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THE   BOOK   OF
PHILOSOPHICAL    EXPERIMENTS.
ILLUSTRATING
             THE PRINCIPAL  FACTS  AND CURIOUS PHENOMENA  OF

ELECTRICITY,  GALVANISM,  MAGNETISM, OPTICS,
                        CHEMISTRY,  HEAT,  ETC.

WITH INTRODUCTORY OBSERVATIONS ON EACH SCIENCE, AND  UPWARDS OF
                                     300  "
BY  J
I  EXPERIMENTS.

.S.DALTON
                TO MY KIND FRIEND, JOHN ALLISTON,  ESQ.,
     One of the Directors of the Provincial Bank of Ireland, a Governor of Ckritt's Hospital, #c, ft.
I MOST RESPECTFULLY DEDICATE THIS LITTLE VOLUME, AS A MARK OP MY HIGH RESPECT
        AND ESTEEM FOR HIS CHARACTER, A\D AS A SLIGHT REMEMBRANCE OF HIS KINDNESS TO
                                                                         THE AUTHOR.
  $&-  This work has two objects in view.  First, to provide young persons with the means of obtaining a knowledge
of some of the most important phenomena of nature, and the applications of science to purposes of utility ; and secondly,
to furnish them with an almost inexhaustible fund of amusement for winter evenings, and other occasions, when ex-
ercises in the open air are obliged to make way for in-door recreations.
  There is no person, however illiterate, but experiences some  degree of pleasure on witnessing the performance of scien-
tific experiments ; and young persons are more particularly delighted with them. It isjioped, therefore, that the volume
now offered to their notice will be accepted with pleasure, as it will enable thein, not only to understand many curious facts
in nature, but will instruct them also how to perform a great variety of beautiful experiments themselves, without risk or
danger.
  For conveying instruction in the sciences, experiment is superior to every  other method.  A person who performs an ex-
periment, and thoroughly understands the  nature of it, will hardly ever forget the principle it illustrates ; because the fact
will be  impressed upon the memory in the strongest manner.  Young persons may  learn, by experiment, what cannot be
taught them by mere description ; and the natural curiosity of youth leads them to desire  earnestly to become acquainted
with the principal laws of nature. No amusement is more gratifying to them than to be engaged in this way ; and hence-
tha pleasure with which they peruse such works as the " Endless Amusements," and others  of a similar description.
  These works, however, have been generally deficient in an important particular :  they do not explain the causes of the
effects they describe, and they are, consequently, much less  interesting and instructive than they might be made.  This
fault has been remedied in the present  work ; and such directions are given, that no possible danger can be incurred in per-
forming the experiments.  Some elementary  books on science contain a number of scientific illustrations, that cannot be
performed without considerable risk, even  by persons who  are familiar  with the subject:  what must be the danger, then,
when ignorant persons attempt to perform them, without having received the least caution ?  Serious accidents are  frequent-
ly the result.  In this work, experiments that cannot be performed without danger are omitted, and cautionary remarks are
tiven whenever necessary, in order to prevent the. possibility of accident.
  As the " Book of Experiments" is intended chiefly for the amusement and instruction of young persons, it will not be
expected to contain any elaborate views of science.   It has been the endeavor of the Editor to collect and arrange only  such
experiments as might easily be understood, and such as explain, in a pleasing manner, the principles of many of the plienom
ena of daily life.  Some of the illustrations are derived from sources not easily accessible ; others are those which the most
eminent scientific lecturers of the present day are in the habit of employing ; and the remainder, it is believed, have never be-
fore been published. The whole ore simple and striking, and will, it is hoped, have  the effect of stimulating the minds of
those who practice them to become still better acquainted with " divine philosophy ;" and, by observing the beautiful har-
mony that pervades the whole material universe, " Look through nature up to Nature's God."
               ELECTRICITY.
