.17 3 eq.-f 135.17 1 eq.=217.34 3HCy+Fe^Cy3. Ditto of iron 163.17 3 eq. +270.34 2 eq.=433.51 3FeCy+2FeaCya Ferro-sesquicyanuret of Potassium.—L. Gmelin, who discovered this family of salts, prepares this compound by transmitting through a rather dilute solution of ferro-cyanuret of potassium a current of chlorine gas, stirring the liquid continually to prevent an excess of chlorine in any one part of the solution. The process is known to be complete by the solution ceasing to give a blue precipitate with a persalt of iron which is quite free from the protoxide, or by the liquid, which at first becomes green, appearing red by transmitted light, as when held between the eye and a burning can- dle. The reaction of the materials is such that 2 eq. ferro-cyan. of pot. -3 1 eq. ferro-sesquicyan. of potassium 2(2KCy + FeCy) ."3 3KCy+Fe2Cy3. and 1 eq. chlorine CI ^ and 1 eq. chlo. of potassium K + Cl. The ferro-sesquicyanuret contdns all the elements of two eq. of the ferro- cyanuret of potassium, except one equivalent of potassium abstracted by the chlorine; but an excess of the gas is to be avoided, as it would speedily decompose the ferro-sesquicyanuret by virtue of the same affinity which caused its production. The solution after concentration deposites acicdar crystals, which by solution in pure water and spontaneous evaporation crys- tallize in transparent anhydrous prisms of a ruby-red colour. The ferro-sesquicyanuret of potassium is unchanged by exposure to the air, and is very sparingly soluble in alcohol. It does not precipitate salts of the peroxide of iron ; but it indicates the presence of protoxide of iron dis- solved in 90,000 parts of water, striking a blue or green precipitate accord- ing to the strength of the solution. When the crystals are heated, cyanogen and nitrogen gases are disengaged, and ferro-cyanuret of potassium, with a little carburet of iron, is reproduced. The ferro-sesquicyanurets of sodium, barium, and calcium are prepared by a similar process as the foregoing salt, which they resemble in pro- perties. . Ferro-sesquicyanuret of Hydrosen.—When the ferro-sesquicyanuret ot potassium is mixed with a salt ofprotoxide of lead, no immediate precipi- tate ensues ; but after a time reddish-brown crystals of ferro-sesquicyanuret of lead arc deputed. On decomposing these crystals with hydrosulphuric acid gas, sulphuret of lead and ferro-sesquicyanuret of hydrogen are gene- rated The latter is left as a red solution, and crystallizes by spontaneous evaporation in needles of a brownish-yellow colour. This compound has an acid taste and reaction like the ferro-cyanuret of hydrogen, and ought to be viewed ns a hydracid if the latter is so. Ferro-sesquid/anuret of Iron.—This compound was accidentally discover- ed at Berlin in 1710, from which, and from its beautiful blue colour, it be- came generally known under the name of Berlin or Prussian blue. It has of late been considered as a ferrocyanate (page 489) of the peroxide of iron, but our improved knowledge of the haloid salts has caused Berzelius w place it among the double cyanurets: it may be viewed as a ferro-cyanurei of the scsquicyanuret of iron, or as a ferro-sesquicyanuret of proloeyanurei of iron. To its production, therefore, the protocyanuret and sesquicyanuret of iron are both necessary. A ferro-cyanuret gives Prussian blue with a salt of the peroxide of iron, because by interchange of their elements ses- quicyanuret of iron is generated: thus 3 eq. ferro-cyan. of pot. ) ^ 1 eq Prussian blue 3FeCy+2Fe2Cy3. and 2 eq. peroxide of j ■% and 6 eq. potassa 6(K+0). iron 2(2Fe+JO) 3 Here the exchange lies between the peroxide of iron and cyanuret of po- tassium, four eq. of iron taking the place of six eq. of potassium, while six eq. of potassa unite with the acid which had been united with two eq. of peroxide of iron. Protoxide of iron does not produce the same effect, be- cause when it exchanges elements with cyanuret of potassium, protocyanu- ret and not sesquicyanuret of iron is generated. When the ferro-sesquicyanuret of potassium (3KCy+Fe2Cy3) is mixed with a persalt of iron, Prussian blue is not generated, because an exchange of elements between peroxide of iron and cyanuret of potassium gives rise to sesquicyanuret and not protocyanuret of iron. But if a salt of the prot- oxide is present, then 2eq.ferro.ses-) x of Pmssian blue 3FeCy+ quicyan. of>2(3KCy+Fe2Cy3) 2Fe2Cy3. potassium j ~3 md 6t ^ °rl atv _l.™ * 6 «!■ °f P°tassa 6(K+0) protoxide of V 6(Fe+0) and 3 eq. proto-cy. of iron 3(Fe+Cy). Here six eq. of protocyanuret of iron and six eq. of potassa are generated; the latter unite with the acid previously combined with the protoxide of iron, three out of the six eq. of protocyanuret of iron enter into the forma- tion of Prussian blue, and the remainder subsides along with it, or is dis- solved if an excess of acid be present To prepare pure Prussian blue, add ferro-cyanuret of potassium to sul- phate of peroxide of iron, the latter being in excess and acidulated with sul- phuric acid, and both largely diluted. After the blue precipitate has sub- sided, draw off the clear liquor with a syphon, fill up the vessel with cold water slightly acidulated with sulphuric acid, and after subsidence again draw off the water. After repeating this process several times, first with acidulated and then with pure water, dry the precipitate in a warm place. It is then obtained in smali lumps of an intensely blue colour, with a copper- red shine on its surface,. The necessity for careful washing arises from the tendency of Prussian blue to combine as a double salt with ferro-cyanuret of potassium, a tendency which is observable in the ferro-cyanurets gene- rally. Thus, Mosander finds that on mixing a concentrated solution of ferro-cyanuret of potassium with a salt of baryta, lime, or magnesia, com- pounds result which consist of one eq. of ferro-cyanuret of potassium with one eq. of ferro-cyanurets of barium, calcium, and magnesium. In like man- ner does Prussian blue combine with ferro-cyanuret of potassium, probably in two or more proportions depending on the relative quantities of the mate- rials. If a persalt of iron is gradually mixed with a solution of ferro-cyanu- ret of potassium in excess, the latter falls with Prussian blue as a double salt, which is insoluble in a saline fluid, but forms a blue solution with pure water. Hence, as soon as the saline matters of the solution are washed away, the precipitate begins to dissolve, and by continually supplying pure water nearly the whole of it is removed. From some late experiments of Berzelius this soluble compound appears to consist of one eq. of Prussian blue with one eq. of ferro-cyanuret of potassium, its formula being (2KCy+ FeCy) + (3FeCy+ 2Fe2Cy3). When the persalt of iron is mixed in excess with ferro-cyanuret of potassium, some of the latter still falls as a double salt, which, however, is insoluble even in pure water, and contains a larger 492 ZINCO-CYANURETS. proportional quantity of Prussian blue. On washing with cold water this double salt is gradually decomposed by atmospheric oxygen: a portion of proto-cyanuret of iron is resolved into peroxide and sesquicyanuret: the lat- ter unites with cyanuret of potassium, and is dissolved as the red ferro- sesquicyanuret, while the peroxide remains with the Prussian blue. (Berze- lius in An. de Ch. et de Ph. Ii. 357.) Prussian blue often contains peroxide of iron, perhaps in combination, as well as ferro-cyanuret of potassium. An instance of this has just been men- tioned, and the same impurity always exists in the Prussian blue of com- merce. When a protosalt of iron is precipitated with cyanuret of potassium the orange protocyanuret quickly undergoes the same change as protochlo- ride of iron (page 338): the iron of a portion of the protocyanuret is divided between cyanogen and oxygen from the air, forming sesquicyanuret and peroxide of iron, and the former instantly unites with the protocyanuret to constitute Prussian blue. The test for hydrocyanic acid mentioned at page 270, as well as the manufacture of Prussian blue, depends on this change. This pigment is prepared for the arts by fusing animal matters with pearl. ash so as to form cyanuret of potassium, which is mixed in solution with green vitriol and alum. A dirty green precipitate ensues, consisting of black oxide of iron, protocyanuret of iron, and alumina, which by exposure to the air becomes blue. Hence the Prussian blue of commerce contains as impurities hydrated alumina and peroxide of iron, combined usually with a little sulphuric acid, and ferro-cyanuret of potassium: this foreign matter may be removed by maceration in water strongly acidulated with sulphuric acid, and common Prussian blue be thus fitted for the preparation of bi- cyanuret of mercury and for other chemical purposes. Pure Prussian blue is insipid and inodorous, insoluble in water and alco- hol, and is not decomposed by dilute hydrochloric or sulphuric acids. Strong hydrochloric acid abstracts its iron, and causes the separation of ferro-cyanuret of hydrogen. By nitric acid it is oxidized and decomposed. Strong sulphuric acid dissolves it, forming a white compound of the aspect of gelatinous starch, from which water throws down Prussian blue in its origind state. Subjected moist to the action of hydrosulphuric acid, iron filings, or powdered tin, air being excluded, it is reduced with loss of cyano- gen to protocyanuret of iron; but it recovers its blue colour on exposure to the air. The alkalies exchange elements with the iron of the sesquicyanu- ret, separating peroxide of iron, and forming a ferro-cyanuret (page 487). Prussian blue has a strong affinity for moisture, and when dried to the ut- most temperature which it can bear without decomposition, it still retains a considerable quantity of combined water. By destructive distillation it gives off water, hydroeyanate of ammonia, and carbonate of ammonia, while carburet of iron is left in the retort ZINCO-CYANURETS. L. Gmelin obtained a double cyanuret of zinc and potassium in which the zinc acts as iron in the ferro-cyanurets. The zinco-cyanuret of potassium is obtained by dissolving cyanuret of zinc in a solution of cyanuret of potas- sium, or by dissolving hydrated oxide of zinc in a solution of cyanuret of potassium, and neutralizing with hydrocyanic acid. It is said to crystallize in large regular octohedrons, which are colourless and anhydrous. A series of zinco-cyanurcts may doubtless be formed, including an acid zinco-cyanu- ret of hydrogen. A cobalto-cyanuret of potassium may be prepared in the same manner as the preceding salt, and Gmelin procured it in yellow crystals similar in com- position and form to ferro-cyanuret of potassium. Gmelin has also obtained similar double cyanurets with nickel and copper, and his observations have been confirmed by a late pupil of mine, Mr. F Rodgers, and his brother (Phil. Mag. iv. 96, for 1834). From the facts collected by these chemiss i is obvious that the double cyanurets offer a wide field of research, and that the ferro-cyanurets merely constitute one family of a large group of salts. PART 111. ORGANIC CHEMISTRY. The department of organic chemistry comprehends the history of those compounds which are of animal or vegetable origin, and which are hence called organic substances. These bodies, viewed collectively, form a re- markable contrast with those of the mineral kingdom. Such substances in general are characterized by containing some principle peculiar to eacm Thus the presence of nitrogen in nitric, and of sdphur in sulphuric acid, establishes a wide distinction between these acids; and although in many instances two or more inorganic bodies consist of the same elements, as is exemplified by the compounds of sulphur and oxygen, or ot nitrogen ana oxygen, they are dways few in number, and distinguished by a well-marked difference in the proportion in which they are united. The products of ani- mal and vegetable life, on the contrary, consist essentially of the same ele- mentary principles, the number of which is very limited. They are nearly dl composed of carbon, hydrogen, and oxygen, in addition to which some of them contain nitrogen. Besides these, portions of phosphorus, sulphur, iron, silicic acid, potassa, lime, and other substances of a like nature, may sometimes be detected; but their quantity is exceedingly minute when com- pared with the principles above mentioned. In point of composition, there- fore, most organic substances differ only in the proportion of their consti- tuents, and on this account may not unfrequently be converted into one another. . The elements of organic bodies are united with each other in definite proportion, and, therefore, the same laws of combination which regulate the composition of mineral substances, must likewise influence that.of organic compounds. In the latter, however, the modes of combination are generally of a complex kind. A single molecule of a metallic oxide or a chloride, as determined by its combining weight, consists of two elements, and usually of two or three atoms only. Thus, in an equivalent of potassa, the chemist has to do with a single equivalent of potassium and of oxygen, between which one kind of combination only is practicable. In an equivalent of sulphuric acid there are four atoms, which admit of three different arrange- ments; for S and 30 may be united as S + 30, or S + 20, or S + 0. But here, guided by the third law of combination (page 137), and by the other compounds of sulphur and oxygen, chemists infer with" confidence that S + 30 is the true mode in which the elements of that acid are united. But in organic compounds a single combining molecule is often made up of so many elementary particles, that one is bewildered by the multiplicity of possible modes of combination. An equivalent of tartaric acid contains four eq. of carbon, two eq. of hydrogen, and five eq. of oxygen, wliich it is obvious may be arranged in a variety of ways; and an equivalent of quinia is composed of twenty-one eq. of carbon, twelve eq. of hydrogen, one eq. of nitrogen, and two eq. of oxygen. The difficulty of assigning any one ar- rangement of so many elements in preference to others equally probable, or sometimes of agreeing on any arrangement which is probable, formerly led chemists to suspect that the modes of combination in organic and inorganic bodies were essentially distinct. But the progress of analytical chemistry has in a great degree corrected this opinion, and is daily destroying the dis- 42 494 ORGANIC CHEMISTRY. tinction between these two classes of substances. The late researches of Liebig and Wohler on the radieal of benzoic acid, and those of Dumas on camphene, have indisputably established the existence of compound inflam- mable or electro-positive substances, which like cyanogen are susceptible of uniting with oxygen, chlorine, sulpbur, and other energetic principles, and of being transferred from one to the other without a change in their own constitution. Many vegetable substances appear to consist of electro-posi- tive compounds of carbon and hydrogen, which in some bodies exist mere- ly as oxides, and in others act as bases in relation to water, carbonic oxide, carbonic acid, or similar compounds. In the constitution of animal mat ters, cyanogen appears to act an important function, as may be inferred from the history of urea and uric acid. When organic substances are heated to redness with pure potassa or soda, they invariably yield alkaline carbonates; but at a temperature of about 400° or 450°, many of them are decomposed with formation of oxalic acid. This fact has been noticed by Gay-Lussac, who observed it with cotton, saw-dust, sugar, starch, gum, sugar of milk, and tartaric, citric, and mucic acids. The other products of course vary with the nature of the substance; but water and acetic acid are generally formed. (Quarterly Journal of Sci- ence, N. S. vi. 413.) Organic substances, owing to the energetic affinities with which their elements are endowed, are very prone to spontaneous decomposition. The prevailing tendency of carbon and hydrogen is to appropriate to themselves so much oxygen as shall convert them into carbonic acid and water: and hence, in whatever manner these three elements may be mutually combined in a vegetable substance, they are always disposed to resolve themselves into the compounds just mentioned. If, at the time this change occurs, there is an insufficient supply of oxygen to oxidize the hydrogen and carbon completely, then, in addition to carbonic acid and water, carbonic oxide and carburetted hydrogen gases will probably be generated. One or both of these combustible products must in every case be formed, except when oxy- gen is freely supplied from extraneous sources; because organic bodies are so constituted that their oxygen is never in sufficient quantity for convert- ing the carbon into carbonic acid, and the hydrogen into water. If substances composed of oxygen, hydrogen, and carbon, are liable to spontaneous decomposition, that tendency should become much stronger when nitrogen is also present Other and powerful affinities are then super- added to those above enumerated, and especially that of hydrogen for nitro- gen. Such compounds are very prone to decomposition, and the usual pro- ducts are water, carbonic acid, hydrocyanic acid, and ammonia. Another circumstance which is characteristic of organic products is the impracticability of forming them artificially by direct union of their ele- ments, the tendency of those elements being to unite so as to form water and carbonic acid. Some organic bodies are developed during the decompo- sition of others, as, for instance, oxalic acid from most substances when digested in nitric acid, alcohol from sugar, and acetic acid in fhe distillation of wood; but these results do not strictly form exceptions to the preceding remark. Animal and vegetable substances are decomposed by a red heat, and nearly all are partially affected by a temperature far below ignition. When heated in the open air, or with substances which yield oxygen freely, they burn, and are converted into water and carbonic acid; but if exposed to heat in vessels from which atmospheric air is excluded, very complicated products ensue. A compound, consisting of carbon, hydrogen, and oxygen, yields water, carbonic acid, carbonic oxide, carburetted hydrogen of various kinds, and probably pure hydrogen. Besides these products, some acetic acid is commonly generated, together with a volatile oil which has a dark colour and burnt odour, and is hence called empyreumatic oil. An azotized substance, in addition to these, yields ammonia, cyanogen, and probably free nitrogen. VEGETABLE CHEMISTRY. 495 From the foregoing remarks, it appears that organic products are char- acterized by the following circumstances:—1st, by being composed of the Bame elements; 2nd, by the facility with which they undergo spontaneous decomposition; 3rd, by the impracticability of forming them by the direct union of their elements; and 4th, by being decomposed at a red heat. VEGETABLE CHEMISTRY. All bodies which are of vegetable origin are termed vegetable substances. They are nearly ay, composed of oxygen, hydrogen, and carbon, and in a few of them nitrogen is likewise present Every distinct compound which exists ready formed in plants, is called a proximate or immediate principle of vegetables. Thus sugar, starch, and gum are proximate principles. Opium, though obtained from a plant, is not a proximate principle; but consists of several proximate principles mixed more or less intimately with each other. The proximate principles of vegetables are sometimes distributed over the whole plant, while at others they are confined to a particular part. The methods by which they are procured are very variable. Thus gum exudes spontaneously, and the saccharine juice of the maple-tree is obtained by in- cisions made in the bark. In some cases a particular principle is mixed with such a variety of others, that a distinct process is required for its sepa- ration. Of such processes consists the proximate analysis of vegetables. Sometimes a substance is separated by mechanical means, as in the prepa- ration of starch. On other occasions, advantage is taken of the volatility of a compound, or of its solubility in some particular menstruum. Whatever method is employed, it should be of such a nature as to occasion no change in the composition of the body to be prepared. The reduction of the proximate principles into their simplest parts consti- tutes their ultimate analysis. By this means chemists ascertain the quan- tity of oxygen, carbon, and hydrogen present in any compound. The former method of performing this operation was by what is termed destructive dis- tillation ; that is, by exposing the compounds to a red heat in close vessels, and collecting all the products. So many different substances, however, are procured in this way, such as water, carbonic acid, carbonic oxide, carbu- retted hydrogen, and the like, that it is almost impossible to arrive at a satis- factory conclusion. A more simple and effectual method was proposed by Gay-Lussac and Thenard in the second volume of their celebrated Re- cherches Physico-Chimiques. The object of their process, which is applica- ble to the ultimate analysis of animal as well as vegetable substances, is to convert the whole of the carbon into carbonic acid, and the hydrogen into water, by meai-.s of some compound which contains oxygen in so loose a state of combination as to give it up to those elements at a red heat The agent first employed by these chemists was chlcrate of potassa. This substance, however, is liable to the objedion, that it not onlv gives oxygen to the substance to be analyzed, but is itself decomposed by heat. On this account it is now very rarely employed in ultimate analysis, oxide of cop- per, proposed by Gay-Lussac, having been substituted for it This oxide, if alone, may be heated to whiteness without parting with oxygen; whereas it yields oxygen readily to any combustible substance with which it is ignited. It is easy, therefore, by weighing it before and after the analysis, to discover the precise quantity of oxygen-which has entered into union with the car- bon and hydrogen of the substances submitted to examination. The ultimate analysis of organic bodies is one of the most delicate opera- turns with which the analytical chemist can be engaged. The chief cause ot uncertainty in the process arises from the presence of moisture, which is retained by some animal and vegetable substances with such force that it can be expelled only by a temperature which endangers the decomposition ot the compound itself. The best mode of drying organic matters for the purpose, is by confining them wilh sulphuric acid under the exhausted re- 496 VEGETABLE CHEMISTRY. ceiver of an air-pump, and exposing them at the same time to a temperature of 212°,—a method adopted by Berzelius, and for which a neat apparatus has been described by Dr. Prout. (An. of Phil. vi. 272.) Another source of difficulty is occasioned by atmospheric air within the apparatus, owing to the presence of which nitrogen may be detected in the products, without having been contained in the substance analyzed. But though the ultimate analysis of organic substances is difficult in practice, in theory it is exceedingly simple. It consists in mixing three or four grains of the body to be analyzed with about two hundred grains of oxide of copper, heating the mixture to redness in a gla^s tube, and collect- ing the gaseous products in a graduated glass jar over mercury. From the quantity of carbonic acid procured by measure, its weight may readily be inferred (page 187); and from this, the quantity of carbonaceous matter may be calculated, by recollecting that every 22.12 grains of the acid con- tain 16 of oxygen and 6.12 of carbon. In order to ascertain the quantity of hydrogen, the gaseous products are transmitted through a tube filled with fragments of fused chloride of cal- cium, which absorbs all the watery vapour; and by its increase in weight indicates the precise quantity of that fluid generated. Every 9 grains of water thus collected correspond to 1 grain of hydrogen and 8 of oxygen. If the quantity of oxygen contained in the carbonic acid and water cor- responds precisely to that lost by the oxide of copper, it follows that the or- ganic substance itself was free from oxygen. But if, on the other hand, more oxygen exists in the products than was lost by the copper, it is obvious that the difference indicates the amount of oxygen contained in the subjeet of analysis. If nitrogen enter into the constitution of the organic substance, it will pass over in the gaseous state, mixed with carbonic acid; and its quantity may be ascertained by removing the carbonic acid by means of a solution of pure potassa. In order to prevent the production of binoxide of nitrogen, which is otherwise apt to be generated, the oxide should be mixed with some metallic copper; or the latter may be placed on the surface of the oxide, and be kept at a red heat, in order that any oxide of nitrogen, in passing through the metallic mass, should be decomposed. The copper for the purpose should be in a state of fine division, and is best prepared from the oxide by means of hydrogen gas. It need scarcely be observed, that if the analysis has been successfully performed, the weight of the different products, added together, should make up the exact weight of the organic substance employed. In analyzing an animal or vegetable fluid, the foregoing process will re- quire slight modification. If the fluid is of a fixed nature, it may be made into a paste with oxide of copper, and heated in the usual manner. But if it is volatile, a given weight of its vapour is conducted over oxide of copper heated to redness in a glass tube. The constitution of vegetable substances is not yet sufficiently known to admit of their being classified in a purely scientific order. I have arranged them into seven sections: the first includes the vegetable acids ; the second, the vegetable alkalies; the third comprises neutral substances, the oxygen and hydrogen of which are in the same ratio as in water; the fourth con- tains oleaginous, resinous, and bituminous principles; the fifth, spirituous and ethereal principles; the sixth, colouring matters; and the seventh com- prehends substances not referable to preceding sections. VEGETABLE ACIDS. SECTION I. VEGETABLE ACIDS. Those compounds are regarded as vegetable acids which possess the pro- perties of an acid, and are peculiarly the product of vegetation. These acids, like all organic principles, are decomposed by a red heat. 1 hey are in gene- ral less liable to spontaneous decomposition than other vegetable substances. They are nearly all decomposed by concentrated hot nitric acid, by which they are converted into carbonic acid and water. They do not possess any constant analogy in composition. It was thought at one time that their oxy- gen was always more than sufficient to convert all their hydrogen into water ; but several acids are known in which, as in sugar, the oxygen und hydro- gen are in the same ratio as in water, and in benzoic acid the hydrogen, is actually in excess. It seems, however, to be true that all vegetable sub- stances are acid, which contain more oxygen than suffices to form water with their hydrogen. The following table exhibits the constituents of the principal vegetable acids, and will serve to show analogies of composition^ The formula? merely express the elements contained in an equivalent 01 each acid, without indicating the order in which they are arranged. The composition, where not otherwise expressed, indicates the anhydrous acids, such as they exist in combination with an alkali: the numeral attached to each symbol expresses the number of equivalents of that element contained in the acid. Names, of acids. Carb. Hyd.Oxyg. Equiv. Formula;. Oxalic . . • 12.24+ 0+24= 36.24 CO'. Do. sublimed with 9 parts or 1 eq. of water = 45.24 Do. in crystals from solution with 27 parts or 3 eq. of water= 63.24 Mellitic . . . 24.4S+ 0+24= 48.48 C*Os. Croconic . . . 30.6 + 0+32= 62.6 C*0*. Acetic . . . 24.48+ 3+24= 51.48 C*H Os. Do. in crystals with 9 parts or 1 eq. of water = 60.48 Lactic (in lactate of oxide" of zinc) 36.72+5+40=81.72 CbH«0»;;. , Do. dried in vacuo from solution with 9 parts or 1 eq. of water= 90.72 Do. sublimed . . 36.72+ 4+32= 72.72 C«H*04. Kinic .... 91.8 +10+80=181.8 C>.Oi°. Malic .... 24.48-f 2+32= 58.48 C«H30*. Citric .... 24.48+ 2+32= 58.48 C+II^O*. Pyrocitric . . . 61.2 + 2+24= 87.2 C^'MaOa. Tartaric . . . 24.48-j- 2+40= 66.48 G»H*0\ Do. in crystals with 9 parts or 1 eq. ofwater = 75.48 Racemic . . . 24.43+ 2+40= 66.48 C«HaOs. Do. in crystals wilh 18 parts or 2 eq. ofwater = 84.48 Benzoic . . . 85.68+ 5+24=114. GS C»«H«0-i. Do. in crystals with 9 parts or 1 eq. of water =123.68 Meconic . . . 42.84+ 2+56=100.84 C7H O?. Metameconic . . 73.44+ 4+80=157.41 C'aH<0"\ Tannic (from catechu) . 110.16+ 9+64=183.16 Ci8lI90«. -----(from gall-nuts) . 110.16+ 9+96=215.16 CWIrO»8. Gallic .... 42.84+ 3+40= 85.84 C7H*08. Do. in crystals with 9 parts or 1 eq. ofwater = 94.84 Pyrogallic . . . 36.72-f 3+24= 63.72 C^O' Metagallic . . . 73.44+ 3+24=100.44 C^HsO' EUagic . . . 42.84+ 2+32= 76.84 C^HaO-*. Succinic . . • 24.48+ 2+24= 50.48 C4H»0* 42* 498 OXALIC ACID. Names of acids. Carb. Hyd.Oxyg. Equiv. Formulas. Mucic .... 36.72+ 5+-64=l 05.72 C»HsO<'. Camphoric . . . 122.4 +16+40=178.4 C20H1EO5. Valerianic . . . 61.2 + 9+24= 94.2 CioHiKK Rocellic . . . 97.92+16+32= 145.92 CieHicO". Oxalic Acid. Oxalic acid exists ready formed in several plants, especially in the rumex acelosa or common sorrel, and in the oxalis acetosella or wood sorrel; but it almost always occurs in combination either with lime or potassa. These plants contain binoxalate of potassa; and the oxalate of lime has been found in large quantity by M. Braconnot in several species of lichen. Oxalic acid is easily made artificially by digesting sugar in five or six times its weight of nitric acid, and expelling the excess of that acid by dis- tillation, until a fluid of the consistence of syrup remain in the retort The residue in cooling yields crystals of oxalic acid, the weight of which amounts to rather more than half the quantity of the sugar employed. They should be purified by repeated solution in pure water, and re-crystal- lization ;• for they are very apt to retain traces of nitric acid, the odour of which becomes obvious when the crystals are heated. In the conversion of sugar into oxalic acid, changes of a very complicated nature ensue, during which,a large quantity of binoxide of nitrogen with some carbonic acid is disengaged: water is freely generated at the same time, and a small quan- tity of malic and acetic acids is produced. As oxalic acid does not contain any hydrogen, and has a smaller proportional quantity of carbon than sugar, there can be no doubt that the production of this acid essentially depends upon the sugar being deprived of all its hydrogen and a portion of its car- bon by oxygen derived from the nitric acid. Many organic substances besides sugar, such as starch, gum, most of the vegetable, acids, wool, hair, and silk, are converted into oxalic by the action of nitric acid;—a circumstance which is explicable on the fact that oxalic acid contains more oxygen than any other principle, whether of animal or vegetable origin. It is also generated by heating organic substances with potassa. (Page 494.) Oxalic acid crystallizes in slender, flattened, four and six-sided prisms ter- minated by two-sided summits; but their primary form is an oblique rhombic prism. It has an exceedingly sour taste, reddens litmus paper strongly, and forms neutral salts with dkalies. The crystals, which consist of one equivalent of real acid and three of water, undergo no change in ordinary states of the air; but when the atmosphere is very dry, or the temperature slightly raised, as to 703 or 80°, partial efflorescence ensues, and at 212° they lo e two equivalents of water, which on exposure to the air while cold they soon recover. Heated in a tube to 209° they fuse in their water of crystallization, and are hence soluble in boiling water with- out limit: at 50° they dissolve in 15.5 times their weight of water, and in 9.5 times at 573; but the solubility is increased by the presence of nitric acid. They are dissolved also by alcohol, though less freely than in water. Oxalic acid possesses considerable volatility. Mr. Faraday has shown that a very slow sublimation of oxalic acid takes place at common tempe- ratures. At 212° its vaporization goes on in appreciable quantities; and at 330° the acid, when deprived of two equivalents of its water of crystalliza- tion, sublimes rapidly, and without any decomposition. The sublimed acid crystallizes in transparent acicular crystals, which contain one equivalent of water; but by exposure to the air they rapidly absorb moisture, and become opaque. (An. of Phil. N. S. x. 318.) When fully hydrated oxalic acid is suddenly heated to about 300°, it undergoes decomposition, and^ yields water, carbonic acid gas mixed with carbonic oxide in the ratio oi 6 to 5, OXALIC ACID. 499 and formic acid. To this change, which has been lately studied by Gay- Lussac, the water of crystallization essentially contributes, the elements ot formic acid being such, that it may be considered a compound of two equivalents of carbonic oxide with one equivalent of water. The crystals, when deprived of 2-3ds of their water, are much more stable, not suffering the same decomposition until the heat exceeds 330°, and even then a consi- derable portion is sublimed. „ , . . c * • i , Oxalic acid is a powerful and rapidly fatal poison; and frequent accidents have occurred from its being sold and taken by mistake lor Epsom salt, with the appearance of which its crystals have some resemblance. Ihese substances may be easily distinguished, however, by the strong acidity of oxalic acid, which may be tasted without danger, while sulphate of magnesia is quite neutral, and has a bitter saline taste. In cases of poisoning with this acid, chalk mixed with water should be administered as an antidote, an insoluble oxalate being formed, which is inert. Chalk was first suggested for this purpose by my colleague, Dr. A. T. Thomson; and his opinion has been since fully confirmed by the experiments of Drs. Christison and Coin- det, who have recommended the use of magnesia with the same intention. (Christison on Poisons.) Oxalic acid is easily distinguished from all other acids by the form of its crystals, and by its solution giving with lime-water a white precipitate, which is insoluble in an excess of the acid. When the acid is contained in mixed fluids, it may be conveniently precipitated by nitrate of oxide of lead, care being taken beforehand to neutralize the solution with a little carbonate of soda. The precipitated oxalate of oxide of lead, after being well washed, and while yet moist, is suspended in water, and decomposed by a current of hydrosulphuric acid: the clear liquid is poured off or filteied from the sul- phuret of lead, and concentrated by evaporation that crystals may form. These may be purified by solution in pure water and a second crystalli- zation. As an equivalent of oxalic acid contains two. eq. of carbon and three of oxygen, it may be regarded either as a direct compound of carbon and oxygen, indicated by the formda C or C Gs, or as a compound of carbonic oxide and carbonic acid, denoted by C + C. The latter is supported by the fact observed by Dobereiner, that oxalic acid is converted into carbonic oxide and carbonic acid gases by the action of concentrated sulphuric acid. The decomposition takes place slowly at 212°, and rapidly at 230°. The neutral oxalates of protoxides consist of 36.24 parts or one eq. of oxdic and one eq. of the base, the general formula of such salts being M+C, or M+O (page 152). Most of these compounds are either insoluble or sparingly soluble in water; but they are all dissolved by the nitric, and also by the hydrochloric acid, except when the latter precipitates the base of the salts. The only oxalates which are remarkable for solubility are those of potassa, soda, lithia, ammonia, alumina, and peroxide of iron. A soluble oxalate is easily detected by adding to its solution a neutral salt of lime or oxide of lead, when a white oxalate of those bases will be thrown down. On digesting the precipitate in a little sulphuric acid, an in- soluble sulphate is formed, and the solution yields crystals of oxalic acid on cooling. All insoluble oxalates, the bases of which form insoluble com- pounds with sulphuric acid, may be decomposed in a similar manner. All other insoluble oxalates may be decomposed by potassa, by which means a soluble oxalate is procured. The oxalates, like all salts which contain a vegetable acid, are decom- posed by a red heat, a carbonate being left, provided the oxide can retain carbonic acid at the temperature which is employed. As oxalic acid is so highly oxidized, its salts leave no charcoal when heuted in close vessels. 500 OXALIC ACID. Several oxalates are reduced to the metallic state, with evolution of pure carbonic acid, when heated to redness in close vessels. (Pages 348 and 351.) The peculiar constitution of oxalic acid accounts for this change; for one equivalent of the acid, to be converted into carbonic acid, requires precisely one equivalent of oxygen, which is the exact quantity contained in the oxide of a neutral protoxdate. Oxalates of Potassa.—Oxalic acid forms with potassa three compounds, which were first described by Wdiaston. (Philos. Trans. 1808.) The first is the neutral oxalate, which is formed by neutralizing carbonate of potassa with oxalic acid. It crystallizes in oblique quadrangdar prisms, which have a cooling bitter taste, require about twice their weight of water at 60° for solution, and contain 36.24 parts or one eq. of oxalic acid, 47.15 or one eq. of potassa, and 9 parts or one eq. ofwater. This salt is much employed as a reagent for detecting lime. The binoxdate is contained in sorrel, and may be procured from that plant by solution and crystallization. It crys- tallizes readily in small rhomfcdds, which are less soluble in water than the neutral oxalate, and consist of 72.48 parts or two eq. of acid, 47.15 or one eq. of potassa, and 18 or two eq. of water. It is often sold under the name of essential salt of lemons for removing iron moulds from linen;—an effect which it produces by half of its add uniting with the peroxde of iron and forming a soluble oxalate. The quadroxalate is the least soluble of these salts, and is formed by digesting the binoxalate in nitric acid, by which it is deprived of half of its base. The crystals consist of 144.96 parts or four eq. of acid, 47.15 or one eq. of potassa, and 63 or seven eq. of water. Oxalate of Soda may be made in the same manner as oxalate of potassa. It likewise forms a binoxalate, but no quadroxalate is, known. Oxalate of Ammonia.—This salt, prepared by neutralizing ammonia with oxalic acid, is much used as a reagent It is very soluble in hot water, and is deposited in acicular crystals when a saturated hot solution is allow- ed to cool. The crystals contain two equivalents of water. Dr. Thomson has likewise described a binoxalate of ammonia, which is less soluble than the preceding, and contains three equivalents of water. During the decomposition of oxalate of ammonia by heat an interesting compound is generated, which was discovered and described by Dumas, who has given it the name of oxalammide or oxamide, compounded of the words oxalic and ammonia. (An. de Ch. et de Ph. xliv. 129.) On putting oxalate of .ammonia into a retort and applying heat the crystals at first lose water and become opaque; then the salt, where directly in contact with the hot glass, fuses, boils, and disappears; and this action goes on successively through the mass, until, excepting traces of a light carbonaceous matter, the whole is expelled. During the whole course of the distillation gas is disengaged: at first ammonia appears, then a mixture of carbonic acid and carbonic oxide, the former of which unites with the ammonia, and towards the close of the process cyanogen gas is generated. The oxamide, which constitutes but a small part of the products, is found as a thick deposite, of a dirty white colour, in the neck of the retort, and partly floating in flakes in the water of the recipient. It is separated from adhering carbonate of ammonia by being well washed with cold water. Oxamide is insoluble in cold water: at 212° it is dissolved, and is depo- sited unchanged on cooling in the form of flocks of a dirty white colour, and of a confused crystalline appearance. Heated gently in an open tube it speedily rises in vapour, and is condensed again on the cold part of the tube ; but when sharply heated in a retort, it enters into fusion, and while part sublimes, another portion yields cyanogen gas, and leaves a very bulky car- bonaceous residue. The elements contained in oxamide arc expressed by the formula 2C+ N+ 211+ 20. Nothing certain is known respecting the mode in which these elements are arranged ; but the three following hypotheses are most consistent with known affinities:—it may be a compound of OXALIC ACID. 501 Cyanogen and water .... (2C+N)+2(H+0) Or binoxide of nitrogen and olefiant gas (2H+2C)+(N+20) Or dinituret of hydrogen and carbonic oxide (2H+N)+2(C+0). It can scarcely be thought to contain either oxalic acid or ammonia. But when boiled with a solution of potassa, ammonia after a short time is evolved, and oxalate of potassa generated ; and when heated with a large excess of strong sulphuric acid, a mixture of carbonic oxide and carbonic acid gases, in the ratio to form oxalic acid, is evolved, and sulphate of ammonia is pro- duced. Under the influence of the attraction of potassa for oxalic acid, or of sulphuric acid for ammonia, oxamide and water interchange elements; so that 1 eq. oxamide 2C+N+2H+20 2 1 eq. oxalic acid 2C+30 and 1 eq. water H+O -^ and 1 eq, ammonia 31I+N. Oxalate of Lime.—This salt, like all the insoluble oxalates, is easily pre- pared by way of double decomposition. It is a white finely divided powder, wliich is remarkable for its extreme insolubility in pure water. On this ac- count a soluble oxalate is an exceedingly delicate test for lime. It is soluble, however, in hydrochloric and nitric acids. It is composed of 36.24 parts or one eq. of oxalic acid, and 28.5 or one eq. of lime. It may be exposed to a temperature of 560° without decomposition, and is then quite anhydrous. No binoxalate of lime is known. This sdt is interesting in a pathological point of view, because it is a fre- quent ingredient of urinary concretions, being the basis of what is cdled the mulberry calculus. Oxalate of Magnesia may be prepared by mixing oxalate of ammonia with a hot concentrated solution of sulphate of magnesia. It is a white pow- der, which is very sparingly soluble in water; but, nevertheless, when sd- phate of magnesia is moderately diluted with cold water, oxalate of ammonia occasions no precipitate. On this fact is founded the best andytic process for separating lime from magnesia. Oxalate of Chromium and Potassa.—This sdt was discovered by my bro- ther during the winter of 1830-31, by adding oxalic acid to a solution of bichromate of potassa until effervescence ceased, and then evaporating. The same salt has been prepared independently, and by a better process, by Dr. Gregory, who employs 190 parts of bichromate of potassa, 157.5 of oxalic acid in crystals, and 517.5 of crystals of binoxalate of potassa, pours hot wa- ter over the materials, and when effervescence has ceased concentrates very considerably. This beautiful salt crystallizes in thin elongated prisms, which appear black by reflection, blue by transmitted light, and green when reduced to powder : its solution is green and red at the same time, except by candlelight, when it is of a pure red. Dr. Gregory considers it a com- pound of three eq. of oxalic acid, two of potassa, one of green oxide of chro- mium, and six ofwater. Mellitic Acid.—-This acid is contained in the rare substance called honey- stone, which is occasionally met with at Thuringia in Germany. The honey- stone, according to Klaproth, is a mellitate of alumina, and on boiling it in a large quantity of water, the acid is dissolved, and the alumina subsides. On concentrating the solution, mellitic acid is deposited in minute acicular crystals. From its rarity it has been little studied. Liebig and Wohler have shown (page 497) that it has the same composition as succinic acid, hydrogen excepted. (An. de Ch. et de Ph. xliii. 200.) Croconic Acid.—-In the preparation of potassium from cream of tartar (page 294), the principal products are potassium and carbonic oxide gas; but these are accompanied with dense fumes, which in cool vessels deposite a gray flaky substance. On the addition of water this matter becomes red, and on exposure to the air a reddish-yellow solution is formed, which by gentle evaporation yields croconate of potassa in crystals of the same colour as the solution: the residud liquid contains bicarbonate and oxalate of po- 502 ACETIC ACID. tassa. In order to separate croconic acid, the crystals, purified by a second crystallization and reduced to fine powder, are put into absolute alcohol, to which sulphuric acid of specific gravity 1.78, in quantity insufficient for combining with all the alkali of the croconate, is added. The mixture is gen- tly warmed during several hours, and frequently shaken, until a drop of the solution, mixed with chloride of barium, causes no turbidity. The yellow alcoholic solution of croconic acid is then separated from the sulphate of potassa by filtration, and the acid obtained by expelling the alcohol. (Gme- lin's Handbuch.) Croconic acid, by solution in water and spontaneous evaporation, yields transparent prismatic crystals of a yellow colour, which are inodorous, have an acid astringent taste, redden litmus, and neutralize alkaline bases. It bears a heat of 212° without decomposition, but at a higher temperature it is decomposed, giving a deposite of charcoal. A similar facility of decomposi- tion is conspicuous in all its salts: when, for instance, croconate of potassa is heated, it takes fire at a temperature below ignition, the whole mass blackens, and is found to be a mixture of charcoal and carbonate of potassa. As it consists of carbon and oxygen in the ratio of five eq. of carbon to four eq. of oxygen, its origin seems referable to the deoxidizing action of potas- sium on carbonic acid and carbonic oxide. Acetic Acid. Acetic acid exists ready formed in the sap of many plants, either free or combined with lime or potassa: it is generated during the destructive dis- tillation of vegetable matter, and is an abundant product of the acetous fer- mentation. Common vinegar, the acidifying principle of which is acetic acil, is com- monly prepared in this country by fermentation from an infusion of malt, and in France from the same process taking place in weak wine. Vinegar, thus obtained, is a very impure acetic acid, containing the saccharine, mu- cilaginous, glutinous, and other matters existing in the fluid from which it was prepared. It is separated from these impurities by distillation. Distilled vinegar, formerly called acetous acid, is real acetic acid merely diluted with water, and commonly containing a small portion of empyreumatic oil, formed during the distillation, and from which it receives a peculiar flavour. It may be rendered stronger by exposure to cold, when a considerable part of the water is frozen, while the acid remains liquid. The acid now generally employed for chemical purposes is prepared by the distillation of wood, and is sold under the name of pyroligneous acid. When first made it is very impure, and of a dark colour, holding in solution tar and volatile oil. In this state it is mixed'with chalk, and obtained in the state of acetate of lime, which is decomposed by digestion with sulphate of soda: the resulting acetate of soda is then fused at a high temperature, insufficient to decompose the salt, but sufficient to expel or char the impuri- ties. The acetate of soda is thus obtained pure and in crystals, and is de- composed by sulphuric acid. Concentrated acetic acid is best obtained by decomposing the acetates either by sulphuric acid, or in some instances by heat. A convenient pro- cess is to distil acetate of potassa with half its weight of concentrated sul- phuric acid, the recipient being kept cool by the application of ice. The acid is at first contaminated with sulphurous acid ; but by mixing it with a little peroxide of manganese, and redistilling, it is rendered quite pure. A strong acid may likewise be procured from acetate of oxide of copper by the sole action of heat The acid when first collected has a greenish tint, owing to the presence of copper, from which it is freed by a second distilla- tion. Pyro-acetic ether is formed towards the close of the process. Strong acetic acid is exceedingly pungent, and even raises a blister when kept for'somo time in contact with the skin. It has a very sour taste and an agreeable refreshing odour. Its acidity is well marked, as it reddens litmus ACETIC ACID. 503 Acid. Water. 1 aton i + 1 atom . 1 + 2 1 + 3 1 + 4 1 + 5 1 + 6 1 + 1 1 + 8 1 + 9 1 +10 paper powerfully, and forms neutral salts with the alkalies. It is exceed- ingly volatile, rising rapidly in vapour at a moderate temperature without undergoing any change. Its vapour is inflammable, and burns with a white light In its most concentrated form it is a definite compound of one equi- valent of water, and one equivalent of acid ; and in this state it crystallizes when exposed to a low temperature, retaining its solidity until the thermo- meter rises to 50° F. It is decomposed by being passed through red-hot tubes; but owing to its volatility, a large quantity of it escapes decompo- sition. The only correct mode of estimating the strength of acetic acid is by its neutralizing power. Its specific gravity is no criterion, as will appear from the following table (Thomson's First Principles, vol. ii. p. 135), exhibiting the density of acetic acid of different strengths. Sp. gr. at 60° F. . 1.06296 . . . 1.07060 - 1.07080 . 1.07132 . 1.06820 . 1.06708 . 1.06349 . 1.05974 . 1.05794 . 1.05439 The acetic is distinguished from all other acids by its flavour, odour, and volatility. Its salts, which are called acetates, are all soluble in hot and most of them in cold water, are destroyed by a high temperature, and are decom- posed by sulphuric acid. The neutral acetates of protoxides consist of 51.48 parts or one eq. of anhydrous acetic acid and one eq. of base, so that the ge- neral formula of such salts is M+C*Hs03, or M+A, A being used as the symbol of one equivalent of acetic acid, and M of any electro-positive metal. Acetate of Potassa.—This salt is made by neutralizing the carbonate with acetic acid, or by decomposing acetate of lime with sulphate of potassa. When cautiously evaporated, it forms irregular crystals, which are obtained with difficulty, owing to the deliquescent property of the salt. According to Dr. Thomson, the crystals are composed of 98.63 parts or one eq. of neutral acetate of potassa, and 18 or two eq. of water. It is commonly prepared for pharmaceutic purposes by evaporating the solution to dryness, and heating the residue so as to cause the igneous fusion. On cooling it becomes a white crystalline foliated mass, which is generally alkaline. This salt is highly soluble in water, and requires twice its weight of boil- ing alcohol for solution. Dr. Thomson procured a binacetate by mixing acetic acid and carbonate of potassa in the proportion of two equivalents of the former to one of the latter. On confining the solution along with sulphuric acid under the ex- hausted receiver of an air-pump, the binacetate was deposited in large trans- parent flat plates. The crystals contain six equivalents of water, and deli- quesce rapidly on exposure to the air. Acetate of Soda is prepared in large quantity by manufacturers of pyro- ligneous acid by neutralizing the impure acid with chalk, and then decom- posing the acetate of lime by sulphate of soda. It crystallizes readily by gentle evaporation, and its crystals, which are not deliquescent, are composed of 51.48 parts or one eq. of acetic acid, 31.3 parts or one eq. of soda, and 54 parts or six equivalents of water. (Berzelius and Thomson.) The form of its crystals is very complicated, and derived from an oblique rhombic prism (Brooke.) When heated to 550°, it is deprived of its water, and undergoes the igneous fusion without parting with any of its acid. At 600° decomno sition takes place. y 504 ACETIC ACID. Acetate of soda is much employed for the preparation of concentrated acetic acid. Acetate of Ammonia is made by neutralizing the carbonate with acetic acid. It crystallizes with difficulty in consequence of being deliquescent and highly soluble. It has been long used in medicine as a febrifuge under the name of spirit of Mindererus. The Acetates of Baryta, Strontia, and Lime are of little importance. The former, which is occasionally employed as a reagent, crystallizes in irregular six-sided prisms terminated by dihedral summits, the primary form of which is a right rhomboidal prism. The latter crystallizes in very slender acicular crystals of a silky lustre, and is chiefly employed in the preparation of acetate of soda. Acetate of Alumina is formed by adding acetate of oxide of lead to sd- phate of alumina, when the sulphate of oxide of lead subsides, and acetate of alumina remains in solution. It is used by dyers and calico printers as a basis or mordant. Acetates of Protoxide of Lead.—The neutral acetate, long known by the names of sugar of lead (saccharum Saturni) and cerussa acetata, is made by dissolving either the carbonate or litharge in distilled vinegar. The solution has a sweet, succeeded by an astringent taste, does not redden litmus paper, and deposites shining acicular crystals by evaporation. When more regu- larly crystallized, it occurs in six-sided prismatic crystals, cleaveable parallel to the lateral and terminal planes of a right rhombic prism, which may be regarded as its primary form. (Mr. Brooke.) The crystals effloresce slowly by exposure to the air, and require about four times their weight ofwater at 60° for solution. They consist of 51.48 parts or one eq. of the acid, 111.6 parts or one eq. of protoxide of lead, and 27 parts or three eqdvalents of water. This salt is partially decomposed, with formation of carbonate of oxide of lead, by water which contains carbonic acid, or by exposure to the air; but a slight addition of acetic acid renders the solution quite clear. It is much used in the arts, in medical and surgicd practice as a sedative and astrin- gent, and in chemistry as a reagent. The subacetate, commonly called extraclum Saturni, is prepared by boil- ing 1 part of the neutrd acetate, and 2 parts of litharge, deprived of carbonic acid by heat, with 25 parts of water. This sdt is less sweet and more soluble in water than the neutral acetde, and has an alkaline reaction. Thenard has obtained it by evaporation in opaque white tabular crystals, but it crystallizes with difficulty. It is decom- posed by a current of carbonic acid, with production of pure carbonate of oxide of lead; and forms a turbid solution, owing to the formation of the carbonate, when it is mixed with water in which carbonic acid is present. It consists of one equivalent of acid, and three equivalents of protoxide of lead, and is, therefore, a triacetate. (Berzelius.) A diacetate may be formed by boiling with water a mixture of litharge and acetate of oxide of lead in atomic proportion. (Thomson.) Acetates of Protoxide of Copper.