I ntrodnctiou to the science—Theor.es of electricity—Conduc-
  tors and non-conductors—Ri;> n transmission of electri-
  city—Galvanism—Magnetism—Description of an electrical
  machine—a Leyden jar—How  to make cheap electrical
  machines of paper—Ditto of glass—How  an electrical
  machine acts.
   1.  The word electricity is used to express the
cause of a great  variety of phenomena^ which
take  place  when certain substances  are gabbed
against each  other; and it is found, by experi-
ment, to be identical with  lightning.  It is sup.
posed to be an extremely subtle fluid, which near-
ly all bodies are capable of producing under cer-
tain circumstances.  It is commonly obtained.
for the purpose of experiment, by the friction oc.
casioned by rubbing glass with silk.  An arrange,
ment of these substances,  in a peculiar manner,
constitutes an electrical machine, by which large
^13 
40                               Philosophical Experiments.
 quantities of the electric  fluid may be obtained.
 When  thus accumulated, it is found to  possess
 the  power  of passing through some  substances
 with extreme facility, which are, therefore, term-
 ed good conductors;  while other bodies retain  it
 entirely, or very much impede  its progress ; such
 substances  are  termed non-conductors, and are
 usually employed  for the purpose of obtaining
 electricity by friction, as before mentioned.  The
 electric fluid obtained by the electrical machine,
 and that obtained from the clouds, which is light-
 ning, are identical in their properties; and the
 electricity procured by either method is found to
 cause attraction and repulsion between bodies ;
 to be capable of decomposing water and other
 chemical compounds, into  their  elements;  and
 when passed through living bodies, in considera-
 ble quantities, of destroying life.   It travels with
 an  inconceivable velocity, like  light,  through
 good conductors, passing  only along their  sur-
 faces;  and it is, undoubtedly, the cause of many
 vital and chemical phenomena, as well as being
 closely identified with galvanism and magnetism.
   It is  wonderful to trunk that  " the forked light-
 ning,"  which, at all times, presents an appearance
 that  cannot be contemplated without some de-
 gree of awe, and which, occasionally, is terrific,
 is identical in all respects, except that it is more
 powerful, with the electric spark that may be ob-
 tained from a common sheet of paper, as describ-
 ed hereafter.  Yet  such is undoubtedly the case;
 for  Franklin,  and other philosophers since his
 time, have  drawn lightning from the clouds by
 means of silk kites, and found  it resemble, in ev-
 ery respect, that which  they  obtained from an
 electrical machine.
   Some philosophers have supposed that there
 are  two kinds  of electricity,   which  they have
 termed  resinous, because it is produced abundantly
 from resinous  substances, and vitreous, because
 procured from glass.  Dr. Franklin supposed  that
 there were not two kinds of electricity, but  that
 it existed in two different conditions, which he
 termed  positive and  negative.  These distinc-
 tions will  be best explained by the experiments:
 it is only necessary now to mention that,  which-
 ever theory of electricity is preferred, the same
 facts are explicable by both of them, since sub-
 stances, in a similarly electrical condition, al-
 ways repel each other, and   when in opposite
 states,  they attract each other.  This is illustra-
 ted by experiment 18.
   3.  Electricity is  not produced by the friction
 of two  portions of one substance, but when  dif-
 ferent substances are rubbed together, the electric
 fluid is  obtained ; and, if the bodies employed are
 bad  conductors, it accumulates, according to the
 time the friction is continued; while, if the sub-
 stances are good conductors,  it passes away as
 quickly as it is obtained,  and the usual phenom-
 ena does not take place.
   The worst conductors are,  therefore, the best
 substances  from which  electricity can be pro-
 duced ;  amber,  wax,  glass,  silk, hair, and dried
 wood, are a few of these.  Steam  and vapor,
 smoke,  living animals, vegetables, water, and the
 different metals, are good conductors; and little
 or no electricity can therefore  be procured from
 them, since they conduct it away as quickly as
it is obtained.   Bodies having points on their  sur-
face readily  give off, as well  as  receive, electri-
city ;  and are, therefore, unfit to  be used for the
purpose of retaining it.