—These salts have been carefully studied by Berzelius and Phillips. (An. of Phil. N. S. i. ii. iv.and viii.) The neutral acetate may be formed either by dissolving protoxide of copper or common verdigris in acetic acid, or by decomposing the sulphate by an equivalent quantity of acetate of oxide of lead. On evaporation it readily crystallizes in rhombic octohedrons of a dark green colour, which are soluble in 20 times their weight of cold water, in 5 of boiling water, and in 14 of boiling alcohol. The crystals consist of 39.6 parts or one eq. of the black oxide, 51.48 parts or 1 eq. of acetic acid, and 9 parts or one equivalent ofwater. When copper plates are covered with a layer of the neutral acetate, made into a thin paste with water, and are then exposed for about two months to a moist atmosphere, a sub-salt is generated, which appears in crystalline blue scales and needles of a silky lustre. It is a diacetate, consisting of two equivdents of the black oxide and one of acetic acid, united with six equiva- LACTIC ACID. 505 lents ofwater. At 212° it loses part of its water and acquires a pretty green tint. When freely mixed with water it is converted into the soluble neutral acetate, and into an insoluble triacetate. The diacetate is the principal in- gredient of the blue-coloured varieties of verdigris. The triacetate, besides being formed by the action of water on the diace- tate, is obtained as a light green powder by digesting the hydrated oxide with the neutral acetate. It is also generated when ammonia is cautiously added to a solution of the neutral acetate: in a cold solution the precipitate is uncrystalline and of a green colour, wliich by washing passes into blue; while in a hot solution the precipitate is granular, and of a dirty grayish- green tint. By the continued action of water, freely employed, it may be resolved into oxide and neutral acetate of copper. It consists of three equiva- lents of oxide, one of acid, and one and a half of water. Another sub-sdt, composed of three equivalents of oxide, two of acid, and six ofwater, is generated by adding to a strong boiling solution of the neu- tral acetate, a small quantity of ammonia insufficient to produce a perma- nent precipitate of the triacetate. The sub-salt in question, which is sparingly soluble in cold water, separates on cooling, and should be washed with alcohol. It is considered by Berzelius as the principal ingredient of the green varieties of verdigris. The pigment, verdigris, which is a variable mixture of the subacetates, is prepared in large quantity in the south of France, by covering copper with the refuse of the grape after the juice has been extracted for making wine: the saccharine matter contained in the husks furnishes acetic acid by fermen- tation, and in four or six weeks the plates acquire a coating of the acetate, A purer and better article is prepared in this country by covering copper plates with cloth soaked in pyroligneous acid. Acetate of Oxide of Zinc—This salt may be prepared by way of double decomposition by mixing sulphate of oxide of zinc with acetate of oxide of lead in equivalent proportions. When made in this way, it is very apt to retain some sulphate of oxide of lead in solution. It is obtained pure by suspending metallic zinc in a dilute solution of acetate of oxide of lead, until all the lead is removed, or by dissolving oxide of zinc in acetic acid. This salt is frequently employed as an astringent collyrium. Acetate of Protoxide of Mercury.—The only interesting compound of mercury and acetic acid is the acetate of the protoxide, which is sometimes employed in the practice of medicine. It is prepared by mixing crystallized protonitrate of mercury with neutral acetate of potassa in the ratio of one equivalent of each. If both salts are dissolved in a considerable quantity of hot water the solutions retain their transparency after being mixed; but on cooling, the acetate of protoxide of mercury is deposited in white scales of a silky lustre. It is easily decomposed; and it should be dried by a verv gentle heat, and washed with cold water slightly acidulated with acetic acid. Lactic Acid. Lactic acid, so named from being first noticed in sour milk, was disco- vered by Scheele in 1780, has been detected by Berzelius in most of the animal fluids, and in the beet-root by Braconnot, who termed it nanceic rbltcdr^0 aSl°ng-eX1S(ed0f its king acetic acid, modified in its Lussac Jl P Jmng.°Tg*T mattef; but the recent experiments of J. Gay. ^ar:ddn^^^ct;tdfienS^4T^a11 doubt about «-•—-^ The mode of preparation, employed by J. Gay-Lussac and Pelouze was for Tr fd gC- qUafty °f thf Juke °f beet-root to a temperate of 8<* for several days in order to establish a brisk fermentation, and set it at rest for some weeks unt, the fermentation has ceased. The liquid is then eva porated to the consistence of syrup, the lactic acid taken up in alcohol which separates a large quantity of vegetable matter, and thedcoholic ex- 43 506 KINIC ACID. tract, redissolved in water, is neutralized by carbonate of oxide of zinc, by which means additional impurities are precipitated. The soluble lactate of oxide of zinc is obtained in crystals by evaporation, is then boiled with ani- mal charcoal freed from phosphate of lime, filtered while hot, and again crystallized. The salt is then perfectly white. The oxide of zinc is thrown down by pure baryta, and the lactate of baryta decomposed by sulphuric acid. If any portions of the acid are not quite white, they may be convert- ed into lactate of lime, purified by animal charcoal, crystallized, dissolved in alcohol, and lastly decomposed by oxalic acid. A similar process applied to milk after long fermentation gives pure lactic acid; and the same acid has been extracted from rice and from nux vomica. When a solution of lactic acid is concentrated in vacuo with sulphuric acid, it acquires the consistence of syrup, has a density of 1.215, is colour- less and inodorous, and has an extremely acid taste. It attracts moisture on exposure to the air, dissolves in all proportions in water and alcohol, and is soluble though to a less degree in ether. When mixed with a strong solution of acetate of potassa, acetic acid is disengaged, and with a strong solution of the acetates of magnesia and oxide of zinc, the lactates of those bases subside. It yields no precipitate with baryta, strontia, or lime. When the syrupy lactic acid is gradually heated, it becomes coloured and yields inflammable gases, acetic acid, a white crystalline sublimate, and a residue of charcoal. The sublimed matter, freed from adhering moisture by bibulous paper, dissolves freely in boiling alcohol, and separates on cooling in white rhomboidal tables. These crystals fuse at 225°, and the liquid boils at 472°, yielding an inflammable irritating white vapour, which crystallizes as it condenses: it may be volatilized repeatedly without the least decomposition. This sublimed acid, as stated at page 497, differs from the liquid lactic acid in containing two eq. less of water; and though when first put into water the crystals dissolve slowly, yet when once in solution they possess all the characters of the liquid acid. Lactic acid in uniting wilh metallic oxides carries into the salt one eq. of water or its elements, which cannot be expelled without decomposing the salt itself. The neutral lactates of protoxides in their dryest state, and ex- clusive of water of crystallization, consist of one eq. o' the oxide and 81.72 parts or one eq. of the acid, the general formula being M + C6H50s or M+L. None of the lactates hitherto examined are insoluble in water. The lactates of baryta, strontia, potassa, soda, ammonia, dumina, and oxide of lead are very soluble, and crystallize with difficulty. That of oxide of silver is white and crystallizes in needles. The salt of zinc is sparingly soluble in cold water, and crystallizes from its hot solution in four-sided prisms, which are insoluble in alcohol, and contain four eq. of water of crystallization. The lactate of magnesia dso crystdlizes readily, and also with four eq. of water. Kinic Acid. This acid, noticed in 1790 by Hofmann, and studied in 1806 by Vau- quelin, exists in cinchona bark in combination with lime, quinia, and cin- chonia. On evaporating an infusion of bark to the consistence of an ex- tract, and treating the residue with alcohol, a viscid matter remains, con- sisting of kinate of lime and mucilaginous matters. On dissolving it in water, and allowing the concentrated solution to evaporate spontaneously in a warm place, the kinate crystallizes in rhombic prisms with dihedral summits, and sometimes in rhomboidal plates. From a solution of this salt, exactly decomposed by oxalic acid, kinic acid separates in foliated crystals by evaporation in a warm atmosphere (An. de Ch. lix.). This acid and its combinations have been studied by Henry and Plisson, Liebig, and Baup. (An. de Ch. et de Ph. xli. 325, xlvii. 191, and li. 56). Baup recommends that the acid should be prepared from kinate of lime by means of sulphuric acid. MALIC ACID. 507 Kinic acid possesses strong acidity, dissolves in 2£ times its weight of water at 483, and is also soluble in alcohol, but appears to form an ethereal compound with it. The crystals have a density of 1.637, and consist of 181.8 parts or one eq. of the anhydrous acid, and 9 parts or one cq. ofwater. When strongly heated it yields a volatile acid called pyrokinic acid. Kinic acid forms very soluble compounds with the alkalies, alkaline earths, and oxides of mercury, lead, and silver. These neutral kinates are so constituted that one eq. of a protoxide is united with 181.8 parts or one eq. of kinic acid, the general formula of^these salts being M+C^HmOio or M+K~. The rhombic crystals of kinate of lime contain ten eq. ofwater- Kinate of soda crystallizes in very fine six-sided prisms with four eq. of water; that of strontia in efflorescent prisms or tables with ten eq. ofwater; and that of baryta with six eq. of water. Kinate of oxide of silver yields by evaporation a white anhydrous salt The oxides of copper and lead form with it neutrd and sub-salts. Malic Acid. This acid is contained in most of the acidulous fruits, being frequently associated with tartaric and citric acids. Grapes, currants, gooseberries, and oranges contain it. Vauquelin found it in the tamarind, mixed with tartaric and citric acids, and in the house-leek (sempervivum tectorum), combined with lime. It is contained in considerable quantity in apples, a circumstance to which it owes its name. It is almost the sole acidifying principle of the berries of the service-tree (sorbus aucuparia), in which it was detected by Mr. Donovan, and described by him under the name of sorbic acid in the Philosophical Transactions for 1815; but it was after- wards identified with the mdic acid by Braconnot and Houton-Labillardiere. (An. de Ch. et de Ph. viii.) Malic acid may be formed by digesting sugar with three times its weight of nitric acid; but the best mode of procuring it is from the berries of the service-tree. The juice of the unripe berries is diluted with three or four parts of water, filtered, and heated ; and while boiling, a solution of acetate of oxide of lead is added as long as any turbidity appears; when the colour- ing matter of the berry is precipitated, while the malate of that oxide re- mains in solution. The liquid, while at a boiling temperature, is then filter- ed. At first a smdl quantity of dark-coloured salt subsides ; but on decant. ing the hot solution into another vessel, the malate of oxide of lead is gra- dudly deposited, in cooling, in groups of brilliant white crystals. The malate is then decomposed by a quantity of dilute sulphuric acid, insuffi- cient for combining with all the oxide of lead; by which means a solution is procured containing malic acid together with a little lead. The latter is afterwards precipitated by hydrosulphuric acid. The colouring matter is more easily separated if the juice is allowed to ferment for a few days be- fore being used. Another process has been minutely described by Liebig, in order to ensure ihe absence of citric and tartaric acids with wliich it is sometimes associated. (An. de Ch. et de Ph. Iii. 434.) Malic acid possesses strong acidity, and a pleasant flavour when diluted. It crystallizes with difficulty, attracts moisture from the air, and is very so- luble in water and alcohol. Its aqueous solution is decomposed by keeping, and it is converted into the oxalic by digestion in strong nitric acid. Heated in close vessels it yields a volatile acid called pyromalic, the properties of which are not yet known. From the late analysis of Liebig it seems that malic and citric acids are isomeric. Most of the salts of malic acid are more or less soluble in water. The malates of soda and potassa are deliquescent The bimalate of ammonia crystallizes very readily. The most insoluble of the malates are those of ba- ryta, lime, and the oxides of lead and silver; but these, excepting the first, are freely soluble in boiling water. These salts are composed of one eq. of 508 CITRIC ACID. base and 58.48 parts or one eq. of acid, the general formula being M+ CiHaO*, or M +M. Pure malate of oxide of lead crystallizes in cooling from Us hot solution in small brilliant scales of extreme whiteness. The sil- ver salt also crystallizes from its solution in boiling water; but the liquid darkens from the reduction of oxide of silver. Citric Acid. This acid is contained in many of the acidulous fruits, but exists in large quantity in the juice of the lime and lemon, from which it is procured by a process very similar to that described for preparing tartaric acid. To any quantity of lime or lemon juice, finely powdered chalk is added as long as effervescence ensues; and the insoluble citrate of lime, after being well washed with water, is decomposed by digestion in dilute sulphuric acid. The insoluble sulphate of lime is separated by a filter, and the citric acid obtained in crystals by evaporation. They are rendered quite pure by being dissolved in water and recrystallized. The proportions required in this pro- cess are 86.98 parts or one eq. of dry citrate of lime, and 49.1 parts or one eq. of strong sulphuric acid, which should be diluted wilh about 10 parts of water. Citric acid crystallizes in cooling from a hot saturated solution in crystals which consists of 58.48 parts or one eq. of the anhydrous acid, and 9 parts or one eq. ofwater, their formula being H+C. These crystals fuse in their water at a heat a little higher than 212°, but without loss of weight. By spontaneous evaporation it crystallizes in large transparent rhomboidal prisms^which differ from the former in composition, their formula heing 4H4-3C: they undergo no change in the air at common temperatures; but at 82° they effloresce and part with exactly half their water, retaining the remainder until the heat is so high that the acid itself is destroyed. Citric acid has a strong sour taste, with an agreeable flavour when di- luted, reddens litmus paper, and neutralizes alkalies. In a dry state it may be preserved for any length of time, but the aqueous solution is gradually decomposed by keeping. The crystals are soluble in an equal weight of cold and in half their weight of boiling water, and are also dissdved by alco- hol. It is converted into the oxalic by digestion in nitric acid. Citric acid is characterized by its flavour, by the form of its crystals, and by forming an insoluble salt with lime, and a deliquescent soluble one with potassa. It does not render lime-water turbid unless the latter is in excess, and fully saturated with lime in the cold. Citric acid is employed in calico-printing, and in medicinal and domestic purposes instead of lemon juice. Tartaric acid is often substituted for it, as being similar in flavour, and less expensive. Of the citrates, those of potassa, soda, ammonia, magnesia, and oxides of iron are soluble in water; while those of lime, baryta, strontia, and the oxides of lead, mercury, and silver are very sparingly soluble in hot water, though dissolved by exoess of their own acid. They are decomposed by sul- phuric acid. No general rule can be stated in respect of their constitution, since, from late observations of Berzelius (An de Ch. et de Ph. liii. 424), it appears that citric acid exhibits very unusual modes of combination, and is prone to form sub-salts of a complex composition. Thus, on mixing neutrd citrate of potassa with acetate or nitrate of oxide of lead, it is impracticable to procure the neutral citrate, Pb+C, because il falls in mixture with an- other salt, the formula of which is Pb2C+2H, and which, digested with very dilute ammonia, abandoned its water and half of its acid forming the salt 4Pb+3TJ. He obtained the neutral citrate in the purest state by adding an alcoholic solution of citric acid to a solution of acetate of oxide of load, and washing the precipitate with alcohol; for water deprives it of acid, and conyerts it into a sub-salt. On digesting the neutral citrate with acetate of TARTARIC ACID. 509 oxide of lead, he obtained a dicitrate, of which the formula is 2Pb+C. He found it very difficult to obtain neutral citrates of lime or baryta, as they both resolve themselves readily into acid and sub-salts of complex composi- tion. The most uniform of the citrates is that of oxide of silver, the neutral citrate Ag+C, being readily obtained by double decomposition. Pyrocitric Acid.—When the crystals of citric acid are subjected to de- structive distillation, three volatile products are obtained, namely, a peculiar acid called pyrocitric, water, and a spirituous matter which has not been ex- amined. Pyrocitric acid forms a sparingly soluble salt with oxide of lead; but the pyrocitrates generally have not yet been studied. It appears to form a hydrate with one equivalent of water. Tartaric Acid. This acid exists in the juice of several acidulous fruits, but it is almost dways in combination with lime or potassa. It is prepared by mixing cream of tartar and chalk in the ratio of their equivalents, 18.). 11 to 50.62, and boiling them in 10 times their weight ofwater for about half an hour. The carbonic acid of the chalk is displaced with effervescence, one eq. of in- soluble tartrate of lime subsides, and one eq. of neutral tartrate of potassa re- mains in solution. On washing the former, and then digesting it diffused through a moderate portion of water, with one equivalent of sulphuric acid, the tartaric acid is set free ; and after being separated from the sulphate of lime by a filter, it may be procured by evaporation in prismatic crystals, the primary form of which is a right rhombic prism. Tartaric acid has a sour taste, which is very agreeable when diluted with water. It reddens litmus paper strongly, and forms with alkalies neutral sdts, to which the name of tartrates is applied. It requires five or six times its weight of water at 60° for solution, and is much more soluble in boiling water. It is dissolved likewise, though less freely, in alcohol. The aqueous solution is gradually decomposed by keeping, and a similar change is expe- rienced, under the same circumstances, by most of the tartrates. The crys- tals may be exposed to the air without change. They are converted into the oxalic by digestion in nitric acid. Tartaric acid cannot be deprived of its water of crystallization, except by uniting with an alkaline base: on attempting to expel it by heat the acid fuses, and is decomposed, yielding, if air is excluded, the usual products of destructive distillation, together with a distinct acid, to which the term pyro- tartaric acid is applied. A large residue of charcoal is obtained. Tartaric acid is distinguished from other acids by forming a white pre- cipitate, bitartrate of potassa, when mixed with any of the salts of that al- kali. This acid, therefore, separates potassa from other acids. It occasions with lime-water a white precipitate, which is very soluble in an excess of the acid. Tartaric acid is remarkable for its tendency to form double salts, the pro- perties of which are often more interesting than the simple salts. The most im- portant of these double salts, and the only ones which have been much studied, are those of potassa and soda, and of oxide of antimony and potassa. The neutral tartrates of the alkalies, of magnesia, and protoxide of copper, are soluble in water; but most of the tartrates of the other bases, and especially those of lime, baryta, strontia, and oxide of lead, are insoluble. All these neutral tartrates, however, which are insoluble in pure water, are soluble in an excess of their acid. They are decomposed by digestion in carbonate of potassa ; and when an acid is added in excess, the bitartrate of potassa is pre- cipitated. All the insoluble tartrates are easily procured from neutral tar- trate of potassa by way of double decomposition. The neutrd tartrates of protoxides consist of 66.48 parts or one eq. of the acid, and one eq. of base, so that the general formula is M+C*H 0s, or M+T. Tartrates of Potassa.—The neutral tartrate, frequently called soluble tar. 43* 510 TARTARIC ACID. tar, is formed by neutralizing a solution of the bitartrate with carbonate of potassa : and it is a product of the operation above described for making tar- taric acid. Its primary form is a right rhomboidal prism; but it o!len occurs in irregular six-sided prisms with dihedral summits. Its crystals are very soluble in water, and attract moisture when exposed to the air. They con- sist of 113.63 parts or one eq. of the neutral tartrate, and 18 or two eq. of water. They are rendered quite anhydrous by a temperature not exceeding 248° F. 6 Of the bitartrate an impure form, commonly known by the name of tartar is found encrusted on the sides and bottom of wine casks, a source from which all the tartar of commerce is derived. This salt exists in the juice of the grape, and, owing to its insolubility in alcohol, is gradually deposited during the vinous fermentation. In its crude state it is coloured by the wine from which it was procured; but when purified, it is quite white, and in this state constitutes the cream of tartar of the shops. Bitartrate of potassa is very sparingly soluble in water, requiring 60 parts of cold and 14 of boiling water for solution, and is deposited from the latter on cooling in small crystalline grains. Its crystals are commonly irregular six-sided prisms, terminated at each extremity by six surfaces; and its primary form is either a right rectangular, or a right rhombic prism. It has a sour taste, and distinct acid reaction. It consists of 47.15 parts or one eq. of potassa, 132.96 or two eq. of acid, and 9 parts or one equivalent of water. Its water of crystallization cannot be expelled without decom- posing the salt itself. Bitartrate of potassa is employed in the formation of tartaric acid and all the tartrates. It is likewise used in preparing pure carbonate of potassa. When exposed to a strong heat, it yields an acrid empyreumatic oil, some pyrotartaric acid, together with water, carburetted hydrogen, carbonic oxide, and carbonic acid, the last of which combines with the potassa. The fixed products are carbonate of potassa and charcoal, which may be sepa- rated from each other by solution and filtration. When deflagrated with half its weight of nitre, by which part of the charcoal is consumed, it forms black flux; and when an equal weight of nitre is used, so as to oxidize all the carbon of the tartaric acid, a pure carbonate of potassa, called white flux, is procured. Tartrate of Potassa and Soda.—This double salt, which has been long employed in medicine under the name of Siegnette or Rochelle salt, is pre- pared by neutralizing bitartrate of potassa with carbonate of soda. By evaporation it yields prismatic crystals, the sides of which often amount to ten or twelve in number; but the primary form, as obtained by cleavage, is a right rhombic prism. (Brooke.) The crystds are soluble in five parts of cold and in a smaller quantity of boiling water, and are composed of 113.63 parts or one eq. of tartrate of potassa, 97.78 parts or one eq. of tartrate of soda, and 72 or eight equivalents of water. Tartrate of Soda is frequently used as an effervescing draught, by dis- solving equal weights of tartaric acid and bicarbonate of soda in separate portions of water, and then mixing the solutions. Soda is better adapted for this purpose than potassa, because the former has little or no tendency to form an insoluble bitartrate Tartrate of Oxide of Antimony and Potassa.—This compound, long cele- brated as a medicinal preparation under the name of tartar emetic, is made by boiling sesquioxide of antimony with a solution of bitartrate of potassa. The oxide of antimony is furnished for this purpose in various ways. Some- times the glass or crocus of that metal is employed. The Edinburgh col- lefrc prepare an oxide by deflagrating sulphuret of antimony with an equal weight of nitre; and the college of Dublin employ the oxy-chloride. Mr. Phillips recommends that 100 parts of metallic antimony in fine powder should be boiled to dryness in an iron vessel with 200 of sulphuric acid, and that the residual subsiilphate, after washing with water, be boiled with an equal weidit of cream of tartar. The solution of the double salt, how. RACEMIC ACID. 511 ever made, should be concentrated by evaporation, and allowed to cool in order that crystals may form. Tartar emetic yields crystals, which are transparent when first formed, but become white and opaque by exposure to the air. Its primary form had been correctly described by Mr. Brooke as an octohedron with a rhombic base (An. of Phil. N. S. vi. 40.); but the edges of the base are frequently replaced by planes which communicate a prismatic form, and its summits are generally formed with an edge instead of a solid angle, which edge is frequently truncated, presenting a narrow rectangular surface. It frequent- ly occurs in segments, having the outline of a triangular prism, a form which has deceived 'many into the belief, that the tetrahedron or regular octohedron is the primary form of tartar emetic. It has a styptic metallic taste, reddens litmus paper slightly, and is soluble in 15 parts of water at 60°, and in 3 of boiling water Its aqueous solution, like that of all the tartrates, undergoes spontaneous decomposition by keeping; and therefore, if kept in the liquid form, alcohol should be added in order to preserve it. From the analysis of Thomson, Phillips, and Wallquist, it may be consider- ed a compound of 1 eq. 1 eq. Base. Acid. Equiv. Formulas. Tartrate of sesquioxide of antimony 153.2 +66.46=219.68 Sb+Y. Tartrate of potassa . . 47.15 + 66.48=113.63 K + T\ Water of crystallization, 2 eq. , . , =18 351.31 5(KT+SbT) i +2H. Tartar emetic is decomposed by many reagents. Thus alkaline sub- stances, from their superior attraction for tartaric acid, separate the oxide of antimony. The pure alkalies, indeed, and especially potassa and soda, pre- cipitate it imperfectly, owing to their tendency to unite with and dissolve the oxide; but the alkaline carbonates throw down the oxide much more completely. Lime-water occasions a white precipitate, which is a mixture of oxide, or tartrate of the oxide, and tartrate of lime. The stronger acids, such as the sulphuric, nitric, and hydrochloric, cause a white precipitate, consisting of bitartrate of potassa and a subsalt of the oxide of antimony. Decomposition is likewise effected by several metallic salts, the bases of which yield insoluble compounds with tartaric acid. Hydrosulphuric acid throws down the orange sesquisulphuret of antimony. It is precipitated by many vegetable substances, especially by an infusion of gall-nuts, and other similar astringent solutions, with which it forms a dirty white precipitate which is a tannate of sesquioxide of antimony. This combination is inert and, therefore, a decoction of cinchona bark is recommended as an antidote to tartar emetic. Heated before the blowpipe, metallic antimony is readily brought into view; and if decomposed by heat in close vessels, a very in- flammable pyrophorus is formed. Jiacemic Acid. (Traubensdure, or Acid of Grapes of the Germans.) This acid was first noticed by Mr. Kestner, chemical manufacturer at Thann in the Upper Rhine, who met with it in the preparation of tartaric acid, with which it is associated in the juice of the grape. Kestner per- ceiving it to be different from tartaric acid, considered it to be the oxalic- John in 1819 declared it to be distinct from both of those acids, and termed it acid of the Vosges; and in 1326 Gay-Lussac and Walchner, receiving a BENZOIC ACID. supply from Kestner, made a careful examination of its principal characters (Jour, de Ch. Med. ii. 335, and Gmelin's Handbuch, ii. 53.) An account of Us properties has since been given by Berzelius, who has suggested for it the name of paratartaric acid. (An. de Ch. ct de Ph. xlvi. 128.) Racemic acid is associated with tartaric acid, apparently as a biracemate of potassa, in the grape of the Upper Rhine, and subsides during the fer- mentation of the juice along with cream of tartar: it is probably contained m the juice of all grapes. It is readily obtained by neutralizing the cream ot tartar of that district with carbonate of soda, separating the double tar trate of potassa and soda by crystallization, throwing down the racemic acid by a salt of hme or oxide of lead, and decomposing the precipitate by dilute sulphuric acid On concentrating the solution, the racemic acid crystallizes, and is thus completely separable from any remaining tartaric acid, the latter being much more soluble in water than the former. I he racemic and tartaric acids, besides being associated in nature, afford a most interesting instance of isomerism. Gay-Lussac showed that the equivalents of these acids are represented by the same number; and Berze- ius has not only confirmed this fact, but proved that their composition is likewise identicd. There is also a close analogy in their chemical rela- jS:Tf mS lnsoluble salts with the same bases, as with lime, baryta, and oxide of lead; biracemate of potassa is a sparingly soluble salt analo- gous to cream of tartar; and wilh sesquioxide of antimony the biracemate of potassa yields a double salt, similar in many respects to tartar emetic, though different in the form of its crystals. Nevertheless, the two acids are essentially distinct. The racemic is much less soluble than tartaric acid; the form of its crystals is different, being an oblique rhombic prism; it con- tains two equivalents of water of crystallization, one of which is given out at 212°, and the other when it unites with alkalies; and it does not yield a double salt with potassa and soda. The racemate of lime, too, is less solu- ble than the tartrate, and is but sparingly dissolved by excess of its acid: a solution of gypsum is not affected by tartaric acid, whereas a little racemic acid after an interval of about an hour causes turbidity: racemate of lime dissolved in dilute hydrochloric acid is almost immediately thrown down by the addition of ammonia; while tartrate of lime under the same treatment does not subside so as to cause turbidity, but after a time slowly separates in octohedral crystals with a square base, which are found adhering to the sides of the glass. It thus appears that racemic and tartaric acids have the same atomic weight, the same composition, and in several respects the same chemical properties; and yet on closely investigating all their charac- ters, they are found to be essentially distinct. Moreover, the different ar- rangement of the atoms in these two acids is indicated by the different form of similar combinations. Benzoic Acid. Benzoic acid exists in gum benzoin, in storax, in the balsams of Peru and Tolu, and in several other vegetable substances. M. Vogel has detected it in the flowers of the trifolium melilotus officinalis. It is found in considerable quantity in the urine of the cow and other herbivorous animals, and is per- haps derived from the grasses on wliich they feed. It has also been detected in the urine of children. This acid is commonly extracted from gum benzoin. One method con- sists in heating the benzoin in an earthen pot, over which is placed a cone of paper to receive the acid as it sublimes; but since the product is always impure, owing to the presence of empyreumatic oil, it is better to extract the acid by means of an alkali. The usual process consists in boiling finely powdered gum benzoin in a large quantity ofwater along with lime or car- bonate of potassa, by which means a benzoate is formed. To the solution, after being filtered and concentrated by evaporation, hydrochloric acid is MEC0NIC ACID. added, which unites with the base, and throws down the benzoic acid. It is then dried by a gentle heat, and purified by sublimation. Benzoic acid has a sweet and aromatic rather than a sour taste; but it reddens litmus paper, and neutralizes alkalies. It fuses readily by heat, and at a temperature a little above its point of fusion, it is converted into vapour, emitting a peculiar, fragrant, and highly characteristic odour, and condens- ing on cool surfaces without change. When strongly healed it takes fire, and burns with a clear yellow flame. It undergoes no change by exposure to the air, and is not decomposed by the action even of nitric acid. It re- quires about 24 parts of boiling water for solution, and nearly the whole of it is deposited on cooling in the form of minute acicular crystals of a silky lustre. It is very soluble in alcohol, especially by the aid of heat. Agreeably to the late admirable researches of Liebig and Wohler on the oil of bitter almonds, benzoic acid must be regarded as the oxide of a com- pound inflammable body which will be treated of in the third section, and to which they have applied the name of benzule, composed of benz, and vKh matter or principle. Benzule is composed of 85.68 parts or fourteen eq. of carbon, 5 parts or five eq. of hydrogen, and 16 parts or two eq, of oxygen, its equivalent being 106.68. The composition of benzoic acid may be thus stated, an equivalent being represented by Bz:—(An. de Ch. et de Ph. Ii. 273.) Anhydrous benzoic acid. Benzule 106.68+Oxygen 8=114.68 Bz+OorBz. The crystallized acid contains one eq. of water, its formula being Bz+H, and it cannot be deprived of its water by heat. Its water of crystallization is also contained in benzoate of oxide of lead, PbBz+H, and is not expelled by heat until the salt itself is decomposed. Benzoate of oxide of silver is an- hydrous, its formula being Ag+Bz: it falls as a white powder when nitrate of oxide of silver is mixed in solution with benzoate of ammonia, is com- pletely dissolved by boiling water, and is deposited on cooling in brilliant foliated crystals. The composition of neutrd benzoates of protoxides is represented by the general formula M+Bz. Most of these are soluble in water; the most in- soluble arc the benzoates of the oxides of lead, mercury, and silver, and per- oxide of iron, Meconic Acid. This acid, which derives its name from Mxnay, poppy, has hitherto been found only in opium, where it exists in combination with morphia ; and it is best prepared from the mixed sulphate and meconate of lime obtained in Gregory's process for hydrochlorate of morphia. The impure meconate of lime is put into ten times its weight ofwater at 200°, and hydrochloric acid is added until the meconate is dissolved: the solution is then filtered, and on cooling deposites bimeconate of lime in the form of light white scaly or acicular crystals. These are again dissolved by hot dilute hydrochloric acid as before, in order to separate the remainder of the lime; and on cooling crystals of meconic acid subside. If they leave a residue after calcination they should be again dissolved in acid and re-crystallized. To procure the acid quite white, it is saturated with potassa diluted with the smallest quan. tity ofwater required by the aid of heat to dissolve the meconate, and when cold the fluid part, which retains the colouring matter, is separated from the crystals by filtration and pressure in cloth or bibulous paper. The meconate of potassa is then decomposed in the same way as the meconate of lime when meconic acid is obtained perfectly white and in transparent micaceous scales. Meconic acid has an acid taste and reaction, and is soluble both in alco- hol and water, requiring only four times its weight of boiling water for so- 514 TANNIC ACID. thev beeJmp ! S ™ unchanged ™ the air, but when heated to 212° beiL 27 ^U e^PaqUe and ,ose 21-5 Per cent of water of crystallization, The! stilf"\™ three eq> t0 100-84 Parts or one e* of the anhydrous acid! water S"''C0?tlnu.e iconic acid even when heated to 248°; but if boiled in acid dSnn CdC ,gaS 1S diseni?aeed< *he liquid becomes brown, meconic acid disappears, and metameconic acid is generated. IVleconic acid is remarkable for forming with persalts of iron a purple and detect' Wh'Ch -^ ■' g°f ^ H the add "hether free °r -mTne with it rBt".mec°n'c ^Cld ln s°lutlon9 of opium, when the morphia combined With t could not be discovered ; the colour is very like that produced by hy. dros, Iphocyanic acid. Meconic acid forms insoluble white sails with lime baryta, and the oxides of lead and silver, which are soluble in nitric acidTit' is very prone to form bisalts, as the bimeconate of potassa and lime which are of sparing solubility. The meconate of oxide of silver consists of 116 oarf^ one eq. of that base and 100.84 parts or one eq. of meeon^cid Us Cula composition H°7' ThC ne"tral meconates of protoxides are analogous in Metameconic Acid. Robiquet,who has described the preparation and properties of meconic acid, also observed its conversion into metameconic acid by boilinjr its aqueous solution; and Liebig, by his skilful analysis of both acids, has proved (An. de Ch. et de Ph. Ii. 241 and liv. 26) that 2 l%r^,9H0?i7CnCi<1 ] yield \ 1^. of metameconic acid I2C+4H+10O «V^+<:tl4-/U; S I and 2 eq. carbonic acid 2(C+20). iThuC bTuWn maMer formed when a solution of meconic acid is boiled, and which adheres to the acid, does not seem essential to the change, since it takes place in a very slight degree when a meconate is boiled with hydro- chloric acid, the resulting metameconic acid being then nearly colourless. Metameconic acid is soluble in 16 times its weight of boiling water, and separates on cooling in anhydrous hard crystalline grains, wholly different from meconic acid. It reddens persalts of iron, but its other properties have not yet been studied. When anhydrous meconic acid is distilled, it yields among other products a crystalline sublimate, which till lately was regarded as pure meconic acid, though in reality it is not so. It reddens persalts of iron; but whether it is metameconic acid, or some other compound, has not been determined. Tannic Acid (Tannin.) This substance exists in an imnure state in the excrescences of several species of oak, called gall-nuts ; in the bark of most trees; in some inspis- sated juices, such as kino and catechu; in the leaves of the tea-plant, su- mach, and whortleberry (uva ursa), and in astringent plants generally, be- ing the chief cause of the astringency of vegetable matter. It is frequently associated with gallic acid, as in gall-nuts, in most kinds of bark, and in tea; but in kino, catechu, and cinchona bark, little or no gallic acid is present. Several processes are recommended for the preparation of tannic acid. Thus, it may be precipitated from a solution of gall-nuts by chloride of tin, and the precipitate after washing be decomposed by hydrosulphuric acid; or it may be thrown down by concentrated sulphuric acid in the cold, and the precipitate, when dried in folds of bibulous paper, be decomposed by di- gestion with carbonate of oxide of lead. But in these and similar processes the tannic acid, which is very prone to change, is more or less modified in its character; and, therefore, the process with ether lately recommended by Pelouze is preferable. (An. de Ch. et de Ph. liv. 337.) The lower aper- TANNIC ACID. ture of an elongated narrow glass vessel is loosely clssed by a piece of linen; on this is laid some gall-nuts in fine powder, which is gently pressed together, and then sulphuric ether is poured upon the powder: the upper aperture is closed by a stopper to prevent loss of ether by evaporation, and the apparatus fixed in the mouth of a bottle, which receives the ether as it slowly percolates through the powder. It appears that the small quantity of water contained in ether as sold in the shops, combines with tannic acid to the exclusion of the other ingredients of the gall-nuts, and forms a saturated aqueous solution, which collects as a distinct stratum below the ether. After exhausting the gall-nuts by repeated portions of ether, the ethereal part is drawn off and purified by distillation, and the aqueous solution of tannic acid, after being washed with ether, is evaporated to dryness at a very gen- tle heat or in vacuo with sulphuric acid. The tannic acid is left as a uni- form spongy mass, not crystallized, either quite white or with a slightly yellow tint. Its quantity varies from 35 to 40 per cent, of the gdl-nuts em- ployed. Pure tannic acid is colourless and inodorous, has a purely astringent taste without bitterness, and may be preserved without change in the solid state. It is very soluble in water, and the solution reddens litmus, and de- composes alkaline carbonates with effervescence, thus leaving no doubt of its acidity. Alcohol and ether also dissolve tannic acid, but more sparingly than water, especially when anhydrous. Solutions of tannic acid do not affect pure protosalts of iron, but strike a deep blue precipitate with the persalts: a strong solution of it yields a copious white precipitate with the sulphuric, nitric, hydrochloric, phosphoric, and arsenic acids, but none with the oxdic, tartaric, lactic, acetic, citric, succinic, and selenious acids. It is precipitated also by the carbonates of potassa and ammonia, by the alkaline earths, alumina, and many solutions of the second class of metals. With cinchonia, quinia, brucia, strychnia, codeia, narcotina, and morphia, it yields white tannates, which are sparingly soluble in pure water, but are dissolved readily by acetic acid. By digestion with nitric acid it yields oxalic acid. A solution of tannic acid may be preserved without change, provided it be excluded from oxygen gas; but in open vessels it gradually absorbs oxy- gen, an equal volume of carbonic acid is evolved, it becomes turbid, and deposites a crystalline matter of a gray colour, nearly all of which is gallic acid. After digestion with a little animal charcoal, the gallic acid is perfect- ly white and pure. There is no doubt, therefore, of the conversion of tannic into gallic acid. Tannic acid is distinguished from all substances, except gallic acid, by forming a deep blue precipitate with persalts of iron, which is the basis of writing-ink and the black dyes; and from gallic acid, by yielding with a solution of gelatin a white flaky precipitate, which is soluble in a solution oi gelatin, but insoluble in water and gallic acid. This substance, to which the name of tanno-gelatin has been applied, is the basis of leather, being always formed when skins are macerated in an infusion of bark. When dried it becomes hard and tough, and resists putrefaction. Its composition is apt to vary, according to the relative quantities of the materials used in its formation; but if the gelatin is in slight excess only, the resulting com- pound contains 54 per cent, of tannic acid. This acid cannot be so wholly separated by gelatin, that the remaining solution shall not be affected by a persalt of iron; but Pelouze finds that every trace of it is removed by a piece of skin previously cleansed by lime as in the manufacture of leather, leaving any gallic acid which may have been present. From the experiments of Davy, it appears that the inner cortical layers of bark are the richest in tannic acid. Its quantity is greatest in early spring when the buds begin to open, and smallest during winter. Of all the varieties of bark which he examined, that of the oak contains the largest quantity of tannic acid. Tannic acid dried at 240° is anhydrous, such as it exists in the white tannate of oxide of lead when dried at 248°. Pelouze found this salt to 516 OALUC ACID. consist of 111.6 parts or one eq. of protoxide of lead, and 215.16 parts or one eq. of tannic acid, its formula being I'b+Tn. The other tannates of protoxides have most probably a similar composition. The white tannate of sesquioxide of antimony and the blue tannate of peroxide of iron are thus constituted:— Base. Acid. Equiv. Formulae. Tannate of sesquiox. of antimony 153.2+645.48=798.68 Sb+3Tn. Tannate of peroxide of iron . 80 +645.48=725.48 Fe+3Tn. The various kinds of tannic acid obtained from cinchona bark, kino, and othei> sources, correspond in most respects with that above described; but at the same time some difference is observable, some kinds striking a green instead of a deep blue colour with the persalts of iron. The tannic acid from catechu is less highly oxidized than that from gall nuts (page 498). Artificial Tannic Acid.—This substance was discovered by Mr. Hatchett, and is best prepared by the action of nitric acid on charcoal (Phil. Trans. 1805-6). For this purpose 100 grains of charcoal in fine powder are digest ed in an ounce of nitric acid, of density 1.4, diluted with two ounces of water, with a gentle heat until the charcoal is dissolved. The reddish- brown solution is then evaporated to dryness, in order to expel the nitric acid, the temperature being carefully regulated towards the close of the process, so that the product may not be decomposed. Artificial tannic acid is a brown fusible substance of a resinous fracture, astringent taste, and acid reaction. It is soluble even in cold water and in dcohol. With a salt of iron and solution of gelatin it acts precisely in the same manner as natural tannic acid. It differs, however, from that sub- stance in not being decomposed by the action of strong nitric acid. Artificial tannic acid may be prepared in several ways. Thus it is gene- rated by the action of nitric acid, both on animal or vegetable charcoal, and on pit-coal, asphaltum, jet, indigo, common resin, and several other resinous substances. It is also procured by treating common resin, elemi, assa- foetida, camphor, bdsams, &c., first with sulphuric acid, and then with dcohol. Gallic Acid. This acid, discovered by Scheele in 1786, exists in the bark of many trees, in gall-nuts, and in most substances which contain tannic acid, being probably developed bv the oxidation of that acid. The best process for pre- paring it is that of Scheele as modified by Braconnot (An. de Ch. et de Ph. ix) Any quantity of gall-nuts, reduced to powder, is infused for a few davs in four times its weight of water, and the infusion, after being strained through linen, is kept for two months in an open vessel and a moderately warm atmosphere. During this period the surface of the liquid becomes mouldv and tannic acid is converted by the oxygen of the air into gallic acid which subsides as a yellowish crystalline matter, mixed with ellagic ■cid' On evaporating the solution to the consistence of syrup, an additional ouantity subsides on cooling. The object of this process is to give, an op- nortunity for the oxidation of tannic acid, which disappears more or less completely, and is the principal source of the gallic acid, since the alter usually exists in gall-nuts in very small quantity. The impure gallic is separated from the insoluble ellagic acid by boiling water; and it is ren- Hpred white by digestion with animal charcoal deprived of its phosphate ot lime bv dilute nitric acid. When the colourless solution is concentrated by evaporation, the gallic acid is deposited in small white acicular crystals of a sZ lustre. Some crystals prepared by Mr. Phillips, and examined by Mr. Brooke, were in the form of an oblique rhombic prism. Pure gallic acid has a weak acid taste, accompanied with sight astrin- gency, and reddens litmus. In boiling water it is free y soluble bu it re- quires 100 parts of cold water for solution; and it is soluble in ether. Its aqueous solution continues unchanged if protected from oxygen gas ; bu tin ooen vessels it is gradually decomposed, acquires a yellow tint, and deposites Sk brown matter. With sulphate of the protoxide of iron, it produces LaJcely^y change; but with the persulphate it gives a dark blue precipi- tate which is mori soluble than the tannate of that oxide, and slowly dis- so yes after its formation. When the recent precipitate is boiled in water the sulphuric acid gradually reunites with the iron, which is reduced to the sTate of protoxide at the expense of gallic acid, carbonic acid being disen gaged. The same action ensues slowly in the cold, and the tannate of the peroxide is liable to a similar change. Both the tannate and gdlate of the peroxide exist in ink. . Gdlic acid does not precipitate solutions of gelatin, or salts of the vegeta- ble dkalies. With lime-water it gives a brownish-green precipitate, which is redissolved by an excess of the alkali, and acquires a reddish tint, owing it is said to the decomposing agency of the air. Crystallized gdlic acid loses its water at 243°, and then has the same composition (page 497) as the acid when united with oxide of lead. 1 his sdt fdls as a white precipitate when gallic acid is mixed in solution with the acetate or nitrate of oxide of lead ; and Pelouze states that when dried at 2483 it consists of 111.6 parts or one eq. of protoxide of lead, and 85.84 parts or one eq. of gallic acid; whence its formula is Pb+C7H303, or Pb+G. The gallates in general have been but little examined; but M+G may be taken as the general formula of the constitution of neutral gallates of protoxides. The gallates of potassa, soda, and ammonia are soluble in water; but the gallates of most other metallic oxides are insoluble. Pyrogallic Acid. Braconnot first noticed that the crystals obtained by subliming gallic acid are not, as was thought, gdlic acid, but an acid of a peculiar kind which he has termed the pyrogallic, to indicate its igneous origin. Pelouze has con- firmed this observation, and finds that if pure anhydrous gallic acid be heat- ed in a dry retort plunged in oil which is kept at a temperature varying from 410° to 419°, it is resolved entirely into carbonic acid which is evolved as gas, and pyrogallic acid which condenses in the upper part of the retort in numerous 6caly crystals of brilliant whiteness. The decomposition is such that (An. de Ch. et de Ph. xlvi. 206, and liv. 352.) 1 eq. g.lUc acid 7C+3H+50-yi«ld, { ^f^S^uS^^ Pyrogallic acid has a faintly bitter astringent taste without acidity, and barely reddens litmus paper. At 247° it fuses and at 410° boils, yielding a colourless vapour of a faint odour, somewhat resembling that of benzoic acid. At 480° it blackens, and is resolved into metagallic acid and water, 2rr/,l2(6c+3H+3°)^^)^3Ts'w,S12c3fH+^0 Pyrogallic acid dissolves in two or three times its weight of cold water and is very soluble in alcohol and ether. Its aqueous solution, at first co- lourless, becomes brown by exposure to the air, and in a few days is de- composed. With the nitrates of the oxides of silver and mercury and a persalt of iron, it is decomposed, and reduces the two former oxides to the metallic state, and the latter to the state of protoxide, the iron solution ac quiring a brown colour. With a protosalt of iron it strikes a blackish-blue co our, and a similar tint is developed with a persalt of iron and a nvro gallate. " 44 518 METAGALLIC ACID.—ELLAGIC ACID.—SUCCINIC ACID. Metagallic Acid. If instead of decomposing gallic acid at 419°, it is suddenly heated to 480°, carbonic acid and water are disengaged, and instead of a sublimate of pyrogallic acid, a black shining insoluble matter like charcqal remains in the retort, to which Pelouze has given the name of metagallic acid. Its formation obviously depends on the decomposition of pyrogallic acid at the instant of its formation, agreeably to the formula above given. The same products are obtained by distilling tannic acid at 480°, in addition to which pyrogallic acid is formed if the distillation is conducted at 410°. Metagallic acid is dissolved by the alkalies, forming neutral metagd- lates, from which it is precipitated in black flakes by acids; and it de- composes alkaline carbonates with effervescence. The metagallate of po- tassa gives black precipitates with soluble salts of baryta, strontia, lime, magnesia, and the oxides of iron, zinc, copper, lead, and silver, showing that the metagallates of those bases are insoluble. The silver salt con. sists of 116 parts or one eq. of oxide of silver, and 100.44 or one eq. of metagallic acid, its formula being Ag + C^H^Os. The acid as first formed, or as precipitated from its salts by acids, is a hydrate with one eq. of water, of which the formula is H+C12Hs03. Ellagic Acid. This acid, called ellagic by Braconnot, from the word galle read back- wards, is left in the process for gallic acid in the form of a gray pow- der, insoluble in water, but soluble in a solution of potassa, and precipi- tated by acids as a yellowish-gray powder. Ellagic acid, when dried at 248°, was found by Pelouze to have the same composition as anhydrous gallic acid, minus one eq. of water; and the acid before being so dried is a hydrate identical in its ingredients with anhydrous gallic acid. Succinic Acid. This acid is procured by heating powdered amber in a retort by a regu latcd temperature, when the succinic acid, which exists ready formed in amber, passes over and condenses in the receiver. As first obtained, it has a yellow colour and peculiar odour, owing to the presence of some empy- reumatic oil; but it is rendered quite pure and white by being dissolved in nitric acid, and then evaporated to dryness. The oil is decomposed, and the succinic acid is left unchanged. Succinic acid has a sour taste, and reddens litmus paper. It is soluble both in water and alcohol, and crystallizes by evaporation in anhydrous prisms. When briskly heated, it fuses, undergoes decomposition, and in part sublimes, emitting a peculiar and very characteristic odour. The salts of succinic acid have been little examined. The succinates of the alkalies are soluble in water. That of ammonia is frequently employed for separating iron from manganese, persuccinate of iron being quite inso- luble in cold water, provided the solutions are neutral. Succinate of prot- oxide of manganese, on the contrary, is soluble. By reference to the table (page 497) it will be seen that succinic acid dif- fers in composition from the acetic only in containing one equivalent less of hydrogen. Muck or Saccholactic Acid was discovered by Scheele in 1780. It is ob- tained by the action of nitric acid on certain substances, such as gum, manna, and sugar of milk. The readiest and cheapest mode of forming it is by digesting gum with three times its weight of nitric acid. On applying heat, effervescence ensues, and three acids—the oxalic, malic, and saccho- lactic—are the products. The latter, from its insolubility, subsides as a ^CAMPHORIC ACID?—VALERIANIC ACID.—ROCELLIC ACID, &C. 519 white powder, and may be separated from the others by washing with cold •. In this state Dr. Prout says it is very impure. To purify it he di- water water, in mis siaie w.i"""""r ;- ,•>,. '. ., ■ ,.•- „n n gests with a slight excess of ammonia, and dissolves the resulting salt n boiling water. It is filtered while hot, and the solution evaporated slowly almost to dryness. The saccholactate of ammonia is thus obtained in crys- tdTwlteh a?e to be washed with cold distilled water, until they become quite white They are then dissolved in boiling water, and the saturated hot solution dropped into cold dilute nitric acid. The sacchokctic is a weak acid which is insoluble in alcohol andIre. quires sixty times its weight of boiling water for solution. iM en heated in a retort it is decomposed ; and in addition to the usual products, yields a volatile white substance, to which the name of pyromucic acid has been ap- P 'camphoric Acid.—This compound has not hitherto been found in any plant, and is procured only by digesting camphor for a considerable time in a large excess of nitric acid. As the solution cools, the camphoric acid sepa- rates out in crystals; but it appears from some observations of Lieb!g (An. de Ch. et de Ph. xlvii. 95), that so long as it retains the odour of camphor, as it is apt to do, its freedom from that substance is incomplete, and it re- quires renewed digestion with nitric acid. It is sparingly soluble in water, fuses at 145°, and sublimes at a temperature by no means elevated. Its taste is rather bitter, and when quite pure has probably no odour. It reddens lit- mus paper, and combines with alkaline bases, forming salts which are called camphorates : those with the alkalies are very soluble and even deliquescent, but with oxide of lead it forms an insoluble compound. Camphoric acid is now to be regarded as an oxide of the compound radical, camphene. (Page 256.) Valerianic Acid.—The existence of this acid was observed by M. Grote, and since studied by Penz and Trommsdorff (An. de Ch. et de Ph. liv. 208.) This acid exists in the root of valerian (Valeriana officinalis) and passes over along with an essential oil when that root is distilled : on agitating the pro- duct with carbonate of magnesia and water, and distilling, the essential oil is expelled, and vderianate of magnesia is left. On decomposing this salt by dilute sulphuric acid, the valerianic acid may be obtained by distillation, partly dissolved in the water which passes over with it, and partly as an oleaginous matter floating on its surface. Pure valerianic acid is a colourless oleaginous fluid, of density 0.944, an odour like that of valerian, and a strong, acid, disagreeable taste. It boils at 270°, but distils along with aqueous vapour at a much lower temperature: the oily stain which it gives to paper disappears entirely by heat It burns with a vivid flame without residue. It dissolves in every proportion in alco- hol, and in 30 parts of cold water. It has a strong acid reaction, and forms salts with alkaline bases, most of which are soluble in alcohol and water, are fatty to the touch, and when decomposed emit the peculiar odour of the acid. From the analysis of Ittner (page 497) the general formula of the neu- tral valerianates of protoxides is M+C*oHsOs, and the oily acid is a hy- drate wilh one eq. ofwater. Rocellic Acid.—This acid was discovered by Heercn in the rocella tincto- ria, and is separated by acting on the lichen with a solution of ammonia, pre- cipitating with chloride of calcium, and decomposing the rocellate of lime by hydrochloric acid, when rocellic acid subsides. It is purified by solution in ether, and is deposited by evaporation in minute white crystals of a silky lustre. This acid fuses at 266°, is very soluble in alcohol and ether, and is insoluble in water. Moroxylie Acid.—This compound, which was first discovered by Klap- roth, is found in combination with lime on the bark of the morus alba or white mulberry, and has hence received the appellation of moric or moroxylie acid. It is obtained by decomposing moroxylate of lime by acetate of oxide of lead, and then separating that oxide by means of sulphuric acid. 520 CHLOROXALIC ACID.—BOLET1C ACID.—IGASURIC ACIDj &C. Hydrocyanic or prussic acid, which is not an unfrequent production of plants, has already been described. The sorbic, as already mentioned, has been shown to be malic acid. Rheumic Acid.—This name was applied to the acid principle contained in the stem of the garden rhubarb; but M. Lassaigne has shown it to be oxalic acid. Chloroxalic Acid.—When crystallizable acetic acid is put into a glass ves- sel full of dry chlorine, and exposed for a day to bright sunshine, hydro- chloric acid gas is generated, and during the night chloroxalic acid is depo- sited in dendritic crystals or small rhombic scales. In order to obtain it pure the chlorine should be in excess, and the gases subsequently expelled from the flask by dry air. The new acid is very volatile and] deliques- cent, and when evaporated in vacuo, yields rhombic crystals. Its elements are in such proportion that it may be regarded as a compound of one equiva- lent of hydrochloric and one equivalent of oxalic acid. These observations were made by Dumas. (Pog. Annalen, xx. 166.) Boletic Acid was discovered by M. Braconnot in the juice of the Boletus pseudo-igniarius. As it is a compound of little interest, I refer the reader to the original paper for an account of it. (Annals of Phil. vol. ii.) Igasuric Acid.—Pelletier and Caventou have proposed this name for the acid which occurs in combination with strychnia in the nux vomica and St. Ignatius's bean. It may be conveniently obtained by adding acetate of oxide of lead to the aqueous solution of nux vomica prepared as in the preparation of strychnia, when the igasurate of that oxide subsides r the precipitate, after being washed, is put into water and decomposed by a current of hydrosul- phuric acid gas. The solution of igasuric acid is then separated from sul- phuret of lead by filtration, and may be purified either by digestion with animal charcoal, from which phosphate of lime has been removed by an acid, or by a second precipitation with acetate of oxide of lead. On concen- trating the purified solution to the consistence of thin syrup, and placing it in a warm situation, the acid separates in crystals which are commonly indistinct in their form. Igasuric acid forms soluble sdts with the alkalies, baryta, and the oxides of iron, silver, and mercury. With oxide of lead, lime, and magnesia, it yields sparingly soluble compounds; but the two latter are dissolved by hot wa- ter. With sulphate of oxide of copper in neutral solutions, it occasions, either immediately or after a short interval, a light green precipitate, which is very characteristic of igasuric acid. Suberic Acid is produced by the action of nitric acid on cork. Its acid properties are very feeble. It is very soluble in boiling water, and the greater part of it is deposited from the solution in cooling in the form of a white powder. Its salts, which have been little examined, are known by the name of suberates. Zumic Acid.—This compound, procured by Braconnot from several vege- table substances which had undergone the acetous fermentation, appears from the observations of Vogel to be lactic acid. (Annals of Philosophy, V°'pfctic Acid.—This substance, distinguished by its remarkable tendency to gelatinize, a property from which its name is derived (from ™xtk, co- agulum), was originally described by Braconnot; and it has since 1»en ex- amined by the late celebrated Vauquelin. (An. de Ch. et de Ph. xxvin. 173, and xli. 46.) Braconnot believed it to be present in all plants ; but tie ex- tracted it chiefly from the carrot. For this purpose the carrot is made into a pulp, the juice is expressed, and the solid part well washed with distilled water It is then boiled for about ten minutes with a very dilute solution of pure potassa, or, as Vauquelin advised, with bicarbonate of potassa m the ratio of 5 parts to 100 of the washed pulp, and the chloride of calc uni is added to the filtered liquor. The precipitate, consisting of peeti° «ad «a lime, is well washed, and the lime removed by water acidulated witn nyuru- chloric acid. LACTUCIC ACID.—CRAMERIC ACID.—CAINCIC AMD, &C. 521 Pectic acid, as thus procured, is in the form of jelly. It is insoluble in cold water and acids, and nearly so in boiling water. It has a slight acid reac tion, and a feeble neutralizing power with alkahes, with which it forms so- luble compounds. The earthy pectates are very insoluble, and on this ac- count, in preparing pectic acid, pure water must be used; tor the process always fails, when water containing earthy salts is employed. By digestion in a strong solution of potassa, pectic acid disappears, the liquid becomes brown, and oxalate of potassa is obtained by evaporation. This fact excites some suspicion that pectic acid may be a compound of oxalic acid with a vegetable principle analogous to gum ; but the conversion of organic substances in general into oxalic acid by the action of potassa, as already noticed at page 494, diminishes the force of this objection. Lactucic Acid.—This acid was obtained by Pfaff from the juice of the lactuca virosa^ who throws down the acid by a salt of copper or lead, and then separates the metallic oxide by hydrosulphuric acid. It is said to differ from oxalic acid, which in most respects it resembles, by giving a green pre- cipitate with the protosalts of iron, and a brown with sulphate of protoxide of copper; but its properties are imperfectly known. Crameric Acid.—This acid was discovered by M. Peschier of Geneva in the extract of Rhatany root, Crameria triandra. After separating from an aqueous solution of the extract all the tannic acid by means of gelatin, and then neutralizing by ammonia, acetate of oxide of lead is added as long as it occasions a precipitate : the cramerate of that oxide is decomposed either by sulphuric acid or hydrosulphuric acid; and the solution is concentrated in order that the crameric acid may crystallize. This acid forms a sparingly soluble sdt with baryta; and it is singular that the small quantity which is dissolved, is not precipitated by sulphuric acid, though the baryta may be thrown down by an alkaline carbonate. Caincic Acid.—This acid, discovered by MM. Francois, Caventou, and Pelletier, is the bitter principle of the bark of the cainca root, a Brazilian shrub which is employed for the cure of intermittent fever. (Journ. de Pharm. xvi. 465.) It crystallizes in delicate white needles arranged in tufts like hydrochlorate of morphia, has a remarkably bitter taste, and an acid reaction. It is sparingly soluble in water and ether, but is abqndantly dis- solved by alcohol, especially when heated. It unites with the alkalies form- ing soluble salts which do not crystallize, and from which acids throw down caincic acid. It forms soluble neutral salts with baryta and lime, but the caincatc of oxide of lead and the subcaincate of lime are insoluble in water. It is decomposed by the concentrated mineral acids; and the hydrochloric, even in the cold, converts it into a gelatinous matter which is nearly in- sipid. Its equivalent has not yet been ascertained; but Liebig finds that the crystallized acid loses 9 per cent, of water at 212°, and that 100 parts of the acid thus dried contain 57.38 parts of carbon, 7.43 of hydrogen, and 35.14 of oxygen. Caincic acid is prepared by exhausting the bark with hot alcohol, and evaporating the solution to the consistence of an extract, which is then boiled in water, and the hot aqueous solution, after filtration, is decomposed by an excess of lime. The precipitate treated with oxalic acid yields oxalate of lime and free caincic acid, which are both insoluble in cold water. Hot dcohol takes up the latter, which is decolorized by animal charcoal. Indigotic Acid.—This acid has been studied by Buff, and recently ana- 17Zcd ? ^Umas- (An'de Ch'etde Ph-xxxvii> 160>xxxix- 290>Ji-174» andliii 17b.) It is generated, with evolution of carbonic acid and binoxide of ni- trogen gases in equal measures, but without the production of any carba- zotic acid by boiling indigo in rather dilute nitric acid, formed by mixing nitric acid of sp gr. 1.2 with an equal weight of water. To the solution kept boiling, md.go in coarse powder is gradually added, as long as efTer' vescence continues; and hot water is occasionally added to supply loss bv evaporation The impure indigotic acid, deposited in cooling, i boiled witJ oxide of lead and filtered, ,n order to separate resin; and the clear yeUow 522 CARBAZOTIC ACID. solution is decomposed by sulphuric acid, and again filtered at a boiling temperature. On cooling, the acid crystallizes in yellowish-white needles. In order to purify them completely, they were digested in water with carbo- nate of baryta; and the indigotate of baryta, deposited from the hot filtered solution in cooling, was dissolved in hot water, and decomposed by an acid. Indigotic acid was thus obtained in acicular crystals of snowy whiteness, which contracted greatly in drying, and lost their crystalline aspect; but the dry mass was dazzling white, and had a silky lustre. Indigotic acid decomposes carbonates,'but is a feeble acid, and reddens litmus faintly. It requires 1000 times its weight of cold water for solution, but is soluble to any extent in hot water and alcohol. Heated in a tube it fuses, and sublimes without decomposition; and the fused mass, in cooling, crystallizes in six-sided plates. When heated in open vessels, it is inflamed, and burns with much smoke. Dumas has compared together the constitution of indigogen, indigo-blue, and indigotic acid, and finds that they may be viewed as oxides of the same compound radical: thus Carb. Hyd. Nitr. Oxy. Equiv. Formulae. Indigogen . 275.4 +15 +42.45+ 32 =364.85 C«Hi6N3+40. Indigo-blue . 275.4 +15 +42.45+ 48 =380.85 C^IDsNa+eO. Do. in 100 parts 72.8 + 4.04+10.8 + 12.36 Indigotic acid 275.4 +15 +42.45+240 =572.85 C^sH^Na+SOO. Do. in 100 parts 48.23+ 2.76+ 7.73+ 41.23 The equivalent of indigotic acid as here represented has not been proved by the composition of its salts, great uncertainty existing as yet respecting the composition of a neutral indigotate. Carbazotic Acid.—This name has been applied by Liebig to a peculiar acid formed by the action of nitric acid on indigo. It was first noticed by Hausmann, and subsequently examined by Proust, Fourcroy and Vauquelin, Chevreu), and Liebig. It is made by dissolving small fragments of the best indigo in 8 or 10 times their weight of moderately strong nitric acid, and boiling as long as nitrous acid fumes are evolved. During the action, car- bonic, hydrocyanic, and nitrous acids are evolved; and in the liquid, besides carbazotic acid, is found a resinous matter, artificial tannin, and indigotic acid. On cooling, carbazotic acid is freely deposited in transparent yellow crystals; and on evaporating the residual liquid, and adding cold water, an additional quantity of the acid is procured. To render it quite pure, it should be dissolved in hot water, and neutralized by carbonate of potassa. As the liquid cools, carbazotate of potassa crystallizes, and may be purified by repeated crystallization. The acid may be precipitated from this salt by sulphuric acid. Carbazotic acid is sparingly soluble in cold water; but it is dissolved much more freely by the aid of heat, and on cooling yields brilliant crystal- line plates of a yellow colour. Ether and alcohol dissolve it readily. It is fused and volatilized by heat without decomposition; but when suddenly exposed to a strong heat, it inflames without explosion, and burns with a yellow flame, with a residue of charcoal. Its solution has a bright yellow colour, reddens litmus paper, is extremely bitter, and acts like a strong acid on metallic oxides. It is said to be poisonous. (Journal of Science, ii. 210, and iii. 490.) The salts of carbazotic acid are for the most part crystallizable, of a yel- low colour, and brilliant lustre. They have the property, when rapidly heated, either of detonating like fulminating silver, or of burning rapidly with scintillations. The sparing solubility of carbazotate of potassa is the cause of carbazotic acid being used as a test of that alkali. According to the analysis of Liebig, carbazotic acid contains no hydro- gen, and its ingredients are expressed by the formula C'«NsOi«, its equiva- lent being 254.25, or the sum of the equivalents of its elements. Buff and Dumas give different proportions, and the latter found 1.4 per cent, of VEGETABLE ALKALIES. 523 h,nWPn It is certain, however, that it is a highly oxidized body, which Suits for'tUdebating property of its.salts. ^»» "f^^?^ in representing the ratio of oxygen and nitrogen to be the same as in nitric ^Carbazotic acid is generated by the action of nitric add on many sub- stances both admal and vegetable, especially on those, whic, eo^jn n. ro- » and 1 eq. benzamide C«Hfi03+H2N. It is obvious that 1 eq. benzamide C^HeOa+HaN 2 1 eq. anhyd. benzoic acid C**H«02+0 and 1 eq. water H+O ~ and 1 eq. ammonia 3H+N. I have theoretically represented benzamide as a compound of benzule with dinituret of hydrogen; but other hypotheses may be formed respecting its constitution, as in the case of oxamide. Benzoine.—The oil of bitter almonds, like several other essential oils, has been observed to deposite a crystalline matter, which is formed abundantly when the oil is left for a few weeks in a corked bottle in contact with a strong solution of pqtassa. It has at first a yellow tint; but by solution in boiling alcohol, and digestion with animal charcoal, it may be obtained pure and white. It crystallizes from its alcoholic solution in transparent pris- matic crystals, which fuse at 248=, and may be distilled without change. It has neither taste nor odour. Liebig and Wohler found its elements to coincide exactly with those of the original oil which yielded it, though doubtless their arrangement is very different. Coumarin.—This name was first applied to the odoriferous principle of the lonka bean by M. Guibourt, and has since been adopted by MM. Boul lay and Boutron-Charlard. (Journ. de Pharmacie, 1825.) It is derived ■0Im if ^Tm Coumarouna odorata, given by Aublet to the plant which yields the bean. r 46* 546 RESINOUS SUBSTANCES. Coumann is white, of a hot pungent taste, and distinct aromatic odour It crystallizes sometimes in square needles, and at other times in short prisms. It is moderately hard, fracture clean, lustre considerable, and den- sity greater than that of water. It fuses at a moderate temperature into a transparent fluid, which yields an opaque crystalline mass on cooling. Heated in close vessels, it is sublimed without change. It is sparingly Boluble in water; but is readily dissolved by ether and alcohol, and the solu- Uons crystallize by spontaneous evaporation. It is very soluble in fixed and volatile oils. Vogel mistook coumarin for benzoic acid: Boullay and Boutron-Charlard maintain that it has neither an acid noT alkaline reaction, and that it is a pecdiar independent principle, nearly allied to the essential oils. These chemists did not find any benzoic acid in the Tonka bean, and consider coumann as the sole cause of its odour. RESINOUS SUBSTANCES. Resins.—Resins are the inspissated juices of plants, and commonly occur either pure or in combination with an essential oil. They are solid at com- mon temperatures, brittle, inodorous, and insipid. They are non-conductors of electricity, and when rubbed become negatively electric. They are gene- rally of a yellow colour, and semi-transparent. Resins are fused by the application of heat, and by a still higher tempera- ture are decomposed. In close vessels they yield empyreumatic oil, and a large quantity of carburetted hydrogen, a small residue of charcoal remain- ing. In the open air they burn with a yellow flame and much smoke, being resolved into carbonic acid and water. Resins are dissolved by alcohol, ether, and the essential oils, and the alco- holic and* ethereal solutions are precipitated by water, a fluid in which they are quite insoluble. Their best "solvent is pure potassa and soda, and they are also soluble in the alkaline carbonates by the aid of heat. The product is in each case a soapy compound, which is decomposed by an acid. Concentrated sulphuric acid dissdves resins; but the acid and the resin mutually decompose each other, with disengagement of sulphurous acid, and deposition of charcod. Nitric acid acts upon them with violence, con- verting them into a species of tannin, which was discovered by Mr. Hatchett. No oxalic acid is formed during the action. The uses of resin are various. Melted with wax and oil, resins constitute ointments and plasters. Combined with oil or alcohol, they form different kinds of oil and spirit varnish. Sealing-wax is composed of lac, Venice turpentine, and common resin. The composition is coloured black by means of lampblack, or red by cinnabar or red lead. Lampblack is the soot of imperfectly burned resin. Of the different resins the most important are common resin, copal, lac, sandarach, mastich, elerni, and dragon's blood. The first is procured by heating turpentine, which consists of oil of turpentine and resin, so as to expel the volatile oil. The common turpentine obtained by incisions made in the trunk of the Scotch fir-tree (Pinus sylveslris) is employed for this purpose; but the other kinds of turpentine, such as Venice turpentine, that from the larch (Pinus larix), Canadian turpentine from the Pinus balsamea, or the Strasburgh turpentine from the Pinus picea, yield resin by a similar treatment. When turpentine is extracted from the wood of the fir-tree by heat, par- tial decomposition ensues, and a dark substance, consisting of resin, empy- reumatic oil, and acetic acid is the product. This constitutes tar; and when inspissated by boiling, it forms pitch. Common resin fuses at 276°, is com- pletely liquid at 306°, and at about 316° bubbles of gaseous matter escape, giving rise to the appearance of ebullition. By distillation it yields empy- reumatic oils: in the first part of the process a limpid oil passes over, which RESINOUS SUBSTANCES. 547 rises in vapour at 300°, and boils at 360°; but subsequently the product becomes less and less limpid, till towards the close it is very thick. This matter becomes limpid when heat is applied, and boils at about 500° F. At a red heat resin is entirely decomposed, yielding a large quantity of com- bustible gas, which is employed for the purpose of artificial illumination. Amfcer.—This substance is brought chiefly from the southern coast of the Baltic, occurring sometimes in beds of bituminous wood, and at others on the shore, being doubtless washed out from strata of brown coal by the action of water. Its vegetable origin is amply attested by the substances with which it is associated, by its resinous nature, and by the vegetable matters which it frequently envelopes. It is commonly met with in trans- lucent pieces of various shades of yellow and brown; but it is sometimes transparent. Its specific gravity varies from 1.065 to 1.07. It may be re- garded as a mixture of several substances; namely, a volatile oil, succinic acid, separable like the former by heat, two different modifications of resin both soluble in alcohol and ether, and a peculiar bituminous matter, which is insoluble in alcohol and ether, and is the most abundant principle in amber. (Berzelius.) Balsams.—The balsams are native compounds of resin and benzoic acid, and issue from incisions made in the trees which contain them, in the same manner as turpentine from the fir. Some of them, such as storax and ben- zoin, are solid; while others, of which the balsams of Tolu and Peru are examples, are viscid fluids. Gum-resins.—The substances to which this name is applied are the con- Crete juices of certain plants, and consist of resin, essential oil, gum, and extractive vegetable matter. The two former principles are soluble in alco- hol, and the two latter in water. Their proper solvent, therefore, is proof spirit. Under the dass of gum-rcsins are comprehended several valuable medicines, such as aloes, ammoniacum, assafcetida, euphorbium, galbanum, gamboge, myrrh, scammony, and guaiacum. Caoutchouc, commonly called elastic gum or Indian rubber, is the con- crete juice of the Hoevea caoutchouc and Iatropa elastica, natives of South America, and of the Ficus Indica and Artocarpus integrifolia, which grow in the East Indies. It is a soft yielding solid, of a whitish colour when not blackened by smoke, possesses considerable tenacity, and is particularly re- markable for its elasticity. It is inflammable, and burns with a bright flame. It is insoluble in water and alcohol; but it dissolves, though with some difficulty, in pure ether. It is very sparingly dissolved by the alkalies, but its elasticity is destroyed by their action. By the sulphuric and nitric acids it is decomposed, the former causing deposition of charcoal, and the latter formation of oxalic acid. Caoutchouc is soluble in the essential oils, ether, naphtha, cajeput oil, and in the volatile liquid obtained by distilling caoutchouc; and from all these solvents, except the essential oils, it is left on evaporation without loss of its elasticity. Before actually dissolving, the caoutchouc swells up remarkably, and acquires a soft gelatinous aspect and consistency: in this state it is used for rendering cloth and leather impervious to water, and, as suggested by Dr. Mitchell of Philadelphia, may be cut with a wet knife into thin sheets or bottles, and be extended to a great size.* * Dr. Mitchell has favoured me with the following detailed description of his peculiar mode of preparing bags of caoutchouc of large size :—" Soak Se COmT£\ g^ in su*Phuric ether» SP- gr- 0.753, at a temperature not less L .? n v I a period of time not less than one week (the ,onger the Delter). fcmpty the bag, wipe it dry, put into it some dry powder, such as starch insert a tube into the neck, and fasten it by a broad soft band slight- Jy applied, and then commence by mouth or bellows the inflation. If the bag be unequal in thickness, restrain by the hand the bulging of the thinner 548 RESINOUS SUBSTANCES. When caoutchouc is. cautiously heated, it fuses without decomposition; but at a higher temperature it is resolved into a volatile liquid of a brown colour, which amounts to 8-I0ths of the original caoutchouc. When care- fully rectified, a very volatile liquid of sp. gr. 0.64 is obtained, which is very combustible and burns with a bright flame, mingles with alcohol, and dis- solves copal and other resins. It is manufactured in large quantity by Messrs. Enderby, of London, and is very useful as a solvent for caoutchouc and for the preparation of varnishes. Wax.—This substance, which partakes of the nature of a fixed oil, is an abundant vegetable production, entering into the composition of the pollen of flowers, covering the envelope of the plum and other fruit, especially the berries of the Myrica cerifera, and in many instances forming a kind of varnish to the surface of leaves. From this circumstance it was long sup. posed that wax is solely of vegetable origin, and that the wax of the honey- comb is derived from flowers only; but it appears from the observations of Huber that it must likewise be regarded as an animal product, since he found bees to deposite wax, though fed on nothing but sugar. Consistently with this remark it has been proved by Oppermann that pure vegetable wax differs from bees-wax in the ratio of its elements. (An. de Ch. et de Ph. xlix. 240.) Common wax is always more or less coloured, and has a distinct peculiar odour, of both which it may be deprived by exposure in thin slices to light, air, and moisture, or more speedily by the action of chlorine. At ordinary temperatures it is solid, and somewhat brittle; but it may easily be cut with a knife, and the fresh surface presents a characteristic appearance, to which the name of waxy lustre is applied. Its specific gravity is 0.96. At about 150° it enters into fusion, and boils at a high temperature. Heated to redness in close vessels it suffers complete decomposition, yielding products very similar to those which are procured under the same circumstances from oil. As it burns with a clear white light, it is employed for forming candles. Wax is insduble in water, and is only sparingly dissolved by boiling alcohol or ether, from which the greater part is deposited on cooling. It is readily attacked by the fixed alkalies, being converted into a soap which is soluble in hot water; and according to Pfaff, the action is attended, as in oils, with the formation of an acid, to which the name of eerie acid is ap- plied. It unites by the aid of heat in every proportion with the fixed and volatile oils, and with resin. With different quantities of oil it constitutes the simple liniment, ointment, and cerate of the Pharmacopcda. Wax, according to the observations of John, consists of two different principles, one of which is soluble, and the other insoluble in alcohol. To the former he has given the name of cerin, and to the latter of myricin. It has heen thought that these principles are generated in the wax by the alcohol used in separating them; but the opinion of John is supported by a fact mentioned to me by Dr. Christison, namely, that the suetty matter of the cinnamon berry consists, with the exception of a little oil, entirely of cerin, without any myricin. parts, until the thicker have been made to give way a little. When the bag has become by such means nearly uniform, inflate a little more, shake up the included starch, and let the bag collapse. Repeat the inflation, and carry it to a greater extent, again permit the collapse, again inflate still more extensively, and so on, until the bag is sufficiently distended. Mere gas holders are thus easily made, but it requires some dexterity and expe- rience to make them thin enough for balloons. The whole experiment should not occupy more than from five to twenty minutes of time; and the picpared bag shodd be closed and hung up to dry for a day or two."—Ed. BITUMINOUS SUBSTANCES. BITUMINOUS SUBSTANCES. Under this title are included several inflammable substances which, though of vegetable origin, are found in the earth, or issue from its surface. Thelmav be conveniently arranged under the two heads of bitumen and ScLr The fost comprehends naphtha, petroleum mineral tar, asphaltum l\Zai pitch, ana retinasphaltum, of which the three «"* ""^^ liquid, and the others solid. The second comprises brown coal, the different varieties of common or black coal, and glance coal. BITUMEN. The characters of naphtha, the purest form of bitumen, have already been described (page 252). _____i_„r Petroleum is much less limpid thap naphtha, has a reddish-brown colour and is unctuous to the touch. It is found m several parts of Britain and the continent of Europe, in the West Indies, and m Persia It occurs particu- larly in coal districts. Mineral tar is very similar to petroleum but is more viscid and of a deeper colour. Both these species become thick by exposure to the atmosphere, and in the opinion of Mr. Hatchett pass into solid '^SSoftton is a solid brittle bitumen, of a black colour, vitreous lustre, and conchoidd fracture. It melts easily, aDd is very inflammable. It emits a bituminous odour when rubbed, and by distillation yields a fluid like naphtha. It is soluble in about five times its weight of naphtha, and the solution forms a good varnish. It is rather denser than water. Asphaltum is found on the surface and on the banks of the Dead bea, and occurs in large quantity in Barbadoes and Trinidad. It was employed by the ancients in building, and is said to have been used by the Egyptians in embalming. „ Mineral Pitch or Maltha is likewise a solid bitumen, but is much softer than asphaltum. Elastic bitumen, or mineral caoutchouc, is a rare variety of mineral pitch, found only in the Odin mine, near Castleton in Derby- shire. Retinasphaltum is a peculiar bituminous substance, found associated with the brown coal of Bovey in Devonshire (Phil. Trans. 1804). It con- sists partly of bitumen, and partly of resin, a composition which led Mr. Hatchett to the opinion that bitumens are chiefly formed from the resinous principle of plants. Inflammable Principles of Tar. Among the products of the destructive distillation of vegetable and ani- mal substances is a black inflammable liquid called tar, which in aspect resembles the tar from fir (page 546). A large quantity is formed during the distillation of wood, and in the preparation of coal gas. The tar obtain- ed from such sources has been the subject of an elaborate inquiry by Dr. Reichenbach, of Blansko, who has discovered in it no fewer than six new principles; namely, paraffine, eupione, creosote, picamar, capnomor, and pittacfil. A description of the two former will be found at pages 251-2, and the latter will be described in this place. The original essays of Reichen- bach appeared in Schweigger-Seidel's Journal for 1830 and the following years. Creosote.—This substanee exists in solution in crude pyroligneous acid; but it is best prepared from those portions of the oil distilled from wood-tar, which are heavier than water. The oil is first freed from adhering acetic acid by carbonate of potassa, and, after separation from the acetate, is dis- tilled. A little phosphoric acid is mixed with the product to neutralize ammonia, and another distillation resorted to. It is next mixed with a 550 BITUMINOUS SUBSTANCES. strong solution of potassa, which combines with creosote, allows any eupione which may be present to collect on its surface, and by digestion decomposes other organic matter: the alkaline solution is then neutralized by sulphuric acid, and the oil which separates is collected and distilled. ror the complete purification of the creosote, this treatment with potassa, followed by neutralization and distillation, requires to be frequently re- peated. J Creosote is a colourless transparent liquid of an oily consistence, which retains its fluidity at—17°, has a sp. gr. of 1.037 at 68°, boils at 397°, is a non-conductor of electricity, and refracts light powerfully. It has a burn- mg taste followed by sweetness, and its odour is like that of wood-smoke or rather of smoked meat. It is highly antiseptic to meat: the antiseptic vir- tue of tar, smoke, and crude pyroligneous acid seems owing to the presence of creosote. Its name, from K^td; flesh, and o-«£» / save, was suggested by this property. Creosote requires about 80 parts of water for solution, and is soluble in every proportion in alcohol, ether, sulphuret of carbon, eupione, and naphtha. It has neither an acid nor alkaline reaction with test-paper, but combines both with acids and alkalies. With potassa, soda, lime, and baryta it forms compounds soluble in water; but the creosote is separated even by feeble acids. Of the acids, it unites most readily with the acetic, dissolving in every proportion: by strong nitric and sulphuric acid it is decomposed. Creosote unites also with chlorine, iodine, bromine, sulphur, and phosphorus. Creosote acts powerfully in coagdating albumen, this effect being pro- duced by a solution of one drop in a large quantity of water. It acts with energy on living beings. Insects and fish thrown into the aqueous solution of creosote are killed, and plants die when watered with it. It appears use- ful in medicine: it is said to be very efficacious as a topical application in toothach, ulcers, and cutaneous diseases; and it probably admits of many other applications. Creosote is a compound of carbon, hydrogen, and oxygen, but the ratio of its elements is not yet known. Picamar.—This substance is the bitter principle of tar, whence it derives its name (in pice amarum). It is present in the heaviest portions of the rec tified oil of tar, and when these are treated by potassa, a crystalline com- pound of the alkali and picamar is formed: this compound, when purified by repeated solution in water and crystallization, is decomposed by phospho- ric acid, and the picamar separated by distillation. Picamar is an oily colourless liquid, of a peculiar odour and very bitter taste. Its sp. gr. is 1.100, and it boils at 545°, being considerably less vola- tile than creosote. It is insoluble in eupione and sparingly soluble in water; but it dissolves without limit in alcohol and ether. It has no action on test- paper; but it unites with potassa as above mentioned, and strong sulphuric acid dissolves it without decomposition. From its permanence in the air, its fixity when heated, and its oily nature, it is well adapted for greasing machinery and protecting it from rust. Its composition is unknown. Capnomor.—This substance occurs along with creosote, picamar, and pit- tacal in the heavy oil of tar. On digesting that oil with solution of potassa, the three latter principles are dissolved, and the capnomor collects on the surface, combined with a little eupione. The capnomor is then dissolved by sulphuric acid, in which eupione is insoluble; and from the solution, on being neutralized with carbonate of potassa, capnomor separates, and is puri- fied by distillation. Its name is derived from xzirvos smoke, and //oi^* part, because it is one of the ingredients of smoke. Capnomor is a colourless transparent liquid, of a pungent taste and rather pleasant odour, has a sp. gr. of 0.975, and refracts light dmost as power- fully as creosote. It boils at 365°. It is insoluble in water and solution of potassa, and is soluble in alcohol, ether, and eupione. It has the property of dissolving caoutchouc, especially when heated, and is the only ingredient of tar which does so: its presence in coal naphtha is the cause of the sol- BITUMINOUS SUBSTANCES. 551 vent action of that liquid on caoutchouc. The composition of capnomor rasnofbTen ascertained, though doubtless carbon and hydrogen are its '"S^Iw^ieTihe heavy oil of tar is digested with a solution of ba- ryta a fine blue colour appeal, which is due to pittacal, from «T« pitch tlZ^ZmlZ He mode of preparing ithas ~t been described It is a solid of a beautiful blue colour, which admxt of bang 1fixed as a dye. It is very permanent, contains nitrogen as one of its elements, and appears to belong to the same class of bodies as indigo. PIT-COAL. Brown Coal is characterized by burning with a peculiar bituminous odour, like that of peat. It is sometimes earthy, but the fibrous structure of the wood from which it is derived is generally more or less distinct, and hence this variety is called bituminous wood. Pitch coal or jet, which is employed for forming ear-rings and other trinkets, is intermediate between brown and black coal, but is perhaps more closely allied to the former than the a Brown coal is found at Bovey in Devonshire (Bovey coal), in Iceland, where it is called surturbrand, and in several parts of the continent, espe- cially at the Meissner in Hessia, in Saxony, Prussia, and Stvria. Common or Black Coal—Of the common or black coal there are severd varieties, which differ from each other, not only in the quantity of foreign matters, such as sulphuret of iron and earthy substances, which they con- tain, but also in the proportion of what may be regarded as essential consti- tuents. Thus some kinds of coal consist dmost entirely of carbonaceous matters, and, therefore, form little flame in burning; while others, of which cannel cod is an example, yield a large quantity of inflammable gases by heat, and consequently burn with a large flame. Dr. Thomson has arranged the different kinds of coal which are met with in Britain into four subdivi- sions. (An. of Phil, xiv.) The first is caking coal, because its particles are softened by heat and adhere together, forming a compact mass. The coal found at Newcastle, around Manchester, and many other parts of England, is of this kind. The second is termed splint coal, from the splintery appear- ance of its fracture. The cherry coal occurs in Staffordshire, and in the neighbourhood of Glasgow. Its structure is slaty, and it is more easily broken than splint coal, which is much harder. It easily takes fire, and is consumed rapidly, burning with a clear yellow flame. The fourth kind is cannel coal, which is found of peculiar purity at Wigan in Lancashire. In Scotland it is known by the name of parrot coal. From the brilliancy of the light which it emits while burning, it is sometimes used as a substitute for candles, a practice which is said to have led to the name of cannel coal. It has a very compact structure, does not soil the fingers when handled, and admits of being polished. Snuff-boxes and other ornaments are made with this coal; and it is peculiarly well fitted for forming coal gas. According to the experiments of Thomson, these varieties of coal are thus constituted :— Caking Coal. Splint Coal. Cherry Coal. Cannel Coal. Carbon 75.28 75.00 74.45 64.72 Hydrogen 4.18 6.25 12.40 21.56 Nitrogen 15.96 6.25 10.22 13.72 Oxygen 4.58 12.50 2.93 0.00 100.00 100.00 100.00 100.00 Judging from the quantity of oxidized products (water, carbonic acid, and carbonic oxide) which are procured during the distillation of coal, Henry in- fers that coal contains more oxygen than was found by Thomson. (Elements 11th Edit. ii. p. 348.) This opinion is supported by the analysis of Ure, who found 26,66 per cent, of oxygen in splint, and 21.9 in cannel coal. When 552 SPIRITUOUS AND ETHEREAL SUBSTANCES. coal is heated to redness in close vessels, a great quantity of volatile matter is dissipated, and a carbonaceous residue, called coke, remains in the retort. The volatile substances are coal tar, acetic acid, water, hydrosulphuric acid and hydrosulphate and carbonate of ammonia, together with the several gases formerly enumerated. The greater part of these substances, even most of the bitummous principles, are real products, that is, are generated during the distillation. Glance Coal.—Glance coal, or anthracite, differs from common coal which it frequently accompanies, in containing no bituminous substances' and in not yielding inflammable gases by distillation. Its sole combustible' ingredient is carbon, and consequently it burns without flame. It commonly occurs in the immediate vicinity of basalt, under circumstances which lead to the suspicion that it is coal from which the volatile ingredients have been expelled by subterranean heat. At the Meissner, in Hessia, it is found be- tween a bed of brown coal and basalt Kilkenny coal appears to be a va- riety of glance coal. (Thomson, An. of Phil. vol. xv.) SECTION V. SPIRITUOUS AND ETHEREAL SUBSTANCES. Alcohol. Alcohol is the intoxicating ingredient of all spirituous and vinous liquors. It does not exist ready formed in plants, but is a product of the vinous fer- mentation, the theory of which will be stated in a subsequent section. Common alcohol or spirit of wine is prepared by distilling whisky or some ardent spirit, and the rectified spirit of wine is procured by a second distillation. The former has a specific gravity of about 0.867, and the lat- ter of 0.835 or 0.84. In this state it contains a quantity of water, from which it may be freed by the action of substances which have a strong affinity for that liquid. Thus, when carbonate of potassa heated to 300° is mixed with spirit of wine, the alkali unites with the water, forming a dense solution, which, on standing, separates from the alcohol, so that the latter may be removed by decantation. To the alcohol, thus deprived of part of its water, fresh portions of the dry carbonate are successively added, until it fdls through the spirit without being moistened. Other substances, which have a powerful attraction for water, may be substituted for carbo- nate of potassa. Gay-Lussac recommends the use of pure lime and baryta (An. de Ch. lxxxvi.); and dry alumina may also be employed. A very con- venient process is to mix the alcohol with chloride of calcium in powder, or with quicklime, and draw off the stronger portions by distillation. An- other process, which has been recommended for depriving alcohol of water, is to put it into the bladder of an ox, and suspend it over a sand-bath. The water gradually passes through the coats of the bladder, while the pure al- cohol is retained; but though this method answers well for strengthening weak spirit, its power of purifying strong alcohol is very questionable. (Journal of Science, xviii.) The strongest alcohol which can be procured by any of these processes has a specific gravity of 0.796 at 60° F. This is called absolute alcohol, to denote its entire freedom from water. An elegant and easy process for procuring absolute alcohol has been pro- posed by Mr. Graham. (Edin. Philos. Trans. 1828.) A large shallow basin is covered to a small depth with quicklime in coarse powder, and a smaller one containing three or four ounces of commercial alcohol is supported just ALCOHOL. 553 above it. The whole is placed upon the plate of an air-pump, covered by a low receiver, and the air withdrawn until the dcohol evinces signs of ebul- lition. Of the mingled vapours of water and alcohol which fill the receiver, the former alone is absorbed by the quicklime, while the latter is unaffected. Now it is found that water cannot remain in alcohol, unless covered by an atmosphere of its own vapour; and consequently the water continues to evaporate without interruption, while the evaporation of the alcohol is en- tirely arrested by the pressure of the vapour of alcohol on its surface. Common alcohol is in this way entirely deprived of water m the course of about five days. The temperature should be preserved as uniform as possi- ble during the process. Sulphuric acid cannot be substituted for quicklime, since both vapours are absorbed by this liquid. Alcohol is a colourless limpid fluid, of a penetrating odour, and burning taste. It is highly volatile, boiling, when its density is 0.820, at the tempe- rature of 176° F. The sp. gravity of its vapour, according to Gay-Lussac, is 1.613. Like volatile liqdds in general, it produces a considerable degree of cold during evaporation. It has hitherto retained its fluidity under every degree of cold to which it has been exposed. Alcohol is highly inflammable, and burns with a lambent yellowish-blue flame. Its colour varies considerably with the strength of the alcohol, the blue tint predominating when it is strong, and the yellow when it is diluted. Its combustion is not attended with the least degree of smoke, and the sole products are water and carbonic acid. When transmitted through a red- hot tube of porcelain, it is resolved into carburetted hydrogen, carbonic oxide, and water, and the tube is lined with a small quantity of charcoal. Alcohol unites with water in every proportion. The act of combining is usually attended with diminution of volume, so that a mixture of 50 mea- sures of alcohol and 50 ofwater occupies less than 100 measures. Owing to this circumstance, the action is accompanied with increase of temperature. Since the density of the mixture increases as the water predominates, the strength of the spirit may be estimated by its sp. gravity. Equal weights of absolute alcohol and water constitute proof spirit, the density of which is 0.917; but the proof spirit employed by the colleges for tinctures has a sp. gravity of 0.930, or 0.935. Of the salifiable bases alcohol can alone dissolve potassa, soda, lithia, am- monia, and the vegetable alkalies. None of the earths or other metdlic oxides are dissolved by it. Most of the acids attack it by the aid of heat, giving rise to a class of bodies to which the name of ether is applied. All the salts which are either insoluble, or sparingly soluble in water, arc insoluble in alcohol. The efflorescent salts are, likewise, for the most part insoluble in this menstruum; but, on the contrary, it is capable of dissolving nearly all the deliquescent salts, except carbonate of potassa. Many of the vegeta- ble principles, such as sugar, manna, camphor, resins, balsams, and the es- sential oils, are soluble in alcohol. The solubility of certain substances in alcohol appears owing to the for- mation of definite compounds, which are soluble in that liquid. This has been proved of the chlorides of calcium, manganese, and zinc, and of the nitrates of lime and magnesia, by Mr. Graham in the essay above cited. It appears from his experiments that all these bodies unite witli alcohol in de- finite proportion, and yield crystalline compounds, which are deliquescent and soluble both in water and alcohol. From their analogy to hydrates, Mr. Graham has applied to them the name of alcoates. These are formed by dissolving the substances in absolute alcohol by means of heat, when on cooling a group of crystals more or less irregular is deposited. The salt and alcohol employed for the purpose should be quite anhydrous; for the crys- talhzation is prevented by a very small quantity of water. Estimating the combining proportion of alcohol at 23.24, the alcoate of chloride of caldum is composed of one equivalent of chloride of calcium, and three equivalents and a half of alcohol. Nitrate of magnesia crystallizes with nine equivalents of dcohol; nitrate of lime with two and a half equivalents ; protochloride of 554 ETHER. manganese witli three equivalents; and chloride of zinc with hdf an equiva- lent of alcohol. The following statement of the components of alcohol is drawn from an analysis by Saussure (An. dc Ch. Ixxxix.) :— Carbon 52.17 12.24 2C or 24.48 4C Hydrogen 13.04 3 3H 6 6H Oxygen 34.79 8 O 16 20 100.00 23.24 C2H30 i6A8 C^O*. Chemists are not agreed about the equivalent of alcohol. It is usually con- sidered as 23.24 or 23, an opinion supported by the composition of the al- coates above mentioned; but some circumstances favour the adoption of twice that number, agreeably to the second of the preceding formula?. Admitting 23.24 as its equivalent, 100 measures of the vapour of alcohol may be viewed as consisting of 100 measures of olefiant gas and 100 of aqueous vapour; since 0.9808 (sp. gr. of olefiant gas)+0.6202 (sp. gr. of aqueous vapour) =1.6009, which closely coincides with the observed sp. gravity of the vapour of alcohol. The formula of alcohol will in this case be H'O+H; and alco- hol is really resolved into olefiant gas and water by the action of galvanism (Ritchie). Adopting 46.48 as the eqdvalent of dcohol, then it may be viewed as a compound of etherine (page 250) and aqueous vapour, H*C4+2H; or as a hydrate of ether formed of equd volumes of the vapours of ether and wa- ter, united without condensation. On both hypotheses, the calcdated density of the vapour of dcohol agrees with observation. Alcohol exists ready formed in wine and spirituous liquors, and may be collected without heat Mr. Brande (Phil. Trans. 1811 and 1813) procured it from wine by precipitating the acid and extractive colouring matters by subacetate of lead, and then depriving the alcohol ofwater by dry carbonate of potassa: the pure alcohol may then be measured in a graduated tube. Gay-Lussac obtained alcohol from wine by distilling it in vacuo at the tem- perature of 60° F. He also succeeded in separating the alcohol by the me- thod of Mr. Brande; but he suggests the employment of litharge in fine powder, instead of subacetate of lead, for precipitating the colouring matter. (Mem. d'Arcueil, vol. iii.) The preceding researches of Mr. Brande led him to examine the quantity of alcohol contained in spirituous and fermented liquors. According to his experiments, brandy, rum, gin, and whisky, contain from 51 to 54 per cent. of alcohol, of specific gravity 0.825. The stronger wines, such as Lissa, Raisin wine, Marsala, Port Madeira, Sherry, Teneriffe, Constantia, Malaga, Bucellas, Calcavella, and Vidonia, contain from between 18 or 19 to 25 per cent, of alcohol. In Claret, Sauterne, Burgundy, Hock, Champagne, Her- mitage, and Gooseberry wine, the quantity is from 12 to 17 per.cent. In cider, perry, ale, and porter, the quantity varies from 4 to near 10 per cent. In all spirits, such as brandy or whisky, the alcohol is simply combined with water ; whereas in wine it is in combination with mucilaginous, sac- charine, and other vegetable principles, a condition which tends to diminish the action of the alcohol upon the system. This may, perhaps, account for the fact that brandy, which contains little more than twice as much real al- cohol as good port wine, has an intoxicating power which is considerably more than double. Ether. Most of the stronger acids, when heated with alcohol, give rise to the formation of a volatile inflammable liquid called ether, the different kinds of which arc further distinguished by the name of the acid with which they were prepared. Thus by sdphuric, hydrochloric, and oxalic ether are meant the ethers formed by means of sulphuric, hydrochloric, and oxdic acid. Most of these ethers actudly contain the acid, or the elements of the ETHER. 555 acid, employed in their production, and, therefore, differ essentially from each other; but that which is formed by means of sulphuric and phosphoric acid is a definite compound of carbon, hydrogen, and oxygen, and is alto- gether free from acid. It is to this compound the generic term ether will always be applied. In the usual process for preparing ether, strong sulphuric acid is gently poured upon an equal weight of rectified spirit of wine contained in a thin glass retort, and after mixing the fluids together by agitation, which occa- sions a sudden rise of temperature, the mixture is heated as rapidly as pos- sible until ebullition commences. At the beginning of the process nothing but alcohol passes over; but as soon as the liquid boils, ether is generated, and condenses in the recipient, wliich is purposely kept cool by the applica- tion of ice or moist cloths. When a quantity of ether is collected, equal in general to about half of the alcohol employed, white fumes begin to appear in the retort At this period, the process should be discontinued, or the re- ceiver changed; for although ether does not ceaso to be generated, its quan- tity is less considerable, and several other products make their appearance: thus on continuing the operation, sulphurous acid is disengaged, and a yel- lowish liquid, commonly called ethereal oil, or oil of wine, passes over into the receiver. If the heat be still continued, a large quantity of olefiant gas is disengaged, and all the phenomena ensue which were mentioned in the de- scription of that compound. (Page 247.) Ether, thus formed, is dways mixed with dcohol, and generally with some sdphurous acid. To separate these impurities the ether should be agitated with a strong solution of potassa, which neutralizes the acid, while the water unites with the alcohol. The ether is then distilled by a very gentle heat, and may be rendered still stronger by distillation from chloride of calcium. Pure ether is a colourless limpid liquid, of a hot pungent taste, and fra- grant odour. Its sp. gr. is 0.700, or 0.632 according to Lovitz; but that of the shops is 0.74, or even greater, owing to the presence of some alcohol and water. Its volatility is very great: under the atmospheric pressure, ether of sp. gr. 0.720 boils at 96° or 98°, and at —40° in a vacuum. Its evaporation, from the rapidity with which it occurs, occasions intense cold, sufficient in a cool atmosphere for freezing mercury. The sp. gr. of its vapour was found by Gay-Lussac to be 2.586. At —46° it is congealed. Ether combines with alcohol in every proportion, but is very sparingly soluble in water. When agitated with water, the greater part separates on standing, a small quantity being retained, which imparts an ethereal odour to the water. The ether so washed is very free from alcohol, which com- bines by preference with the water. Ether is highly inflammable, burning wilh a yellow flame, and formation of water and carbonic acid. With oxygen gas its vapour forms a mixture, which explodes violently on the approach of flame, or by the electric spark! On being transmitted through a red-hot porcelain tube it undergoes decom- position, and yields the same products as alcohol. When a coil of platinum wire is heated to redness, and then suspended above the surface of ether contained in an open vessel, the wire instantly be- gins to glow, and continues in that state until all the ether is consumed. During this slow combustion, pungent acrid fumes are emitted, which if received in a separate vessel, condense into a colourless liquid possessed'of acid properties. Professor Daniell, who prepared a large quantity of it was at first inclined to regard it as a new acid, which he described under the name of lampic acid; but he has since ascertained that its acidity is owing to acetic acid, which is combined with some compound of carbon and hydrogen different both from ether and alcohol. (Journal of Science vi. andI x.i.) Alcohol when similarly burned, likewise yields acetic add If ether is exposed to light in a vessel partially filled, and which is fre quently opened ,t gradually absorbs oxygen, and abortion of aceticacid is generated. This change was first noticed by M. Planche, and L bee" 556 ETHER. confirmed by Gay-Lussac. (An. de Ch. et de Ph. ii. 98 and 213.) M. Henry of Paris attributes its developement to acetic ether, which he believes to be always contained in sulphuric ether. The solvent properties of ether are less extensive than those of alcohol. It dissolves the essential oils, resins, and most of the fatty principles. Some of the vegetable alkalies are soluble in it, and it dissolves ammonia; but the fixed alkalies are insoluble in ether. From the analyses of Saussure, and Dumas and Boullay, the composition of pure ether is admitted to be as follows:— Carbon . . 64.96 24.48 4 eq. 4C Hydrogen . 13.47 5 5 eq. 5H Oxygen . . 21.57 8 1 eq. O 100.00 37.48 1 eq. C«H«0. On comparing the composition of ether with that of alcohol, it will be obvious that these compounds may be regarded as hydrates of etherine (page 244):—thus Alcohol. Ether. Etherine . . 28.48 H*Ci 28.48 HO Water ... 18 2H 9 H 46.48 HO+2H 37.48 HC-4-H. Recent discoveries in organic chemistry have induced Berzelius to regard ether as the oxide of a compound inflammable body called ethule or ethyle (from ether and v\» principle); and his opinion has been ably supported by Liebig (An. de Ch. et de Ph.lv. 113). On this supposition ethule consists of 4 eq. of carbon and 5 eq. of hydrogen, H6C*, so that the formula of ether is HbC4+0, a constitution analogous to that of camphor. It may be urged against this view that ethule has not been obtained in a separate form, and that the sp. gr. of the vapours of dcohol and ether is exactly such as would be expected from hydrates of etherine; but as a theory it applies at least as well as the former theory. This will appear from the following comparative view, which represents the composition of the prin- cipal compounds described in this section, agreeably to both theories. Etherine is expressed symbolically by En, and ethule by El:— Alcohol . . . En+2H or EnH* El+H or E1H. Ether . . . En+H or EnH El+O or El. Sulpho-vinic acid . EnHa+2S E1H+2S. Ethero-sulphuric acid EnH+2S E1+2S. Ethero-phosphoric acid EnH+J^ El+P. Oil of wine . . 2En+2S Hydrochloric ether En+HCl El+Cl. Hydriodic ether . En+HI El+I. Hydrobromic ether En+HBr El+Br. Nitrous ether . EnH+N El+N. Oxalic ether EnH+Ox El+Ox. 55* Benzoic ether . EnH+B El+B. Acetic ether . . EnH+A El+A. Tartaric ether . EnH+T El+T. Pyroxylic spirit . En+H El+20 Sulpho-vinic Acid.—This acid was first noticed in the year 1800 by M- Dabit, and has since been studied by Sertuerner, Vogel, and Gay-Lussac, and more lately by Serullas, Hennell, Wohler and Liebig, and Magnus. (An. de Ch. et de Ph. xlvii. 421; Phil. Trans. 1826 and 1828; and Pog. Annalen, xxvii. 367.) It is formed by the action of strong sulphuric acid on alcohol, and plays an essential part in the formation of ether. When the ingredients for forming ether are intermixed, and before heat is applied, nearly half of the acid exists in the state of sulpho-vinic acid, and may be separated from sulphuric acid by neutralizing the mixture with carbonate of baryta, when an insoluble sulphate and a soluble sulpho-vinate of baryta are generated, the latter of which by evaporation may be obtained in crys- tals. Soluble sulpho-vinates of lime and oxide of lead may be obtained in the same manner; and by decomposing a solution of these salts by car- bonate of potassa or soda, a sulpho-vinate of these alkalies is formed. Sulpho-vinic acid may be procured in solution by exactly decomposing sulpho-vinate of baryta with sulphuric acid; but it has not been obtained in a dry state except when combined with a base. Hennell considers the acid to be a compound of sulphuric acid with olefiant gas or etherine; but the analyses of sulpho-vinate of baryta by Wohler, Liebig, and Magnus indicate that it consists of sulphuric acid and alcohol. The crystals of this salt consist of Probable Formula. Ba+(EnH*+2S)+H. On heating the crystals to 122°, one equivalent of water of crystallization is expelled, and the anhydrous sulpho-vinate remains; but on raising the heat dcohol is evolved, and sulphate of baryta generated. The salt cannot exist as a sulpho-vinate without possessing sufficient water to convert all its etherine into Alcohol. Sulpho-vinic acid may be viewed as a bisulphate of alcohol, and sulpho-vinate of baryta as a double sulphate of baryta and alco- hol, in which one hdf of the acid is neutralized by alcohol and the other half by baryta. Theory of the Formation of Ether.