  Lightning  conductors—the  long, pointed rods
of iron  that  are  placed against  high chimneys
and buildings, in order to prevent their being
struck by the electric fluid—are examples of the
manner  in wliich a knowledge of  the facts just
related may be applied to useful purposes.  Iron,
being one  of the metals, is a  good conductor j
and when, therefore, a cloud, charged  with elec-
tricity, passes near it, it  establishes a communica-
tion between the earth and the  cloud, and thus
prevents the  serious  consequences which ensue
when the electric fluid endeavors to find a passage
through a bad conductor.  As just mentioned,
living vegetables are  good  conductors; and  it
therefore frequently happens that a cloud dischar-
ges itself by  means of a tree;  but  in passing
through it, the vitality of the tree is destroyed, in
the same way that animals may be  killed by hav-
ing a powerful  shock  passed through them.   It
is extremely dangerous for persons to take shelter
under a tree during a thunder storm; because, if
the tree be  struck by lightning, it may proba-
bly pass through their bodies in descending to the
earth, as they are as good conductors as the tree.
  4. The rapidity with which lightning travels
is inconceivable;  and it is the same with the
electric  fluid  procured  artificially.   A wire  has
been attached to a portion of  an electrical batte-
ry, and   after being extended  for  two or three
miles, in folds,  the other end has been  made to
communicate with  an explosive compound, in or-
der to ascertain how much time would expire be-
tween the discharge from the conductor and the
explosion.  In every case they appeared to take
place at the same moment; and all other experi-
ments that have been performed for a similar pur-
pose,  have tended to prove that the passage of the
electric  fluid  is instantaneous.
  The principal phenomena of electricity will be
found described and illustrated  in  the following
experiments; among them, also, are a few in gal-
vanism  and  magnetism, wliich  must, therefore,
be briefly alluded to.
  5.  Galvanism is so  termed from the nature
of the person who  first  noticed  some of the re-
markable effects it is capable of producing. Gal-
vani found, that when he brought the  point of a
knife  in contact with the nerve of a frog, a por-
tion of whose body was in contact with the prime
conductor of an electrical machine, that the ani-
mal was violently convulsed; and following up
these  experiments, by the assistance of a friend,
he constructed what is termed  the galvanic batte.
ry.   This consists of a number of plates of zinc
and copper, placed side  by side, and immersed in
a dilute  acid,  which, acting on the surface of the
metals,  produce  a  current of galvanism, that,
passing  off at  each end by means  of  two wires,
may be  made to form a circuit through any sub-
stance.   All  the metals, when  exposed to gal-
vanic influence in  this way, become liquid, and
compound  bodies are decomposed; the elements
in a  positive state of  electricity passing  off to
one pole, and those  in a negative state to the
other.   Illustrations of these facts will be found
in the ensuing pages, where a description is like-
wise given of the best means of constructing  a 
Philosophical Experiments.                              41
galvanic battery, and  a voltaic pile, and how to
illustrate their effects.
  6. Magnetism is the term used to express the
property of the  loadstone, with Vhich most  per-
sons are familiar.  It is well known that the load-
stone will attract iron  and steel, and that the lat-
ter, by being rubbed on the loadstone, becomes a
magnet; one end of which will always  point to
the north pole, and  the other to the south.  The
advantages that inin has derived from this circum.
Ftance in being able,  at  all times, to determine
with exactness  his  situation at sea,  when  sur-
rounded on all  sides with an  apparently bound-
less ocean, arc  too  well known to require com-
ment.
  It has only quite  lately,  however, been deter.
mined  that the direction of the  magnet is occa-
sioned  by currents of electricity, which arc con-
stantly passing across the earth from west to east,
and that the magnet is, therefore, always at right
angles  to  this  current.  That this is the case,
however, is clearly proved by experiment 79.  It
is also capable of proof that magnetism is a mod-
ification of  electricity.  By experiment 77,  it
will be seen that, by passing a current of elcctri-
city round a steel bar,  it becomes a magnet, pos-
sessing  all the  properties of one formed by fric-
tion on a loadstone;  and  electro-magnetic  ma-
chines may now be seen in the opticians' shops,
by wliich the electric spark can be procured from
magnets themselves.

         SIMPLE ELECTRICAL MACHINES.
  7. A few of the more important effects of elec-
tricity,  can be exhibited without the assistance of
an electrical machine ; yet  it is so very expensive
a piece  of apparatus,  and so liable to accident,
that fe\v*of our readers will probably  be induced
to purchase one from a mathematical  instrument
maker.   They  may occasionally  be met  with
cheap, second-hand; but as few pereons have the
opportunity of procuring them in  this way,  wc
have  described, in  the following  pages, several
methods by which, with a little ingenuity, and at
a trifling expense, any one may make a good elec-
trical machine for himself.
  The  principal parts of a good machine are, a
cylinder, or a plate of glass, from which electri-
city is to be obtained by causing it to rub against
a piece of silk, covered with  an amalgam; the
method of preparing which will be described here-
after.
  As the electricity accumulates, it is passed to a
hollow  cylinder of metal,  supported  on a  glass
leg, so that the electricity cannot escape from it,
which is called  the  prime  conductor; and from
this reservoir, such  quantities of the electric fluid
as  may be required, can be obtained.  It is  ne-
cessary that the rubbing  surfaces should be  con-
nected, by a  chain,  with the  ground, or  other-
wise :  in a short time, the silk is incapable of
giving off anymore electricity, being, in fact, in-
sulated by the dry wood of the table on which it
may be standing, just  as the  prime conductor is
 by its glass supporter.
   8. A Leyden jar  is used to  contain electricity,
 so that when many  of them, of large size, are at-
 tached together, a great quantity of the fluid may
 be accumulated, and  then the most  powerful ef-
 fects can be produced.  The jar  is formed of
 lass, and is coated, inside  and out, with tin foil,
 y which the electricity is diffused over both sur-
faces.  A description of the best method of mak-
ing one  cheaply wdl be found in a subsequent
page.
  9. It has  been previously mentioned, that not
only glass, but all substances that are bad con-
ductors of electricity, are the best from which to
obtain the electric fluid.  Accordingly,  brown
paper, being  a non-conductor,  may be used for
the purpose; and  the following  is a simple plan
for making an electric machine with it:—Take
a circular piece of wood,  about one inch thick,
and of convenient diameter, and  paste over it a
sheet of brown paper, cutting the edges even;
then paste a strip all round the edge of the cir-
cle, and when  quite dry, paste on another coat-
ing of the brown paper in the same manner ; then
cut a square hole in the centre, and pass through
it the axle, wliich  mount on two  pieces of wood
as pillars.  A piece of wood, staple-shaped, and
covered with silk, or woolen cloth, will  do for the
rubber ; and a  cylindrical  piece  of wood, like a
rolling-pin, with rounded ends, covered with tin
foil, and mounted on a wine bottle, serves for the
prime conductor ; three or four  wires or needles,
being inserted  in the wood,  to  collect the elec-
tricity from  the  two  sides  and edges  of the
wheel.