—It was formerly thought, as first sug- gested by Fourcroy and Vauquelin, that the sole principle concerned in the formation of ether was the attraction of sulphuric acid for water, by which the alcohol was directly converted into ether. It is apparently in this way that certain substances which are very greedy of water, such as the arsenic and fluoboric acids, and the chlorides of tin and arsenic, effect the conver- sion of alcohol into ether; but it is now obvious that the ordinary process of forming ether by sulphuric acid, is of a more complex nature. It consists of two distinct parts, namely, the formation of sulpho-vinic acid, and the subsequent decomposition of that acid under the joint agency of heat and sulphuric acid. The affinities which determine the formation of sdpho- vinic acid are the attraction of sulphuric acid for water and for alcohol. The anhydrous acid is divided between the water and alcohol in such a manner that 47* Sulphuric acid . 80.2 2S Baryta . 76.7 Ba Alcohol 46.48 EnHa Water . . 9 H 558 ETHER. 4 eq. protohydrate of sulphuric acid 4(H+S) 2 (En+2H)+2S 13 , and 1 eq. of dcohol . . En+2H '>* and 2(2H+S). The actud composition of the hydrated acid varies with the strength of the acid and alcohol which are used: the diagram is only intended to indi- cate that one-hdf of the acid yields its water to the other half, and com- bines with alcohol. Any excess of alcohol remains uncombined in the mixture, or rises in vapour. If the materials are kept cool, no ether is enerded ; but as soon as, on applying heat, the temperature rises to about 60°, the sulpho-vinic acid is resolved into hydrated sulphuric acid and ether. The sulpho-vinic acid gradually disappears, and all the sulphuric acid returns to its original state of a hydrate, except that it is rather more diluted than at first The same acid may be employed to convert succes- sive portions of alcohol into ether, until it bocomes too dilute to form sulpho- vinic acid. Consistently with the foregoing explanation, Mr. Hennell ob- tained ether by heating sulpho-vinate ofpotassa along with strong sulphuric acid. On distilling sulpho-vinate of potassa with sulphuric acid, not con- centrated, but diluted with half its weight of water, he procured alcohol instead of ether. Hence it seems that the first effect of heat on sulpho-vinic acid is to expel alcohol, which, if too much water is not present, is convert- ed into ether in the act of separation. Ethero sulphuric Acid.—When the vapour of anhydrous sulphuric acid is introduced into absolute alcohol preserved at a low temperature, a yellow- ish oily liquid is formed, which consists of hydrated sulphuric and ethero- sulphuric acid, the latter consisting of sulphuric acid and ether, in the ratio indicated by the formula EnH+2S. Its production is owing to the water and ether, which constitute dcohol, uniting with separate portions of anhy- drous sulphuric acid. On diluting the mixed acids with water, and neu- tralizing with carbonate of baryta, an insoluble sulphate and soluble ethero- sulphate of baryta are obtained. The latter salt does not crystallize, and is insoluble in alcohol, in which respects it differs from sulpho-vinate of baryta. The ethero-sulphate of baryta, when dried in vacuo along with sulphuric acid, is anhydrous, and consists of one cq. of baryta and one eq. of ethero- sulphuric acid. Its formula is Ba + (EnH+2S). Magnus, who discovered this acid (Pog. Annalen, xxvii. 378), also formed it by the action of anhydrous sulphuric acid on ether; but in this case some oil of wine was generated at the same time, indicating the separation of a portion of ether into etherine and water. He likewise obtained, by partially decomposing ethero-sulphuric acid by heat, another acid of* a similar com- position, the nature of which is not yet determined. Ethero-phosphoric Acid.—It has been known for years that ether, identi- cal with that above described, may be formed by heating alcohol with con- centrated phosphoric acid; and Pelouze has lately proved that its formation is preceded by the production of an acid similar to the foregoing. (An. de Ch. et de Ph. Iii. 37.) Phosphoric acid of a sp. gr. less than 1.2 has no effect on alcohol; and if in a slate of great concentration, phosphoric acid decomposes the alcohol entirely, olefiant gas is disengaged, a liquid like oil of wine passes over, and charcoal is set free,—phenomena similar to those produced by an excess of strong sulphuric acid, and referable to the strong affinity of those acids for water. To prepare ethero-phosphoric acid, mix equal weights of dsolute alcohol and phosphoric add in the state of syrup, heat the mixture for a few minutes to 160°, set aside for twenty-four hours, and then dilute wilh seven parts of water: the solution, neutralized by car- bonate of baryta, boiled to expel free alcohol, and filtered, yields on cooling a white ethero-phosphate of baryta in hexagonal laminae. A solution of this salt exactly decomposed by sulphuric acid, yields ethero-phosphoric acid. ETHER. Ethero-phosphoric acid forms soluble salts with soda, potassa, ammonia, baryta, and magnesia, and insoluble ones with lime, and the oxides of lead, mercury, and silver. A moderately dilute solution suffers very partial de- composition when boiled, which explains why phosphoric acid and alcohol yield but a small portion of ether; but if the concentrated acid is boiled, then ether, traces of an oily matter like oil of wine, and olefiant gas are evolved, leaving at length phosphoric acid and charcoal. Pelouze considered ethero-phosphoric acid as a compound of phosphoric acid and dcohol, analogous to sulpho-vinic acid; but from a subsequent analysis by Liebig, it appears that it is a compound of phosphoric acid and ether (page 556). The ethero-phosphate of baryta in crystals contains twelve eq. of water of crystallization, which may be expelled by heat, leav- ing one eq. of acid and two eq. of baryta, 2Ba+(EnH+P). If the ether and baryta were regarded as two bases united with phosphoric acid, the resulting salt, (2Ba-|-EnH)+P, would be analogous in composition to the anhydrous rhombic phosphate of soda (page 452). Oil of Wine.—In the process above given for preparing ether, the quantity of alcohol perpetually diminishes, and that of free sulphuric acid augments, the temperature of the mixture rises, and at length the same changes ensue as in the preparation of olefiant gas (page 247). The energetic affinity for water exerted by the sulphuric acid decomposes the ether which would otherwise be formed, olefiant gas is evolved, and the oil of wine collects in the receiver in form of a yellowish fluid. It may be obtdned in greater quantity by distilling dcohol with an equal measure of sulphuric acid. The oil of wine should be purified from adhering sulphurous acid and ether by washing with water, and be then dried in vacuo along with sulphuric acid and fused potassa. Oil of wine is also formed when sulpho-vinic acid or a sulpho-vinate is distilled, some dcohol and dher being first disen- gaged. The oil of wine has an oily consistence, an aromatic odour, and a bitter- ish pungent taste. Its sp. gr. is 1.133. It is neutral to test paper, is sparingly soluble in water, but more freely in dcohol and ether, from which it may be recovered by evaporation. It is said by Hennell to consist of sulphuric acid, united with twice as much carbon and hydrogen as in sdpho-vinic acid, and may be hence regarded as a sulphate of etherine, the formula of which is En+S or 2En + 2S. According to Serullas it also contains hdf as much water as is associated in ether with etherine, its for- mula being (2En + H) + 2S. When evaporated with an alkali, or simply boiled along wilh a little water, it is converted into olefiant gas and sulpho- vinie acid. In the preparation of ether, the last portions of that fluid contain oil of wine in solution. On distilling such ether from lime, the oil of wine is changed into sulpho-vinic acid, which unites with the lime, and another oily matter is left, which is lighter than water and contains no sulphuric acid. The source and nature of this oil have not been determined. Hydrochloric Ether.—This compound is generated by the action of hydro- chloric acid on alcohol, and may be prepared by several processes:—by dis- tilling alcohol previously saturated with hydrochloric acid gas, or mixed with an equal volume of strong hydrochloric acid; by heating a mixture of 5 parts of alcohol, 5 of strong sulphuric acid, and 12 of fused sea-salt in fine powder; or by distilling alcohol with the chlorides of tin, bismuth antimony, or arsenic. The products are transmitted through tepid water' by which free alcohol and acid are absorbed, and the pure hydrochloric ether is then received in a vessel surrounded by ice or a freezing mixture. Hydrochloric ether is a colourless liquid, of a penetrating, somewhat allia- ceous, ethereal odour, and a strong rather sweet taste. Its sp. gr. at 4P is 560 ETHER. 0.774, and it is so volatile that it boils at about 54°. It is neutral to test paper. When inflamed, as it issues from a small aperture, it burns with an emerald-green flame without smoke, yielding abundant vapours of hydro- chloric acid. The elements of this ether are in such a ratio that it may be viewed either as a hydrochlorate of etherine or a chloride of ethule (page 556). Hydriodic Ether.—This ether, analogous in composition to the foregoing, is most conveniently generated by distilling at a gentle heat 2£ parts of iodide of phosphorus with 1 part of alcohol of sp. gr. 0.845. The product is washed with water, and then distilled from chloride of calcium. This ether is colourless, and has a strong ethereal odour, has a sp. or of 1 9206 at 72°, and boils at 148°. S Hydrobromic Ether.—This ether is analogous to the preceding, and is prepared by introducing into a retort 40 parts of alcohol of sp. gr. 0.84, and 1 of phosphorus, to which is added drop by drop 7 or 8 parts of bromine, and then distilling. By the reaction of the phosphorus and bromine, phos- phorous and hydrobromic acids are generated; and the latter in the nascent state unites with etherine. The product should be washed with water, to which a little potassa is added, in order to separate adhering acid and al- cohol. Nitrous Ether.—The three foregoing ethereal fluids may be viewed as compounds of a hydracid with etherine; but the ether now to be described consists of an acid in combination with sulphuric ether. Nitrous ether is prepared by distilling a mixture of concentrated nitric acid with an equal weight of alcohol; but as the reaction is apt to be exceedingly violent, the process should be conducted with extreme care. The safest method is to add the acid to the alcohol by small quantities at a time, allowing the mix- ture to cool after each addition before more acid is added. The distilla- tion is then conducted at a very gentle temperature, and the ether collected in Woulfe's apparatus. During the process there is a disengagement of nitrogen, protoxide and binoxide of nitrogen, and carbonic acid gases, from which it is apparent that nitric acid is deoxidized at the expense of the dco- hol. According to the researches of Dumas and Boullay, its elements are in such proportion that it may be viewed as a compound of 37.48 parts or one eq. of ether, and 38.15 or one eq. of hyponitrous acid (An. de Ch. et de Ph. xxxii. 26.). The nitrous agrees with sulphuric ether in its leading properties; but it is still more volatile. When recently distilled from quicklime by a gentle heat, it is quite neutral; but it soon becomes acid by keeping. The pro- ducts of its spontaneous decomposition are alcohol, nitrous acid, and a little acetic acid. A similar change is instantly effected by mixing the ether with water, or distilling it at a high temperature. It is also decomposed by potassa, and, on evaporation, crystals of the nitrite or hyponitritc of that al- kali are deposited (Memoires d'Arcueil, i.). Oxalic Ether.—This ether is generated by the action of oxalic acid on common ether in its nascent state, and is conveniently prepared by distil- ling 1 part of alcohol, and 1 of binoxalate of potassa, with 2 parts of sul- phuric acid. At first alcohol and then ether pass over, and lastly the oxalic ether collects as an oily liquid at the bottom of the recipient. The alcohd which collects is repeatedly returned to the retort and redistilled. The oxalic ether is quickly washed with water, and boiled along with litharge in fine powder until the boiling point of the liquid descends to 362°. Adhering wa- ter and alcohol are thus dissipated, and free oxalic acid combines with oxide of lead : the oxalic ether is then decanted and distilled. It is apt to contain a small quantity of oil of wine. Oxalic ether is a colourless fluid of an oily aspect, of an aromatic allia- ceous odour, and a sp. gr. of 1.0929 at 45°. It boils at 362°. It is neutral to test paper when pure, is sparingly soluble in water, and dissolves in every proportion in alcohol. When kept for some time it is decomposed, and oxdic acid separates in crystals; and with alkalies it readily yields alcohol ETHER. JU1 and oxalic acid. According to the analysis of Dumas and Boullay, it is composed of 36.24 parts or one eq. of oxdic acid, and 37.48 parts or one eq. ° When oxalic ether is briskly agitated with an aqueous solution of pure ammonia, a white precipitate is formed, which, after being washed succes- sively wih water and alcohol, was found by Liebig to be pure oxamide (page 500). Alcohol is generated at the same time, the interchange of ele- ments being such, that 1 eq. oxalic ether (En+H)+(2C+30) 2 1 eq. oxamide 2C+N+2H+20 and leq. of ammonia 3H+N. >> and 1 eq. alcohol En+2H. When oxalic ether is agitated with alcohol saturated with ammonia, no oxamide is formed, and the solution on evaporation yields crystals, which consist of one eq. of oxalic ether and one eq. of oxalate of ammonia. This compound may be viewed as an ether-oxalate of ammonia, ether-oxahc acid consisting of two eq. of oxalic acid and one eq. of ether, a constitution similar to ethero-sulphuric acid. This compound and oxamide are both generated when dry ammoniacal gas is transmitted over oxdic ether. ^Icert'c Ether.—This compound may be formed by distilling strong acetic acid with an equal weight of alcohol, but it is more readily produced by dis- tilling to dryness 3 parts of acetate of potassa and 3 of alcohol with 2 parts of sulphuric acid. In each case a considerable quantity of free acetic acid and alcohol arc commonly present, and hence the product should be re- peatedly returned into the retort and redistilled. Fragments of chloride of calcium are then introduced, which gradudly combines with free alcohol, forming, if in quantity, a dense stratum, from which the acetic ether may be removed by a syphon. If acetic acid is present, it should be neutralized by potassa, and the acetic ether again distilled. Acetic ether is a colourless liquid, of an agreeable but burning taste, and a very fragrant odour. Its density is 0.882 at 65 ^ and it boils at 165°. It is soluble in 7 or 8 times its weight of water at 60°, and, in all proportions in alcohol. It may be preserved without change; but if distilled with pure potassa or lime, an acetate is formed, and alcohol passes over. It consists of one eq. of acetic acid and one eq. of ether. Formic acid, like the acetic, forms an ether with alcohol without the aid of any other add. Tartaric, Citric, and Malic Ether, Sec.—These ethereal fluids may be ob- tained by distilling their respective acids with sulphuric acid and alcohol, and in composition are analogous to oxalic and acetic ethers. Kinic and benzoic ethers have also been formed, and the former is solid at common temperatures. Cyanuric Ether.—This ether was formed by Wohler by mixing the va- pours of anhydrous cyanuric acid and alcohol, and cdlects as a white pow- der, nearly insoluble in cold alcohol. It is soluble by aid of heat in a mix- ture of alcohol and ether, and crystallizes in colourless prisms by evapora- tion. When heated in open vessels, it fuses and then passes off in vapour. In addition to cyanuric acid and ether, it contains the elements ofwater; so (hat it is not analogous to the ethers previously described. Sulphocyanic Ether.—The substance so termed, Liebig discovered by dis- tilling a mixture of 1 part of sulphocyanuret of potassium, 2 of sulphuric acid, and 3 of strong alcohol. He believes it to be a compound of sulphuret of cyanogen and a carburet of hydrogen. (An. deCh. et de Ph. xli. 202.) Chloric Ether.—This name is sometimes applied to the compound of olefiant gas and chlorine (page 248), and sometimes to an oily liquid pre- pared either by distilling a mixture of sulphuric acid, peroxide of manga- nese, sea-salt, and alcohol, or by directly transmitting chlorine gas into cold alcohol. It is probable, from the mode of preparation, that the liquid ob- tained from chlorine and alcohol is identical with chloral (page 222).* * Besides those mentioned in the text, there is still another variety of chloric ether, formed by the reciprocal action of alcohol and chloride of 562 ETHER. Oxidized Ether.—This name has been applied by Dobereiner to a sub- stance obtained by distilling alcohol with sulphuric acid and peroxide of manganese; and a similar compound is formed by placing alcohol in a dish covered with an open jar, and setting just over the surface of the spirit some moist spongy platinum contained in watch-glasses. The substance obtained by the latter process has been termed acetal by Liebig (Pog. Ann. xxvii. 605); but the nature and properties of these products have not yet been de- termined, nor is it certain that they are definite compounds. Pyroacetic Spirit.—This compound is generated by distilling the salts of acetic acid, and is one of the ingredients of wood tar. It was obtained by Derosne from acetate of copper, and was called by him pyroacetic ether. Mr. Chevenix formed it by distilling the acetates of manganese, zinc, and lead. It is purified from acetic acid and empyreumatic oil, which pass'over at the same time, by admixture with a solution of potassa, and redistillation; and it is subsequently rendered anhydrous by distillation from dry carbo-' nate of potassa or chloride of calcium. It has been examined by Macaire and Marcet (An. of Phil. N. S. viii. 69), and its constitution has been deter- mined by Liebig and Dumas (An. de Ch. et de Ph. xlix). Pyroacetic spirit, when carefully purified from water, acid, and oil, is a colourless limpid liquid, highly volatile and inflammable, of a peculiar pene- trating odour different from both alcohol and ether, and has a density of 0.7921 at 64°. It boils at 132°, and the density of its vapour is 2.019. It unites with water, alcohol, ether, and oil of turpentine in every proportion. It may be exposed for months to the dr without change, and be distilled from the alkalies without decomposition; but it is entirely decomposed by sulphuric acid without the formation of ether. When pyroacetic spirit is distilled from a solution of chloride of lime, the chloride of carbon already described is generated (page 220). By the action of chlorine gas hydrochloric acid is formed, together with a peculiar oily fluid, which has a density of 1.331, and of which 100 parts contain 52.6 of chlorine, 28 of carbon, 2.8 of hydrogen, and 16.6 of oxygen. (Liebig.) According to the analysis of Liebig, which is confirmed by Dumas, pyro- acetic spirit is composed of three eq. of carbon, three eq. of hydrogen, and one eq. of oxygen. These quantities are such that one equivdent of acetic acid, 4C+3H+30, exactly corresponds to one equivalent of pyroacetic spirit, 3C+3H+0, and one equivalent of carbonic acid, C+20. Accord- ingly both Liebig and Dumas have observed that dry acetate of baryta is converted by heat into pyroacetic spirit and carbonate of baryta. Pyroxylic Spirit.—When wood is heated in close vessels, it yields a large quantity of impure acetic acid (pyroligneous acid), and charcoal of great lime. It was discovered by Mr. Samuel Guthrie, of the state of New York, who prepares it by distilling a gallon from a mixture of three pounds of chloride of lime and two gallons of alcohol of sp: gr. 0.844, and rectifying the product by redistillations, first from a great excess of chloride of lime, and afterwards from strong sulphuric acid. About the same time that Guthrie was making his researches in this country, the same ether was dis- covered by Soubeiran in France, who obtained it from the same materials which had been employed by Guthrie. This ether, as obtained by Mr. Guthrie's process, is an extremely volatile liquid, of a sweetish, ethereal odour, and hot, aromatic, peculiar taste. It boils at 166°, and has the sp. gr. of 1.486. It is not acted on by the minerd acids, is slightly soluble in water, and soluble in all proportions in alcohol. When sufficiently diluted with water, It forms an aromatic and saccharine liquid, very grateful to the taste, and acting as a diffusible stimulus, appli- cable to several states of disease. According to Soubeiran, it consists of two eq. of chlorine 70.84, and one eq. of light carburetted hydrogen 8.12=78.96. As this ether does not contain chloric acid, I have elsewhere proposed for it the name of chlorine ether, as being more appropriate.—Ed. COLOURING MATTERS. 563 purity remains in the retort During this process a peculiar spmtuous liquid is formed, which was discovered in 1812 by Mr. P. Taylor* and soon afterwards examined by Macaire and Marcet,t who proposed for it the name of pyroxylic spirit. This liquid is similar to alcohol in several of its proper- ties, and may be substituted for it in spirit-lamps, and frequently as a sol- vent for chemical compounds; but it differs essentially from alcohol in not yielding ether by the action of sulphuric acid. It has a strong penetrating ethereal odour, with a flavour like the oil of peppermint, and a pungent pep- per-like taste. Its sp. gr. at 65° is 0.804, and it boils at 150°. It burns with a blue flame, without residue. According to Liebig, pyroxylic spirit contains exactly one more equiva- lent of oxygen than ether (page 556); but Rcichenbach has raised a doubt about the existence of pyroxylic spirit as a definite compound, maintaining it to be a mixture of alcohol and pyroacetic spirit. SECTION VI. COLOURING MATTERS. Infinite diversity exists in the colour of vegetable substances; but the prevailing tints are red, yellow, blue, and green, or mixtures of these co- lours. Colouring matter rarely or never occurs in an insulated state, but is always attached to some other proximate principle, such as mucilaginous, extractive, farinaceous, or resinous substances, by which some of its proper- ties, and particularly that of solubility, is greatly influenced. Nearly all kinds ot* vegetable colouring matter are decomposed by the combined agency of the sun's rays and a moist atmosphere; and they are all, without exception, destroyed by chlorine (page 211). Heat, likewise, has a similar effect, even without being very intense; for a temperature between 300° or 400°, aided by moist air, destroys the colouring ingredient. Acids and alka- lies commonly change the tint of vegetable colours, entering into combina- tion with them, so as to form new compounds. Several of the metallic oxides, and especially alumina and the oxides of iron and tin, form with colouring matter insoluble compounds, to which the name of lakes is applied. Lakes are commonly obtained by mixing alum or pure chloride of tin with a coloured solution, and then by means of an al- kali precipitating the oxide, which unites with the colour at the moment of separation. On this property are founded many of the processes in dyeing and calico-printing. The art of the dyer consists in giving a uniform and permanent colour to cloth. This is sometimes effected merely by immers- ing the cloth in the coloured solution; whereas in other instances the affinity between the colour and the fibre of the cloth is so slight, that it ody re- ceives a stain which is removed by washing with water. In this case some third substance is requisite, which has an affinity both for the cloth and co- louring matter, and which, by combining at the same time with each, may cause the dye to be permanent A substance of this kind was formerly called a mordant; but the term basis, introduced by the late Mr. Henry of Manchester, is now more generdly employed. The most important bases, and indeed the only ones in common use, are alumina, oxide of iron, and oxide of tin. The two former are exhibited in combination either with the sulphuric or acetic acid, and the latter most commonly as the chloride. * Quarterly Journal, xiv. 436. f An. of Phil. N. S. viii. 69. 564 COLOURING MATTERS. Those colouring substances that adhere to the cloth without abasia are called substantive colours, and those which require a basis, adjective colours. Various as are the tints observable in dyed stuffs, they may all be produced by the four simple ones, blue, red, yellow, and black; and hence it will be convenient to treat of colouring matters in that order. Blue Dyes.—Indigo is chiefly obtained from an American and Asiatic plant, the Indigofera, several species of which are cultivated for the purpose. It is likewise extracted from the Nerium tinctorium; and an inferior sort is prepared from the /satis tinctoria or wood, a native of Europe. Two differ- ent methods are employed for its extraction. In one, the recent plant, cut a short time before its flowering, is placed in bundles in a steeping vat, where it is kept down with cross bars of wood, and covered to the depth of an inch or two with water. In a short time fermentation sets in, carbonic acid gas is freely disengaged, and a ydlow solution is formed. In the course of ten or twelve hours, when its surface begins to look green from the mixture of blue indigo with the yellow solution, it is drawn off into the beating vat where it is agitated with paddles, until all the colouring matter is oxidized by absorbing oxygen from the atmosphere, and is deposited in the form of blue insoluble indigo. The other method consists in drying the leaves like hay, removing the leaf from its stalk by threshing, and grinding the former into powder, in which state it is preserved for use. The dye is then extracted either by maceration in water at the temperature of the air, and fermentation; or by digestion in water at 150° or 180°, without being fermented. In either case it is beaten with paddles as before. (Ure in Journ. of Science, N. S. vi. 259.) The process of fermentation, by some thought essential, may be dispensed with. According to Mr. Weston, how- ever, the dye, as contained in the plant, is insoluble in cold water; but by exposure to the air it undergoes a change, in which oxygen acts a part, and by which it is rendered soluble in water. (Journ of Science, N. S. v. 296.) The indigo of commerce, which occurs in cakes of a deep blue colour and earthy aspect, is a mixed substance, containing, in addition to salts of magnesia and lime, the four following ingredients:—1. a glutinous matter; 2. indigo-brown; 3. indigo-red; 4. indigo-blue. (Berzelius in Lehrbuch, iii. 679.) 1. The gluten is obtained by digesting finely pulverized indigo in dilute sulphuric acid, neutralizing with chdk, and evaporating the filtered solution to dryness. The gluten is then taken up by alcohol, and on evaporation is left with the appearance of a yellow or yellowish-brown, transparent, shining varnish. Its odour is similar to that of broth, and it contains nitro- gen as one of its elements. It differs, however, from common gluten in its free solubility both in alcohol and water. 2. Indigo-brown has not been obtained in a perfectly pure state, owing to its tendency to unite both with acids and alkalies. With the former it yields in general sparingly soluble, and with the latter very soluble com- pounds, which have a deep brown colour. From indigo, freed from gluten by dilute acid, it is separated by a strong solution of potassa aided by gentle heat; and after dilution with water, without which it passes with difficulty through paper, the liquid is filtered. The solution has a green tint, owing to some indigo-blue being dissolved, and with sulphuric acid yields a bulky semi-gelatinous precipitate of a blackish colour. By dissolving it in solution of carbonate of ammonia, evaporating to dryness, and removing the soluble parts by a small quantity of water, the brown matter is freed from indigo- blue and sulphuric acid. It still, however, contains ammonia, and though this alkali may be expelled by means of hydrated lime or baryta, the indigo- brown is still impure, since it retains some lime or baryta in combination. Like indigo-gluten it contains a considerable quantity of nitrogen as one of its elements. The indigo-green of Chevreul is probably a mixture of this substance with indigo-blue. 3. Indigo-red is obtained by boiling indigo, previously purified by potassa, in successive portions of strong alcohol as long as a red solution is obtained. 565 The alcoholic solutions are then concentrated by evaporation, during which th« indigo-red is deposited as a blackish-brown powder. The concentrated solution, of a deep red colour, yields by evaporation a compound of indigo- red and indigo-ljfqwn with alkali, which is soluble in water. Indigo-red is insoluble in water and alkahes; but it is soluble, hough sparingly, in hot alcohol, and rather more freely in ether. It dissolves in strong sulphuric acid, and forms a dark yellow liquid; and with nitric acid it yields a beautiful purple solution, which speedily becomes yellow by de- composition. When heated in vacuo, it yields a gray crystalline sublimate, which, when purified by a second sublimation, is obtained m minute trans- parent needles, shining and white. This substance, in its relation to re- agents, resembles indigo-red; and especially by yielding with nitric acid a similar purple-red solution, which subsequently becomes yellow. 4. Indigo-blue.—This term is applied to the real colouring matter of in- digo, which is left, though not quite pure, after acting on common indigo with dilute acid, potassa, and alcohol. It is conveniently prepared from the greenish-yellow solution, which dyers make by mixing indigo with green vitriol, hydrate of lime, and water; when the indigo is deoxidized by the protoxide of iron, and yields a soluble compound with lime. On pouring this solution into an excess of hydrochloric acid, while freely exposed to the air, oxygen gas is absorbed, and the indigo is obtained in the form of a blue powder. It may also be procured in a state of great purity by subli- mation ; but this process is one of delicacy, from the circumstance that the subliming and decomposing points of indigo are very near each other; and minute directions have been given by Mr. Crum for conducting it with suc- cess. (An. of Phil. N. S. v.) To be sure of obtaining it quite pure by either process, the indigo should first be purified by the action of dilute acid, potassa, and alcohol. Pure indigo sublimes at 550°, forming a violet vapour with a tint of red, and condensing into long flat acicular crystals, which appear red by reflect- ed, and blue by transmitted light. It has neither taste nor odour, and it is insoluble in water, alkalies, and ether. Boiling alcohol takes up a trace of it, and acquires a blue tint; but it is generally deposited again on standing. Nitric acid produces a change which has already been described (page 522). Concentrated sulphuric acid, especially that of Nordhausen, dissolves it readily, forming an intensely deep blue solution, commonly termed sulphate of indigo, which is employed by dyers for giving the Saxon blue. The in- digo during solution undergoes a change, and in this modified state it has received the name of cerulin from Mr. Crum, who regards it as a compound of one equivdent of indigo and four of water. According to Berzelius the solution is of a more complicated nature, and contains the three following substances: 1. indigo-purple; 2. sulphate of indigo; 3. hyposulphate of indigo. Indigo-purple is chiefly formed when indigo is dissolved in English oil of vitriol, and subsides when the solution is diluted with from 30 to 50 times its weight of water. It was first described under the name of phevecin, from qoht!;, purple, by Mr. Crum, who considers it a hydrate of indigo with two equivalents of water. Into the dilute solution, after phenecin is separated, Berzelius inserts fragments of carefully washed flannel, until all the colour is withdrawn from the liquid. The dyed flannel, after the ad- hering acid is entirely removed, is digested in water, with a little carbonate of ammonia, by which means a blue solution is obtained, consisting of ammonia in combination with sulphate and hyposulphate of indigo. The solution is evaporated to dryness at 140°, and to the residue is added alcohol of 0.833, which dissolves only the hyposulphate. The compounds of indigo with sulphuric and hyposulphuric acid are considered by Berzelius, not as salts in which indigo acts as a base, but as distinct acids of which indigo is an essential ingredient. Indigo-sulphuric acid, as sulphate of indigo may, therefore, be called, is prepared by mixing indigo-sulphate of ammonia with acetate of oxide of lead, when indiiro- 48 s 566 COLOURING MATTERS. sulphate of that oxide subsides. This salt is suspended in water, and de- composed by hydrosulphuric acid: the sulphuret of lead is collected on a filter; and the filtered solution, at first colourless or nearly so, owing to de- oxidation of indigo by hydrosulphuric acid, but which soon becomes blue by the action of the air, is evaporated at a temperature not exceeding 122° F. The acid is left as a dark blue solid, of a sour astringent taste, soluble in water and alcohol, and capable of forming a distinct group of salts with alkalies. Indigo-hyposulphuric acid may be prepared by a similar process. One of the most remarkable characters of indigo-blue is its susceptibility of being deoxidized, and thus returning to the state in which it appears to exist in the plant, and of again recovering its blue tint by subsequent oxi- dation. The change is effected by various deoxidizing agents, such as hy- drosulphuric acid, hydrosulphate of ammonia, hydrated protoxide of iron, or solution of orpiment in potassa. In the deoxidized state it readily unites with alkaline substances, such as potassa or lime, and forms compounds which are very soluble in water. The method by which dyers prepare their blue vat is founded on these properties. A portion of indigo is put into a tub with about three times its weight of green vitriol and an equal quantity of slaked lime, and water is added. The protoxide of iron, precipitated by lime, gradually deoxidizes the indigo, and in the course of a day or two a yellow solution is obtained. When cotton cloth is moistened with this liquid and exposed to the air, it speedily becomes green from the mixture of colours, and then blue; and as the blue indigo is insoluble, and unites chemically with the fibre of the cloth, the dye is permanent. Deoxidized indigo has been obtained in a separate state by Liebig. A mixture is made with 1.5 parts of indigo, 2 of green vitriol, 2.5 of hydrate of lime, and 50 or 60 of water; and after an interval of 24 hours the yellow solution is carefully drawn off by a syphon, and mixed with dilute hydro- chloric acid. A thick white precipitate falls, which remains without change if carefully excluded from oxygen, and may even be exposed to the air when quite dry; but it rapidly becomes blue by exposure to the atmosphere while moist, or by being covered with aerated water. To this substance Liebig has applied the name of indigogen; and he has ascertained that, in passing into blue indigo, it absorbs 11.5 per cent, of oxygen. The necessity for perfectly excluding every source of oxygen renders the preparation of indi- gogen difficdt All the vessels employed in the process should be filled with hydrogen gas, the water be freed from air by boiling, and as a further protection a little sulphate of ammonia is added both to the acid by which the precipitate is made, and to the water with which it is washed. The composition of indigogen and indigo-blue will be found at page 522. Red Dyes.—The chief substances which are employed for the red dye are cochineal, lac, archil, madder, Brazil wood, logwood, and safflower, all of which are adjective colours. The cochineal is obtained from an insect which feeds upon the leaves of several species of the Cactus, and which is supposed to derive this colouring matter from its food. It is very soluble in water, and is fixed on cloth by means of alumina or oxide of tin. Its natu- ral colour is crimson; but when bitartrate of potassa is added to the solu- tion, it yields a rich scarlet dye. The beautiful pigment called carmine is a lake made of cochineal and alumina, or oxide of tin. The dye called archil is obtained from a peculiar kind of lichen, (Lichen roceella,) which grows chiefly in the Canary Islands, and is employed by the Dutch in forming the blue pigment called litmus or turnsol. The co- louring ingredient of litmus is a compound of the red colouring matter of the lichen and an alkali; and hence, on the addition of an acid, the colour- ing matter is set free, and the red tint of the plant is restored. Litmus is not only used as a dye, but is employed by chemists for detecting the pre- sence of a free acid. The colouring principle of logwood has been procured in a separate state by Chevred, who has applied to it the name of hematin. (An. de Ch. Ixxxi.) COLOURING MATTERS. 567 It is obtdned in crystds by digesting the aqueous extract of logwood in dcohol, and dlowing the alcoholic solution to evaporate spontaneously. Safflower is the dried flowers of the Carthamus tmctorius, which is culti- vated in Egypt, Spain, and in some parts of the Levant. The pigment called rouge is prepared from this dye. .... , c Madder, extensively employed in dyeing the Turkey red, is the root of the Rubia tinctorum. A red substance, supposed to be the chief colouring principle of the plant has been obtained in an insulated state by Robiquet and Colin, who have termed it alizarine, from Ali-zan, the commercial name by which madder is known in the Levant. Their process has received the following modification by Zenneck. Ten parts of madder are digested in four of ether, the solution is evaporated to the consistence of syrup, and then allowed to become dry by spontaneous evaporation. The residue is pulverized, and sublimed by a gentle heat from a watch glass. The sub- limate, which is collected by covering the watch glass with a cone of paper, is deposited in the form of yellowish-red, brilliant, diaphanous, acicular crystals, wliich are soft, flexible, and heavier than water. They soften when heated, and sublime at a temperature between 500 and 600=, causing an aromatic odour. They are nearly insoluble in cold and very sparingly soluble in hot water. They require for solution 210 times their weight of alcohol, and 160 of ether at 60°. According to Zenneck the acidity of dizarine is very decisive, both in its sour taste, and its power of neutralizing dkalies. It consists, in 100 parts, of 18 of carbon, 20 of hydrogen, and 62 of oxygen. (Journal of Science, N. S. v. 198.) Yellow Dyes.—The chief yellow dyes are quercitron bark, turmeric, wild American hiccory, fustic, and saffron; all of which are adjective colours. Quercitron bark, which is one of the most important of the yelloyy dyes, was introduced into notice by Dr. Bancroft. With a basis of alumina, the decoction of this bark gives a bright yellow dye. With oxide of tin it com- municates a variety of tints, which may be made to vary from a pale lemon colour to deep orange. With oxide of iron it gives a drab colour. Turmeric is the root of the Curcuma longa, a native of the East Indies. Paper stained with a decoction of this substance constitutes the turmeric or curcuma paper employed by chemists as a test of free alkali, by the action of which it receives a brown stain. The colouring ingredient of saffron (Crocus sativus) is soluble in water and alcohol, has a bright yellow colour, is rendered blue and then lilac by sdphuric acid, and receives a green tint on the addition of nitric acid. From the great diversity of colours which it is capable of assuming under different circumstances, Bouillon Lagrange and Vogel have proposed for it the name polychroite. (An. de Ch. Ixxx.) Black Dyes.—The black dye is made of the same ingredients as writing ink, and, therefore, consists essentially of a compound of oxide of iron with gallic acid and tannin. From the addition of logwood and acetate of copper, the black receives a shade of blue. By the dexterous combination of the four leading colours, blue, red, yel- low, and black, all other shades of colour may be procured. Thus green is communicated by forming a blue ground with indigo, and then adding a yellow by means of quercitron bark. The reader who is desirous of studying the details of dyeing and calico- printing, a subject which does not fall within the plan of this work, may consult Berthollet's Elemens de VArt de la Teinture; the treatise of Dr. Bancroft on Permanent Colours; a paper by Mr. Henry in the third volume of the Manchester Memoirs; and the Essay of Thenard and Board in the 74th volume of the Annales de Chimie, 568 VEGETABLE ALBUMEN.--GLUTEN. SECTION VII. SUBSTANCES WHICH, SO FAR AS IS KNOWN, DO NOT BELONG TO EITHER OF THE PRECEDING SECTIONS. Vegetable Albumen.—Under this name is distinguished a vegetable prin- ciple which has a close resemblance to animal albumen, especially in the cha- racteristic property of being coagulable by heat. This substance was found by Vogel in the bitter almond, and in the sweet almond by Boullay; it appears to be an ingredient of emulsive seeds generally ; and it exists in the sap of many plants. Einhof detected it in wheat, rye, barley, peas, and beans. Vegeta- ble albumen is soluble in cold water, but by a boiling temperature it is co- agulated, and thus completely deprived of its solubility. It is insoluble in alcohol, and very sparingly soluble in acids. Alkalies dissolve it readily, and it may be precipitated from them by acids; but the albumen falls in combination with a portion of the acid. Ferrocyanate of potassa and corro- sive sublimate act upon it as on solutions of animal albumen. Vegetable albumen contains nitrogen as one of its elements, and is very prone, when kept in the moist state* to undergo the putrefactive fermenta- tioa, emitting an offensive odour, with disengagement of ammonia and for- mation of acetate of ammonia. During a certain period of putrefaction it has the odour of old cheese. (Berzelius.) Gluten.—In the separation of starch from wheat flour, as already de. scribed (page 536), a gray viscid substance remains, fibrous in its texture, and elastic. Beccaria, who first carefully examined its properties, was struck with its analogy to glue, both in its viscidity as well as its tendency to putrefy like animal matter, and gave it the name of vegetable gluten. Einhof has since shown that this gluten is a mixed substance, containing gluten and vegetable albumen. Pure gluten is obtained by washing dough in water until the starch and soluble parts are removed, and treating the residue with boiling alcohol. On mixing the alcoholic solution with water, and distilling off the spirit, the gluten is deposited in large coherent flakes. As thus obtained it has a pde yellow colour, and a peculiar odour, but no taste, adheres tenaciously to the, fingers when handled, and has considerable elasticity. It is insoluble in water and ether, but dissolves readily in hot alcohol, apparently without any change of property ; but if the alcoholic solution is evaporated to dryness, the gluten is left as a transparent varnish. It swells up and softens with acetic acid, forming a compound which is soluble in water. It unites also with the mineral acids; and these compounds, excepting that with sulphuric acid, dissolve readily in pure water, but are insoluble when there is an ex- cess of acid. It is dissolved by a dilute solution of potassa, apparently with- out being decomposed; for the gluten, after being thrown down by the min- eral acids, retains its viscidity. In this state, however, it is combined with some of the acid. (Berzelius.) When gluten is kept in a warm moist situation it ferments, and an acid is formed; but in a few days putrefaction ensues, and an offensive odour, like that of putrefying animal matter, is emitted. According to Proust, who made these spontaneous changes a particular object of study, the process is divisible into two distinct periods. In the first, carbonic acid and hydrogen gases arc evolved; and in the second, besides acetic and phosphoric acids and ammonia, two new compounds are generated, for which he proposed the names of caseic acid and caseous oxide. These are the same principles which arc generated during the fermentation of the curd of milk, and their real nature will be considered in the section on milk. It is apparent from these circumstances that gluten contains nitrogen as one of its elementsy and thai YEAST.—ASPARAGIN. 569 it approaches closely to the nature of animal substances. It has hence been ^SSA^ISfc beat, it contracts in volume, becomes hard and bSe and may in th?s state be preserved without change. Exposed to a strong heat, it yields, in addition to the usual inflammable gases, a thick fetid oil and carbonate of ammonia. GlutenTs present in most kinds of grain, such as whea , barley, rye, oats, peas and beans; but the first contains it in by far the largest proportion. ffiSdie reason that wheaten bread is more nutritious than that made with other kinds of flour; for of all vegetable substances gluten appears.to be the most nutritive. It is to the presence of gluten that wheat flour owes its property of forming a tenacious paste with water. To the same cause is owing the formation of light spongy bread; the carbonic acid which u dis- engaged during the fermentation of dough, being detained by the viscid glu- ten, distends the whole mass, and thus produces the rising of the dough. From the experiments of Davy, it appears that good wheat flour contains from 19 to 24 per cent, of gluten. The wheat grown in the south of Europe is richer in gluten than that of colder climates. M Taddei, an Italian chemist has given an account of two principles se- parable from the gluten of Beccaria by means of boiling alcohol. To the substance soluble in alcohol he has applied the name of ghadme, j \ia, alu- ten; and to the other that of zymome, from £t/,uii, a ferment. (An. of Phil. xv.) For the latter he has discovered a delicate test in the powder of guaia- cum, which when rubbed in a mortar with moist zymome, instantly strikes a beautiful blue colour: and the same tint appears, though less rapidly, when it is kneaded with gluten or dough made with good wheat flour. But with bad flour, the gluten of which has suffered spontaneous decomposition, the blue tint is scarcely visible; and accordingly M. Taddei conceives that useful inferences as to the quality of flour may be drawn from the action of guaiacum. These views have been criticised by Berzelius, who declares that the sub- stances described by Taddei are nothing else than the gluten and vegetable albumen already described; and the habitual accuracy of Berzelius leaves little chance of error in his statement. The blue tint, above alluded to, must have arisen from the action of guaiacum either on vegetable albumen itself, or on some substanee by which it is accompanied in wheat (An. of Phil. iv. 69, or Lehrbuch, iii. 362.) Yeast.—This substance is always generated during the vinous fermenta- tion of vegetable juices and decoctions, rising to the surface in the form of a frothy, flocculent, somewhat viscid matter, the nature and composition of which are unknown. It is insoluble in water and alcohol, and in a warm, moist atmosphere gradually putrefies, a sufficient proof that nitrogen is one of its elements. Submitted to a moderate heat, it becomes dry and hard, and may in this state be preserved without change. Heated to redness in close vessels, it yields products similar to those procured under the same circum- stances from gluten. To this substance, indeed, yeast is supposed by some chemists to be very closely allied. The most remarkable properly of yeast is that of exciting fermentation. By exposure for a few minutes to the heat of boiling water, it loses this pro- perty, but after some time again acquires it. Nothing conclusive is known concerning either the nature of these changes, or the mode in which yeast operates in establishing the fermentative process. Asparagin.—This principle was discovered by Vauquelin and Robiquet in the juice of the asparagus, from which it is deposited in crystals by evapo- ration. It is dso contained in the root of the marsh-mallow and liquorice. Robiquet, who first procured it from the juice of the recent liquorice root, doubted its identity with asparagin, and gave it the name of agedoite; but the mistake has been corrected by Al. Plisson. Asparagin crystallizes in the form of a rectangular octohedron, six-sided prism, or right rhombic prism, is inodorous, and has a cool slightly nauseous 570 BASSORIN.,—CAFFEIN. taste. In ether and pure alcohol it is insoluble, and requires for solution 58 parts of cold water, but is more freely dissolved by the aid of heat. It has neither an acid nor alkaline reaction with test-paper. Plisson has noticed that when boiled for some time with hydrated oxide of lead or magnesia, it is re- solved into ammonia, and a new acid, called the aspartic. The possibility of such a change is intelligible from the late analysis of asparagin and aspar- tic acid by Liebig, as subjoined:—(An. de Ch. et de Ph. liii. 416.) Anhydrous Asparagin. Anhydrous Aspartic Acid. Carbon 36.74 48.96 8C 42.16 48.96 8C Hydrogen 5.94 8 8H 4.37 5 5H Nitrogen 21.27 28.3 2N 12.20 14.15 N Oxygen 36.05 48 60 41.27 48 60 100.00 133.26 CsHsNaOs 100.00 116.11 C^HeNOs. It is obvious that one eq. of anhydrous asparagin contains one eq. of aspar- tic acid and one eq. of ammonia. And yet asparagin is not aspartate of ammonia, since acids do not eliminate aspartic acid nor alkalies ammonia with the facility which would be expected on that supposition. The crys- tals of asparagin lose 12.133 per cent, of water when heated, or consist of 133.26 parts or one eq. of anhydrous asparagin and 18 or two eq. of water. Aspartic acid is prepared by decomposing the aspartate of oxide of lead by hydrosulphuric acid, and evaporating the filtered solution, when the acid is obtained as a colourless powder composed of minute prismatic crystals, which, like asparagin, contain two eq. of water of crystallization. It has little taste, is sparingly soluble in cold water, and still less so in alcohol. Its aqueous solution is not precipitated by the soluble salts of baryta, lime, lead, magnesia, copper, mercury, or silver. The aspartates, when the taste of the base does not interfere, have the taste of the juice of meat. It yields am- monia when decomposed by heat. (An. de Ch. et de Ph. xxxv. 175, and xl. 309.) Bassorin was first noticed in gum Bassora by Vauquelin. It is an ingre- dient of gum tragacanth (page 538), and probably occurs in other gums. Salep, from the experiments of Caventou, appears to consist almost totally of bassorin. Bassorin is characterized by forming with cold water a bulky jelly, which is insoluble in that menstruum, as well as in alcohol and ether. Boiling water does not dissolve it except by long-continued ebullition, when the bas- sorin,at length disappears, and is converted into a substance similar to gum- arabic. Caffein was discovered in coffee by Robiquet in the year 1821, and was soon after obtained from the same source by Pelletier and Caventou, with- out a knowledge of the discovery of Robiquet. (An. of Phil. N. S. xii.) It is best prepared by making an aqueous decoction of bruised raw coffee, add- ing subacetate of oxide of lead as long as a precipitate falls, by which means a large quantity of extractive and colouring matter is thrown down, and then precipitating the excess of oxide of lead by hydrosulphuric acid. The caffein, which is left in solution, is ultimately obtained in crystals by evapo- ration. Pfaff and Liebig recommend that it should be decolorized by digestion with animal charcod together with some moist hydrated oxide of lead. Caffein is a white crystalline volatile matter, sparingly soluble in cold wa- ter, but very soluble in boiling water and alcohol, and is deposited from these solutions as they cool, in the form of silky filaments like amianthus. Pelle- tier, contrary to the opinion of Robiquet, at first regarded it as an alkdine base; but he now admits that it does not affect the vegetable blue colours, nor combine with acids. From a late analysis by Pfaff and Liebig, made with a purer specimen than that analyzed by Pelletier and Dumas, caffein is composed of 48.96 parts or eight eq. of carbon, 5 parts or five eq. of hy- drogen, 28.3 or two eq. of nitrogen, and 16 or two eq. of oxygen. (An. de CATHARTIN.—-FUNGIN.—SUBERIN.—ULMIN.—LUPULIN, &C. 571 Ch. et de Ph. xlix. 303.) Though it contains more nitrogen than most ani- mal substances, it does not, under any circumstances, undergo the putrefac- tive fermentation. . . In the precipitate caused in a decoction of coffee by acetate of oxide ot lead, tannic acid is present together with another acid supposed to be pecu- liar to coffee, and termed by Pfaff caffeic acid. Catharlin.—This name has been applied by Lassaigne and 1-tnculle to the active principle of senna. (An. de Ch. et de Ph. vol. xvi.) A similar bit- ter purgative principle, called Cytisin, has been prepared irom the Cytisus Fungin.—This name is applied by Braconnot to the fleshy substance of the mushroom, purified by digestion in hot water, to which a little alkali is added. Fungin is nutritious in a high degree, and in composition is very analogous to animal substances. Like flesh it yields nitrogen gas when di- gested in dilute nitric acid. Suberin.—This name has been applied by Chevred to the cellular tissue of the common cork, the outer bark of the cork-oak, (quercus suber), after the astringent, oily, resinous, and other soluble matters have been removed by the action ofwater and alcohol. Suberin differs from all other vegetable principles by yielding the suberic when treated by nitric acid. Ulmin, discovered by Klaproth, is a substance which exudes sponta- neously from the elm, oak, chesnut, and other trees ; and according to Ber- zelius is a constituent of most kinds of bark. It may be prepared by acting upon dm-bark by hot alcohol and cold water, and then digesting the residue in water, which contains an alkaline carbonate in solution. On neutralizing the alkali with an acid, the ulmin is precipitated. Ulmin is a dark brown, nearly black substance, is insipid and inodorous, and is very sparingly soluble in water and alcohol. It dissolves freely, on the contrary, in the solution of an alkaline carbonate, and is thrown down by an acid. Ulmin is regarded as an acid by M. P. Boullay, who has pro- posed for it the name of ulmic acid. He found that 100 parts of it contain 56.7 of carbon, and 43.3 of oxygen and hydrogen, in the ratio to form wa- ter. According to him it is an ingredient of vegetable mould and turf, and contributes much to the growth of plants. The black matter deposited during the decomposition of prussic acid, supposed by Gay-Lussac to be a carburet of nitrogen, is an acid very similar to the ulmic, and to which he has given the name of azulmic acid. (An. de Ch. et de Ph. xliii. 273.) Lupulin is the name applied by Dr. Ives to the active principle of the hop,' but which has not yet been obtained in a state of purity. Inulin is a white powder like starch, which is spontaneously deposited from a decoction of the roots of the Inula helenium or elecampane. This substance is insoluble in cold, and soluble in hot water, and is deposited from the latter as it cools, a character which distinguishes it from starch. With iodine it forms a greenish-yellow compound of a perishable nature. Its solution is somewhat mucilaginous; but inulin is distinguished from gum by insolu- bility in cold water, and in not yielding the saccholactic when digested in nitric acid. Medullin.