   10. A glass electrical machine, of the cylin-
drical  form, may be made at a  very trifling ex-
pense.  The following is an extremely  ingenious
method of constructing one from the most simple
materials:—
   First, drill a sufficient hole through the bottom
of a common wine bottle, opposite the mouth ;  or
take off the bottom, by igniting a piece of worsted
tied round it dipped in  turpentine, which will  do
this.  Through this hole and the  mouth, pass a
spindle, as represented  in  the engraving  (b, c,)
                   v
          Cfjm'T:SS^v::'J"*°gsM|||  ,w, (i, "rc
The end of b should be squared, to fix a handle
on, and the spindle should be fixed firmly in the
bottle.  The bottle is then to be fixed in a frame
(as represented at page 42,) in the following man-
ner :—the end of the spindle  c passes through a
hole, the diameter of which is the  same as that
of the spindle in  the  upright d, and the end b
slides down the grove in the other upright, f, the
bottom of which is the same hight as  the hole
in the upright c, so as to keep the bottle in a hori-
zontal position ;  the spindle is kept from starting
up by passing a  pin through the upright, in the
direction of the  line a.  Next, make a cushion of
wash leather, stuffed  with  wool,  and  fasten it
with glue on the top of a frame, f, as   S^&f f
represented in the annexed cut.  This  (T^,, -
frame is to be  of such a hight, that   k,
the  cushion shall  press against the   'jy    ,
side  of the  bottle; and  a piece of-,f|i..Ui
black silk  (which,  for the  sake of*  |
clearness is omitted in  this engraving,   &''[.. j
but is marked a, in the one  at page  I *??,  '^_j
42,) i6 to  be sewn on the top of the   L^J
cushion, and hang over the  bottle, as represented
in page 42.  The method of keeping this cushion 
42
Philosophical Experiments.
a
 d
                   pressing  against the  bottle
                   will be  better understood  by
                   reference  to  the  following
                   figure, which is a plan of the
                   board, k,  in which the up-
                   rights are fixed; a  and b
                   are the places for the up-
                   rights; c and d are the
                   holes in which the ends .'Xj
                   of the cushion frame are
                   put.  A long wedge (see^
                   cut on the right) is put
in the hole e, and another wedge  is then put be-
tween the cushion frame  and the upright wedge.
This is better explained by the following engra-
            ving, where f, is  a side view of the
            cushion from l, the upright wedge,
            and  m,  the wedge which is  put be-
            tween the cushion frame and the up-
            right wedge.  The cushion  should
            be smeared with an amalgam, formed
            by melting together, in the bowl of
            a tobacco-pipe, or in a crucible, one
part of tin,  with two of zinc; to which,  while
fluid, ssix parts of mercury should be added, and
stirred about till cold, when it is to be reduced to
fine powder in  a mortar, and mixed with a  suffi-
cient quantity of lard to  form a paste.  If this
cannot, however, be easily obtained, a little quick-
silver,  scraped from the back of a broken piece of
looking-glass,  and mixed with tallow, may  be
used  instead.   When this is  done, the machine
will be complete, and its appearance is represented
in the annexed engraving.  The letters refer to the
part before described.  If the cushion be pressed
                         A       C
 against the bottle with the hand, it will cause the
 machine to work better; and before it is used, it
 should be held before the fire for a minute or two,
 in order that the bottle  may  be perfectly dry, as
 moisture conducts the electricity away.  This is
 the reason why many experiments, which suc-
 ceed on a clear, dry day, fail when the atmos-
 phere is filled with moisture.
   A prime conductor for the above machine may
 be made thus :—at right angles to one end of a
 cylinder of wood, about two inches and a half in
 diameter, and six inches long, fix a small wooden
 cylinder, about  three quarters of an inch in  di-
 ameter, and three  inches long, rounded at both
 ends ; the other end of the larger cylinder is also
 to  be rounded.  Cover the whole with tin foil,
 and mount it on a stand on a glass rod.  When
 used, it is to be placed with  the cross-piece in a
 line even with, and about half an  inch from the
 bottle ;  and it should be of such a hight as just to
 come below the silk apron.  When it is wished
 to charge a Leyden  jar, it is to  be placed at the
 round end of the conductor.
  11. The mode in wliich the electrical machine
just  described, and others, act, will be easily un-
 derstood  from the  following  description:—The
 friction of the cushion against the glass cylinder
 (bottle,) produces a transfer of the electnc fluid
 from the cushion to the bottle;  that is, the cushion
 becomes  negatively, and the glass positively elcc.
 trifled. The fluid which thus adheres to the glass,
 is carried round by the revolution of the cylinder,
 and its escape is at first prevented by the silk flap
 which covers the cylinder, untd it comes to the
 immediate vicinity of the part that projects from
 the prime conductor ; and which, being placed at
 a small distance from the cylinder, absorbs nearly
 all the electricity as it passes near it,  and trans-
 fers it to  the  prime  conductor.  Positive  electri.
 city is thus accumulated  in the prime conductor,
 while the cushion, being deprived of its electricity,
 is negatively electified.
   12. An electrical  machine may be made, with-
 out the trouble of  forming a hole through the bot-
 tom of a bottle,  by attending  to the  following
 directions :—The rubber is to be glued to a piece
 of wood, which is then  to be inserted into the
 neck of a small bottle, as shown by a  in the en-
 graving annexed, and secured by sealing-wax. A
 piece of leather should  then be tied tightly near
 the bottom of the bottle, at back, as at  b, the end
 of which is to be nailed to the stand to secure it,
 as a hinge.  In front of the bottle is then to be
 tied a piece of Indian rubber,  the end  of  which
 is also to be  made fast
 to the stand, as at c, so
 that when the rubber is ,
 pressing against the cyl-1
 inder,  the  elasticity of
 the Indian rubber per-
 mits the cushion to yield
 to the inequalities of the
 cylinder,andthe pressure
 is always nearly equal.
 d, represents the cylin-
 der ; and above a, is a
 knob (a piece of bent wire may be used instead,)
 to receive the spark from.  If the  cylinder  is
 made  of a  green  glass bottle, the positive spark
 will be given out by the rubber, and the negative
 by the conductor; but if of white glass, the con-
 trary will take place.
   13.  One  of the greatest difficulties in con-
 structing  the machine first described, is making
 a  hole in the bottom of the bottle,  whick is  to
 serve as a cylinder. This difficulty may be avoided
 by  fixing a piece of wood in  the centre  of the
 concavity at the bottom of the bottj^  while melted
 sealing-wax is dropped in untiWthe stick is well
surrounded, as in the accompanying figure.  On
the wax, cooling, the stick will remain quite firm.
Care should be taken to fix it exactly opposite the
spindle that is  to be fixed  in the neck  of the
bottle.
  Thus, with  a little ingenuity,  any one  who
wishes  to study  the  science  of electricity may
easily make himself a cheap machine for the pure
pose; and by doing so, he will have the addi-
tional  advantage of perfectly understanding th-
different parts of which it is formed. -■ 
Philosophical Experiments.
43
              EXPERIMENTS.

Electrical attraction-Repulsion-Electrified ribbons-De Luc's
  column—Identity of the electric fluid—Positive and nega-
  tive electricity—The discharging rod—Working power
  of electricity—Electricity from a cat's back—Electrome-
  ters—To draw sparks of fire from the body—To ignite
  ether by the touch—Electrified  head of hair—An elec-
  trified  kiss—The ringing belU—The jumping balls—The
  The. sportsman—Luminous figures by electricity—To im-
  itate the sound of thunder—Imitation thunder clouds—
  The cause of thunder—Place of safety in a thunder storm
  —Electrified sheet  of  paper—The  electrophorus—To
  charge  the.  electrophone—Description of  the electric
  spark.
               THE LEYDEN JAR.
   14. Tuis is one of the most useful pieces of
electrical apparatus.   It is employed for  the pur-
pose of containing a quantity of electricity, which
may thus be  applied  to any
substance.   It consists  of  a
glass jar, coated,  both inside
and out, nearly to the top, with
tin  foil, by which the electri-
city is  equally distributed.   A
krTob rises  through a wooden
top,  communicating with  the
inside of a jar.   When it is
wished to change  it, this knob is applied  to  the
prime conductor of an electrical machine in  ac-
tion, and the jar will  remain charged till a con-
nection is made, by  some good  conductor,  be-
tween the knob and the outside tin foil.