—This name was applied by John to the pith of the sun-flower, but its existence as an independent principle is somewhat dubious. The term pollenin has been given by the same chemist to the pollen of tulips. Piperin is the name which is applied to a white crystalline substance ex- tracted from black or white pepper. It is tasteless, and is quite free from pungency, the stimulating property of the pepper being found to reside in a fixed oil. (Pelletier, in An. de Ch. et de Ph. vol. xvi.) Dr. A. T. Thomson has extracted it from chamomile flowers. A process recommended for its preparation by Vogel consists in digest- mg for two days 16 ounces of black pepper in coarse powder in twice its weight ofwater, five times in succession ; and digesting the insoluble parts previously well pressed and dried, for three days in 24 ounces of dcohol' I he solution is pressed through linen cloth, filtered, and evaporated to the 572 OLIVILE.—SARCOCOLL.—RHUBARBARIN.—RHEIN, &C. consistence of syrup; and the impure crystals of piperin, deposited by cool- ing, are freed from adhering resinous matter by ether, and further purified by animal charcoal, re-solution in alcohol, and a second crystallization. Piperin crystallizes in four-sided prisms, which have commonly a yellow colour, owing to adhering oil or resin. It is insoluble in cold, and sparingly soluble in hot water; but it is very soluble in alcohol, and less so in ether. Acetic acid also dissolves it, and leaves it by evaporation in feathery crys- tals. It fuses at 212°, and according to Gobel consists of 80.95 parts of car- bon, 8.13 of hydrogen, and 10.92 of oxygen. Olivile.—When the gum of the olive-tree is dissolved in alcohol, and the solution is allowed to evaporate spontaneously, a peculiar substance, appa- rently different from the other proximate principles hitherto examined, is deposited either in flattened needles or as a brilliant amylaceous powder. To this Pelletier, its discoverer, has given the name of Olivile. (An. of Phil, xii.) Sarcocoll is the concrete juice of the Pencea sarcocolla, a plant which grows in the northern parts of Africa. It is imported in the form of small grains of a yellowish or reddish colour like gum-arabic, to which its proper- ties are similar. It has a sweetish taste, dissolves in the mouth like gum, and forms a mucilage with water.' It is distinguished from gum, however by its solubility in alcohol, and by its aqueous solution being precipitated by tannin. Dr. Thomson, who has given a full account of sarcocoll in his System of Chemistry, considers it closely allied to the saccharine matter of liquorice. Rhubarbarin is the name employed by Pfaff to designate the principle in which the purgative property of the rhubarb resides. Al. Nani of Alilan re- gards the active principle of this plant as a vegetable alkdi; but he has not given any proof of its alkaline nature. (Journal of Science, xvi. 172.) Rhein.—M. Vaudin has applied this name to a substance which he obtained by gently heating rhubarb in powder with eight times its weight of nitric acid of 1.375, evaporating to the consistence of syrup, and diluting with cold water. Rhein, which is then deposited, is inodorous, has a slightly bitter taste, and an orange colour. It is sparingly soluble in cold water; but it dissolves in alcohol, ether, and hot water, and its solutions are rendered pale yellow by acids, and rose-red by dkalics. It may be extracted from rhubarb by ether, a fact which proves that it exists ready formed in the plant; and its mode of preparation shows that it possesses unusual permanence, power- fully resisting the action of nitric acid. Rhaponticin.—This substance was obtained by Hornemann from the Rheum Rhaponticum. It was obtained in the form of yellow scales, which are taste- less and inodorous, insoluble in cold water and ether, and requires for solu- tion 24 times its weight of boiling water, and only twice its weight of anhy- drous alcohol. Colocyntin.—This name was applied by Vauquelin to a bitter resinous matter extracted from colocynth by the action of alcohol, and left by evapo- ration as a brittle substance of a golden-yellow colour. It is slightly soluble in water, is freely dissolved by alcohol and alkalies, and possesses the purga- tive properties of colocynth. (Journal of Science, xviii. 400.) Berberin.—This is a yellow bitter principle contained in the alcoholic ex- tract of the root of the barberry. Bryonin.—This is a bitter, rather poisonous principle, obtained first by Vauquelin and afterwards by Brandes from the root of the Brionia alba. Gentianin is the name applied to the bitter principle of the root of the gentian. Zanthopicrin is a bitter principle obtained from the bark of the Zanthoxy- lum caribii um by Chevallier and Pelletan. It is of sparing solubility in water, insoluble in ether, very soluble in alcohol, from which it crystdlizes by evaporation in yellow acicular crystals of a silky lustre. Cctrarin is the name applied by Herberger to the bitter principle of Ice- land moss. SCILLITIN.—SENEGIN.—SAPONIN.—ARTHANATIN, &C. 57 Scillitin is the name applied by Vogel to the bitter medicinal principle of squills (Scilla maritima). It is prepared by evaporating the juice expressed from the fresh root of squills to the consistence ot an extract, forming a solu- tion of it in alcohol, which is then evaporated to dryness, and the soluble parts taken up by water. The solution contains scillitin associated with tannic acid and saccharine matter: the former is separated by means of subacetate of oxide of lead; and the latter by forming an alcoholic solution of the scillitin and sugar, freed from tannic acid, and adding ether, which throws down the sugar, leaving most of the scillitin in solution. By evapo- ration the scillitin remains in the form of a white friable mass of a resinous ^SeZgin is the name given by Gehlen to the bitter acrid principle contained in the root of the Polygala Senega. Saponin.—This substance is contained in the root of the Saponana offici- nalis, and is the cause of the lather which that root forms when agitated with water. It is prepared by evaporating to dryness an aqueous solution of the alcoholic extract of the root, or an alcoholic solution of the aqueous extract. Both solvents are requisite, in order to separate the saponin from resinous and gummy matter wilh which it is associated in the root. Arthanatin.—This name was applied by Saladin to a colourless crystalline matter, which is extracted by alcohol from the root of the Cyclamen Eu- ropium. Extractive Matter.—This expression, if applied to one determinate princi- ple supposed to be the same in different plants, is quite inapplicable. It is indeed true that most plants yield to water a substance which differs from gum, sugar, or any proximate principle of vegetables, which, therefore, con- stitutes a part of what is called an extract in pharmacy, and which, for want of a more precise term, may be expressed by the name of extractive. It must be remembered, however, that this matter is always mixed with other proximate principles, and that there is no proof whatever of its being identical in different plants. The solution of saffron in hot water, said to afford pure extractive matter by evaporation, contains the colouring matter of the plant, together wilh all the other vegetable principles of saffron, which happen to be soluble in the menstruum employed. Plumbagin, extracted by Dulong from the root of the Plumbago Europaa, is soluble in water, alcohol, and ether, and crystallizes from its solutions in acicular crystals of a yellow colour. Its aqueous solution is made cherry- red by alkalies, subacetate of oxide of lead, and perchloride of iron ; but acids restore the yellow tint, and the plumbagin is found unaltered. Its taste is at first sweet, but is subsequently sharp and acrid, extending to the throat (Journal of Science, N. S. vi. 191.) Chlorophyle.—This name has been applied by Pelletier and Caventou to the green colouring matter of leaves. It is prepared by bruising green leaves into a pulp with water, pressing out all the liquid, and boiling the pulp in alcohol. The solution is mixed with water, and the spirit driven off by dis- tillation, when the chlorophyle is left floating on the surface of the water. As thus obtained, it appears to be wax stained with the green colour of the leaves; and from some late observations of Macaire Prinsep, the wax may be removed by ether, and the colouring matter left in a pure state. The red autumnal tint of the leaves, according to the same observer, is the effect of an acid generated in the leaf. The green tint may be restored by the action of an alkali. Amygdalin.—This substance was extracted in 1830 by Robiquet and Boutron-Charlard from the bitter almond. (An. de Ch. et de Ph. xli v. 352.) The almond, reduced to a pulp, is digested with ether in order to separate its fixed oil, after which it is boiled in 3 or 4 successive portions of alcohol, and the alcoholic solution is distilled in a water bath to the consistence of syrup. This residue is then briskly agitated with ether, and set at rest in a tube placed perpendicularly, when three distinct strata are gradually formed: the upper one is nearly pure ether, the lower is viscid and consists of saccharine 574 SALICIN.—POPULIN.—MECONIN. matter, and the intermediate stratum, white and semi-solid, contains the amygdalin. Amygdalin, insoluble in water, dissolves freely in hot alcohol, and crystal- lizes as the solution cools in white needles, which are not volatile, and remain unchanged in the open dr. Its taste is sweet followed by a bitter flavour, analogous to that of the bitter almond, the peculiar flavour of which appears owing to amygdalin. This principle has neither acid nor dkaline properties. It contains nitrogen as one of its elements, and emits a strong odour of am- monia when it is boiled in a solution of potassa. Heated with nitric acid it yields benzoic acid. It certainly has a close relation to benzule, but the na- ture of that connexion has not yet been traced. Salicin.—-This principle was discovered in 1830 in the bark of the willow (Salix helix) by M. Leroux, who announced it, along with attestations of its virtues from Majendie and other medical authorities, as a cure for intermit- tent fever of sufficient power to become a substitute for quinia; and obser- vations on its preparation and properties have since been made by Braconnot, Pelouze and J. Gay-Lussac, and Peschier. (An. de Ch. et de Ph. xliii. 440, and xliv. 220, 296, and 418.) It exists in several species of the willow, and Braconnot has met with it in the bark of the poplar, especially of the Popu- lus tremula. The most approved method of preparation consists in forming an aqueous decoction of the willow bark, adding subacetate of oxide of lead as long as a precipitate fdls, in order to remove colouring matter, boiling with chalk to throw down the excess of oxide of lead, and evaporating the solution. The salicin is deposited in crystals, which may be purified by solution in alcohol and digestion with animal charcoal. Pure salicin is perfectly white, crystallizes in delicate prisms or needles, and has a very bitter taste. In cold water it is sparingly soluble; but it is freely taken up both by water and alcohol at a boiling temperature, and is insoluble in ether. By strong sulphuric acid in the cold it is decomposed, and the acid acquires a purple tint. Heated with sulphuric acid somewhat diluted, or with strong hydrochloric acid, it is converted into a white insolu- ble matter of the nature of resin. When digested with eight times its weight of nitric acid, salicin yields a large quantity of carbazotic acid. Salicin has neither acid nor alkaline properties, and according to Pelouze and J. Gay-Lussac consists of carbon, hydrogen, and oxygen, in the ratio of two equivalents of the first element, two equivalents of the second, and one equivalent of the third. Populin.—A substance, described under this name, was found by Bra- connot in the bark of the Populus tremula during his search for salicin. It exists still more plentifully in the leaves of the same tree, and is obtained by throwing down the colouring and extractive matter of an aqueous decoc- tion of the leaves by subacetate of lead, as in the process for salicin, and then evaporating to the consistence of thin syrup: the impure crystals are pressed within linen, mixed with a little animal charcoal, and dissolved in 160 times their weight of boiling water. The filtered solution deposites, in cooling, white, silky, acicular crystals of populin. Populin requires 2000 times its weight of cold, and 70 of boiling water for solution; but it is much more soluble in hot alcohol. Acids act upon it exactly in the same manner as on salicin, showing that these two sub- stances, if essentially distinct, are very analogous in properties and com- position. Meconin.—This principle was discovered separately by Al. Couerbe in 1830, and before that period by Al. Dublanc; but we are indebted to the former for a knowledge of its composition and properties (An. de Ch. et de Ph. 1. 337). At common temperatures it is a white solid, inodorous, and of a rather acrid taste after some time, though tasteless at first It begins to liquefy at 190°, and is a limpid liquid at 195°, and may be kept fluid till the temperature falls to 167°. At 311° it sublimes, and condenses on cool- ing into a white matter like fat. It requires 266 parts of cold, and 18.5 of boiling water for solution, and is very soluble in alcohol, ether, and the es- COLUMRIN.--ELATIN.--SINAPISIN. 575 gential oils. It crystdlizes from these solutions in six-sided prisms, two pardlel faces of the prism being larger than the rest, and forming a dihedra summit. When a crystal is put into water which is then heated, the crystal becomes softer, then forms a liquid globule like oil at the bottom of the flask, and then, as the water boils, it soon disappears entirely. Meconin has neither acid nor alkaline properties. The sulphuric and nitric acids and chlorine decompose it, and produce compounds of a charac- teristic nature. In sulphuric acid diluted with a quarter or halt its weight ofwater it forms in the cold a colourless solution; but on concentrating to a certain point, the solution acquires a deep blue tint: in this state the me- conin is wholly decomposed, and on dilution with water a chestnut matter falls, which is soluble in warm strong sulphuric acid, dkalies, alcohol, and ether, reproducing the green solution with the first, and a rose-red with the two last. Digested in nitric acid it yields a yellow solution, which yields by evaporation elongated crystals of the same colour. Chlorine transmitted over fused meconin gives rise to a fused mass of a reddish-yellow colour, con- sisting principally of chlorine in union with a new acid, which Couerbe has termed mechloic acid. Meconin is a constituent of opium, and is procured in the process for preparing narceia already described (page 528). By ether narceia is puri- fied in that process from narcotina, fat, and meconin; and on treating the ethereal extract with hot water the meconin is separated from narcotina and fat. It shodd be purified by a second crystdlization, a little animal char- cod being added, Some meconin is likewise present in the impure mass of morphia when precipitated by ammonia (page 525), and is taken up by the dcohol used in its purification. Meconin is present in very small quantity in opium, one pound containing about 2^ grains, so that it is hopeless to search for it except in large manufacturing operations. According to Couerbe, 100 parts of meconin contain 60.247 of carbon, 4.756 of hydrogen, and 34.997 of oxygen. Columbin.—This is a bitter crystalline principle, obtained by Al. Witt- stock from an alcoholic decoction of columbo root: the solution is concen- trated to about a third of its volume, and is then left in a warm place, when yellowish-brown crystals are gradudly deposited. It is purified in the usud manner by animal charcoal and solution in hot alcohol, from which it is afterwards obtained in colourless prismatic crystds. (Royal Inst. Jour- nd, N. S. i. 630.) Elatin.—This matter has been described by Mr. Hennell (R. Inst. Jour- nal, N. S. i. 532), and is prepared by forming an alcoholic decoction of elaterium, distilling off most of the alcohol, and setting aside the remainder for spontaneous evaporation. The residual mass consists of a green resin, in which the medical qualities of elaterium appear to reside, and a crystal- line matter: the former is readily taken up by sulphuric ether, and the lat ter left in a nearly pure state. It is deposited in colourless acicular tufts when its solution in hot alcohol is allowed to cool. In water it is nearly insoluble, and is very slightly dissolved by ether. It has neither acid nor alkaline properties, fuses at a heat between 300° and 400°, and has a bitter taste. According to the analysis of Hennell, 46 parts contain 17 of carbon, 11 of hydrogen, and 18 of oxygen. Elaterium contains 40 per cent, of elatin, and 21 per cent of the green resin, the remainder being ligneous fibre, earthy matter, and starch. Sinapisin.—A peculiar principle, called sulpho-sinapisin, sinapisin, or sinapin, has been extracted from mustard seed (sinapis alba) by MM. Henry, jun., and Garot who at first supposed it to be an acid, but have since cor- rected their mistake (Phil. Mag. and An. ix. 390). They believe it to con- tain the elements of sulphuret of cyanogen united with a peculiar organic matter from which the volatile oil of mustard may be developed. It is ob- tained by forming an aqueous decoction of mustard seed, adding subacetate of oxide of lead as long as a precipitate falls, removing the excess of that 576 SACCHARINE FERMENTATION. oxide by hydrosulphuric acid, and concentrating the filtered solution. The first crop of crystals is purified by a second crystallization. Pure sinapisin is white and inodorous, has a bitter taste, accompanied with a flavour of mustard, is more soluble in hot water or alcohol than when they are cold, and crystallizes in pearly needles or small prisms ar- ranged in tufts. Heated with hydrochloric acid it is decomposed, emitting an odour of hydrocyanic acid; and when distilled with sulphuric or phos- phoric acid, sulphocyanic acid is generated. By the fixed alkalies it is also decomposed: evaporated with potassa, the sulphocyanuret of potassium is generated, and a strong odour of the volatile oil of mustard may be per- ceived. With persalts of iron a solution of sinapisin strikes a deep red colour. The ultimate elements contained in 100 parts of sinapisin are 50.504 carbon, 7.795 hydrogen, 4.94 nitrogen, 9.657 sulphur, and 27.104 oxygen. SECTION VIII. SPONTANEOUS CHANGES OF VEGETABLE MATTER. Vegetable substances, for reasons already explained in the remarks in- troductory to the study of organic chemistry, are very liable to spontaneous decomposition. So long, indeed, as they remain in connexion with the living plant by which they were produced, the tendency of their elements to form new combinations is controlled; but as soon as the vital principle is extinct, of whose agency no satisfactory explanation can at present be afforded, they become subject to the unrestrained influence of chemical affinity. To the spontaneous changes which they then experience from the operation of this power, the term fermentation is applied. As might be expected from the difference in the constitution of different vegetable compounds, they are not all equally prone to fermentation ; nor is the nature of the change the same in all. Thus alcohol, the oxalic, acetic, and benzoic aeds, and probably the vegetable alkalies, may be kept for years without change, and some of them appear unalterable; while others, such as gluten, sugar, starch, and mucilaginous substances, are very liable to decomposition. In like manner, the spontaneous change sometimes tor- minates in the formation of sugar, at another time in that of alcohol, at a third in that of acetic acid, and at a fourth in the totd dissolution of the substance. This has led to the division of the fermentative processes into four distinct kinds, namely, the saccharine, vinous, acetous, and putrefactive fermentation. Saccharine Fermentation. The only substance known to be subject to the first kind of fermentation is starch. When gelatinous starch, or amidine, is kept in a moist state for a considerable length of time, a change gradually ensues, and a quantity of sugar, equal to about half the weight of the starch employed, is generated. Exposure to the atmosphere is not necessary to this change, but the quantity of sugar is increased by access of air. The germination of seeds, as exemplified in the mdting of barley, is like- wise an instance of the saccharine fermentation; but as it differs in some respects from the process above mentioned, being probably modified by the vitality of the germ, it may with greater propriety be discussed in the fol- lowing section. VINOUS FERMENTATION. 577 The ripening of fruit has also been regarded as an example of the saccha- rine fermentation, especidly since many fruits, of which the pear and apple are examples, if gathered before their maturity, ripen by keeping; and this view is contended for by M. Couverchel as an inference from his experi- mental inquiry on the maturation of fruits. (An. de Ch. et de Ph. xlvi. 147.) Proust, who examined the unripe grape in its different stages towards matu- rity, found that the green fruit contains a large quantity of free acid, chiefly the citric, which gradually disappears as the grape ripens, while its place is occupied by sugar. Couverchel examined the grape, peach, apricot, and pear. He found that the acid and mucilaginous matters of the fruit arc diminished, while carbonic acid, water, and sugar are generated: these changes he found to be independent of the oxygen of the air, and to occur whether the fruit is on the tree or removed from it: they arise from reaction among the ingredients of the fruit, rendered operative by heat, but indepen- dent of vitality. He considers the developement of sugar and disappearance of acid, wliich occur during the process of ripening, to be a change purely chemical. Vinous Fermentation. The conditions which are required for establishing the vinous fermentation are four in number; namely, the presence of sugar, water, yeast, or some ferment, and a certain temperature. The best mode of studying this pro- cess, so as to observe the phenomena and determine the nature of the change, is to place five parts of sugar with about twenty of water in a glass flask furnished with a bent tube, the extremity of which opens under an inverted jar full of water or mercury; and after adding a little yeast, to expose the mixture to a temperature of about 60° or 70°. In a short time bubbles of gas begin to collect in the vicinity of the yeast, and the liquid is soon put into brisk motion, in consequence of the formation and disengagement of a large quantity of gaseous matter; the solution becomes turbid, its tempera- ture rises, and froth collects upon its surface. After continuing for a few days, the evolution of gas begins to abate, and at lengtli ceases altogether; the impurities gradually subside, and leave the liquor clear and transparent The only appreciable changes which are found to have occurred during the process are the disappearance of the sugar, and the formation of dcohol, which remains in the flask, and of carbonic acid gas, which is collected in the pneumatic apparatus. A small portion of yeast is indeed decomposed; but the quantity is so minute that it may without inconvenience be left out of consideration. The yeast indeed appears to operate only in exciting the fermentation, without further contributing to the products. The atmospheric air, it is obvious, has no share in the phenomena, since it may be altogether excluded without affecting the result. The theory of the process is founded on the fact that the sugar, which disappears, is almost precisely equal to the united weights of the alcohol and carbonic acid; and hence the former is supposed to be resolved into the two latter. The mode in which this change is conceived to take place has been ably explained by Gay-Lussac, an expla- nation which will easily be understood by comparing the composition of sugar with that of alcohol. The elements of sugar, which consist of car- bon, hydrogen, and oxygen, in the ratio of one equivdent of each (page 535), are multiplied by six, in order to equalize the quantity of hydrogen contained in the two compounds. (An. de Ch. xcv. 317.) „ , Sugar. Alcohol. Carbonic acid. Carbon . . 36.72 6 eq. 24.48 4 eq. 12.24 2 eq. Hydrogen .6 6 eq. 6 6 eq. Oxygen . . 48 6 eq. 16 2 eq. 32 4 eq. °0.72 46.48 1 eq. 4454 2 eq It hence appears that 90.72 parts of sugar are capable of supplying 46.48 578 VINOUS FERMENTATION. of alcohol and 44.24 of carbonic acid, nearly equal weights, without any other products. It admits of doubt whether any substance besides sugar is capable of un- dergoing the vinous fermentation. The only other principle which is sup- posed to possess this property is starch, and this opinion chiefly rests on the two following facts. First, It is well known that potatoes, which contain but little sugar, yield a large quantity of alcohol by fermentation, during which the starch disappears. And, secondly, Al. Clement procured the same quantity of alcohol from equal weights of malted and un malted barley. (An. de Ch. et de Ph. v. 422.) Nothing conclusive can be inferred, however, from these data; for, from the facility with which starch is converted into sugar, it is probable that the saccharine may precede the vinous fermentation. This view is, indeed, justified by the practice of distillers, who do not ferment with unmalted barley only, but are obliged to mix with it a certain propor- tion of malt, which appears to act as a ferment to the unmalted grain. Though a solution of pure sugar is not susceptible of the vinous fermen- tation without being mixed with yeast, or some such ferment, yet the sac- charine juices of plants do not require the addition of that substance; or in other words, they contain some principle which, like yeast, excites the fer- mentative process. Thus, must or the juice of the grape ferments sponta- neously; but Gay-Lussac has observed that these juices cannot begin to ferment unless they are exposed to the air. By heating must to 212°, aDd then corking it carefully, the juice may be preserved without change; but if it be exposed to the air for a few seconds only, it absorbs oxygen, and fer- mentation takes place. From this it would appear that the must contains a principle which is convertible into yeast, or at least acquires the character- istic property of that substance, by absorbing oxygen. It appears from the experiments of Al. Colin, that various substances are capable of acting as a ferment. This property is possessed by gluten and vegetable albumen, caseous matter, albumen, fibrin, gelatin, blood, and urine. In general they act most efficaciously after the commencement of putrefac- tion ; and indeed exposure to oxygen gas seems equally necessary for ena- bling these substances to act as ferments, as to the principle contained in the juice of fruit. The various kinds of stimulating fluids, prepared by means of the vinous fermentation, are divisible into wines which are formed from the juices of saccharine fruits, and the various kinds of ale and beer produced from a decoction of the nutritive grains previously malted. The juice of the grape is superior, for the purpose of making wine, to that of all other fruits, not merely in containing a larger portion of saccharine matter, since this deficiency may be supplied artificially, but in the nature of its acid. The chief or only acidulous principle of the mature grape, ripened in a warm climate, such as Spain, Portugal, or Aladeira, is bitartrate of po- tassa. As this sdt is insoluble in alcohol, the greater part of it is deposited during the vinous fermentation; and an additional quantity subsides, con- stituting the crust, during the progress of wine towards its point of highest perfection. The juices of other fruits, on the contrary, such as the goose- berry or currant, contain malic and citric acids, which are soluble both in water and alcohol, and of which therefore they can never be deprived. Con- sequently these wines arc only rendered palatable by the presence of free sugar, which conceals the taste of the acid: and hence it is necessary to arrest the progress of fermentation long before the whole of the saccharine matter is consumed. For the same reason, these wines, unless made very sweet, do not admit of being long kept; for as soon as the free sugar is con- verted into alcohol by the slow fermentative process, which may be retarded by the addition of brandy, but cannot be prevented, the wine acquires a strong sour taste. Ale and beer differ from wine in containing a large quantity of mucilagi- nous and extractive matters derived from the malt with which they are made. From the presence of these substances they always contain a free acid, and ACETOUS FERMENTATION. 579 are greatly disposed to pass into the acetous fermentation. The sour taste is concealed partly by free sugar, and partly by the bitter flavoured the hop, the presence of which diminishes the tendency to the formation of an acid. The fermentative process which takes place in dough mixed with yeast, and on which depends the formation of good bread, has been supposed to be of a peculiar kind, and is sometimes designated by the name of panary fcr- mentation. The ingenious researches of Dr. Colquhoun, however, leave no doubt that the phenomena are to be ascribed to the saccharine matter ot the flour undergoing the vinous fermentation, by which it is resolved into alcohol and carbonic acid. (Brewster's Journal, vi.) Mr. Graham first procured alcohol by distillation from fermented dough, and a Company was form?d in London for collecting the spirit emitted by dough in the process of baking. Acetous Fermentation. When any liquid which has undergone the vinous fermentation, or even pure alcohol diluted with water, is mixed with yeast, and exposed in a warm place to the open air, an intestine movement speedily commences, heat is developed, the fluid becomes turbid from the deposition of a peculiar fila- mentous matter, and in general carbonic acid is disengaged. Oxygen is ab. sorbed from the atmosphere. These changes, after continuing a certain time, cease spontaneously; the liquor becomes clear, and instead of alcohol, it is now found to contain acetic acid. This process is cdled the acetous fermentation. The vinous may easily be made to terminate in the acetous fermenta- tion ; nay, the transition takes place so easily, that in many instances, in wliich it is important to prevent it, this is with difficulty effected. It is the uniform result, if the fermenting liquid be exposed to a warm tempera- ture and to the open air; and the means by which it is avoided is by ex- cluding the atmosphere, or by exposure to cold. For the acetous fermentation a certain degree of warmth is indispensable. It takes place tardily below 60° F.; at 50° it is very sluggish; and at 32°, or not quite so low, it is wholly arrested. It proceeds with vigour, on the contrary, when the thermometer ranges between 60° and 80°, and is even promoted by a temperature somewhat higher. The presence of water is likewise essentid; and a portion of yeast, or some analogous substance, by which the process may be established, must also be present The information contained in chemical works relative to the substances susceptible of the acetous fermentation is somewhat confused, a circum- stance which appears to have arisen from phenomena of a totally different nature being included under the same name. It seems necessary to dis- tinguish between the mere formation of acetic acid, and the acetous fer- mentation. Several or perhaps most vegetable substances yield acetic acid when they undergo spontaneous decomposition. Mucilaginous substances in particular, though excluded from the air, gradually become sour; and consistently with this fact, inferior kinds of ale and beer are known to ac- quire acidity in a short time, even when confined in well-corked bottles. In like manner, a solution of sugar, mixed with water, in which the gluten of wheat has fermented, and kept in close vessels, was found by Fourcroy and Vauquelin to yield acetic acid. All these processes, however, appear essen- tially different from the proper acetous fermentation above described, be- ing unattended with visible movement in the liquid, with absorption of oxy- gen, or disengagement of carbonic acid. The acetous fermentation, in this limited sense, consists in the conver- sion of alcohol into acetic acid. That this change does really take place is inferred, not only from the disappearance of alcohol and the simulta- neous production of acetic acid, but also from the quantity of the latter be. ing precisdy proportional to that of the former. The nature of the chemi- cd action requires to be elucidated by future researches. The production of carbonic acid was long considered an essential part of the change: but 580 PUTREFACTIVE FERMENTATION. as it is now known that the acetification of pure alcohol may occur with- out the formation of carbonic acid, it is probable that the appearance of that gas during the acetous fermentation of vinous liquors is referable to the si- multaneous evolution of alcohol from sugar contained in the solution. Look- ing to the composition of alcohol and acetic acid, and to the fact that dco- hoi may be acetified by atmospheric oxygen without any carbonic acid being formed, the most feasible theory is, that a certain portion of alcohol and oxygen are resolved into water and acetic acid, the proportions being leq. of alcohol 4C+6H-|-20 S 1 eq. of acetic acid 4C+3H+30 and 4 eq. of oxygen . . 40 ~ and 3 eq. ofwater 3(11+0). This requires, however, a more direct experimental proof than it has hitherto received. The acetous fermentation is conducted on a large scale for yielding the common vinegar of commerce. In France it is prepared by exposing weak wines to the air during warm weather; and in this country it is made from a solution of brown sugar or molasses, or an infusion of mdt. The vinegar thus obtained always contains a large quantity of mucilaginous and other vegetable matters, the presence of wliich renders it liable to several ulterior changes. Putrefactive Fermentation. By this term is implied a process which is not attended with the pheno- mena of the saccharine, vinous, or acetous fermentation, but during which the vegetable matter is completely decomposed. All proximate principles are not equally liable to this kind of dissolution. Those in which charcoal and hydrogen prevail, such as the oils, resins, and alcohol, do not undergo the putrefactive fermentation; nor do acids, which contain a considerable excess of oxygen, manifest a tendency to suffer this change. Those sub- stances alone are disposed to putrefy, the oxygen and hydrogen of which are in proportion to form water; and such, in particular, as contain nitro- gen. Among these, however, a singular difference is observable. Caffein evinces no tendency to spontaneous decomposition; while gluten, which certainly must contain a smaller proportional quantity of nitrogen, putre- fies with great facility. It is difficult to assign the precise cause of this difference; but it most probably depends partly upon the mode in which the ultimate elements of bodies are arranged, and partly on their cohesive power;—those substances, the texture of which is the most loose and soft, being, caleris paribus, the most liable to spontaneous decomposition. The conditions which are required for enabling the putrefactive process to take place, are moisture, air, and a certain temperature. The presence of a certain degree of moisture is absolutely necessary; and hence vegetable substances, which are disposed to putrefy under fa- vourable circumstances, may be preserved for an indefinite period if care- fully dried, and protected from humidity. Water acts apparently by soften- ing the texture, and thus counteracting the agency of cohesion ; and a part of the effect may also be owing to its affinity for some of the products of putrefaction. It is not likely that this liquid is actually decomposed, since water appears to be a uniform product. The air cannot be regarded as absolutely necessary, since putrefaction is found to be produced by the concurrence of the two other conditions only; but the process is without doubt materially promoted by free exposure to the atmosphere. Its operation is of course attributable to the oxygen com- bining with the carbon and hydrogen of the decaying substance. The temperature most favourable to the putrefactive process is between 60° and 100°. A strong heat is unfavourable, by expelling moisture; and a cold of 32°, at wliich water congeals, arrests its progress altogether. The GERMINATION. 581 mode in which caloric acts is the same as in all similar cases, namely, by tending to separate elements from one another which are already combined. The products of the putrefactive fermentation may be divided into the solid, liquid, and gaseous. The liquid are chiefly water, together with a little acetic acid, and probably oil. The gaseous products arc light car- buretted hydrogen, carbonic acid, and, when nitrogen is present, ammonia. Pure hydrogen, and probably nitrogen, are sometimes disengaged. I nun hydrogen and carbonic acid, according to Proust, are evolved from putrefy- ing gluten; and Saussure obtained the same gases from the putrefaction of wood in close vessels. Under ordinary circumstances, however, the chief gaseous product of decaying plants is light carburetted hydrogen, which is generated in great quantity at the bottom of stagnant pools during summer and autumn (page 245). Another elastic principle, supposed to arise from putrefying vegetable remains, is the noxious miasm of marshes. The origin of these miasms, however, is exceedingly obscure. Every at- tempt to obtain them in an insulated state has hitherto prove d abortive; and, therefore, if they are really a distinct species of matter, they must be regarded, like the effluvia of contagious fevers, as of too subtile a nature for being subjected to chemical analysis. When the decay of leaves or other parts of plants has proceeded so far that all trace of organization is effaced, a dark pulverulent substance re- mains, consisting of charcoal combined with a little oxygen and hydrogen. This compound is vegetable mould, which, when mixed with a proper quantity of earth, constitutes the soil necessary to the growth of plants. Saussure, in his excellent Recherches Chimiques sur la Vegetation, has de- scribed vegetable mould as a substance of uniform composition; and on heating it to redness in close vessels, he procured carburetted hydrogen and carbonic acid gases, water holding acetate or carbonate of ammonia in solu- tion, a minute quantity of empyreumatic oil, and a large residue of charcoal mixed with saline and earthy ingredients. On exposing yegetable mould to the action of light, air, and moisture, a chemical change ensues, the effect of which is to render a'portion of it soluble in water, and thus applicable to the nutrition and growth of plants. SECTION IX. CHEMICAL PHENOA1ENA OF GERMINATION AND VEGETATION. Germination. Germination is the process by which a new plant originates from seed. A seed consists essentially of two parts, the germ of the future plant, en- dowed with a principle of vitality, and the cotyledons or seed-lobes, both of which are enveloped in a common covering of cuticle. In the germ two parts, the radicle and plumula, may be distinguished, the former of which is destined to descend into the earth and constitute the root, the latter to rise into the air and form the stem of the plant. The office of the seed-lobes is to afford nourishment to the young plant, until its organization is so far ad- vanced, that it may draw materials for its growth from extraneous sources. For this reason seeds are composed of highly nutritious ingredients. The chief constituent of most of them is starch, in addition to which they fre- quently contain gluten, gum, vegetable albumen or curd, and sugar. The conditions necessary to germination are three-fold; namely, moisture 49* 582 GERMINATION. a certain temperature, and the presence of oxygen gas. The necessity at moisture to this process has been proved by extensive observation. It is well known that the concurrence of other conditions cannot enable seeds to germinate provided they are kept quite dry. A certain degree of warmth is not less essential than moisture. Germi- nation cannot take place at 32° ; and a strong heat, such as that of boiling water, prevents it altogether, by depriving the germ of the vital principle. The most favourable temperature ranges from 60° to 80°, the precise degree varying with the nature of the plant, a circumstance that accounts for the difference in the season of the year at which different seeds begin to eer- minate. 6 & That the presence of air is necessary to germination was demonstrated by several philosophers, such as Ray, Boyle, Muschenbroeck and Boerhaave, before the chemical nature of the atmosphere was discovered; and Scheele, soon after the discovery of oxygen, proved that beans do not germinate without exposure to that gas. Achard afterwards demonstrated the same fact with respect to seeds in general, and his experiments have been fully confirmed by subsequent observers. It has even been shown by Humboldt, that a dilute solution of chlorine, owing to the tendency of that gas to de- compose water and set oxygen at liberty, promotes the germination of seeds. These circumstances account for the fact that seeds, when buried deep in the earth, are unable to germinate. It is remarkable that the influence of light, which is so favourable to all the subsequent stages of vegetation, is injurious to the process of germina- tion. Ingenhousz and Sennebier have proved that a seed germinates more rapidly in the shade than in light, and in diffused daylight quicker than when exposed to the direct solar rays. From the preceding remarks it is apparent that when a seed is placed an inch or two under the surface of the ground in spring, and is loosely cover- ed with earth, it is in a state every way conducive to germination. The ground is warmed by absorbing the solar rays, and is moistened by occa- sional showers; the earth at the same time protects the seed from light, but by its porosity gives free access to the air. The operation of malting barley, in which the grain is made to germi- nate by exposure to warmth, air, and humidity, affords the best means of studying the phenomena of germination. In preparing malt, the grain passes through four distinct stages, called steeping, couching, flooring, and kiln-drying. In the first it is steeped in water for about two days, when it absorbs moisture, softens, and swells considerably. It is then removed to the couch-frame, where it is laid in heaps 30 inches in depth for from 26 to 30 hours. In this situation the grain becomes warm and acquires a dispo- sition to germinate; but as the temperature, in such large heaps, would rise very unequally, and germination consequently be rapid in some portions and slow in others, the process of flooring is employed. This consists in laying the grain in strata a few inches thick on large airy but shaded floors, where it remains for about 12 or 14 days until germination has advanced to the extent desired by the malstcr. During this interval the grain is fre- quently turned, in order that the temperature of the whole mass should be uniform, that each grain should be duly exposed to the air, and that the radicles of contiguous grains should not become entangled with each other. As soon as saccharine matter is freely developed, germination must be ar- rested ; since otherwise, being taken up as nutriment by the young plant, it would speedily disappear. Accordingly, the grain is removed to the kiln, where it is exposed to a temperature gradually rising from 100° to 160°, or rather higher; the object being, first, to dry the grain completely, and then to provide against any recurrence of germination by destroying the vitality of the plant. The most convenient mode of applying the heat is to place the grain on a metallic net-work, through wliich passes hot air issuing from a fire made with good coke. The process of malting is not conducted during summer, because in hot weather the grain is apt to become mouldy. GROWTH OF PLANTS. The difference between malted and unmalted barley is readily perceived by the taste; but it will be more correctly appreciated by inspecting the re- sult of Proust's comparative andysis of malted and unmdted barley. (An. de Ch. et de Ph. v.) In 100 In 100 parts of barley. parts of malt . . 1 .... 1 4 ... 15 : : s .... 15 3 ..-• 1 . 32 .... 56 . .55 .... 12 Resin Gum Sugar Gluten Starch Hordein It is hence apparen into starch, gum, and that, during germination, the hordein is converted u ..ugar; so that from an insoluble material, which could not in that state be applied to the uses of the young plant, two soluble and highly nutritive principles result, which by being dissolved in water are readily absorbed by the radicle. The chemical changes which take place during germination have been ably investigated by Saussure, whose experiments are detailed in the work to which I have already referred. The leading facts which he determined are the following:—that oxygen gas is consumed, that carbonic acid is evolved, and that the volume of the latter is precisely equal to that of the former. Now since carbonic acid gas contains its own volume of oxygen, it follows that this gas must have united exclusively with carbon. It is likewise obvious that the grain must weigh less after than before germina- tion, provided it is brought to the same state of dryness in both instances. Saussure indeed found that the loss is greater than can be accounted for by the carbon of the carbonic acid which is evolved; and hence he concluded that a portion of water, generated at the expense of the grain itself, is dis- sipated in drying. According to Proust, the diminution in weight is about a third; but Dr. Thomson affirms that in fifty processes, conducted on a large scale under his inspection, the average loss did not exceed one-fifth. Growth of Plants. While a plant differs from an animal in exhibiting no signs of perception or voluntary motion, and in possessing no stomach to serve as a receptacle for its food, there exists between them a close analogy both of parts and functions, which, though not discerned at first, becomes striking on a near examination. The stem and branches act as a frame-work or skeleton for the support and protection of the parts necessary to the life of the individual The root serves the purpose of a stomach by imbibing nutritious juices from the soil, and thus supplying the plant with materids for its growth. The sap or circulating fluid, composed of water holding in solution saline, extractive, mucilaginous, saccharine, and other soluble substances, rises up- wards through the wood in a distinct system of tubes called the common vessels, which correspond in their office to the lacteals and pulmonary arte- ries of animals, and are distributed in minute ramifications over the surface of the leaves. In its passage through this organ, which may be termed the lungs of a plant, the sap is fully exposed to the agency of light and air, ex- periences a change by which it is more completely adapted to the wants of the vegetable economy, and then descends through the inner layer of the bark in another system of tubes called the proper vessels, yielding in its course all the juices and principles peculiar to the plant. The chemical changes which take place during the circulation of the sap are in general of such a complicated nature, and so much under the control of the vital principle, as to elude the sagacity of the chemist One part of the subject, however, namely, the reciprocal agency of the atmosphere and 584 GROWTH OF PLANTS. growing vegetables on each other, falls within the reach of chemical inqdry, and has accordingly been investigated by several philosophers. For the leading facts relative to what is called the respiration of plants, or the chemical changes which the leaves of growing vegetables produce on the atmosphere, we are indebted to Priestley and Ingenhousz, the former of whom discovered that plants absorb carbonic acid from the air under certain rarcumstances, and emit oxygen in return; and the latter ascertained that this change occurs only during exposure to the direct rays of the sun. When a healthy plant, the roots of which are supplied with proper nourish- ment, is exposed to the direct solar beams in a given quantity of atmo- spheric air, the carbonic acid after a certain interval is removed, and an equal volume of oxygen is substituted for it. If a fresh portion of carbonic acid is supplied, the same result will ensue. In like manner, Sennebier and Woodhouse observed, that when the leaves of a plant are immersed in water, and exposed to the rays of the sun, oxygen gas is disengaged. That the evolution of oxygen in this experiment is accompanied with a proportional absorption of carbonic acid, is proved by employing water deprived of car- bonic acid by boiling, in which case little or no oxygen is procured. Such are the changes induced by plants when exposed to sunshine; but in the dark an opposite effect ensues. Carbonic acid gas is not absorbed under these circumstances, nor is oxygen gas evolved; but, on the contrary, oxygen disappears, and carbonic acid gas is evolved. In the dark, therefore, vegetables deteriorate rather than purify the air, producing the same effect as the respiration of animals. The cause of these opposite effects has been lately discussed by Professor Burnet, who has offered an ingenious explanation, supported by experi- ments wliich appear to me satisfactory. (R. Inst. Journ. N. S. i. 83.) He considers that the influence of vegetation on the atmosphere is owing not to one but to two functions, digestion and respiration: the latter is believed to proceed at all times as in animals without intermission, and its uniform effect is the production of carbonic acid; while the former takes place only under the influence of light, and gives rise to evolution of oxygen gas, and the abstraction of carbonic acid. A plant, exposed to sunshine, purifies the air, by absorbing carbonic acid from the atmosphere, as well as that emitted by its own respiration, and emits oxygen gas in return. In the dark, diges- tion is at a stand, and respiration continuing without intermission, carbonic acid accumulates. From several of the preceding facts, it is supposed that the oxygen emit- ted by plants while under the influence of light is derived from the carbonic acid which they absorb, and that the carbon of that gas is applied to the purposes of nutrition. Consistently with this view it has been observed that plants do not thrive when kept in an atmosphere of pure oxygen; and it was found by Dr. Percival and Air. Henry, that the presence of a little car- bonic acid is even favourable to their growth. Saussure, who examined this subject minutely, ascertained that plants grow better in an atmosphere which contains about one-twelfth of carbonic acid than in common air, pro- vided they are exposed to sunshine; but if that gas be present in a greater proportion, its influence is prejudicial. In an atmosphere consisting of one- half of its volume of carbonic acid, the plants perished in seven days; and they did not vegetate at all when that gas was in the proportion of two- thirds. In the shade, the presence of carbonic acid is always detrimental. He likewise observed that the presence of oxygen is necessary, in order that a plant should derive benefit from admixture with carbonic acid. Saussure is of opinion that plants derive a large quantity of their carbon from the carbonic acid of the atmosphere, an opinion which receives great weight from the two following comparative experiments. On causing a plant to vegetate in pure water, supplied with common air and exposed to light, the carbon of the plant increased in quantity ; but when supplied with common air in a dark situation, it even lost a portion of the carbon which it had previously possessed- FOOD OF PLANTS. 585 Light is necessary to the colour of plants. The experiments of Sennebier and Mr. Gough have shown that the green colour of the leaves is not de- veloped, except when they are in a situation to absorb oxygen and give out carbonic acid. .... .. Though the experiments of different philosophers agree as to the in- fluence of vegetation on the air in sunshine and during the night, very dif- ferent opinions have been expressed both as to the phenomena occasioned by diffused daylight and concerning the total effect produced by plants on the constitution of the atmosphere. Priestley found that air, vitiated by combustion or the respiration of animals, and left in contact for several days and nights with a sprig of mint, was gradually restored to Us original purity; and hence he inferred that the oxygen gas, consumed during these and various other processes, is restored to the mass of the atmosphere by the agency of growing vegetables. This doctrine was confirmed by the re- searches of Ingenhousz and Saussure, who found that the quantity of oxy- gen evolved from plants by day exceeds that of carbonic acid emitted during the night; and Davy arrived at the same conclusions as Priestley. But an opposite opinion has been supported by Mr. Ellis, who, from an extensive series of experiments, contrived with much sagacity, inferred that growing plants give out oxygen only in direct sunshine, while at other times they absorb it; that when exposed to the ordinary vicissitudes of sunshine and shade, light and darkness, they form more carbonic acid in the period of a day and night than they destroy; and, consequently, that the general effect of vegetation on the atmosphere is the same as that produced by animals. (Ellis's Researches and Farther Inquiries on Vegetation, &c.) The recent experiments of Dr. Daubeny appear decisive of this question. He has convinced himself that in fine weather a plant consisting chiefly of leaves and stems, if confined in the same portion of air night and day, and duly supplied with carbonic acid gas during the sunshine, will go on adding to the proportion of oxygen present, so long as it continues healthy, at least up to a certain point, the slight diminution of oxygen and increase of car- bonic acid which takes place during the night, bearing no considerable ratio to the degree in which the opposite effect occurs by day. He accounts for the discordance between his own results and those of -Mr. Ellis, by his hav- ing carefully removed the plants from the experimenting jar immediately they began to suffer from the heat or confinement, and conducted the expe- riments on a larger and more suitable scde. (Reports of the British Asso- ciation for 1834, 436.) Food of Plants. The chief source from which plants derive the materials for their growth is the soil. However various the composition of the soil, it consists essen- tially of two parts, so far as its solid constituents are concerned. One is a certain quantity of earthy matters, such as siliceous earth, clay, lime, and sometimes magnesia; and the other is formed from the remains of animal and vegetable substances, which, when mixed with the former, constitute common mould. A mixture of this kind, moistened by rain, affords the pro- per nourishment of plants. The water, percolating through the mould, dis- solves the soluble salts with which it comes in contact, together with the gaseous, extractive, and other matters which are formed during the decom- position of the animal and vegetable remains. In this state it is readily ab- sorbed by the roots, and conveyed as sap to the leaves, where it undergoes a process of assimilation. But though this is the natural process by which plants obtain the greater part of their nourishment, and without which they do not arrive at perfect maturity, they may live, grow, and even increase in weight, when wholly deprived of nutrition from this source. Thus in the experiment of Saussure, already described, sprigs of peppermint were found to vegetate in distilled water; and it is well known that many plants grow when merely suspended 586 FOOD OF PLANTS. in the air. In the hot-houses of the botanical garden of Edinburgh, for ex- ample, there are two plants, species of the fig-tree, the Ficus australis and Ficus elastica, the latter of which, as Dr. Graham informs me, has been suspended for ten, and the former for nearly sixteen years, during which time they have continued to send out shoots and leaves. Before scientific men had learned to appreciate the influence of atmo- spheric air on vegetation, the increase of carbonaceous matter, which occurs in soane of these instances, was supposed to be derived from water, an opinion naturally suggested by the important offices performed by this fluid in the vegetable economy. Without water plants speedily wither and die. It gives the soft parts that degree of succulence necessary for the perform- ance of their functions;—it affords two elements, oxygen and hydrogen, which either as water, or under some other form, are contained in all vege- table products;—and, lastly, the roots absorb from the soil those substances only, which are dissolved or suspended in water. So carefully, indeed, has nature provided against the chance of deficient moisture, that the leaves are endowed with a property both of absorbing aqueous vapour directly from the atmosphere, and of lowering their temperature during the night by ra- diation so as to cause a deposition of dew upon their surface, in conse- quence of which, during the driest seasons and in the warmest climates, they frequently continue to convey this fluid to the plant, when it can no longer be obtained in sufficient quantity from the soil. But necessary as water is to vegetable life, it cannot yield to plants a principle which it does not possess. The carbonaceous matter which accumulates in plants, under the circumstances above mentioned, may, with every appearance of justice, be referred to the atmosphere; since we know that carbonic acid exists there, and that growing vegetables have the property of taking carbon from that gas. When plants are incinerated, their ashes are found to contain sdine and earthy matters, the elements of wliich, if not the compounds themselves, are supposed to be derived from the soil. Such at least is the view deducible from the researches of Saussure, and which might have been anticipated by reasoning on chemical principles. The experiments of M. Schrader, how- ever, lead to a different conclusion. He sowed several kinds of grain, such as barley, wheat, rye, and oats, in pure flowers of sulphur, and supplied the shoots as they grew with nothing but dr, light, and distilled water. On in- cinerating the plants, thus treated, they yielded a greater quantity of saline and earthy matters than were originally present in the seeds. These results, supposing them accurate, may be accounted for in two ways. It may be supposed, in the first place, that the foreign matters were introduced accidentally from extraneous sources, as by fine particles of dust floating in the atmosphere; or, secondly, it may be conceived, that they were derived from the sulphur, air, and water, with which the plants were sup- plied. If the latter opinion be adopted, we must infer either that the vital principle, which certainly controls chemical affinity in a surprising manner, and directs this power in the production of new compounds from elementary bodies, may likewise convert one element into another; or that some of the substances, supposed by chemists to be simple, such as oxygen and hydro- gen, are compounds, not of two, but of a variety of different principles. As these conjectures are without foundation, and are utterly at variance with the facts and principles of the science, I do not hesitate in adopting the more probable opinion, that the experiments of M. Schrader were influenced by some source of error which escaped detection. 587 ANIMAL CHEMISTRY. All distinct compounds, which are derived from the bodies of animals, are called proximate animal principles. They are distinguished front inor- trade matter by the characters stated in the introduction to organic chcmis- try The circumstances which serve to distinguish them from vegetable matter are, the presence of nitrogen, their strong tendency to putrefy, and the hidily offensive products to which their spontaneous decomposition gives rise, "it should be remembered, however, that nitrogen is likewise a consti- tuent of many vegetable substances; though few of these, the vegeto-ammal principles excepted (page 568), are prone to suffer the putrefactive fermen- tation. It is likewise remarkable that some compounds of animal origin, such as cholesterine and the oils, do not contain nitrogen as one of their elements, and are not disposed to putrefy. The essentid constituents of animal compounds are carbon, hydrogen, oxygen, and nitrogen, besides which some of them contain phosphorus, sul- phur, iron, and earthy and saline matters in small quantity. Owing to the presence of sulphur and phosphorus, the process of putrefaction, which will be particularly described hereafter, is frequently attended with the disengage- ment of hydrosulphuric acid and phosphuretted hydrogen gases. When heated in close vessels, they yield water, carbonic oxide, carburetted hydro- gen, probably free nitrogen and hydrogen, carbonate and hydroeyanate of ammonia, and a peculiarly fetid thick oil. The carbonaceous matter left in the retort is less easily burned, and is more effectual as a decolorizing agent, than charcoal derived from vegetable matter. The principle of the method of analyzing animal substances has already been mentioned (page 495). In describing the proximate animal principles, the number of which is far less considerable than that of vegetable compounds, the arrangement sug- gested by Gay-Lussac and Thenard in their Recherches Physico-Chimiques, and followed by Thenard in his System of Chemistry, has been adopted. The animal compounds are accordingly arranged in three sections. The first contains substances which are neither acid nor oleaginous; the second comprehends the animal acids; and the third includes the animd oils and fats. Several of the principles belonging to the first division, such as fibrin, albumen, gelatin, caseous matter, and urea, were shown by Gay-Lussac and Thenard to have several points of similarity in their composition. They all contain, for example, a large quantity of carbon, and their hydrogen is in such proportion as to convert all their oxygen into water, and their nitrogen into ammonia. No general laws have been established relative to the con- stitution of the compounds comprised in the other sections. PROXIMATE ANIMAL SUBSTANCES. SECTION I. SUBSTANCES WHICH ARE NEITHER ACID NOR OLEAGINOUS. Fibrin. Fibrin enters largely into the composition of the blood, and is the basis of the muscles; it may be regarded, therefore, as one of the most abundant of the animal principles. It is most conveniently procured by stirring recently drawn blood with a stick during its coagulation, and then washing the ad 588 ALBUMEN. hering fibres with water until they are perfectly white. It may also be ob- tained from lean beef cut into small slices, the soluble parts being removed by digestion in several successive portions of water. Fibrin is solid, white, insipid, and inodorous. When moist it is somewhat elastic, but on drying, it becomes hard, brittle, and semi-transparent. In a moist warm situation it readily putrefies. It is insoluble in water at com- mon temperatures, and is dissolved in very minute quantity by the continued action of boiling water. Alcohol, of specific gravity 0.81, converts it into a fatty adipocirous matter, which is soluble in alcohol and ether, but is preci- pitated by water. The action of acids on fibrin has been particularly described by Berzelius. (Medico-Chir. Trans, iii. 201.) Digested in concentrated acetic acid, fibrin swells and becomes a bulky tremulous jelly, which dissolves completely, with disengagement of a little nitrogen, in a considerable quantity of hot water. By the action of nitric acid of specific gravity 1.25, aided by heat, on fibrin, a yellow solution is formed with disengagement of a large quantity of nearly pure nitrogen, in which Berzelius could not detect the least trace of binoxide of nitrogen. After digestion for 24 hours, a pale yellow pulverulent substance is deposited, which Fourcroy and Vauquelin described as a new acid under the name of yellow acid. According to Berzelius, however, it is a compound of modified fibrin and nitric acid, together with some malic and nitrous acids. It likewise contains some fatty matter, which may be re- moved by alcohol. The origin of the nitrogen which is disengaged in the beginning of the process is somewhat obscure. From the total absence of binoxide of nitrogen, it is probable that in the early stages very little, if any, of the nitric acid is decomposed, and that the nitrogen gas is solely or chiefly derived from the fibrin. Dilute hydrochloric acid hardens without dissolving fibrin, and the strong acid decomposes it. The action of sulphuric acid, according to Braconnot, is very peculiar. When fibrin is mixed with its own weight of concentrated sulphuric acid, a perfect solution ensues, without change of colour, or disen- gagement of sulphurous acid. On diluting with water, boiling for nine hours, and separating the acid by means of chalk, the filtered solution was found to contain a peculiar white matter, to which Braconnot has applied the name of leucine. (An. de Ch. et de Ph. xiii.) Digested in strong sul- phuric acid, a dark reddish-brown, nearly black, solution is formed, and the fibrin is carbonized and decomposed. Fibrin is dissolved by pure potassa, and is thrown down when the solution is neutralized. The fibrin thus precipitated, however, is partially changed, since it is no longer soluble in acetic acid. It is soluble likewise in am- monia. The following is a tabular view of the composition in 100 parts of fibrin, albumen, gelatin, and urea:— Carbon. Hydrog. Nitrogen. Oxygen. Fibrin . . 53.36 7.02 19.934 19.685 Gay-L. and Thenard. .,, S 52.883 7.54 15.705 23.872 Ditto. Albumen < 5Q ? 7g 15_5- 2667 prout Gelatin . . 47.881 7.914 16.998 27.207 Gay-L. and Thenard. Urea . . . 19.99 6.66 46.66 26.66 Prout. Albumen. Albumen enters largely into the composition both of animal fluids and solids. Dissolved in water it forms an essential constituent of the serum of the blood, the liquor of the serous cavities, and the fluid of dropsy; and in a solid state it is contained in several of the textures of the body, such as the cellular membrane, the skin, glands, and vessels. From this it appears that albumen exists under two forms, liquid and solid. ALBUMEN. 589 Liqdd albumen is best procured from the white of eggs, which consists dmost solely of this principle, united with water and free soda, and mixed with a small quantity of saline matter. In this state it is a thick glairy fluid, insipid, inodorous, and easily miscible with cold water, in a sufficient quantity of which it is completely dissolved. When exposed in thin layers to a current of air it dries, and becomes a solid and transparent substance, which retains its solubility in water, and may be preserved for any length ot time without change; but if kept in its fluid condition it readily putrefies. From the free soda which they contain, albuminous liquids have always an alkaline reaction. Liquid albumen is coagulated by heat alcohol, and the stronger acids. Undiluted albumen is coagulated by a temperature of 160-, and when diluted with water at 212° F. Water which contains only 1-lOOOth of its weight of albumen is rendered opaque by boiling. (Bostock.) On this pro- perty is founded the method of clarifying by means of albuminous solu- tions ; for the albumen being coagulated by heat, entangles in its substance all the foreign particles which are not actually dissolved, and carries them with it to the surface of the liquid. The character of being coagulated by hot water distinguishes albumen from all other animal fluids. The acids differ in their action on albumen. The sulphuric, hydrochlo- ric, nitric, and tannic acids coagulate it; and in each case, some of the acid is retained by the albumen. It is precipitated also by metaphosphoric acid.^ but not by the phosphoric or pyrophosphoric (page 204). The solution of albumen is not precipitated at all by acetic acid. By maceration in dilute nitric acid for a month, it is converted, according to Mr. Hatchett, into a substance soluble in hot water, and possessed of the leading properties of gelatin. Digested in strong sulphuric acid, the coagulum is dissolved, and a dark solution is formed similar to that produced by the same acid on fibrin; but if the heat be applied very cautiously, the liquid assumes a beau- tiful red colour. This property was discovered some years ago by Dr. Hope, who informs me that the experiment does not always succeed, the result being influenced by very slight causes. Albumen is precipitated by several reagents, especially by metallic salts. Of these the most delicate as a test is corrosive sublimate, which causes a milkiness when the albumen is diluted with 2000 parts of water. The pre- cipitate, as stated at page 3.98, is generally considered as a compound of calomel and albumen; but a late analysis by Rose has proved that it con- sists of oxide of mercury and albumen (Pog. Ann. xxviii. 132). Other metallic solutions, such as the chlorides of tin and iron, subacetate of oxide of lead, and the sulphates of alumina and oxide of copper, also precipitate albumen; the precipitate in every case consists, according to Rose, of a metallic oxide united with albumen. All these precipitates, not excepting that from corrosive sublimate, dissolve in an excess of albumen, and most of them are soluble in an excess of the metallic salt. Ferrocyanuret of potassium is a test for albumen equally delicate as corrosive sublimate, if not more so, provided acetic acid is previously added to neutralize the alkali. When an albuminous liquid is exposed to the agency of galvanism, pure soda makes its appearance at the negative wire, and the albumen coagulates around that which is in connexion with the positive pole of the battery. Mr. Brande,* who first observed this phenomenon, ascribes it to the separa- tion of free soda, upon which he supposes the solubility of albumen in water to depend; but Lassaignet attributes it to the decomposition of chlo- ride of sodium and the developement of acid, which coagulates the albumen. However this may be, galvanism is one of the most elegant and delicate tests which we possess of the presence of albumen in animal fluids. Chemists are not agreed as to the cause of the coagulation of albumen by Philosophical Transactions for 1809. t An. de Ch. et de Fh vol 50 590 GELATIN. alcohol and heat. The explanation usually given is that proposed by Dr. Thomson, who ascribes the solubility of albumen to the presence of free soda, and its coagulation to the removal of the alkali. To this hypothesis Dr. Bostock objects, and with justice, that albuminous liquids do not contain a sufficient quantity of free alkali for the purpose (Medico-Chir. Trans, ii. 175). Were I to hazard an opinion on this subject, it would be the follow- ing:—that albumen combines directly with water at the moment of being secreted, at a time when its particles are in a state of minute division; but as its affinity for that liquid is very feeble, the compound is decomposed by slight causes, and the albumen thereby rendered quite insoluble. Silicic acid affords an instance of a similar phenomenon. (Page 325.) _ Albumen coagulates without appearing to undergo any change of compo- sition, but it is quite insoluble in water, and is less liable to putrefy than in its liquid state. It is dissolved by alkalies with disengagement of ammo- nia, and is precipitated from its solution by acids. In the coagulated state it bears a very close resemblance to fibrin, and is with difficulty distinguish- ed from it. Alcohol, ether, acids, and dkalies, according to Berzelius, act upon each in the same manner. He observes, however, that acetic acid and ammonia dissolve fibrin more easily than coagulated albumen. According to Thenard, they are readily distinguished by means of binoxide of hydro- gen, from which fibrin causes evolution of oxygen, while albumen has no action upon it Gelatin. Gelatin exists abundantly in many of the solid parts of the body, espe- cially in the skin, cartilages, tendons, membranes, and bones. According to Berzelius, it is not contained in any of the healthy animal fluids; and Dr. Bostock, with respect to the blood, has demonstrated the accuracy of this statement. (Medico-Chir. Trans, vol. i. and ii.) Gelatin is distinguished from all animal principles by its ready solubility in boiling water, and by the solution forming a bulky, semitransparent, tremulous jelly as it cools. Its tendency to gelatinize is such, that one part of gelatin, dissolved in 100 parts of water, becomes solid in cooling. This jelly is a hydrate of gelatin, and contains so much water, that it readily liquefies when warmed. On expelling the water by a gentle heat, a brittle mass is left, which retains its solubility in hot water, and may be preserved for any length of time without change. Jelly, on the contrary, soon he- roines acid by keeping, and then putrefies. The common gelatin of commerce is the well-known cement called glue, which is prepared by boiling cuttings of parchment, or the skins, ears, and hoofs of animals, and evaporating the solution: it may also be prepared from bones. Isinglass, which is the purest variety of gelatin, is prepared from the sounds of fish of the genus acipenser especially from the sturgeon. The animal jelly of the confectioners is made from the feet of calves, the tendinous and ligamentous parts of which yield a large quantity of gelatin. Gelatin is insoluble in alcohol, but is dissolved readily by most of the diluted acids, which form an excellent solvent for it. Mixed with twice its weight of concentrated sulphuric acid.it dissolves without being charred; and&on diluting the solution with water, boiling for sevdal hours, separating the acid by means of chalk, and evaporating the filtered liquid, a peculiar saccharine principle is deposited in crystals. This substance has a sweet taste, somewhat like that of the sugar of grapes, is soluble in water, though less so than common sugar, and is insoluble in alcohol. When heated to redness, it yields ammonia as one of the products, a circumstance which shows that it contains nitrogen. Mixed with yeast, its solution does not undergo the vinous fermentation; and it combines directly with nitric acid. It is hence apparent that, though possessed of a sweet taste, it differs en- tirely from sugar. This substance was discovered by Braconnot. (An. de Ch. et de Ph. vol. xiii.) UREA. Gelatin is dissolved by the liquid alkalies, and the solution is not precipi- tated by acids. . • , „- ., ^ Gelatin manifests little tendency to unite with metallic oxides. Corrosive sublimate and subacetate of oxide of lead do not occasion any precipitate in a solution of gelatin, and the salts of tin and silver affect it very slightly. The best precipitant for it is tannic acid. By means of an infusion of gall- nuts, Dr. Bostock detected the presence of gelatin when mixed with oOOO times its weight of water; and its quantity may even be estimated approx- imately by this reagent (page 515). But since other animal substances as for example albumen, are precipitated by tannic acid, it cannot be relied on as a test of gelatin. The best character for this substance is that of solu- bility in hot water, and of forming a jelly as it cools. Urea. Urea is procured by evaporating fresh urine to the consistence of a syrup, and then gradually adding to it, when quite cold, pure concentrated nitric acid, which should be free from nitrous acid, till the whole becomes a dark- coloured crystallized mass, which is to be repeatedly washed with ice-cold water, and then dried by pressure between folds of bibulous paper. To the nitrate of urea, thus procured, a pretty strong solution of carbonate of potassa is added, until the acid is neutralized; and the solution is afterwards concentrated by evaporation, and set aside, in order that the nitre may sepa- rate in crystals. Dr. Prout recommends that the residual liquid, which is an impure solution of urea, should be made up into a thin paste with animal charcoal, and be allowed to remain in that state for a few hours. The paste is then mixed with cold water, which takes up the urea, while the colouring matter is retained by the charcoal; and the colourless solution is evaporated to dryness at a low temperature. The residue is then boiled in pure alco- hol, by which the urea is dissolved, and from which it is deposited in crys- tals on cooling. (Medico-Chir. Trans, viii. 529.) In order to obtain them quite colourless, it is necessary to redissolve in alcohol, and crystallize a second or even a third time. The crvtals of pure urea are transparent and colourless, of a slight pearly lustre, and have commonly the form of a four-sided prism. It leaves a sensation of coldness on the tongue like nitre, and its smell is faint and peculiar, but not urinous. Its specific gravity is about 1.35. It does not affect the colour of litmus or turmeric paper. In a moist atmosphere it deliquesces slightly; but otherwise it undergoes no change on exposure to the air. (Prout.) It is fused at 248°, and at a rather higher temperature it is de- composed, being resolved chiefly into carbonate of ammonia and cyanuric acid, the latter of which, if the heat be not incautiously raised, is left in the retort. (Page 273.) Water at 60° dissolves more than its own weight of urea, and boiling wa- ter takes up an unlimited quantity. It requires for solution about five times its weight of alcohol of specific gravity 0.816 at 60°, and rather less than its own weight at a boiling temperature. The aqueous solution of pure urea may be exposed to the atmosphere for several months, or be heated to the boiling point, without change; but, on the contrary, if the other con- stituents of urine are present, it putrefies with rapidity, and is decomposed by a temperature of 212°, being almost entirely resolved into carbonate of ammonia by continued ebullition. The pure fixed alkalies and alkaline earths decompose urea, especially by the aid of heat, carbonate of ammonia being the chief product. Though urea has not any distinct alkaline properties, it unites with the nitric and oxalic acids, forming sparingly soluble compounds, which crys- tallize in scales of a pearly lustre. This property affords an excellent test of the presence of urea. Both compounds have an acid reaction, and the nitrate consists of 54.15 parts or one equivalent of nitric acid, and 60.54 parts or one eqdvalent of urea. 592 SUGAR OF MILK.--SUGAR OF DIABETES. Urea has been carefully analyzed by Dr. Prout, and the accuracy of his analysis is amply attested by the late researches of Liebig and Wohler. Its equivalent is 60.54; and its elements, as already explained (page 271), are in precisely the same ratio as the hydrated cyanate of ammonia. A singular instance of the artificial production of urea has been noticed by Wohler. It is formed by the action of ammonia on cyanogen, as also by direct contact of cyanic acid and ammonia; but the best mode of pre- paring it is by decomposing cyanate of oxide of silver with hydrochlorate of ammonia, or cyanate of oxide of lead with ammonia, the action being promoted by a gentle heat In the last case, oxide of lead is set free, and the only other product appears in colourless, transparent, four.sided, rect- angular crystals. These crystals, judging by the mode of preparation, must be cyanate of ammonia; but yet no ammonia is evolved from them by the action of potassa: the stronger acids do not, as with other cyanates, cause an evolution of carbonic and cyanic acids; nor do they yield precipitates with salts of lead and silver. In fact, though procured by the mutual ac- tion of cyanic acid and ammonia, and containing the very same elements (page 271), the characters above mentioned do not indicate the presence of either; but on the contrary the crystals agree with urea obtained from urine in composition and in all their chemical properties. (Journal of Science, N. S. iii. 491.) The cyanic acid above referred to is that disco- vered by Wohler. Sugar of Milk and Sugar of Diabetes. Sugar of Milk.—The saccharine principle of milk is obtained from whey by evaporating that liquid to the consistence of syrup, and allowing it to cool. It is afterwards purified by means of albumen and a second crystal- lization. The sugar of milk has a sweet taste, though less so than the sugar of the cane, from which it differs essentially in several other respects. Thus it re- quires seven parts of cold and four of boiling water for solution, and is in- soluble in alcohol. It is not susceptible of undergoing the vinous fermenta- tion ; and when digested with nitric acid it yields saccholactic acid, a property first noticed by Scheele, and which distinguishes the saccharine principle of milk from every other species of sugar. Like starch, it is con- vertible into real sugar by being boiled in water acidulated with sulphuric acid. Sugar of milk contains no nitrogen, and according to the analysis of Gay- Lussac and Thenard, is very analogous to common sugar in the proportion of its elements. Sugar of Diabetes.—In the disease called diabetes the urine contains a pe- culiar saccharine matter, which, when properly purified, appears identical both in properties and composition with vegetable sugar, approaching nearer to the sugar of grapes than that from the sugar-cane. This kind of sugar is obtained in an irregularly crystalline mass by evaporating diabetic urine to the consistence of syrup, and keeping it in a warm place for several days. It is purified by washing the mass with alcohol, either cold or at most gen- tly heated, till that liquid comes off colourless, and then dissolving it in hot alcohol. By repeated crystallization it is thus rendered quite pure. (Prout) A few other principles yet remain to be considered, such as the colouring principle of the blood, caseous matter, and mucus ; but these will be more conveniently studied in subsequent sections. 593 ANIMAL ACIDS. SECTION II, ANIMAL ACIDS. In animal bodies several acids are found, such as sulphuric, hydrochloric, phosphoric, acetic, &c, which belong equally to the mineral or vegetable kingdom, and which have consequently been described in other parts of the work. In this section are included those acids only which are believed to be peculiar to animal bodies. Uric or Lithic Add—This acid is a common constituent of urinary and gouty concretions, and is always present in healthy urine, combined with ammonia or some other alkali. The urine of birds of prey, such as the eagle, and of the boa constrictor and other serpents, consists almost solely of urate of ammonia, from which pure uric acid may be procured by a very simple process. For this purpose the solid urine of the boa constrictor is reduced to a fine powder, and digested in a solution of pure potassa, in which it is readily dissolved with disengagement of ammonia. The urate of potassa is then decomposed by adding acetic, hydrochloric, or sulphuric acid in slight excess, when the uric acid is thrown down, and, after being washed, is collected on a filter. On its first separation from the alkali it is in the form of a gelatinous hydrate, but in a short time this compound is decomposed spontaneously, and the uric acid subsides in small crystals. Pure uric acid is white, tasteless, and inodorous. It is insoluble in alco- hol, and is dissolved very sparingly by cold or hot water, requiring about 10,000 times its weight of that fluid at 60° F. for solution. (Prout.) It red- dens litmus paper, and unites with alkalies, forming salts which are called urates or lithates. The uric acid does not effervesce with alkaline carbo- nates; but Dr. Thomson affirms that when boiled for some time with carbo- nate of soda, the whole of the carbonic acid is expelled. A current of car- bonic acid, on the contrary, throw? down the uric acid when dissolved by potassa. This acid undergoes no change by exposure to the air. Of the acids none exert any peculiar action on the uric excepting nitric acid. When a few drops of nitric acid, slightly diluted, are mixed on a watch-glass with uric acid, and the liquid is evaporated to dryness, a beauti- ful purple colour comes into view, the tint of which is improved by the ad- dition ofwater. This character affords an unequivocal lest of the presence of uric acid. The nature of the change will be considered immediately. Uric acid is decomposed by chlorine. Liebig has observed, that when dry uric acid is heated with dry chlorine, an enormous quantity of cyanic and muriatic acid is generated. If the uric acid is moist, chlorine then gives rise to the disengagement of carbonic and cyanic acids; while in solution there remain hydrochloric acid, ammonia, and much oxalic acid. According to a late analysis by Liebig, uric acid is thus constituted :— (An. de Ch. et de Ph. Iv. 58.) Carbon . . . 36.11 30.6 or 5 eq. Hydrogen . . . 2.34 2 2 eq. Nitrogen . . . 33.36 28.3 2 eq. Oxygen .... 28.19 24 3 eq. 100.00 84.9 1 eq. i to prove that the foregoing consi -, who found six eq. of carbon insiea hve. Crystallized uric acid is considered by most chemists to be anhv- drous; but Dr. Thomson maintains that at 400° it loses precisely two equiva- Liebig has given strong evidence to prove that the foregoing constitution is more correct than that of Prout, who found six eq. of carbon instead of lents ofwater 50* 594 ANIMAL ACIDS. The salts of uric acid have been described by Henry (Manchester Me- moirs, vol. ii. N. S.) The principal ones yet examined are the urates of am- monia, potassa, and soda. Urate of ammonia is soluble to a considerable extent in boiling, but more sparingly in cold water. The urates of soda and potassa, if neutral, are of very sparing solubility; but an excess of either alkali takes up a large quantity of the acid. The former was found by Wol- laston to be the chief constituent of gouty concretions. When uric acid is heated in a retort, carbonate and hydroeyanate of am- monia are generated, and a volatile acid sublimes, called pyro-uric acid which was formerly described by Henry, and has since been studied by Che' valher and Lassaigne, Liebig, and Wohler. The two latter chemists have noticed, that pyro-uric is identical with cyanuric acid (page 273); and Wohler finds that urea, as well as cyanuric acid, is an essential product of the de structive distillation of uric acid. Purpuric Acid.—This compound was first recognized as a distinct acid iXiorli,,.™18, described b7 hira in ^e Philosophical Transactions for 1«18. I hough colourless itself; it has a remarkable tendency to form red or purple-coloured salts with alkaline bases, a character by which it is distin- guished from all other substances, and to which it owes the name of pur- puric acid, suggested by Wollaston. Thus the purple residue above men- tioned, as indicative of the presence of uric acid, is purpurate of ammonia, which is always generated when the uric is decomposed by nitric acid. Purpuric acid may be prepared by the following process, for the outline of which I am indebted to directions kindly given me by Prout. Let 200 grains of uric acid, prepared from the urine of the boa constrictor, be dissolved in 300 grains of pure nitric acid diluted with an equal weight of water, the uric acid being added gradually in order that the heat may not be exces'sive. Effervescence ensues after each addition, nitrous acid fumes appear, heat is evolved, and a colourless solution is formed, which, on standing i'n a cool place for some hours, yields colourless crystals, which have the outline of an oblique rhomboidal prism. By gentle evaporation an additional quantity may be obtained. They contain nitric and purpuric acid and ammonia, should be dissolved in water, and be exactly neutralized by pure ammonia; and the liquid is then digested in a solution of potassa, until the ammonia is wholly expelled. On pouring this solution into dilute sulphuric acid, pur- puric acid is set free, which, being insoluble in water, subsides as a granular powder, of a white colour if pure, but commonly of a yellowish-white tint Considerable uncertainty prevails as to the nature of purpuric acid. Vau- quelin denied that its salts have a purple colour, attributing that tint to some impurity, and Lassaigne is inclined to the same opinion (An. de Ch. et de Ph. xxii. 334); but from the intense colour given even by a very minute quantity of purpuric acid, the opinion of Dr. Prout appears to me the more probable. The composition of the acid is, likewise, unsettled; for Prout has expressed a doubt of the accuracy of the analysis which he formerly published. The name of erythric acid (from eguSga/Wv, to redden) was applied by Brugnatclli to a substance which he procured by the action of nitric on uric acid. It obviously contains purpuric acid, and Prout thinks it probable that it is a supcrsalt, consisting of purpuric and nitric acids, and ammonia, being probably identical with the crystals above mentioned. Rosacic Acid.—This name was applied by Proust to a peculiar acid sup- posed to exist in the red matter, commonly called by medical practitioners the lateritious sediment, which is deposited from the urine in some stages of fover. From the experiments of Vogel it appears to be uric acid, either combined with an alkali, or modified by the presence of animal matter. Prout is of opinion that it contains some purpurate of ammonia; and, as he has detected the presence of nitric acid in the urine from which such sedi- ments were deposited, he thinks it probable that the purpurate may be gene- rated by the reaction of the uric and nitric acids on each other in the uri- nary passages. ANIMAL ACIDS. 595 Hippuric Acid.—Under this name, derived from tvrat a horse, and Iv^n urine, Liebig has described a peculiar compound, which is deposited from the urine of the horse when it is mixed with hydrochloric acid in excess. The deposite, which is crystalline and of a yellowish-brown tint, is boiled with milk of lime, to which small quantities of chloride of lime are added, until the urinous odour ceases. It is then digested with animal charcoal; and on mixing the hot filtered solution with a large excess of hydrochloric acid, hippuric acid is deposited in cooling in rather large prisms, two or three inches in length, and beautifully white. It exists in the urine of most her- bivorous animals united with soda. (An. de Ch. et de Ph. xliii. 188.) Hippuric acid is analogous in its characters to benzoic acid, and was at first supposed to be that acid modified by the presence of animal matter; but Liebig contends that it is clearly disfingubhcd from benzoic acid by the character of its salts, in being less soluble in water, and in containing nitro- gen. It is composed of Carbon. Hydrogen. Nitrogen. Oxygen. In 100 parts 60.76 4.92 7.82 26.5 In equiv. 122.4 or 20 eq.+9 or 9 eq.+14.15 or 1 eq.+48 or 6 eq. = 193.55. Formic Acid.—A sour liquor exists in ants, which they eject when irri- tated, and which may be obtained in solution by bruising these insects into a pulp with water: the liquid was supposed by Fourcroy and Vauquelin to contain acetic and malic acids; but the experiments of Suersen, Gehlen, Ber- zelius, and Dobereiner have proved that it is a mixture of the malic with a peculiar volatile acid, distinct from acetic acid, and easily separable by dis- tillation. The peculiar origin of this compound has suggested for it the name of formic acid. Dobereiner has shown that it is readily generated artificially by distilling a mixture of 1 part of tartaric acid, 1£ of peroxide of manganese, and I£ of sulphuric acid diluted with about 2^ parts ofwater: a capacious retort should be used, as the materials froth up during the pro- cess. The tartaric acid receives oxygen from the manganese, and is resolved into water, carbonic acid, and formic acid. (An. of Phil. N. S. iv. 311.) Lie- big and Gmelin have found that several other substances, such as sugar, starch, sugar of milk, and ligneous fibre, may be substituted for tartaric acid; but the formic acid is then accompanied by some foreign matter, which may be removed by neutralizing with an alkali, and decomposing the formate by sulphuric acid. Even alcohol may be used; but it must be employed in a dilute state, in order to prevent the production of sulphuric or formic ether. Formic acid, indeed, appears to be a frequent result of" chemical changes, of which some examples were formerly mentioned (pages 269 and 499); and hence it is important to be well acquainted with its composition. By these processes formic acid is procured in a very dilute state : it is obtained with less water by distilling a mixture of one part of water, two of strong sulphuric acid, and three of dry formate of potassa. But the free acid in its most concentrated form has not less than 19.6 per cent, of water, and a density of 1.1168. Formic acid is a feeble acid, has a sour taste and reac- tion, and its odour resembles that of acetic acid, but is attended with a pecu- liar pungency. With bases it forms crystallizable salts, by the form of which, especially of the formates of baryta, strontia, lime, and oxide of zinc, it is completely distinguished from acetic acid. According to the analysis of formate of oxide of lead by Berzelius, the equivalent of this acid is inferred to be 37.24; and it is composed of 12.24 parts or two equivalents of carbon, 1 part or one equivalent of hydrogen, and 24 parts or three equivalents of oxygen. From the ratio of these ele- ments, it is manifest that formic acid may be considered a compound of two equivalents of carbonic oxide and one equivalent of water, as expressed bV the formula 2tC+0)+(H+0); and it is actually resolved into these com- pounds by being gently heated with strong sulphuric acid, carbonic oxide gas being disengaged. Warmed with peroxide of mercury it takes oxygen from the metal, and is resolved into water and carbonic acid, the latter of 596 ANIMAL OILS AND FATS. which escapes with effervescence. By these characters formic acid is dis- tinguished, from other acids. Allantoic acid.—This compound, described by Buniva and Vauquelin un- der the name of amniotic acid, and said to exist in the liquor amnii of the cow, was found by Dzondi to be present solely in the liquor of the allantois, and to be in fact the urine of the foetus. The mistake of the discoverers has also been corrected by Lassaigne. (An. de Ch. et de Ph. xxxiii. 279.) The allantoic acid is obtained by gently evaporating the liquid of the al- lantois of the fcetal calf, when the acid is deposited in the form of white acicular crystals. It is very sparingly soluble in water, but yields with the alkalies soluble compounds which are decomposed by most of the acids. According to the analysis of Liebig, its elements are in the ratio of five equivalents of carbon, four of hydrogen, two of nitrogen, and four of oxygen. Several other animal acids, such as the stearic, oleic, margaric, and others, should also be mentioned here; but as they are closely allied to the fatty principles from which they are derived, they will be more conveniently described in the following section. SECTION III. ANIMAL OILS AND FATS. The fatty principles derived from the bodies of animals are very analo- gous in composition and properties to the vegetable fixed oils ; and in Britain, where the latter are comparatively expensive, the former are employed, both for the purpose of giving light, and for the manufacture of soap. Their ultimate elements are carbon, hydrogen, and oxygen; and most of them, like the fixed oils, consist of two or more definite compounds, such as stea- rine, margarine, and oleine, in a state of combination. From a curious experiment of Berard it appears that a substance very analogous to fat may be made artificidly. On mixing together one measure of carbonic acid, ten measures of carburetted hydrogen, and twenty of by- drogen, and transmitting the mixture through a red-hot tube, several white crystals were obtained, which were insoluble in water, soluble in alcohol, and fusible by heat into an oily fluid. (An. of Ph. xii. 41.) Dobereiner pre- pared an analogous substance from a mixture of coal gas and aqueous va- pour. The annexed table shows the composition of several animal fats, and of their products when formed into soap. The analyses are by Cbevreul, ex- cepting that of stearine by Lccanu, of spermaceti by Berard, and of am- breine by Pelletier. Carb. Hyd. Oxy. Formulae. Human fat 79.000-f-ll.4164- 9.584=100 Oleine of human ) 7a566+1L447+ 9.987=100 Hogslard . 79.098+11.1464- 9.756=100 01faT 78.02+12.2 +9.78=100 VC7sH70O7or ton suet ' C 446.76 +70 +56 =572.76) C7"Hs703+C3H30a 0tonesu°eft mUt'( 79.354+11.09 + 9.556=100 ANIMAL OILS AND FATS. Carb. Hyd. Oxy. Formula;. Stearic acid (an-> 80.145+12.478+ 7.377=100 ) C7oHfir0„ hydrous) 428.4 +67 +40 =535.4 I Margaric acid ( 79.053+12.01 + 8.937=100 JC7oH6a00 (anhydrous) (428.4 +65 +48 =541.4 j Oleic acid (an- / 80.942+11.359+7.699=100 ]C7oUb^Oo hvdrous) (428.4 +58 +40 =526.4 $ „403 hydrous; . 40.071+ 8.925+51.004=100 ?C J"4° °r,. Glycerine j 18 36 + 4 +24 = 46.36 \ C»HsO'-' + H Cetine . 8l'.660+12.862+ 4.578= 99.1 Spermaceti 79.5 +11.6 + 8.9 =100 n nr , , V 79.766+13.945+6.289^100 JC«H*-Oor Ethal . j 97.92 +17 +8 =122.92 ) 4CW + H. D, ., S 65.00 + 8.25+26.75=100 SCioh-0' Phocenic acid j gl 2 _J_ 7 +24 = 92.2 ) Cholesterine 85.095+11.88 + 3.025=100 Ambreine 83.37 +13.32 + 3.31 =100 Train Oil—Train oil is obtained by means of heat from the blubber of the whale, and is employed extensively in making oil gas, and for burning in common lamps. It is generally of a reddish or yellow colour, emits a strong unpleasant odour, and has a considerable degree of viscidity, proper- ties which render it unfit for being burned in Argand lamps, and which are owing partly to the heat employed in its extraction, and partly to the pre- sence of impurities. By purification, indeed, it may be rendered more lim- pid, and its odour less offensive; but it is always inferior to spermaceti oil. Spermaceti Oil is obtained from an oily matter lodged in a bony cavity in the head of the Physeter macrocephalus, or spermaceti whale. On subject- ing this substance to pressure in bags, a quantity of pure limpid oil is ex- pressed ; and the residue, after being melted, strained, and boiled with a solution of potassa, is sold under the name of spermaceti. This principle is probably modified in the process by which it is purified. Animal Oil of Dippel—This name is applied to a limpid volatile od, which is entirely different from the oils above mentioned, and is a product of the destructive distillation of animal matter, especially of albuminous and gelatinous substances. When purified by distillation, it is clear and transparent It was formerly much used in medicine, but is now no longer employed. Human Fat.—This variety of fat has a yellowish colour, is inodorous, fuses at a very gentle heat, and retains its fluidity at 59°: that of the loins begins to solidify at 77° and it is quite solid at 63°. It requires for solu- tion 40 parts of hot alcohol, which in cooling deposites stearine of a peculiar kind, leaving oleine in solution. When saponified it yields margaric and oleic acids and glycerine, but no stearic acid. Hogslard.—This fat is of a nearly white colour, and the fusing point of different varieties between 79° and 88°. It probably contains both the stearine and margarine besides oleine. When saponified, it yields marga- ric, stearic, and oleic acids, and glycerine. Suet.—This term is applied to the fat situated about the loins and kid- neys, which is less fusible than the fat from other parts of the same ani- mal. The suet from the ox and sheep is principally used : when separated by fusion from the membrane in which it occurs, it is called tallow, and is extensively employed in the manufacture of soap and candles. Beef and mutton suet resemble hogslard in their constituents and in the products of saponification. Beef suet, when fused, congeals at 102°, and mutton suet at 98° or a few degrees higher. Stearine.—This compound is a constituent of several animal fats, espe- dally of hogslard and suet, the latter of which contains one-fifth of stea- rine. It is prepared by fusing mutton suet in a flask, agitating with its own weight of ether, and when the whole is cold separating the soluble 598 ANIMAL OILS AND FATS. parts by pressure in a cloth and in bibulous paper. This process is repeated until the parts soluble in cold ether are removed. The insoluble part is stearine. It may also be prepared by mixing essence of turpentine with fused mutton suet, and when the mass has cooled pressing out the fluid parts by bibulous paper: the solid part left after repeated fusion with tur- pentine is stearine, which should be purified by solution in hot ether to re- move adhering turpentine. Pure stearine, as separated from its ethereal solution, and dried by com- pression in bibulous paper, occurs in small white brilliant lamina?, and fuses at 129° into a mass like wax, but so friable that it may be reduced to pow- der. It dissolves in boiling strong alcohol, but separates on cooling in snow- white flakes. Boiling ether dissolves it in large proportion, but when cold retains an l-225th of its weight. When sharply heated, it is decomposed, and among other products it yields stearic acid. When digested in pure po- tassa, it is entirely resolved into stearic acid and glycerine, both regarded as anhydrous. This will be obvious by the formulae of pages 596 and 597. Stearine may be regarded as a compound of stearic acid and glycerine, and its conversion into soap as the mere separation of glycerine, which, in the act of quitting the stearic acid, combines with one eq. of water. The fore- going facts are drawn from a late essay by Lecanu. (An. de Ch. et de Ph. Iv. 192.) Margarine.—After the stearine in the foregoing process has been depo- sited, another solid fatty principle, which Lecanu terms margarine, may be obtained by allowing the ethereal solution to evaporate spontaneously. To free it from adhering ether and oleine, it should be pressed between folds of bibulous paper. Margarine is very similar in its properties to stearine, but is distinguished from it by its greater fusibility, its point of fusion being 117£°, and by its ready solubility in cold ether. With an alkdi it yields stearic acid and glycerine. The solid matter of vegetable oils is not identical with the stearine of suet, as was thought by Chevred, but appears from the late experiments of Lecanu to be more closely allied to the margarine of suet. It differs, how- ever, from both by yielding margaric and oleic, and not stearic, acid when converted into soap. Oleine.—The liquid matter of animal fats, obtained by compression in bibulous paper, is very similar to the oleine obtained in a similar manner from frozen vegetable oils. Margaric and Oleic Acids.—When the fats above mentioned, or the fixed vegetable oils, are boiled with a solution of potassa or soda, the elements of the fat or oil undergo a new arrangement, whereby, without losing any gaseous substance or absorbing any from the air, they are converted into one or more fatty acids and glycerine. To this change the elements of water contribute. The new acids combine with the alkali and constitute soap, which after due evaporation collects on the surface of the water, while the glycerine remains in solution. The acids which, in ordinary soap, are united with potassa or soda as compounds soluble in water, form insoluble com- pounds with most other metallic oxides; and hence it is that solutions of soap are precipitated by salts of lime, oxide of lead, and similar metallic so- lutions. These insoluble soaps may be formed directly by digesting oleagi- nous substances with water and metallic oxides. The margaric and oleic acids are best prepared from soap made with po- tassa and fluid vegetable oil. This soap, after being well dried, is treated by successive portions of cold alcohol of sp. gr. 0.821, in which the oleate of potassa is soluble and the margarate insoluble. The two sdts, thus sepa- rated, aro decomposed by means of an acid. Margaric acid, so named from its pearly lustre (from fxngyn^iTiic a pearl) is insoluble in water, and is hence precipitated by acids from the solution of its salts. It is abundantly dissolved by hot alcohol, and is deposited from the saturated solution, on cooling, in a crystalline mass of a pearly lustre. At 140° it is fused, and shoots into brilliant white acicular crystds as it ANIMAL OILS AND FATS. 599 cools. It has an acid reaction, and its salts, those of the alkalies excepted, are very sparingly soluble in water. The crystallized acid contains 3.4 per cent, of water, corresponding to two eq. ofwater, from which it can only be separated by combining with an alkaline base. Oleic acid is best prepared from soap made with linseed oil and potassa, since the greater part of it consists of the oleate of that alkali. This salt is first separlted from margarate of potassa by cold alcohol, and the oleic acid then precipitated from an aqueous solution of the oleate by means of an acid. At the mean temperature oleic acid is a colourless oily fluid, which congeds when it is cooled to near zero. It has a slightly rancid odour and taste, and reddens litmus paper. Its specific gravity is 0.898. It is insolu- ble in water, but is dissolved in every proportion by alcohol. Ut tne neu- tral oleates hitherto examined, those of soda and potassa are alone soluble in water. ,. . , . In its free state it contains 3.8 per cent., corresponding to one equivalent of water, which it loses in uniting with most metallic oxides. Stearic Acid.—This acid is best prepared by converting pure stearine into soap, and decomposing the resulting stearatc with an acid. It is very simi- lar in its appearance and properties to margaric acid, the principal distinc- tion being a difference of fusibility, the fusing point of stearic acid being 158°. The crystallized acid consists of one eq. of anhydrous stearic acid and one eq. of water. , • , • ,. • Sebacic Acid.—Thenard has applied this name to an acid which is obtain- ed by the distillation of hogslard or suet, and is found in the recipient mixed with acetic acid and fat, partially decomposed. It is separated from the latter by means of boiling water, and from the former by acetate of oxide of lead. The sebacate of that oxide, which subsides, is subsequently decom- posed by sulphuric acid. Sebacic acid reddens litmus paper, dissolves freely in alcohol, and is more soluble in hot than in cold water. It melts like fat when heated, and crystallizes in small white needles in cooling. It is not applied to any use. Butyrine.—Butter differs from the common animal fats in containing a peculiar oleaginous matter, which is quite fluid at 10°, and to which Chevreul has applied the name of butyrine. When converted into soap, it yields, in addition to the usual products, three volatile odoriferous com- pounds, namely, the butyric, caproic, and capric acids. Phocenine is a peculiar fatty substance contained in the oil of the por- poise (Delphinus phocama) mixed with oleine. When converted into soap, it yields a volatile odoriferous acid, called the phocenic acid. (Chevreul.) Hircine is contained in the fat of the goat and sheep, and yields the hircic acid when converted into soap. (Chevreul.) Other acids, more or less analogous to those above described, are formed during the conversion of other oleaginous substances into soap. Thus, castor oil yields three acids, to which Bussy and Lecanu have applied the names of margarilic, ricinic, and elaiodic acid. The cevadic acid was pre- pared in a similar manner by Pelletier and Caventou from oil derived from the seeds of the Veratrum sabadilla; and the same chemists have given the name of jatrophic acid (more properly crotonic) to the acid of the soap made from croton oil. This oil is derived from the seeds of the Croton tiglium. Glycerine.—The sweet principle of oils, glycerine of Chevreul, was dis- covered by Scheele. It was originally obtained in the formation of common plaster by boiling oil with oxide of lead and a little water; and Chevreul found that it is produced during the saponification of fatty substances in general. In preparing soap by means of potassa, the glycerine is left in the mother liquor; and on neutralizing the free alkali with sulphuric acid, evaporating to the consistence of syrup, and treating the residue with alco- hol, it is dissolved. The alcoholic solution, when evaporated, yields glyce- rine in the form of an uncrystdlized syrup. It is soluble in water and 600 ANIMAL OILS AND FATS. alcohol, and has a sweet taste, but is not susceptible either of the vinous or acetous fermentation. Spermaceti.—This inflammable substance, which is prepared from the spermaceti whale, as above mentioned, commonly occurs in crystalline Plates of a white colour and silvery lustre. It is brittle, and feels soft and slightly unctuous to the touch. It has no taste, and scarcely any odour. It is insoluble in water, but dissolves in about thirteen times its weight of boihng alcohol, from which the greater part is deposited on cooling in the form of brilliant scales. It is still more soluble in ether. It is exceedingly fusible, liquefying at a temperature which is distinctly below 212°. The spermaceti of commerce always contains some fluid oil, from which it may be purified by solution in boiling alcohol. To the white crystalline scales deposited from the spirit as it cools, and which is spermaceti in a state of perfect purity, Chevreul has given the name of cetine. Ethal.—When cetine is boiled with alkalies it is slowly and incompletely saponified, since, in addition to margaric and oleic acid, a solid fatty princi- ple is generated, amounting to 40.64 per cent, of the cetine used. Chevreul gave it the name of ethal, from the first syllabic of the words ether and alcohol, because of its analogy to those liquids in point of composition (page 597). Ethal congeals after fusion at 86°, and at 124° under water. It is soluble in every proportion in strong alcohol at 130°, and may be distilled without change. Adipocire.—When a piece of fresh muscle is exposed for some time to the action of water, or is kept in moist earth, the fibrin entirely disappears, and a fatty matter called adipocire remains, which has some resemblance to spermaceti. The fibrin was formerly thought to be really converted into adipocire; but Gay-Lussac* and Chevreul maintain that this substance pro- ceeds entirely from the fat originally present in the muscle, and that the fibrin is merely destroyed by putrefaction. Dr. Thomson maintains, how- ever, that the conversion of fibrin into fat does occur in some instances, and has related a remarkable case in proof of his opinion. (An. of Phil. vol. xii. p. 41.) According to M. Chevreul, the adipocire is not a pure fatty princi- pie, but a species of soap, chiefly consisting of margaric acid in combination with ammonia generated during the decomposition of the fibrin. Cholesterine.]—This name is applied by Chevreul to the crystalline mat- ter which constitutes the basis of most of the biliary concretions formed in the human subject Fourcroy, regarding it as identical with spermaceti and the fatty matter just described, comprehended all these circumstances under the general appellation of adipocire; but Chevreul has shown that it is an independent principle, wholly different from spermaceti. Cholesterine is a white brittle solid of a crystalline lamellated structure and brilliant lustre, very much resembling spermaceti; but it is distinguish- ed from that substance by requiring a temperature of 278° for fusion, and by not being convertible into soap when digested in a solution of potassa. It is free from taste and odour, and is insoluble in water. It dissolves freely in boiling alcohol, from which it is deposited on cooling in white pearly scales. When heated with its own weight of concentrated nitric acid, cholesterine is dissolved with disengagement of nitric oxide gas; and in cooling a yellow matter subsides, an additional quantity of which may be obtained by dilution with water. This substance possesses the properties of acidity, and is called cholesteric acid. It is insoluble in water, but dissolves freely in dcohol, especially with the aid of heat. Its taste is slightly styptic, and its odour somewhat like that of butter; it is lighter than water, and fusible at 136£° F. In mass it is of an orange-yellow tint; but when the alcoholic solution is evaporated spontaneously, it is deposited in acicular crystals of a white colour. It reddens litmus paper, and neutralizes alkaline bases. The * An. de Ch. et de Ph. vol. iv. t From xoMl °^ef an<^ rngtoc solid. BLOOD. 601 cholesterates of potassa and soda are deliquescent and very soluble in water, but insoluble in alcohol and ether. The cholesterates of the earths and other metallic oxides are either sparingly dissolved by water or altogether insoluble. Its salts are precipitated by the mineral and most of the vegeta- ble acids; but are not decomposed by carbonic acid. For these facts re- specting the formation and properties of cholestenc acid, we are indebted to the experiments of Pelletier and Caventou. (Journal de Pharmacie, iii. 292.) Cholesterine has been detected in the bile of man, and of several of the lower animals, such as the ox, dog, pig, and bear. This interesting disco- very was made about the same time by Chevreul in Paris, and by Tiede- mann and Gmelin in Heidelberg. Lassaigne has likewise found it in the biliary calculus of a pig. (An. de Ch. et de Ph. xxxi.) It is frequently formed in parts of the body quite unconnected with the hepatic circulation, and appears to be a common product of deranged vascular action. Caventou, in the Journal de Pharmacie for October 1825, states that the contents of an abscess, formed under the jaw, apparently in consequence of a carious tooth, were found by him to consist almost entirely of cholesterine. In the article Calcul of the Nouveau Dictionnaire de Midecine, M. Breschet ob- serves that cholesterine has been found in cancer of the intestines, and in the fluid of hydrocele and ascites in the human subject; and he adds that M. Barruel procured it in large quantity from an ovarian cyst in a mare, and in the fluid drawn off from the ovary of a woman, and scrotum of a man. Breschet has found it also in a tumour under the tongue. Dr. Chris- tison found it in the fluid of hydrocele, taken from a patient in the Royal Infirmary of Edinburgh by the late Dr. William Cullen; in an osseous cyst, into which the kidneys of another patient were converted; and in the mem- branes of the brain of an epileptic patient. Reichenbach has detected it in animal tar, whence it would seem to be a product of the destructive distilla- tion of animd matter. The best method of preparing pure cholesterine is to treat human biliary concretions, reduced to powder, with boiling alcohol, and to filter the hot so- lution as rapidly as possible. As the liquid cools, the greater part of the cholesterine subsides. In this way it is freed from the colouring matter with which it is commonly associated in the gall-stone. Ambergris.—This substance is found floating on the surface of the sea near the coasts of India, Africa, and Brazil, and is supposed to be a concre- tion formed in the stomach of the spermaceti whale. It has commonly been regarded as a resinous principle; but its chief constituent is a substance very analogous to cholesterine, and to which Pelletier and Caventou have given the name of ambreine. By digestion in nitric acid, ambreine is con- verted into a peculiar acid called the ambreic acid. (An. of Phil. vol. xvi.) MORE COMPLEX ANIMAL SUBSTANCES, AND SOME FUNCTIONS OF ANIMAL BODIES. SECTION I. BLOOD, RESPIRATION, AND ANIMAL HEAT. Blood. The blood is distinguished from other animal fluids by its colour, which is a florid red in the arteries and of a dark purple tint in the veins. Its taste is slightly saline, its odour peculiar, and to the touch it seems somewhat unctuous. Its specific gravity is variable, but most commonly it is near 1.05; and in man its temperature is about 98° or 100° F. While flowing 60a BLOOD. in its vessels, or when recently drawn, it appears to the naked eye as a uni- form homogeneous liquid; but if examined with a microscope of sufficient power, numerous red particles of a globular form are seen floating in a nearly colourless fluid. Hence the blood, while circulating, is mechanically distin- guishable into two parts, one essentially liquid, which may be called liquor sanguinis, and the other essentially solid, wliich is merely suspended in the former, and imparts its red colour to the mixture. Both of these constituents of the blood are complex substances. The red globules, discovered by Leuwenhoeck and carefully examined by Hewson, were observed by the late Dr. Young to consist of two parts, a colourless globule insoluble in water, and a red colouring matter soluble in that men- struum. The observations of Mr. Bauer, coinciding with those of Young, led to the belief that the entire red globule consists of a central smaller glo- bule of fibrin, which is surrounded with a pellicle or film of colouring mat- ter : the coloured globules manifest no tendency to cohere; but the nuclei of fibrin, when deprived of their colouring envelope, which they soon lose after the blood is drawn, adhere together with great facility. (Phil. Trans. 1818, p. 172.) These remarks have been corroborated by Prevost and Dumas, who extended their researches to the blood of various animals; and they further observed that in birds and the cold-blooded tribes the globules are elliptical, while they are spherical in mammiferous animals. The liquor sanguinis, considered by some as serum, has been shown by Dr. Benjamin Babington, in a short essay replete with sound observation, to be very similar to chyle, and to consist of fibrin, held in solution, along with albuminous, oleaginous, and saline matter, by the water of the blood. When set at rest the liquor sanguinis coagulates, yielding a uniform jelly of pre- cisely the same volume as when it was liquid, and possessing the exact figure of the containing vessel; and in a short time, by the contraction of the mass of coagulated fibrin, a yellowish liquid appears, which is the serum of the hlood. The microscopical observations of Mr. Bauer, cited by the late Sir E. Home in the Croonian lecture for 1818 (Phil. Trans. 1819, p. i.), prove that, during this process, numerous globules, smaller than the colourless nuclei of the red globules, are generated, and give rise to coagulation by mutual adhesion. Though the production of these globules is most abundant soon after the blood is drawn, yet their developement continues for several days; .Mr. Bauer observed them to appear in the clear serum of a sheep 8 or 10 days after removal from the animal; and Mr. Faraday made the same obser- vation with the serum of human blood. It is the liquor sanguinis, thus shown to be spontaneously separable into fibrin and serum, which forms a yellowish liquid stratum at the surface of blood recently drawn from persons in acute rheumatism or other inflamma- tory fevers. In such affections the liquor sanguinis, from causes not at all understood, "-enerally coagulates with unusual slowness, so that the heavier red globulesliave time to subside to an appreciable extent, leaving an upper stratum of nearly colourless fluid, wliich by the cautious use of a spoon may be removed and collected in a separate vessel. The buffg coat of such blood is the pure fibrin separated by coagulation from the liquor sanguinis. The coagulable lymph of Surgeons, which is thrown out on cut surfaces, appears to be the liquor sanguinis; and this fluid is also not unfrequently exhaled in dropsies, when the fibrin cither constitutes a gelatinous deposite, or appears as white (lakes floating in the serous fluid. It is poured out by the intes- tines during an attack of cholera, the rice-water fluid characteristic of that disease consisting of a saline and albuminous solution, in which numerous shreds of fibrin are suspended. When blood drawn from a healthy person is set at rest, it speedily coagu- lates, and is found after a few hours to have separated itself into two parts, one the scrum identical with that obtained from the liquor sanguinis, and the other a uniformly red coagulum called the clot, cruor, or crassamentum. The uniform redness of the clot is owing to the fibrin coagulating before the red globules have had time to subside. It contains the colouring matter of BLOOD. 603 the blood, together with dl the fibrin, except traces held in solution in the serum, as well that which had formed part of the liquor sanguinis, as the fibrinous nuclei of the red globules. The ratio of the clot and serum is very variable, and by no means represents the quantity of fibrin or colouring mat- ter contained in the blood. Dr. B. Babington has shown, in the essay already referred to, that the ratio materially depends on the figure of the containing vessel:—two portions of blood were drawn from the same person, one being received and allowed to coagulate in a pear-shaped bottle, and the other in a pint basin; and the ratio of serum to clot was as 1000 to 1292 in the tormer, and as 1000 to 1717 in the latter. In fact, when a mass of coagulating blood is contained in a spherical vessel, the particles of fibrin being little removed from a common centre are more powerfully attracted towards each other, yield a denser clot, and squeeze out more serum, than when the coagulation takes place in a shallow wide bisin, where the particles are spread over a large surface. The clot of the former is compact and small; while that of the latter, being spongy, and hence retaining much serum within it, is large and abundant, though the actual quantity of solid matter is the same in both. The following table exhibits the results of two careful analyses of the blood by Lecanu:—(An. de Ch. et de Ph. xlviii. 308.) Water Fibrin Colouring matter Albumen Crystalline fatty mat Oily matter Extractive matter soluble in water and alcohol Albumen combined with soda Chloride of sodium 780.115 785.590 2.100 3.565 133.000 119.626 65.090 69.415 2.430 4.300 1.310 2.270 1.790 1.920 1.265 2.010 -----------potassium Carbonates 1 )> 8.370 7.304 Phosphates > of potassa and soda Sulphates ) Carbonates of lime and magnesia . 1 Phosphates of lime, magnesia, and iron > 2.100 1.414 Peroxide of iron . . . } Loss.....2.400 2.586 1000.000 1000.000 The earthy salts were obtained by incinerating the albumen, and the trace of iron was obviously derived from a little colouring matter of the blood. Fatty matter appears to be always present in scrum, having been found by Dr. B. Babington as well as by Lecanu : the former obtained it by agitating serum repeatedly but gently with ether, which took up the fatty substance, and left it by evaporation ; while the latter mixed the serum with alcohol, and dissolved the fatty principles from the alcoholic extract by means of ether. The oily matter of the serum is very soluble in ether and cold alcohol, is liquid at common temperatures, and is readily converted into soap by potassa. The crystalline fatty matter is similar in appearance to cholesterine, resists the action of potassa, and is soluble in ether and boiling alcohol: it appears to contain nitrogen as one of its elements. The relative proportion of the ingredients of the blood must necessarily vary independent of disease even in the same individual, according as the nutrition is scanty or abundant. According to Lecanu slight variations arise from difference of age and sex, and the following comparative view is the mean of his analysis made with blood drawn from ten women and from ten men. 604 BLOOD. Female. Male. 804.37 789-32 69.72 67.50 9.95 10.69 115.96 132.49 1000.00 1000.00 In addition to the constituents of the blood already enumerated, M. Bar- ruel declares that this fluid contains a volatile principle, peculiar to- each species of animal. This principle has an odour resembling that of the cu- taneous or pulmonary exhalation of the animal, and serves as a distinctive character by which the blood of different animals may be recognized. It is dissolved in the blood, and its odour may be perceived when the blood or its serum is mixed with strong sulphuric acid. The odour is commonly stronger in the male than in the female. In man it resembles the human perspiration; in the ox, it smells like oxen or a cow-house; and the odour from horses' blood is similar to that of their perspiration. (Journ. of Science, vi. N. S. 187.) Should the accuracy of these observations be confirmed, they may be advantageously applied in some cases of legal medicine. Minute portions of alumina, silica, and manganese have been detected in the blood, and Dr. O'Shaughnessy confirms the statement of M. Sarzeau that a trace of copper may likewise be found; but the extremely minute quantity in which these substances occur, renders it doubtful whether they really exist in the blood, or are casually introduced in the course of ana- lysis. Dr. Clanny reports the existence of a large quantity of free carbon in the blood ; but I am not aware that the statement is borne out by experi- ment, and it is entirely opposed to the observation of other chemists. Of the presence of free carbonic acid in the blood, I shall have occasion to speak while discussing the subject of respiration. Serum of the Blood.—This liquid, which separates during the coagulation of the blood, has a yellowish colour, is transparent when carefully collected, has a slightly saline taste, and is somewhat unctuous to the touch. Its average specific gravity is about 1.029. It has a slight alkdine reaction with test paper, owing to the presence of soda, which some chemists believe to be combined with carbonic acid, and others with albumen: the last opinion is the more probable, since serum, when agitated with carbonic acid, absorbs that gas in considerable quantity. Like other albuminous liquids, it is coagulated by heat, acids, alcohol, and all other substances which coagulate albumen. On subjecting the coagulum prepared by heat to gentle pressure, a small quantity of a colourless limpid fluid, called the serosity, oozes out, which contains, according to Dr. Bostock, about l-50th of its weight of animal matter, together with a little chloride of sodium. Of this animal matter a portion is albumen, which may easily be coagulated by means of galvanism ; but a small quantity of some other principle is present, which differs both from albumen and gelatin. (Medico-Chir. Trans, ii. 166.) The composition of the serum, according to the late analysis of Lecanu, may be seen from his analysis of the blood, abstracting the colouring mat- ter and fibrin which are foreign to it (page 603). The late Dr. Marcet found that 1000 parts of the serum of human blood are composed of water 900 parts, albumen 86.8, muriate of potassa and soda 6.6, muco-extractive matter 4, carbonate of soda 1.65, sulphate of potassa 0.35, and of earthy phosphates 0.60. This result agrees very nearly with that obtained by Ber- zelius, who states that the extractive matter of Marcet is lactate of soda united with animal matter. (Medico-Chir. Trans, iii. 231.) Colouring Matter of the Blood.—This substance, to which the term hema- tosine is now applied, is so analogous in most of its chemical relations to albumen, that its complete separation from it is attended with great difficulty. It is obtained nearly pure by cutting the clot of blood into very thin slices with a sharp knife, as advised by Berzelius, soaking them repeatedly in dis- Water Albumen Saline and extractive matter Red globules BLOOD. 605 tilled water, and letting them drain on bibulous paper after each immersion: the slices are then broken up in distilled water with a stick, briskly stirring in order to dissolve the colouring matter; and the filtered solution is evapo- rated to dryness in shallow capsules or dishes at a temperature of 80° or 100° F. As thus prepared the colouring matter, or hematosine, is soluble and possessed of all its characters; but it retains a little serum. It may be still further purified by the method of Engclhart, which is founded on the fict that hematosine is more coagulable by heat than albumen: serum di- luted with ten parts of water does not coagulate at 160°; whereas hema- tosine, dissolved in fifty parts of water, begins to coagulate at 149°, and is thrown down in insoluble flocks of a brown colour. Unfortunately, how- ever, its characters are changed by this operation, uncoagulated and coagu- lated hematosine bearing the same relation to each other as soluble and insoluble albumen. For the purpose of ultimate analysis the pure insolu- ble hematosine should be employed; but for examining the properties of hematosine, its less pare but soluble state is preferable. Soluble hematosine, when quite dry, is black, with a lustre like jet in mass and red in powder or in thin layers, is insipid to the taste, and inodo- rous. In cold water it readily dissolves, forming a red liquid, which may be preserved without change for months. Its solution, like that of albumen, may be coagulated by heat as already mentioned; but when quite dry it may be heated to 212° without being rendered insoluble. Alcohol and acids likewise precipitate it: the latter deepen its tint, and fall in combination with it; but the compounds of hematosine with the muriatic, sulphuric, and acetic acid may be dissolved in water by means of an excess of their acid. The alkalies do not precipitate the aqueous solution of hematosine ; but its solution in hydrochloric acid yields red flocks on the addition of ammonia. With some of the metallic oxides it forms insoluble compounds, especially with the oxides of tin and mercury; and hence the salts of these metals precipitate hpmatosine. In most of these properties, colour excepted, it re- sembles albumen; but Lecanu has indicated two pointed differences:—1. a solution of hematosine is not precipitated by the acetate or subacetate of oxide of lead, while both of these salts throw down albumen ; 2. the pre- cipitate occasioned by hydrochloric acid, when pressed between folds of linen to remove adhering acid and well dried, is soluble in strong boiling alcohol, whereas the corresponding compound of albumen and hydrochloric acid is quite insoluble. By this character Lecanu found that hematosine, carefully prepared from human blood, contains very little albumen ; but that the co- louring matter of blood from the ox and sheep appears to be a compound, in nearly equal parts, of hematosine and albumen. (An.de Ch. et de Ph. xlv. 5.) Hematosine, according to the analysis of Michaelis, consists of carbon, hydrogen, nitrogen, and oxygen, very nearly in the same ratio as in fibrin and albumen; but it differs from both in containing iron. This was an- nounced in 1806 by Berzelius in his comparative examination of the ingre- dients of the blood (M< dico-Chir. Tr-ins. iii. 213): from the albumen and fibrin he could obtain no iron ; but 100 parts of hematosine, when burned in the open dr, left 1.2.5 of ashes, containing 0.625 of oxide of iron, and 0.625 of a mixture of carbonate and phosphate of lime, phosphate of magnesia, and subphosphate of oxide of iron. He was unable to detect the presence of iron by any of the liquid tests. These facts, though questioned by other chemists, were fully established by Dr. Engelhart, who gained the prize offered in the year 1825 by the Medical Faculty of Gottingen for the best Essay on the nature of the colouring matter of the blood (Edinb. Med. and Surg. Journ. for January, 1827). He demonstrated that the fibrin and albu- men of the blood, when carefully separated from colouring particles, do not contain a trace of iron ; and, on the contrary, he procured iron from the red globules by incineration. But he likewise succeeded in proving the exist- ence of iron in the colouring matter of the blood by the liquid tests; for on transmitting a current of chlorine gas through a solution of the red do bules, the colour entirely disappeared, white flocks were thrown down and 606 BLOOD. a transparent solution remained, in which peroxide of iron was discovered by all the usual reagents. The results obtained by Dr. Engelhart, relative to the quantity of the iron, correspond with those of Berzelius. These facts have been since confirmed by Rose, who has accounted in a satisfactory manner for the failure of former chemists in detecting iron in the blood u llCiiin a fluid state' He finds that oxide ofiron cannot be precipitated by the alkalies, hydrosulphuret of ammonia, or infusion of galls, if it is dissolved n a solution which contains albumen or other soluble organic principles. From the presence ofiron in hematosine, and its total absence in the other principles of the blood, chemists were induced to suspect that its peculiar colour was in some way or other produced by that metal, an idea which re- ceived additional support from the known tendency of peroxide of iron to form salts of a red colour. But this view, though plausible on these grounds, is in other respects improbable. The real state in which iron exists in the blood is quite unknown, and its minute proportion seems unequal to produce so intense a colour as that of the blood. The only probable mode of explain- ing the diversity of tints which agents of different kinds produce in the blood, is to adopt the opinion ably maintained by Mr. Brande, who supposes that the tint of hematosine is owing to a peculiar animal colouring principle, capable like cochineal or madder of acting as a dye, and of combining with metallic oxides. He succeeded in obtaining a compound with hematosine and oxide of tin; but it yields the finest lakes with nitrate of peroxide of mercury and corrosive sublimate. Woollen cloths impregnated with either of these compounds, and immersed in an aqueous solution of hematosine, acquired a permanent red dye, unchangeable by washing- with soap. (Phil. Trans. 1812.) 5 ' Fibrin of the Blood.—Fibrin appears to exist in the blood in two states, as already mentioned;—as the central nuclei of the red globules, and in solu- tion in the serum. The fibrin from both sources is obtained by beating up the clot of blood with successive portions of water, so as to dissolve all the serum and colouring matter, leaving the insoluble fihrin quite white; but the best mode of separation is to stir recently drawn blood with a stick during coagulation, when the fibrin adheres to it in strings, which are easily rendered colourless by washing. Coagulation of the Blood.—This phenomenon is occasioned by the agglu- tination of the fibrin of the blood,—both of the nuclei of the red globules, nfter they have lost their colouring envelope, and of that part which is in solution. The time required for coagulation is influenced by temperature, being promoted by heat, and retarded by cold. Sir C. Scudamore finds that blood which begins to coagulate in four minutes and a half in an atmosphere of 53°, undergoes the same change in two minutes and a half at 98°; and that which coagulates in four minutes at 98° will become solid in one minute at 120°. On the contrary, blood which coagulates firmly in five minutes at 60° will remain quite fluid for twenty minutes at the temperature of 40°, and requires upwards of an hour for complete coagulation. (Scudamore on the Blood.) The process of coagulation is influenced by exposure to the air. If atmo- spheric air be excluded, as by filling a bottle completely with recently drawn blood, and closing the orifice with a good stopper, coagulation is retarded. It is singular, however, that if blood be confined within the exhausted re- ceiver of an air-pump, the coagulation is accelerated. (Scudamore.) Recently drawn blood, owing doubtless to its temperature, is known to give off a portion of aqueous vapour, which has a peculiar odour, indicative of the presence of some peculiar principle, but in which nothing but water can be detected. Physiologists are not agreed upon the question whether the act of coagulation is or is not accompanied with disengagement of gase- ous matter. In the experiments of Vogcl, Brande, and Scudamore, blood coagulating in the vacuum of an air-pump was found to emit carbonic acid, and Scudamore even inferred that the evolution of this gas constitutes an essential part of the process. Other experimentalists, however, have obtained BLOOD. 607 a different result Dr. John Davy and the late Dr. Duncan, jum, failed in their attempts to procure carbonic acid from blood during coagulation; and Dr. Christison, in an experiment which I witnessed, was not more success- ful. This evidence seems positive against the notion that the evolution ot carbonic acid gas is an essential part of coagulation. The existence of car- bonic acid in venous blood, is a different question, and this point will be dis- cussed under the head of respiration. ,.,,.. j • j Coagulation is influenced by the rapidity with which the Wood is removed from the body. Sir C. Scudamore observed, that blood slowly drawn from a vein coagulates more rapidly than when taken in a full stream. Experiments are still wanting to show the influence of different gases on coagulation. Oxygen gas accelerates coagulation, and carbonic acid retards but cannot prevent it , Heat is evolved during the coagulation of the blood. The late Dr. Gordon estimated the rise of the thermometer at six degrees; and Dr. Davy, on the other hand, regards the increase of temperature from this cause as very slight. Sir C. Scudamore finds thai the rate at which blood cools is distinctly slower than it would be were no caloric disengaged, and he observed the thermometer to rise one degree at the commencement of coagulation. Some substances prevent the coagulation of the blood. This effect is pro- duced by a saturated solution of chloride of sodium, hydrochlorate of am- monia, nitre, and a solution of potassa. The coagulation, on the contrary, is promoted by alum and the sulphates of the oxides of zinc and copper. The blood of persons who have died a sudden violent death, by some kinds of poison, or from mental emotion, is usually found in a fluid state. Light- ning is said to have a similar effect; but Sir C. Scudamore declares this to be an error. Blood, through which electric discharges were transmitted, coagulated as quickly as that which was not electrified; and in animals killed by the discharge of a powerful gdvanic battery, the blood in the veins was always found in a solid state. The cause of the coagulation of the blood has been the subject of much speculation to physiologists. The tendency of this fluid to preserve the liquid form while contained in a living animal, cannot be ascribed to the motion to wliich it is continually subject within the vessels. It is a familiar fact that blood, though continually stirred out of the body, is not prevented from coagulating; and it has been noticed, that the coagulation of blood which is set at rest within its proper vessels by the application of ligatures, or which has been accidentally extravasated within the body, is materially retarded. It has, indeed, been hitherto found impossible to account in a satisfactory manner for the blood retaining fluidity by reference to motion, temperature, or the operation of any physical or chemical laws; and, conse- quently, it is generally ascribed to the agency of the vital principle. The blood is supposed either to be endowed with a principle of vitality, or to re- ceive from the living parts with which it is in contact a certain vital impres- sion, which, together with constant motion, counteracts its tendency to coagulate. Blood in Disease.—The blood may be diseased either by the excess or de- ficiency of one or more of its proper constituents, or from the presence of substances which are foreign to it. One familiar instance of diseased blood is jaundice, when bile enters the circulation and is distributed to every or- ganized part of the body. Though the presence of bile in the blood during jaundice has been detected, yet its passage into the circulating mass appears so rapidly succeeded by its exit, that its detection in the blood itself is gene- rally difficult. Urea has dso been detected, sometimes in very large quan- tity : it appears to be constantly present in the blood, whenever the secretion of urine is suppressed. Prevost and Dumas wholly arrested the secretion of urine in a dog by tying the renal vessels and excising the kidneys, and two days after the operation they obtained 20 grains of urea from 5 ounces of his blood. (An. de Ch. et de Ph. xxxiii. 90.) Dr. Christison found urea in the blood of persons suffering under renal disease, as will be mentioned in the 608 BLOOD. section on the urine; and Dr. Prout appears also to have made a similar ob- servation. The serum of the blood in this affection has usually a lower sp. gr. than healthy serum, the average of several cases being 1.021 (Gregory); and the albumen is in smaller proportion than natural. Urea has been de- tected by Dr. O'Shaugnessy in the blood of persons labouring under cholera, in which disease the action of the kidneys is generally very much disturbed. The sp. gr. of the serum of diabetic blood is usually above the natural stand- ard, being so high as 1.0354. (Marcet). The serum of blood, instead of being transparent, as it commonly is, has sometimes a cloudy appearance like whey, and in some more rare instances is perfectly opaque and white, as if it had been mixed with milk. The cause of the opacity has been experimentally examined by Drs. Traill and Chris- tison, who have traced it to the presence of oleaginous matter, which the latter has shown to contain both stearine and elaine, and to be very similar to human fat. The milkiness may, therefore, be ascribed to fat being me- chanically diffused through the serum like oil in an emulsion. It may be easily separated by agitating the serum in a tube with half its bulk of sul- phuric ether, when the adipose matter is instantly dissolved, the opacity in consequence disappears, and on evaporating the clear ethereal solution, which rises to the surface of the mixture, the fat is obtained in a separate state. By this means he procured on one occasion five per cent of fat from milky serum, and one per cent, from serum which had the aspect of whey. (Edinb, Med. and Surg. Journal, April, 1830.) The most remarkable kind of diseased blood which has yet been studied by chemists is that which occurs in cholera. During the progress of that disease an enormous discharge takes place of a whitish-coloured fluid simi- lar to a mixture of boiled rice with water, an appearance occasioned by a white flaky matter floating in a nearly colourless liquid. The insoluble part has the character of fibrin: while the liquid portion is a weak solution of albumen, is faintly alkaline, and contains the same kind of salts as exist in the blood. On examining the blood itself, il is found to contain less water and more albumen and hematosine than healthy blood; the density of the serum is consequently greater than usual; its colour is remarkably black even in the arteries; in some cases it is semi-fluid and incapable of coagulating, having the appearance of tar; and the salts of the blood arc often in unusually smdl quantity, being sometimes almost entirely wanting. On comparing the condition of the blood with that of the discharges, it is manifest that the latter contain all the ingredients of the blood except the red globules; but that the aqueous and saline parts pass out of the circula- tion more rapidly than the albuminous. The cause of"the dirk colour of the blood in cholera is a point by no means decided. Dr. Thomson states that the blood in cholera is not ren- dered florid by exposure to atmospheric air, and hence argues that the dark colour is owing to some diseased condition of the blood which unfits it for being duly artcrialized; but this view is opposed to the statement of Dr. O'Shaughnes^y, who declares that the dark blood in cholera is susceptible of artcrialization,—that, at least, on being agitated with air, it is rendered florid, absorbs oxygen, and emits carbonic acid gas. Dr. Stevens, in his treatise on the blood, maintains that the black colour is primarily owing to the contagion of cholera, wliich throws the circulating fluids into a morbid state; and that the effect of the poison is increased by the diminished quantity of saline matter, an opinion intimately connected with his theory of artcrialization, which will shortly be explained. He has observed the blood of persons living in a cholera hospital to be preternaturally dark, though they were not affected with the disease. Perhaps the most correct opinion is, that the blood of persons in cholera, in consequence of deranged arterial action, circulates sluggishly, and is hence imperfectly artcrialized: from this cause the dark colour may arise without any diminution of saline matter; and it may disappear, from an improved circulation, without the administration of salt. But there is no doubt that loss of saline matter in- RESPIRATION. wv creases the dark tint of the blood, and prevents it from acquiring the arterial colour. . .... The substance known under the name of black vomit, ejected by the stomach during the last stage of yellow fever, appears from the observa- tions of Dr. Stevens to be blood blackened and partially coagulated by a free acid. The acidity is suspected by Dr. Prout to arise from the secretion of acetic acid; and the presence of hydrochloric acid is also probable. Respiration. When venous blood is brought into contact with atmospheric air, its sur- face passes from a dark purple to a florid-red colour, oxygen gas disappears, and carbonic acid gas is emitted. The change takes place more speedily when air is agitated with blood ; it is still more rapid when pure oxygen is substituted for atmospheric air; and it does not occur at all when oxygen is entirely excluded. These facts, which were long considered indisputable, have been questioned by Dr. Davy; but they are nevertheless perfectly true, and have been fully established by Dr. Christison. (Edin. Med. and Surg. Journal, Jan. 1831.) The quantity of carbonic acid developed very exactly corresponds with the oxygen which disappears; but when the blood and air are agitated together, part of the carbonic acid which would otherwise be found as gas, is absorbed by the serum. It appears certain, from the expe- riments of Dr. Christison, that the colouring matter is the part of the blood essentially concerned in the phenomenon; an inference which is drawn, not from the mere change of tint, but from the effect of the blood on the air varying with the quantity and condition of the colouring matter. In some fevers, as acute rheumatism, in which the circulation is rapid and the respi- ration free, the venous blood is found to be very florid, and to withdraw very little oxygen from air; and a similar scanty abstraction of oxygen is ob- served in dark venous blood, when its usual proportion of colouring matter is deficient. The conversion of the dark purple colour of venous blood into the florid tint of that contained in the arteries, is familiarly expressed by the term arterialization; or, more strictly, this name is applied to a change in the constitution of the blood, which is accompanied and indicated by change of colour, evolution of carbonic acid, and abstraction of oxygen from the air. Chemists have differed about the origin of the carbonic acid. Some sup- pose that the blood, in circulating through the body, becomes charged with carbon in some unknown mode of combination, which causes the venous character; and that when such blood is exposed to the air, its redundant carbon, by a process of oxidation, unites directly with atmospheric oxygen, the carbonic acid so generated is set free, and the blood unloaded of carbon recovers the arterial character. By others, venous blood is thought to owe its colour to the presence of carbonic acid ready formed within it: they maintain that oxygen gas is directly absorbed into the blood, and displaces the pre-existing carbonic acid. The near coincidence between the quantity of evolved carbonic acid and absorbed oxygen, which has been urged in favour of the former theory, is scarcely an argument either way; for though true, on the one hand, that carbonic acid contains its own volume of oxygen, it is also true, on the other, that if one gas displaces another from a liquid, the volumes of evolved and absorbed gas usually correspond. The strongest evidence is in favour of the latter theory. Carbonic acid has been proved to be capable when absorbed by arterial blood of causing the venous character; and Mr. Hoffman has shown that when venous blood is received from a vein into a vessel full of hydrogen gas, without exposure to the atmosphere, and is then agitated with the hydrogen, carbonic acid gas is evolved. (Medi- cal Gazette, March 30,1833.) But arterialization does not alone depend, as was thought till lately, on the influence of the atmosphere. Dr. Stevens, in his treatise on the blood, has the merit of proving the saline matter of the scrum to be essential to the 610 RESPIRATION. phenomenon. Various salts, such as nitre, chlorate of potassa, sea-salt, and bicarbonate of soda, have the property of giving to hematosine, or the colour- ing principle of the blood, a bright red tint, far more florid, when a strong saline solution is used, than arterial blood. A saline solution of the requi- site strength gives to venous blood the arterial tint, without exposure to the air; but the serum contains so small a quantity of salt that its effect be- comes visible only when aided by the agency of the atmosphere. A clot of venous blood, when carefully separated from its serum, is not brightened by oxygen gas; and arterial blood, when its serum is displaced by pure water, becomes as dark as venous blood. Hence it may be inferred that the change from venous to arterial blood consists of two parts which are essentially distinct: one is attended with the direct absorption of oxygen, and the evo- lution of carbonic acid pre-existing in venous blood, an action essential to life; and the other, the most conspicuous but probably least essentid, is the effect of the saline parts of the serum, which impart a florid tint to the co- louring matter after the former change has occurred. By what means the absorption of oxygen gives rise to the evolution of carbonic acid, is a ques- tion which, like others that will occur to the attentive reader, has not been sufficiently explained, and invites further investigation. The same changes which occur in blood out of the body are continually taking place within it. During respiration, venous blood is exposed in the lungs to the agency of the air and is arterialized, oxygen gas disappears, and carbonic acid is evolved; and it is remarkable that these phenomena ensue not only during life, but even after death, provided the respiratory process be preserved artificially. Since, therefore, all the characteristic phenomena of arterialization are the same in a living and in a dead animal, and whether the blood is or is not contained in the body, it seems legitimate to infer, that this process is not necessarily dependent on the vital principle, but is solely determined by the laws of chemical action. In studying the subject of respiration, the first object is to determine the precise change produced in the constitution of the air which is inhaled. Dr. Black was the first to notice that the air exhaled from the lungs con- tains a considerable quantity of carbonic acid, which may be detected by transmission through lime-water. Priestley, some years after, observed that air is rendered unfit for supporting flame or animal life by the process of respiration ; from which it was probable that oxygen is consumed; and Lavoisier subsequently established the fact, that during respiration oxygen gas disappears, and carbonic acid is disengaged. The chief experimental- ists who have since cultivated this department of chemical physiology are, Priestley, Scheele, Lavoisier, Seguin, Crawford, Goodwin, Davy, Ellis, Allen and Pepys, Edwards, and Despretz. Of these, the results obtained by Messrs. Allen and Pepys (Phil. Trans. 1808), and Dr. Edwards,* are the most conclusive and satisfactory; their researches having been conducted with great care, and aided by all the resources of modern chemistry. One of the chief objects of Allen and Pepys, in their experiments, was to ascertain if any uniform relation exists between the oxygen consumed and the carbonic acid evolved. They found in general that the quantity of the former exceeds that of the latter; but as the difference was very trifling, they inferred that the carbonic acid of the expired air is exactly equal to the oxygen which disappears. The experiments of Dr. Edwards were attended with a remarkable result, which accounts very happily for some of the dis- cordant statements of preceding inquirers. He found the ratio between the gases to vary with the animal. In some animals it might be regarded as nearly equal; while in others the loss of oxygen considerably exceeded the gain of carbonic acid, so that the respired air suffered a material diminution in volume. With respect to the human subject, the statement of Allen and Pepys seems very near the truth. * De ITnfluenco des Agens Physiques sur la Vie, 1824. RESPIRATION. 611 The quantity of oxygen withdrawn from the atmosphere, and of carbonic acid disengaged, is variable in different individuals, and in the same indivi- dual at different times. It is estimated by Allen and Pepys that in every minute during the calm respiration of a healthy man of ordinary stature, 26.6 cubic inches of carbonic acid of the temperature ot 50 > are emitted, and an equal volume of oxygen withdrawn from the atmosphere. Jrom these data it has been calculated, that in an interval of twenty-four hours not less than eleven ounces of carbon are given off from the lungs alone,—an estimate which must surely be inaccurate, the quantity being so great as sometimes to exceed the weight of carbon contained in the food. 1 he same observers have lately found the production of carbonic acid in a pigeon, breathing freely in atmospheric air, to be such that, supposing the same rate to continue, the bird must have thrown off 96 grains of carbon in the space of 24 hours. From the observations of Dr. Prout it appears that the quan- tity of carbonic acid emitted from the lungs is variable at particular periods of the day, and in particular states of the system. It is more abundant during the day than the night; about daybreak it begins to increase, con- tinues to do so till about noon, and then decreases until sunset During the night it seems to remain uniformly at a minimum; and the maximum quantity given off at noon exceeds the minimum by about one-fifth of the whole. The quantity of carbonic acid is diminished by any debilitating causes, such as low diet, depressing passions, and the like. (An. of Phil. xm. 269.) The experiments of Dr. Fyfe, published in his Inaugural Disserta- tion, are confirmatory of those above mentioned. Messrs. Allen and Pepys observed that atmospheric air, when drawn into the lungs, returns charged in the succeeding expiration with from 8 to 6 per cent, of carbonic acid gas; but this estimate is probably too high, since in some recent observations of Dr. Apjohn of Dublin, dr, once respired, con- tained only 3.6 per cent of carbonic acid. When an animal is confined in the same quantity of air, death ensues before dl the oxygen is consumed : when the same portion of air is repeatedly respired until it can no longer support life, it then contains 10 per cent of carbonic acid according to Allen and Pepys, and barely 8 per cent.according to Dr. Apjohn. (Edin. Med. and Surg. Journd, Jan. 1831.) Although in respiration the arterialization of the blood by means of free oxygen is the essential change, without the due performance of which the life of warm-blooded animals cannot be preserved beyond a few minutes, and which is likewise necessary to the lowest of the insect tribe, it is important to determine whether the nitrogen of the atmosphere has any influence in the function. The results of different inquirers differ considerably. In the experiments of Priestley, Davy, Humboldt Henderson, and Pfaff, there ap- peared to be absorption of nitrogen, a less quantity of that gas being exhaled than was inspired. Nysten, Berthollet, and Despretz, on the contrary, re- marked an increase in the bulk of the nitrogen; and from th<; researches of Seguin and Lavoisier, Vauquelin, Ellis, Dalton, and Spallanzani, it was in- ferred that there is neither absorption nor exhalation of nitrogen, the quan- tity of that gas undergoing no change during its passage through the air- cells of the lungs. Allen and Pepys arrived at a similar conclusion; and since the appearance of their Essay, the opinion has prevailed very generally among physiologists, that in respiration the nitrogen of the air is altogether passive. The facts ascertained by Edwards relative to this subject are novel and of peculiar interest. This acute physiologist has reconciled the discordant results of preceding experimenters by showing that, during the respiration even of the same animal, the quantity of nitrogen may one while be in- creased, at another time diminished, and at a third wholly unchanged. He has traced these phenomena to the influence of the seasons; and he sus- pects, as indeed is most probable, that other causes, independently of season, have a share in their production. In nearly all the lower animals which were made the subject of experiment, an augmentation of nitrogen was ob- 612 RESPIRATION. servable during summer. Sometimes, indeed, it was so slight that it might be disregarded. But in many other instances, it was so great as to place the fact beyond the possibility of doubt; and on some occasions it almost equalled the whole bulk of the animal. Such continued to be the result of his inquiries until the close of October, when he observed a sensible diminu- tion of nitrogen, and the same continued throughout the whole of winter and the beginning of spring. There are two modes of accounting for these phenomena. According to one view, the nitrogen which disappears is ascribed to the absorption of what was inhaled, and its increase to direct exhalation, the opposite pro- cesses of absorption and exhalation being supposed not to occur at the same moment. According to the other view, both these processes are always going on at the same time, and the result depends on the preponderance of one over the other. When absorption prevails, a smaller quantity of nitrogen is exhaled than was inspired; when exhalation exceeds absorption, increase of nitrogen takes place; but when absorption and exhalation are equal, the bulk of the inspired air, so far as concerns nitrogen, is unchanged. The lat- ter opinion, which is adopted by Edwards, is supported by two decisive ex- periments performed by Allen and Pepys, in one of which a guinea-pig was confined in a vessel of oxygen gas, and in the other in an atmospere com- posed of 21 measures of oxygen and 79 of hydrogen. In both cases the re- sidual air contained a quantity of nitrogen greater than the bulk of the ani- mal itself; and in the latter a portion of hydrogen had disappeared. Hence it follows that nitrogen may be exhaled from the lungs, and that hydrogen may be absorbed. An account of some interesting researches on the respiration of birds, bearing directly on this subject, was published in 1829 by Allen and Pepys. (Phil. Trans.) The subject of inquiry was the pigeon, and the phenomena attending its respiration were observed under three different circumstances, namely, in atmospheric air, in oxygen gas, and in a mixture of oxygen and hydrogen, in which the former amounted, as in the atmosphere, to 20 per cent. In each case the bulk of the gaseous mixture remained without change. In the experiments with atmospheric air, the oxygen which disap- peared was equd to the carbonic acid evolved; the nitrogen was unaffected, except on one occasion when the bird appeared uneasy, and then there was a slight loss of nitrogen. In oxygen gas the production of carbonic acid was about half the quantity emitted when the pigeon breathed common air ; and the decrease in oxygen was exactly equal to the united volumes of the carbonic acid and nitrogen which were disengaged. When the pigeon was placed in mixed oxygen and hydrogen gases, the production of carbonic acid was rather more abundant than in atmospheric air, and its volume equalled exactly the loss in oxygen; nitrogen, as before, was given out with con- siderable freedom, and its bulk precisely corresponded to the decrease in hydrogen. In the two latter series of experiments, especially in the last, the respiration of the pigeon was at times laborious. The experiments, how- ever, are decisive of the fact, that carbonic acid and nitrogen gases may be thrown off from the lungs, and that oxygen and hydrogen gases may be absorbed. Two theories similar to those of arterialization (page 609) have been pro- posed to explain the phenomena of respiration. According to one theory, the carbonic acid found in the respired air is actudly generated in the lungs themselves ; while, according to the other, this gas is thought to exist ready formed in the blood, and to be merely thrown off from that liquid during its distribution through the lungs. The former theory, which appears to have originated with Priestley, has received several modifications. Priestley imagined that the phenomena of respiration are owing to the disengagement of phlogiston from the blood, and its combination with the air. Dr. Craw- ford modified this doctrine in the following manner. (Crawford on Animal Heat.) He was of opinion that venous blood contains a peculiar compound of carbon and hydrogen, termed hydro-carbon, the elements of which unite in RESPIRATION. 613 the lunffs with the oxygen of the air, forming water with the one, and^car- bonkS^uVthe other; and that the blood, thus purified, regains its florid hue, and becomes fit for the purposes of the animal economy. The hypothesis of Crawford, however, is not merely liable to the objection that theT suDDOsed hydro-carbon, as respects the blood, is quite imaginaryj Sut i wa?Kt JKnco with the kading facts established by Allen and Pepys Bv the elaborate researches of these chemists it was estabhshed, S'carbomc acid gas contains its own volume of oxygen; andthey^ so concluded that air, inhaled into the lungs, re urns chargec^'* tJ^J* of carbonic acid, almost exactly equal ,n bulk to the oxygen whch disap- pears-an inference which, as applied to man and some of the kn er am- Lis, seems very near the truth. A review of these clrcu™stanCXduiSs them to adopt the opinion, that the oxygen of the air combines in the lungs exclusively with carbon; and that the watery vapour, which is ahvays con- tained in the breath, is an exhalation from minute P"1^0"3^^™ They conceived that the fine animal membrane interposed between the blood and the air does not prevent chemical action from taking place be- tween them. ,, _,„. , ,, . .r,- This view has been further modified by Mr. Ellis, who supposes tliat the carbon is separated from the venous blood by a process of secretion,, and that then, coming into direct contact with oxygen, it is converted into car- bonic acid. (Inquiry, &c. Parts i. and ii.) The circumstance which led Mr. EUis to this opinion, was a disbelief in the possibility of oxygen acting upon the blood through the animal membrane in which it is confined. 1 his difficulty, as will immediately appear, no longer exists; and the tree per- meability of membranes by gases is now completely established. According to the second theory, which was supported by Le Grange and Hassenfratz, and has lately been adopted by Edwards, carbonic acid gene- rated during the course of the circulation is given off from venous blood in the lungs, and oxygen gas is absorbed. This doctrine, though generally regarded hitherto as less probable than the preceding, is supported by very powerful arguments. The experiments and observations of Dr. Edwards seem to leave no doubt that the blood, while circulating through the lungs, is capable of absorbing hydrogen, nitrogen, and oxygen gases, and of emitting nitrogen; and he has gone very far towards proving that the car- bonic acid is derived from the same source. On confining frogs and snails for some time in an atmosphere of hydrogen, the residual air was found to contain a quantity of carbonic acid, which wan in some instances even greater than the bulk of the animal; and a similar result was obtained with young kittens. These facts, in proving the possibility of gaseous inhalation and exhala- tion, as well as the evolution of carbonic acid independently of atmospheric air, entitle the theory of Edwards to a preference; and they will go far, when attested by more extensive observation, especially should the constant presence of carbonic acid in venous blood be established, to the rejection of the former theory. The difficulty which formerly stood in the way of both theories of respi- ration, arising from the supposed impermeability of animal membranes by gases, has been entirely removed by the researches of Drs. Faust and Mit chell. (American Journal of the Medical Sciences, No. 13.) It fully ap- pears from their experiments, and of the accuracy of their principal results I am satisfied from personal observation, that animal membranes both in the living and dead subject, both in and out of the body, are freely penetra- ble by gaseous matter;—that the phenomena of endosmose and exosmose, observed in liquids by Dutrochet, are likewise exhibited by gases. If a glass full of carbonic acid be closed by an animal membrane, such as thfc ccecum of a fowl, or the bladder of a sheep, and be then exposed to the at- mosphere, a portion of air will pass into the glass and some of the confined gas escape from it; and if the experiment be reversed by confining air in the glass, which is then placed in an atmosphere of carbonic acid, the latter 52 614 RESPIRATION. passes in and the former out of the glass. Similar phenomena ensue with other gases; so that when any two gases are separated by a membrane, both of them pass through the partition. The permeability of a membrane is greater in a living than in a dead animal; but the property is by no means peculiar to organized matter, since a thin lamina of any substance of organic origin, such as a sheet of caoutchouc, is freely permeable. Water and other liquids transmit gases apparently on the same principle as mem- branes; and porous solid bodies of the mineral kingdom will doubtless be found to possess a similar property. But though all gases pass through membranous septa, they differ remark- ably in the relative rapidity of transmission. Thus Dr. Mitchell found that the time required for the passage of equal volumes of different gases through the same membrane, was 1 minute with ammonia, 2£ minutes with sulphuretted hydrogen, 3$ with cyanogen, 5<| with carbonic acid, 6J with protoxide of nitrogen, 27£ with arseniuretted hydrogen, 28 with olefiant gas, 37«£ with hydrogen, 113 with oxygen, 160 with carbonic oxide, and a much greater time with nitrogen. Hence, when a bladder full of air is surrounded with carbonic acid, the latter enters faster than the former es- capes, and the bladder bursts; but on reversing the conditions of the expe- riment, the bladder becomes flaccid, because the carbonic acid within passes out more rapidly than the exterior air enters. The transmission of gases in some of these experiments takes place in opposition to a pressure equd to several atmospheres. It would perhaps be premature to speculate on the cause of this singular property of gases, nor is it material for my present purpose to do so; but as the reader will naturally desire some explanation, the following, which is nearly that of Dr. Mitchell, may be given as most consistent with the known properties of matter.—The passage of a gas through a membrane or other substance containing within it very fine pores, appears to depend in the first place on the power of the porous body to absorb the gas into its substance. This action is apparently of the same kind as that exerted on gases by charcoal, and seems to depend on the attraction which dl bodies similar or dissimilar exert upon each other, such as is exemplified by the tendency of contiguous floating bodies to approach one another, by the ad- hesion ofwater to the surface of glass, and the ascent of liquids in capillary tubes. The absorbent power of such bodies, which may thus be regarded as aggregates of capillary tubes, will vary with the size and number of the pores. The entrance of a gas into such pores will be promoted by its elas- ticity; but the same force will oppose its retention within the pores, will re- sist its return into the mass of the same particles, and urge it to escape where there is no such resistance. Hence, when two gases are separated by a membrane, each passes through, and mixes wilh the other: the pene- tration of each is arrested as soon as its individual elasticity is the same on both sides of the partition, and therefore that gas which penetrates the membrane the more rapidly, is the first to be stationary. The relative velocity of transmission is doubtless a complex phenomenon, referable to the natural elasticity of each gas, to its diffusiveness, to its affinity for water, and to the size of its atom should that differ in different gases. The power of different liquids to absorb gases with which they have no chemical action, is explicable on the same principles. A gas may be absorbed by such a liquid, entering between its particles as into the pores of a mem- brane : on exposure to a different atmosphere, a portion of the absorbed gas escapes, and about an equal volume of the other enters the space which the former had occupied. But whatever may be thought of these speculations, the facts which they are designed to explain are obviously applicable to the phenomena of respiration. It is clear that oxygen has free ingress to the blood through the fine membrane of the lungs; and carbonic acid, whether f)ie-existing in venous blood or generated during its flow through the lungs, las a free passage outwards. This is a sufficiently direct inference from what has already been mentioned; but it may be added, as additional evi- ANIMAL HEAT. 615 dence, that an aqueous solution of carbonic acid confined in a bladder gives out that gas to the surrounding atmosphere, and that venous blood exposed in a bladder to the air, absorbs oxygen, emits carbonic acid, and acquires the arterial character. * . It appears from the essays of Drs. Faust and Mitchell that their attention was awakened to the permeability of membranes to gases by the endosmosc and exosmose of liquids described by Dutrochet, by an insulated example of a similar phenomenon in gases observed by -Mr. Graham, and by some facts of a like kind noticed long ago by Priestley. Dr. Stevens also, as stated at page 96 of his treatise on the blood, was aware that carbonic acid passes readily through animal membranes when air is on the other side, applied that fact to the theory of respiration, and brought the subject under the notice of several men of science in New York shortly before the publication of Drs. Faust and Mitchell. But the views of Dr. Stevens, though well cal- eulated to elicit inquiry, were vague and partial; and the American philoso- phers are entitled to the merit of advancing from the detached facts of others to the establishment of a principle. The conversion of venous into arterial blood appears not to be confined to the lungs. The disengagement of carbonic acid from the surface of the skin, and the corresponding disappearance of oxygen gas, was demonstrated by the experiments of Jurine and Abernethy; and dthough the accuracy of their results has been doubted by some persons, it has been confirmed by others. However this may be in the human subject, the fact with respect to many of the lower animals is unquestionable. Spallanzani proved that some animals possessed of lungs, such as serpents, lizards, and frogs, pro- duce the same changes on the air by means of their skin, as by their proper respiratory organs; and Dr. Edwards, in a series of masterly experiments, has shown that this function compensates so fully for the want of respira- tion by the lungs, as to enable these animals, in the winter season, to live for an almost unlimited period under the surface of water. Animal Heat. The striking andogy between the processes of combustion and respiration, in both of which oxygen gas disappears, and an oxidized body is substituted for it, led Dr. Black to infer that the heat generated in the animal system, by means of which the more perfect animals preserve their temperature above that of the surrounding medium, is derived from the changes going forward in the lungs. But this opinion is not founded on analogy alone; many circumstances conspire to show that the developement of animal heat is dependent on the function of respiration, although the mode by which the effect is produced has not hitherto been satisfactorily determined. Thus, in all animals whose respiratory organs are small and imperfect, and which, therefore, consume but a comparatively minute quantity of oxygen, and ge- nerate little carbonic acid, the temperature of the blood varies with that of the medium in which they live. In warm-blooded animals, on the contrary, in which the respiratory apparatus is larger, and the chemical changes more complicated, the temperature is almost uniform; and those have the highest temperature whose lungs, in proportion to the size of their bodies, arc largest, and which consume the greatest quantity of oxygen. The temperature of the same animal at different times is connected with the stale of the respi- ration. When the blood circulates sluggishly, and the temperature is low, the quantity of oxygen consumed is comparatively small; but, on the con- trary, a large quantity of that gas disappears when the circulation is brisk, and the power of generating heat energetic. It has also been observed, especially by Crawford and De Laroche, that when an"animal is placed in a very warm atmosphere, so as to require little heat to be generated within his own body, the consumption of oxygen is unusually small, and the blood within the veins retains the arterial character. The connexion between the power of generating heat and respiration has 616 ANIMAL HEAT. been illustrated in a very pointed manner by Dr. Edwards. Some young animals, such as puppies and kittens, require so small a quantity of oxygen for supporting life, that they may be deprived of that gas altogether for twenty minutes without material injury ; and it is remarkable that so long as they possess this property, the temperature of their bodies sinks rapidly by free exposure to the air. But as they grow older they become able to maintain their own temperature, and at the 6ame time their power to endure ■ the privation of oxygen ceases. The same observation applies to young sparrows, and other birds which are naked when hatched; while young partridges, which are both fledged and able to retain their own temperature at the period of quitting the shell, die when deprived of oxygen as rapidly as an adult bird. The first consistent theory of the production of animal heat was proposed by Dr. Crawford. This theory was founded on the assumption that the car- bonic acid contained in the breath is generated in the lungs, and that its for- mation is accompanied with disengagement of caloric. But since the tem- perature of the lungs is not higher than that of other internal organs, and arterial very little if at all warmer than venous blood, it follows that the greater part of the caloric, instead of becoming free, must in some way or other be rendered insensible. Accordingly, on comparing the specific caloric of arterial and venous blood, Dr. Crawford found the capacity of the former to exceed that of the latter in the ratio of 1030 to 892. He, therefore, in- ferred that the dark blood within the veins, at the moment of being arterial- ized, acquires an increase of insensible caloric; and that while circulating through the body, and gradually resuming the venous character, it suffers a diminution of capacity, and evolves a proportional degree of heat. Unfortunately for the hypothesis of Crawford, one of the leading facts on which it is founded has been called in question; Dr. Davy maintaining, on the authority of his own experiments, that there is little or no difference be- tween the capacity of venous and arterial blood. (Philos. Trans, for 1814.) If this be true, the hypothesis itself necessarily falls to the ground. One part of the doctrine of Crawford may, however, in a modified form, be ap- plied to the theory of respiration advocated by Dr. Edwards: For if oxygen be absorbed by the blood in its passage through the lungs, and carbonic acid, ready formed, be emitted in return, it follows that this gas must be generated during the course of the circulation; and it may be inferred that the heat developed in consequence of this chemical change is at once communicated to the adjacent organs. In this way the question concerning the capacity of the blood for caloric may be entirely disregarded. While some physiologists have been disposed to refer the source of animd heat entirely to the alternate changes of venous to arterial, and of arterial to venous blood, others have denied its agency altogether, ascribing the evolu- tion of caloric solely to the influence of the nervous system. The chief foundation for this opinion is in the experiments of Mr. Brodie, who inflated the lungs of animals recently killed by narcotic poisons or division of the spinal marrow. (Phil. Trans, for 1811 and 1812.) In an animal so treated, the blood continued to circulate, oxygen gas disappeared, carbonic acid was evolved, and the usual changes of colour took place with regularity; but notwithstanding the concurrence of all thefe circumstances, the temperature fell with equal if not greater rapidity than in another animal killed at the same time, but in which artificial respiration was not performed. Were these experiments rigidly exact, they would lead to the opinion that no caloric is evolved by the mere process of arterialization. This inference, however, cannot be admitted for two reasons:—first, because other physiolo- gists, in repeating the experiments of Brodie, have found that the process of cooling is retarded by artificial respiration; and, secondly, because it is dif. ficult to conceive why tho formation of carbonic acid, which uniformly gives rise to increase of temperature in other cases, should not be attended within the animal body with a similar effect. It may hence be inferred, that this is one of tho sources of animal heat It is certain, however, that the heat SALIVA. 617 of animals cannot be maintained by the sole process of arterialization. Con- sistently with this fact, the researches of Dulong and Despretz agree in prov- ing, in opposition to the results obtained by Lavoisier and Crawford, that a hedthy animal imparts to surrounding bodies a quantity of heat considera- bly greater than can be accounted for by the combustion of the carbon thrown off during the same interval from the lungs in the form of carbonic acid. (An. de Ch. et de Ph. xxvi.) Though the influence of the nervous system over the developement ot animal heat is no longer doubtful, physiologists arc not agreed as to the mode by which it operates. Its action may either be director indirect; that is, the nerves may possess some specific power of generating heat, or they may excite certain operations by wliich the same effect is occasioned. It is far from improbable, that the nerves act more by the latter than the former mode; that the infinite number of chemical phenomena going on in the mi- nute arterial branches during the processes of secretion and nutrition, pro- cesses which are entirely dependent on the nervous system, are attended with disengagement of caloric. This view has, at least, been ably defended by Dr. Williams in an essay published in the Medico-Chir. Trans, of Edin- burgh. (Vol. ii.) SECTIOX II. SECRETED FLUIDS SUBSERVIENT TO DIGESTION. Saliva. The saliva is a slightly viscid liquor, secreted by the salivary glands. When mixed with distilled water, a flaky matter subsides, which is mucus, derived apparently from the lining membrane of the mouth. .The clear so- lution, when exposed to the agency of galvanism, yields a coagulum, and is hence inferred by Mr. Brande to contain albumen; but the quantity of this principle is so very small that its presence cannot be demonstrated by any other reagent. The greater part of the animal matter remaining in the liquid is peculiar to the saliva, and may be termed salivary matter. It is soluble in water, insoluble in alcohol, and, when freed from the accompany. ing salts, is not precipitated by subacetate of lead, corrosive sublimate, or infusion of gall-nuts. The saliva likewise contains a small quantity of ani- mal matter, which is soluble both in alcohol and water, and which is sup- posed by Tiedemann and Gmelin to be osmazome. The solid contents of the saliva, according to Berzelius, do not exceed seven in 1000 parts, the rest being water. From the analysis of Tiedemann and Gmelin, the chief saline constituent is chloride of potassium ; but seve- ral other salts, such as the sulphate, phosphate, acetate, and carbonate of potassa, are likewise present in small quantity. The saliva of the human subject, according to the same authority, contains very little soda. The pro- perty which the saliva possesses of striking a red colour with a persalt of iron is owing to sulpho-cyanuret of potassium. This sdt exists also in the saliva of the sheep; but it has not been found in that of the dog. The sa- liva of the sheep contains so much carbonate of soda, that it effervesces with acids. The only known use of the saliva is to form a soft pulpy mass with the food during mastication, so as to reduce it into a state fit for being swal- lowed with facility, and for being more readily acted on by the juices of the stomach. Concretions are sometimes found in the salivary glands and ducts. A stone contained in the salivary, gland of an ass was found by M. Caventou to con- 618 FANCREATIC AND GASTRIC JUICE. tain 91.6 parts of carbonate of lime, 4.8 of phosphate of lime, and 3.6 of ani- mal matter. A salivary concretion of a horse was found by M. Henry, jun. to consist of carbonate of lime 85.52, carbonate of magnesia 7.56,-phosphate of lime 4.40, and 2.48 of animal matter. Carbonate of lime is the chief in- gredient of salivary concretions. Pancreatic Juice. This fluid is commonly supposed to be analogous to the saliva, but it ap- pears from the analys is of Tiedemann and Gmelin that it is essentially dif- ferent. The chief animal matters are albumen, and a substance like curd; but it also contains a small quantity of salivary matter and osmazome. It reddens litmus paper, owing to the presence of free acid, which is supposed to be the acetic. Its salts are nearly the same as those contained in the sa- liva, except that sulphocyanic acid is wanting. The uses of this fluid are entirely unknown. Gastric Juice. The gastric juice, collected from the stomach of an animal killed while fasting, is a transparent fluid which has a saline taste, and has neither an acid nor alkaline reaction. During the process of digestion, on the contrary, it is found to be distinctly acid. Thus free hydrochloric acid was detected under these circumstances by Dr. Prout in the stomach of the rabbit, hare, horse, calf, and dog (Phil. Trans. le\24); and he has discovered the same acid in the sour matter ejected from the stomach of persons labouring under indigestion, a fact wliich has since been confirmed by Mr. Children. Tiede- mann and Gmelin observed that the secretion of acid commences as soon as the stomach receives the stimulus of food or any foreign body. This effect is occasioned, for example, by the presence of flint stones or other indigesti- ble matters; but it is produced in a still greater degree by substances of a stimulating nature. According to their observation the acidity is owing to the secretion of free hydrochloric and acetic acids. The gastric juice coagulates milk, apparently in consequence of the acid secreted during digestion. According to the experiments of Spallanzani and Stevens it is highly antiseptic, not only preventing putrefaction, but render- ing meat fresh after it is tainted. But of all the properties of the gastric juice, its solvent virtue is the most remarkable, being that on which depends the first stage of the process of digestion. When the food is introduced into the stomach, it is there intimately mixed with the gastric juice, by the agency of which it is dissolved, and converted into a semifluid matter called chyme. That this change is really owing to the solvent power of the gastric juice fully appears from the researches of Spallanzani, Reaumur, and Stevens. In the experiments of Dr. Stevens, described in his Inaugural Dissertation, the common articles of food were enclosed in hollow silver spheres perfo- rated with holes, and i-.fter remaining for some time within the stomach, completely protected from pressure and trituration, the alimentary sub- stances were found to have been entirely dissolved. A similar effect takes place when nutritious matters, out of the body, are mixed with the gastric fluid, and the mixture is exposed to a temperature of 100°. So great, in- deed, is the solvent power of this fluid, that it has been known to dissolve the coats of the stomach itself; at least the corrosions of this organ some- times witnessed in persons who have died suddenly while fasting, and in good health, were ascribed by the celebrated physiologist, John Hunter, to this cause. That the agent here assigned is adequate to produce such an effect, has been fully proved by my colleague, Dr. Carswell. (Edin. Med. and Surg. Journal, October, 1830.) Rabbits were killed by a blow on the head during digestion, and then suspended for some hours by the hinder legs. The most dependent parts of the stomach, to which the fluids had gravitated, were invariably more or less dissolved: in some cases the tex- BILE. 619 tures were thin, white, soft, and pulpy; and in others, complete perforations existed, and the contiguous viscera were attacked. The blood in the vessels of the corroded part was black and more or less coagulated, an effect ando- gous to that produced by an acid. The corroding fluid, as during healthy digestion, was strongly acid ; and this acid liquor, taken from the stomach of one rabbit and introduced into that of another previously killed, produced corrosion. Great diversity of opinion has prevailed respecting the cause of the sol- vent property of the gastric fluid. It was formerly ascribed to some specific power, and was thought to be inexplicable on any known chemical princi- ples ; but the more precise observation of recent experimentdists has re- moved one great part of the mystery. Tiedemann and Gmelin directly ascribe the solvent action of the gastric juice to the acid which it contains: they found healthy digestion to be invariably attended with the secretion of hydrochloric and acetic acids, and ascertained that these acids, at the tem- perature of the body, are capable of dissolving all the digestible substances employed as food. Similar remarks on the invariable connexion between the acidity and solvent power of the gastric fluids have been made by Dr. Carswell, who informs me of the additional decisive fact, that on neutralizing the gastric juice with magnesia, its solvent property was destroyed. It would thus seem that the stimulus of food causes the neutral salts of the blood circulating in the stomach to be decomposed, either by a purely vital process, or, as Dr. Prout suggests, by a galvanic operation (Bridgewater Treatise); that the dkali remains in the blood, causing the alkalinity of that liquid; and that the acids, passing into the stomach, dissolve the food. Bile. The bile is a yellow or greenish-yellow coloured fluid, of a peculiar sick- ening odour, and of a taste at first sweet and then bitter, but exceedingly nauseous. Its consistence is variable, being sometimes limpid, but more commonly viscid and ropy. It is rather denser than water, and may be mixed with that liquid in every proportion. It contains a minute quantity of free soda, and is, therefore, slightly alkaline; but owing to the colour of the bile itself, its action on test paper is scarcely visible. Of the chemists who have of late years investigated the composition of the bile, Thenard, Berzelius, Tiedemann, and Gmelin deserve particular men- tion. In an elaborate essay published in the Memoires d'Arcueil, vol. i. Thenard endeavoured to show that the bile of the ox consists of three dis- tinct animal principles, a yellow colouring matter, a species of resin, and a peculiar substance, to which, from its sweetish bitter taste, he applied the name of picromel. According to his analysis, 800 parts of bile consist of water 700 parts, resin 15, picromel 69, yellow matter about 4, soda 4, phos- phate of soda 2, muriates of soda and potassa 3.5, sulphate of soda 0.8, phos- phate of lime and perhaps magnesia 1.2, and a trace of oxide of iron. He supposed the resin to be combined with the picromel and soda, and ascribes its solubility in water to this cause. Berzelius takes a totally different view of the constitution of the bile. He denies that this fluid contains any resinous principle, and regards the yd- low matter, resin, and picromel of Thenard, as one and the same substance, to which he applies the name of biliary matter. (Medico-Chir. Trans, iii.) Tiedemann and Gmelin, however, in their recent work on Digestion, admit the existence of picromel and resin as the chief constituents of bile; although it appears from their experiments that the substance described by Thenard as picromel was not pure, but contained a portion of resin. According to the analysis of these chemists, the bile of the ox is a very complex fluid, consisting of the following ingredients:—water to the extent of 91.5 per cent; a volatile odoriferous principle; cholesterine; resin; asparagin- pi cromel; yellow colouring matter; a peculiar azotized substance soluble in water and alcohol; a substance which is soluble in hot alcohol,bat insolu- 620 BILIARY CONCRETIONS. ble in water, supposed to be gluten; osmazome; a principle which emits a urinous odour when heated; a substance analogous to albumen or caseous matter; and mucus. The salts of the bile are the margarate, oleate, acetate, chelate, bicarbonate, phosphate, sulphate, and hydrochlorate of soda, together with a little phosphate of lime. The cholic is a peculiar animal acid, which crystallizes in needles, reddens litmus paper, and is distinguished from analogous compounds by having a sweet taste. The flaky precipitate which ie occasioned by adding acids to bile from the ox, consists of several substances. At first the caseous and colouring mat- ters, along with mucus, are thrown down; and, afterwards, the margaric acid and a compound of picromel and resin with the acid employed are pre- cipitated. When acetate of lead is mixed with this fluid, a white precipi- tate falls, which consists of oxide of lead combined with the phosphoric, sul- phuric, and several other acids, together with a small quantity of the compound of picromel and resin. On adding subacetate of lead to the clear liquid, a copious precipitate ensues, consisting chiefly of picromel, resin, and oxide of lead. If this compound be suspended in water, through which a current of hydrosulphuric acid gas is transmitted, sulphuret of lead and the resin subside, while the picromel remains in solution. By collecting and drying the precipitate, and digesting it in alcohol, the resin is dissolved, and maybe obtained by evaporation. The aqueous solution, when evaporated, yields the picromel of Thenard ; but according to Tiedemann and Gmelin, it still contains a portion of resin. The chief difficulty, indeed, of preparing pure picromel arises from its tendency to dissolve the resin; and the only mode of separation is by throwing them down repeatedly by means of sub- acetate of lead. By this process the affinity of the picromel and resin for each other is gradually lessened, until at length the separation is rendered complete. Pure picromel occurs in opaque, rounded, crystalline particles, is soluble in water and alcohol, but is insoluble in ether. Its taste is sweet without any bitterness; but it cannot be regarded as a species of sugar, because a large quantity of nitrogen enters into its composition. Its aqueous solution is not precipitated by acids, nor by acetate and subacetate of lead. AVhen digested with the resin of bile, a portion of the latter is dissolved, and a solution is obtained, which has both a bitter and sweet taste, and yields a precipitate with subacetate of lead and the stronger acids. This is the com- pound which causes the peculiar taste of the bile. The bile of the human subject has not been studied so minutely as that of the ox. According to Thenard it consists, besides salts, of water, colouring matter, albumen, and a species of resin. Chevallier has since detected picro- mel, and Chevreul cholesterine, in human bile; and both these discoveries have been confirmed by the observations of Tiedemann and Gmelin. The derangement which takes place in the system when the secretion of bile or its passage into the intestines is arrested, is a sufficient indication of the importance of this fluid. It acts as a stimulus to the intestinal canal generally, and produces on the chyme some peculiar change, which is essen- tial to its conversion into chyle. Biliary Concretions. The concretions which arc sometimes formed in the human gall-bladder have been particularly examined by Fourcroy, Thenard, and Chevreul. Four- croy found that they consist chiefly of a peculiar fatty matter, resembling spermaceti, which he included under the name of adipocire (page 600); and the experiments of Thenard tended to confirm this view. According to Chevreul, however, biliary concretions in general are composed of the yellow eolouring matter of the bile and cholsterine, the latter predominating, and being sometimes in a state of purity; and I have had frequent opportunities of satisfying myself of the accuracy of this observation. These substances may easily bo separated frpra each other by boiling alcohol, which dissolves CHYLE. 621 the cholesterine, and leaves the colouring matter; or by digestion in dilute potassa, in which the colouring matter is dissolved, while the cholesterine is insoluble. Gall-stones sometimes contain a portion of inspissated bile; and in some rare instances the cholesterine is entirely wanting. The concretions found in the gdl-bladder of the ox consist almost entirely of the yellow biliary colouring matter, which, from the beauty and perma- nence of its tint, is much valued by painters. This substance is readily dis- tinguished by its yellow or brown colour, by insolubility in water and alco- hol, and by being readily dissolved by a solution of potassa. The solution has at first a yellowish-brown colour, which gradually acquires a green tint, and is precipitated in green flocks by hydrochloric acid. According to the observations of Tiedemann and Gmelin, the colouring matter is influ- enced by the presence of oxygen gas. The yellowish precipitate, occasioned by adding hydrochloric acid to bile, absorbs oxygen by exposure to the air, and its colour changes to green. The action of nitric acid is still more remark- able. By successive additions of this acid, the tint of the colouring matter may be converted into green, blue, violet, and red, in the course of a few seconds. Erythrogen.—This substance was discovered in 1821 by M. Bizio of Venice in a peculiar fluid, quite different from bile, which was found in the gall- bladder of a person who had died of jaundice. It is of a green colour, trans- parent, tasteless, and of the odour of putrid fish. It is unctuous to the touch, may be scratched or cut with facility, and has a specific gravity of 1.57. It does not affect the colour of litmus or turmeric paper. At 110° it fuses, having the appearance of oil, and crystallizes when slowly cooled; and at 122° it rises in the form of vapour. It is insoluble in water and ether, but is dissolved readily by hot alcohol; and the solution, by partial evaporation and cooling, yields crystals in the form of rhomboidal parallelopipedons. When erythrogen is put into nitric acid of the temperature of about 120 or 140° its green tint disappears, effervescence, owing to the escape of oxy- gen gas, ensues, and the solution acquires a deep purple colour. A similar phenomenon takes place, with disengagement of hydrogen gas, when erythro- gen is digested in a solution of ammonia; and when volatilized in the open air, it yields a purple-coloured vapour. M. Bizio is of opinion that the ery thro- gen, under all these circumstances, unites with nitrogen, and that the pro- duct is identical with the colouring matter of the blood. The production of the red compound is characteristic of erythrogen, and suggested the name by which this substance is designated. (Egt/flgif, ruber). (Journal of Science, vol. xvi.) Erythrogen has not been discovered either in bile or in any of the animal fluids. SECTION III. CHYLE, MILK, AND EGGS. Chyle.—-The fluid absorbed by the lacteal vessels from the small intes- tines during the process of digestion is known by the name of chyle. Its appearance varies in different animals; but as collected from the thoracic duct of a mammiferous animal three or four hours after a meal, it is a white opaque fluid like milk, having a sweetish and slightly saline taste. In a few minutes after removal from the duct it becomes solid, and in the course of twenty-four hours separates into a firm coagulum, and a limpid liquid, which may be called the serum of the chyle. The coagulum is an opaque white substance, of a slightly pink hue, insoluble in water, but soluble easily in 622 MILK. the alkalies and alkaline carbonates. Vauquelin* regards it as fibrin in an imperfect state, or as intermediate between that principle and albumen; but Mr. Brandet considers it more closely allied to the caseous matter of milk than to fibrin. The serum of chyle is rendered turbid by heat, and a few flakes of albii, men are deposited; but when boiled after being mixed with acetic acid, a copious precipitation ensues. To this substance, which thus differs slightly from albumen, Dr. Prout has applied the name of incipient albumen. The same chemist has made a comparative analysis of the chyle of two dogs, one of which was fed on animal and the other on vegetable substances, and the result of his inquiry is as follows:—(Annals of Philos. vol. xiii. p. 25.) Vegetable Animal Food. Food. Water ....;. 93.6 89.2 Fibrin ...... 0.6 0.8 Incipient albumen ? . . . .4.6 4.7 Albumen with a little red colouring matter . 0.4 4.6 Sugar of milk ? . . . . a trace — Oily matters .... a trace a trace Saline matters . . . . .0.8 0.7 100.0 100.0 Milk.—This well-known fluid, secreted by the females of the class mam- malia for the nourishment of their young, consists of three distinct parts, the cream, curd, and whey, into which by repose it spontaneously separates. The cream, which collects upon its surface, is an unctuous yellowish-white opaque fluid, of agreeable flavour. According to Berzelius 100 parts of cream, of specific gravity 1.0244, consist of butter 4.5, caseous matter 3.5, and whey 92. By agitation, as in the process of churning, the butter as- sumes the solid form, and is thus obtained in a separate 'state. During the operation there is an increase of temperature amounting to about three or four degrees, oxygen gas is absorbed, and an acid is generated;. but the ab- sorption of oxygen cannot be an essential part of the process, since butter may be obtained by churning, even when atmospheric air is entirely ex- cluded. After the cream has separated spontaneously, the milk soon becomes sour, and gradually separates into a solid coagulum called curd, and a limpid fluid which is whey. This coagulation is occasioned by free acetic acid, and it may be produced at pleasure either by adding a free acid, or by means of the fluid known by the name of rennet, which is made by infusing the inner coat of a calf's stomach in hot water. When an acid is employed, the curd is found to contain some of it in combination, and may, therefore, be re- garded as an insoluble compound of an acid with the caseous matter of milk. The action of rennet requires further examination: it confessedly acts by means of the gastric fluid which it contains, and hence its coagulating power, consistently with the facts stated in the last section, is referable to the aci- dity of that juice. The curd of skim milk, made by means of rennet, and separated from the whey by washing with water, is generally considered to be caseous mat- ter, or the basis of cheese in a state of purity. In this state it is a white, insipid, inodorous substance, insoluble in water, but readily soluble in the alkalies, especially in ammonia. By alcohol it is converted, like albumen and fibrin, into an adipocirous substance of a fetid odour; and, like the same substance, it may be dissolved by a sufficient quantity of acetic acid. * An. de Ch. vol. xxxi. t Philos. Trans, for 1812. MILK. 623 Braconnot maintains that caseum, in its coagulated state, is always com- bined with some foreign substance, generally an earthy salt or an acid, on which its insolubility depends; and that when pure, it is soluble both in hot and cold water, is not coagulated either by heat or air, and when concen- trated becomes viscid like mucilage, being so highly adhesive that it may be usefully employed as a cement. Soluble caseum may be obtained from curd, spontaneously formed in milk as it becomes sour, in which state it is combined with acetic acid, by washing the curd, and digesting it with water, to which so much carbonate of potassa is added as is sufficient to unite with the acetic acid. Acetate of potassa is generated with disengage- ment of carbonic acid, and the caseum is dissolved. In order to separate it from the accompanying acetate, the solution, after separating the cream Wliich collects on its surface by repose, is mixed with a little sulphuric acid, and the precipitated sulphate of caseum, carefully washed, is dissolved in water by means of the smallest" possible quantity of carbonate of potassa. If dcohol is then freely employed, the caseum itself is thrown down; but if the solution is mixed with about its own volume of alcohol, a deposite of sulphate of potassa with some curd and cream takes place, and the filtered liquor contains caseum in a state of great purity. Caseum, as thus prepared, still contains a little potassa; but Braconnot considers its solubility as not dependent on the presence of the alkali. When evaporated to dryness it forms a diaphanous mass which strongly re- sembles gum-arabic, may be long preserved without change, and still retains its solubility in water. It has an acid reaction, and combines readily with the dkalies, forming very soluble compounds. With other metallic oxides, as well as with their salts, it forms sparingly soluble compounds. Its affi- nity for acids is equally marked, and it is precipitated by all the mineral acids, except the phosphoric. Braconnot conceives that soluble caseum may be advantageously employed in a commercial point of view. Its adhesive- ness fits it as a cement for glass, porcelain, wood, and paper. Its solution, flavoured with sugar and aromatics, may be serviceable to convdescents as an article of food. It may be taken in its dry state in long voyages, form- ing together with water, butter, and sugar, an excellent substitute for milk. (An. de Ch. et de Ph. xliii. 337.) Caseum is commonly considered to have a close resemblance to animal albumen, and the analogy is supported by its being coagulated by acids. In other respects, if the remarks of Braconnot prove correct, it resembles gum rather than albumen. It differs from both, however, in the nature of the spontaneous changes to which it is subject; for when kept in a moist state, it undergoes a species of fermentation precisely analogous to that expe- rienced by gluten under the same circumstances (page 568). The accuracy of the remarks made by Proust on this subject has been questioned by Bra- connot (Brewster's Journal, viii. 369). The latter states that, in his experi- ments, the curd from spontaneously coagulated skim milk, covered with water, and kept at a temperature of about 75°, underwent complete putre- faction in the space of a month. The soluble parts were then filtered, and by evaporation yielded a product of a very fetid odour, acetate of ammonia, and acetic acid. The residue, after being reduced to the consistence of syrup, concreted on cooling into a granulated reddish mass like honey, but of a saline bitter taste, and was separated by the action of alcohol into two parts, one soluble and the other insoluble. The former is the caseate of ammonia of Proust, and the latter is his caseous oxide. In order to obtain caseous oxide quite pure, it must be washed carefully with alcohol, treated with animal charcoal, and dissolved repeatedly in boil- ing water, from which it is separated by evaporation. In this state it is a beautiful white powder, inodorous, and of a slightly bitter taste. It is heavier than water, and soluble in 14 parts of that fluid at 72°. On allow- ing the solution to evaporate spontaneously, it crystallizes either in the form of elegant dendritic ramifications, or in rings composed of delicate acicular crystals of a silky lustre. 624 EGGS. Caseous oxide is almost entirely insoluble even in boiling dcohol. Its aqueous solution yields a white flaky precipitate with infusion of gall-nuts, soluble in excess of the precipitant; and subacetate of lead likewise throws down a white precipitate. The crystals, if suddenly heated, volatilize with- out change; but if the heat is gradually raised, decomposition ensues, and a large quantity of carbonate and hydrosulphate of ammonia is generated. When strongly heated in open vessels it takes fire, and burns with flame without residue. The composition of caseous oxide has not been determined, but from the facility with which its aqueous solution putrefies, Braconnot regards it as a highly azotized animal principle. It contains sulphur also. He believes it to be a product of the putrefaction of all animal substances, and proposes for it the name of aposepedine, from ano and rnirtfuy, result of putrefaction, as more appropriate than caseous oxide. Braconnot denies the existence of caseic acid. Proust's caseate of am- monia consists of various substances, such as free acetic acid, aposepedine, animal matter, resin, several salts, and a yellow pungent oil, which is the chief cause of the pungency of old cheese. From 750 parts of curd completely putrefied were obtained 36 of dry matter insoluble in water. These consisted of 14.92 parts of margarate of lime, 2.57 of margaric acid, and 18.51 of oleic acid, retaining margaric acid and a brown animal matter. According to the analysis of Gay-Lussac and Thenard, 100 parts of the caseous matter are composed of carbon 59.781, hydrogen 7.429, oxygen 11.409, and nitrogen 21.381. It yields by incineration a white ash amount- ing to 6.5 per cent, of its weight, the greater part of which is phosphate of lime, a circumstance which renders caseous matter an article of food pecu- liarly proper for young animals. Milk carefully deprived of its cream has a specific gravity of about 1.033; and 1000 parts of it, according to Berzelius, are thus constituted:—water 928.75, caseous matter with a trace of butter 28; sugar of milk 35; muriate and phosphate of potassa 1.95; lactic acid, acetate of potassa, and a trace of lactate of iron 6, and earthy phosphates 0.3. Subtracting the caseous matter, the remaining substances constitute whey. Eggs.—The composition of the recent egg and the changes which it un- dergoes during the process of incubation have been ably investigated by Dr. Prout (Phil. Trans, for 1822). New-laid eggs are rather heavier than water; but they become lighter after a time, in consequence ofwater evapo- rating through the pores of the shell, and air being substituted for it. An egg of ordinary size yields to boiling water about three-tenths of a grain of saline matter, consisting of the sulphates, carbonates, and phosphates of lime and magnesia, together with animal matter and a little free alkali. Of an egg which weighs 1000 grains, the shell constitutes 106.9, the white 604.2, and the yelk 288.9 grains. The shell contains about two per cent, of animal matter, one per cent of the phosphates of lime and mag- nesia, and the residue is carbonate of lime with a little carbonate of mag. When the yelk of a hard-boiled egg is repeatedly digested in alcohol of specific gravity 0.807, until that fluid comes off colourless, there remains a white pulverulent residuum, possessed of many of the properties of albumen, but distinguished from that principle by containing a large quantity of phosphorus in some unknown state of combination. The alcoholic solution is of a deep yellow colour, and on cooling deposites crystals of a sebaceous matter, and a portion of yellow semi-fluid oil. On distilling off the alcohol, the oil is left in a separate state. When the yelk is dried and burned, the phosphorus is converted into phosphoric acid, which, melting into a glass upon the surface of the charcoal, protects it from complete combustion. In the white of the egg, which consists chiefly of albumen, sulphur is present . The obvious use of the phosphorus contdned in the yelk is to supply LIQUIDS OF SEROUS AND MUCOUS SURFACES, &C. 625 phosphoric acid for forming the bones of the chick; but Dr. Prout was un- able to discover any source of the lime with which that acid unites to form the earthy part of bone. It cannot be discovered in the soft parts of the egg; and hitherto no vascular connexion has been traced between the chick and its shell. SECTION IV. LIQUIDS OF SEROUS AND MUCOUS SURFACES, &c, The surface of the cellular membrane is moistened with a peculiar limpid transparent fluid called lymph, which is in very small quantity during health, but collects abundantly in some dropsical affections. Mr. Brande collected it from the thoracic duct of an animal which had been kept without food for twenty-four hours. Its chief constituent is water, besides which it contains muriate of soda and albumen, the latter being in such minute quantity that it is coagulated only by the action of galvanism. Lymph does not affect the colour of test-paper; but when evaporated to dryness, the residue gives a green tint to the syrup of violets. The fluid secreted by serous membranes in general, such as the pericar- dium, pleura, and peritoneum, is very similar to lymph. According to Dr. Bostock, 100 parts of the liquid of the pericardium eonsist of water 92 parts, dbumen 5.5, mucus 2, and muriate of soda 0.5. The serous fluid exhaled within the ventricles of the brain in hydrocephalus internus is composed, in 1000 parts, of water 988.3, albumen 1.66, muriate of potassa and soda 7.09, lactate of soda and its animd matter 2.32, soda 0.28, and animd matter so- luble only in water, with a trace of phosphates, 0.35. (Berzelius, in Medico- Chir. Trans, vol. iii. p. 252.) The liquor of the amnios, or the fluid contained in the membrane which surrounds the foetus in utero, differs in different animals. That of the hu- man femde was found by Vauquelin and Buniva to contain a small quantity of albumen, soda, muriate of soda, phosphate and carbonate of lime, and a matter like ourd which gives it a milky appearance. That of the cow was said by the same chemists to contain a peculiar acid, which has since been recognized as belonging to the allantois (page 596). Dr. Prout found some sugar of milk in the amnios of a woman. (An. of Phil. v. 417.) Humours of the Eye.—The aqueous and vitreous humours of the eye con- tain rather more than 80 per cent, of water. The other constituents are a small quantity of dbumen, muriate and acetate of soda, pure soda, though scarcely sufficient to affect the colour of test-paper, and animal matter solu- ble only in water, but which is not gelatin. (Berzelius.) The crystalline lens, besides the usual salts, contains 36 per cent, of a peculiar animal matter, very analogous to albumen if not identical with it. In cold water it is so- luble, but is coagulated by boiling. The coagulum, according to Berzelius, has all the properties of the colouring matter of the blood excepting its colour. The tears are limpid and of a saline taste, dissolve freely in water, and, owing to the presence of free soda, communicate a green tint to the blue in- fusion of violets. Their chief salts are chloride of sodium and phosphate of soda. According to Fourcroy and Vauquelin the animal matter of the tears is mucus; but it is more probably either albumen, or some analogous prin- ciple. Its precise nature has not however been satisfactorily determined. Mucus.—The term mucus has been employed in very different significa- tions. Dr. Bostock applies it to a peculiar animal matter which is soluble both in hot and cold water, is not precipitated by corrosive sublimate or so- 626 LIQUIDS OF SEROUS AND MUCOUS SURFACES, &C. lution of tannin, is not capable of forming a jelly, and which yields a preci- pitate with subacetate of lead. The existence of this principle has not, how- ever, been fully established; for the presence of muriatic and phosphoric acids, the latter of which is frequently contained in animal fluids, and the former scarcely ever absent, sufficiently accounts for the precipitates occa- sioned in them by the salts of lead or silver. But even supposing the opinion of Dr. Bostock to be correct, it would be advisable to give some new name to his principle, and apply the term mucus solely to the fluid secreted by mucous surfaces. The properties of mucus vary somewhat according to the source from which it is derived; but its leading characters are in all cases the same, and are best exemplified in mucus from the nostrils. Nasal mucus, accord- ing to Berzelius, has the following properties. Immersed in water, it im- bibes so much of that fluid as to become transparent, with the exception of a few particles which remain opaque. When dried on blotting-paper, it loses its transparency, but again acqdrcs it when moistened. It is not co- agulated or rendered horny by being boiled in water; but as soon as the ebullition has ceased, it collects unchanged at the bottom of the vessel. It is dissolved by dilute sulphuric acid. Nitric acid at first coagulates it; but by continued digestion, the mucus gradually softens and is finally dissolved, forming a clear yellow liquid. Acetic acid hardens mucus, and does not dis- solve it even at a boiling temperature. Pure potassa at first renders it more viscid, but afterwards dissolves it. By tannic acid mucus is coagulated, both when softened by the absorption of water, and when dissolved either in an acid or an alkali. Pus.—Purulent matter is the fluid secreted by an inflamed and ulcerated surface. Its properties vary according to the nature of the sore from which it is discharged. The purulent matter formed by an ill-conditioned ulcer is a thin, transparent, acrid, fetid ichor; whereas a healing sore in a sound constitution yields a yellowish-white coloured liquid, of the consistence of cream, which is described as bland, opaque, and inodorous. This is termed healthy pus, and is possessed of the following properties. Though it ap- pears homogeneous to the naked eye, when examined with the microscope it is found to consist of minute globules floating in a transparent liquid. Its specific gravity is about 1.03. It is insoluble in water, and it is thickened, but not dissolved, by alcohol. When recent, it does not affect the colour of test-paper; but by exposure to the air it becomes acid. The dilute acids have little effect upon it; but strong sulphuric, nitric, and muriatic acids dissolve it, and the pus is thrown down by dilution with water. Ammonia reduces it to a transparent jelly, and gradually dissolves a considerable por- tion of it. With the fixed alkalies it forms a whitish ropy fluid, which is decomposed by water. The composition of pus has not been ascertained with precision; but its characteristic ingredient is more closely allied to albumen* than the other animal principles. Several attempts have been made to discover a chemical test for distin- guishing pus from mucus. When these fluids are in their natural state, the appearance of each is so characteristic that the distinction cannot be at- tended with any difficulty ; but, on the contrary, when a mucous surface is inflamed, its secretion becomes opaque, and, as sometimes happens in some pulmonary diseases, acquires more or less of the aspect of pus. Mr. Charles Darwin, who examined this subject, pointed out three grounds of distinc- tion between them. 1. When the solution of these liquids in sulphuric acid is diluted, the pus subsides to the bottom, and the mucus remains sus- pended in the water. 2. When pus and catarrhal mucus are diffused through water, the former sinks, and the latter floats. 3. Pus is precipitated from its solution in potassa by water, while the solution of mucus is not decom- posed by similar treatment Dr. Thomson, in his System of Chemistry, has given the following test on the authority of Grasmeyer. The substance to be examined, after being triturated with its own weight of water, is mixed URINE. 627 with an equal quantity of a saturated solution of carbonate of potassa. If it contain pus, a transparent jelly forms in a few hours; but this does not hap- pen if mucus only is present Dr. Young, in his work on Consumptive Diseases, has given a very elegant character for distinguishing pus, founded on its optical properties. But the practical utility of tests of any kind is rendered very questionable by the fact that inflamed mucous membranes may secrete genuine pus without breach of surface, and that the natural passes into purulent secretion by insensible shades. Sweat.—Watery vapour is continually passing off by the skin in the form of insensible perspiration; but when the external heat is considerable, or violent bodily exercise is taken, drops of fluid collect upon the surface, and constitute what is called sweat. This fluid consists chiefly ofwater; but it contains some muriate of soda and free acetic acid, in consequence of which it has a saline taste and an acid reaction. SECTION V. URINE AND URINARY CONCRETIONS. Urine. The urine differs from most of the animal fluids which have been de- scribed by not serving any ulterior purpose in the animal economy. It is merely an excretion designed for ejecting from the system substances, which, by their accumulation within the body, would speedily prove fatal to health and life. The sole office of the kidneys, indeed, appears to consist in separating from the blood the superfluous matters that are not required or adapted for nutrition, or which have already formed part of the body, and been removed by absorption. The substances which in particular pass off by this organ are nitrogen, in the form of highly azotized products, and various saline and earthy compounds. This sufficiently accounts for the great diversity of different substances contained in urine. The quantity of the urine is affected by various causes, especially by the nature and quantity of the liquids received into the stomach; but on an ave- rage, a healthy person voids between thirty and forty ounces daily. The quality of this fluid is likewise influenced by the same circumstances, being sometimes in a very dilute state, and at others highly concentrated. The urine voided in the morning by a person who has fed heartily, and taken no more fluids than is sufficient for satisfying thirst, may be regarded as afford- ing the best specimen of natural healthy urine. The urine in this state is a transparent limpid fluid of an amber colour, having a saline taste, and while warm emitting an odour which is slightly aromatic, and not at all disagreeable. The average sp. gr. of the urine of fifty healthy men, observed by the late Dr. Gregory in autumn at mid-day, is 1.02246. It gives a red tint to litmus paper, a circumstance which indi- cates the presence either of a free acid or of a supersalt Though at first quite transparent, an insoluble matter is deposited on standing; so that urine, voided at night, is found to have a light cloud floating in it by the following morning. This substance consists in part of mucus from the urinary pas- sages, and partly of superurate of ammonia, which is much more soluble in warm than in cold water. The urine is very prone to spontaneous decomposition. When kept for two or three days it acquires a strong urinous smell; and as the putrefaction proceeds, the disagreeable odour increases, until at length it becomes exceed- 628 URINE. ingly offensive. As soon as these changes commence, the urine ceases to have an acid reaction, and the earthy phosphates are deposited. In a short time, a free alkali makes its appearance, and a large quantity of carbonate of ammonia is gradually generated. Similar changes may be produced in recent urine by continued boiling. In both cases the phenomena are owing to the decomposition of urea, which is almost entirely resolved into carbo- nate of ammonia. The composition of the urine has been studied by several chemists, but the most recent and elaborate analysis of this fluid is by Berzelius. Ac- cording to the researches of this indefatigable chemist, 1000 parts of urine are composed of Water . 933.00 Urea . 30.10 Uric acid . 1.00 Free lactic acid, lactate of ammonia, and ammal matter not separable from them 17.14 Mucus of the bladder , 0.32 Sulphate of potassa 3.71 Sulphate of soda 3.16 Phosphate of soda * 2.94 Phosphate of ammonia • 1.65 Muriate of soda . 4.45 Muriate of ammonia .... 1.50 Earthy matters, with a trace of fluate of lime 1.00 Siliceous earth 0.03 Besides the ingredients included in the preceding list, the urine contains several other substances in small quantity. From the property this fluid possesses of blackening silver vessels in which it is evaporated, owing to the formation of sulphuret of silver, Proust inferred the presence of unoxidized sulphur; and Dr. Prout, from the odour of phosphuretted hydrogen, which he thinks he has perceived in putrefying urine, suspects that phosphorus is likewise present. The urine also contains a peculiar yellow colouring mat- ter which has not hitherto been obtained in a separate state. From the pre- cipitate occasioned in urine by the infusion of gall-nuts, the presence of gelatin has been inferred; but this effect appears owing to the presence not of gelatin but of a small portion of albumen. According to Scheele, the urine of infants sometimes contains benzoic acid, a compound which, when present, may be easily procured by evapora- ting the urine nearly to the consistence of syrup, and adding hydrochloric acid. The precipitate, consisting of uric and benzoic acids, is digested in dcohol, which dissolves the benzoic acid. Notwithstanding the high authority of Berzelius, it is very doubtful if any free acid be present in healthy urine. Dr. Prout, with every appearance of reason, maintains that the acidity of recent urine is occasioned by super- salts, and not by uncombined acid. He is of opinion that the acid reaction is chiefly, if not wholly, to be ascribed to the superphosphate of lime and superurate of ammonia, salts which he finds may co-exist in a liquid without mutual decomposition. A very strong argument, which to me indeed ap- pears conclusive, in favour of this view, is derived from the fact, that on adding hydrochloric acid to recent urine, minute crystals of uric acid are gradually deposited, as always happens when this acid subsides slowly from a state of solution; but, on the contrary, if no free acid is added, an amor- phous sediment, which Dr. Prout regards as superurate of ammonia is obtained. Such is the general view of the composition of human urine in its natural healthy state. But this fluid is subject to a great variety of morbid condi- tions, which arise either from the deficiency or excess of certain principles which it ought to contain, or from the presence of others wholly foreign to its composition. As the study of these affections affords an interesting ex- URINE. 629 ample of the application of chemistry to pathology and the practice of me- dicine, I shall briefly mention some of the most important morbid states of this fluid, referring for more ample detdls to the excellent treatise of Dr. Prout* Of the substances wliich, though naturally wanting, are sometimes con- tained in the urine, the most remarkable is sugar, which is secreted by tho kidneys in diabetes (page 592). Diabetic urine has a sweet taste, and yields a syrup by evaporation, is almost always of a pale straw colour, and in gene- ral has a greater specific gravity than ordinary urine. A specimen ot dia- betic urine, the sp. gr. of which was 1.03626, was found by Mr. Ivane to contain in 1000 parts, 913 ofwater, 60 of sugar, 6.5 of urea, and 20 of salts (Dublin Journal of Science, i. 2in. Owing to the large quantity of sugar and the dilute state of the urea, diabetic urine has little tendency to putrefy, and is susceptible of undergoing the vinous fermentation. The acidifying process which istonstantly going forward in the kidneys, as evinced by the formation of sulphuric, phosphoric, and uric acids, some- times proceeds to a morbid extent in consequence of which two acids, the oxalic and nitric, are generated, neither of wliich exists in healthy urine. The former, by uniting with lime, gives rise to one of the worst kinds of urinary concretions; and the latter, in the opinion of Dr. Prout, leads to the production of purpurate of ammonia, by reacting on uric acid. In severe cases of jaundice, the bile passes from the blood into the kidneys, and communicates a yellow colour to the urine. The most delicate test of its presence is hydrochloric acid, which causes a green tint. Though albumen is contained in very minute quantity in healthy urine, in some diseases it is present in large proportion. According to Dr. Black- dl it is characteristic of certain kinds of dropsy, accompanied with an in- flammatory diathesis, as in that which supervenes in scarlet fever; and Dr. Prout has described two cases of albuminous urine, in which, without any febrile symptoms, albumen existed in such quantity that spontaneous coagu- lation took place within the bladder. Dr. Bright, in his Medical Reports, has shown that dropsical effusions with albuminous urine are often associated with di-ease of the kidney, and his observations have been confirmed by the experience of Dr. Christison and the late Dr. Gregory (Edin. Med. and Surg. Journal for Oct. 18-2'J, Oct. 1831, and Jan. 163:2). In this affection the urine is scanty, and its sp. gravity, from deficient quantity of saline matter and urea, is lower than natural, being in the average of 50 cases 1.01318, and is rarely so high as 1.02: this is so constant, that scanty albuminous urine of a low sp. gravity was considered by Dr. Gregory as nearly a sure indication of renai disease. The albumen is readily detected by the coagulating effect of heat or by ferrocyanuret of potassium (page 589), the urine if not acid, which it frequently is, being always previously acidulated by acetic acid. In the blood of patients suffering under renal disease with albuminous urine, Dr. Bostock detected a crystalline substance resembling urea, and Dr. Chris- tison, pursuing the inquiry, obtained urea with all its characteristic pro- perties. Urea is secreted less abundantly than usual during inflammatory affec- tions of the liver, whether acute or chronic, in the renal disease just men- tioned, and during the hysteric paroxysm, in which latter affection the saline ingredients of the urine are secreted in unusual quantity. Urea is said to be wanting in diabetic urine, an error caused by the presence of sugar dimi- nishing the tendency of nitrate of urea to crystallize. Dr. Henry has shown that urea, when mixed with a considerable proportion of sugar, cannot be discovered by the usual test of nitric acid; and, consequently, though present in diabetic urine, it may easily be overlooked. 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