   15. A Leyden jar may be made out of a com-
mon wine  bottle, with about three inches of iron
filings in it, ftncffilled to the shoulder with water.
Coat it on  the outside with tin foil, and pass  a
wire througli the cork, one end reaching the iron
filings, and the other terminating in a brass knob.
By  this means,  the necessity of coating the in-
side of the jar with tin foil, a work of much diffi-
culty, may be avoided.

            ELECTRICAL ATTRACTION.
   16. If a piece of amber be rubbed on a coat-
 sleeve  for  a  short time, or on  a piece of silk,
 which  is preferable, it becomes electrified, and
 will attract light substances, &c.   It  was  the
 discovery of this peculiar property of  amber that
 first directed the attention of philosophers to elec-
 trical phenomena.
   i7.  If a  piece of sealing-wax  be rubbed in a
 similar manner to amber, it wall exhibit the same
 properties.                    \
             mECTRICAL REnXsiBN.
   18.  Electrify  a smooth  glass  tube with a  silk
 rubber, and hold a small feather at a short  dis-
 tance  from it; the feather will  immediately fly
 to the  tube, and adhere to it for a short time,  and
 then fly off; and  the tube can never be brought
 close to the feather till it has touched the side  of
 the room, or some other body that communicates
 with the ground.  If, therefore, the operator take
 care to keep the tube constantly between the fea-
 ther and the side of the room, he  may drive it
 round to  all parts, without touching  it; and the
 same side of the feather will be constantly oppo-
 sed to the tube.
     19.  If, while the  feather is flying before the
 smooth tube,  an excited rough tube, or a stick of
 sealing wax, be presented to it, it will fly contin-
ually from the wax, or from one tube to the other,
till  the electricity  of both  is discharged.  This
was one of the first, and is one of the most com-
mon experiments  in  electricity: it is, however,
very entertaining,  and well  exemplifies electrical
attraction and repulsion.
  20.  If the feathers be attached by threads of
silk, the experiments may be performed as repre-
sented below, where the left-hand figure repre-
sents the  pith ball attracted by the  glass tube,
and the right-hand  figure  the same ball, when
charged with electricity, and repelled by the tube.
             ELECTRIFIED RIBBONS.
  21. If a white and a black  ribbon, about two
or three feet long, and perfectly dry, be applied to
each  other by  their  smooth surfaces,  and then
drawn frequently between the  finger and thumb,
so as to rub against each  other,  they  will  be
found to adhere together; and if pulled asunder
at one end, will rush together  with great  quick-
ness.   While united,  they exhibit no sign of elec-
tricity,  because  the  operation of the one is just
the reverse of that of the other,  and their power
is neutralized.   If  completely  separated, how-
ever, each will manifest a strong electrical pow-
er ; the  one  attracting  those  bodies which  the
other repels.   One  is  positively electrified,  the
other negatively.
               DE  LUC'S  COLUMN.
  22.  The nearest approach to a perpetual mo-
tion, by means of apparatus, is represented in the
following figure, known as De Luc's column.  It
consists of a glass tube, closed at each  end by a
brass knob, and containing a number of pieces of
Dutch leaf, with paper placed  between them. By
this means electricity is produced,  and may be
made to attract the pith ball, b, which Is suspend-
ed by a silk thread between the  column and c, a
piece of wood, cover-
ed with tin foil, com-
municating by a chain
with the ground.  Af-
ter the  ball has be-
come charged  by con-
tact with the column,
it is repelled, and then
flies to the tin-foil con-
ductor, where  it parts
with its excess  of elec-
tricity, and  becomes
negative, returning to
its perpendicular posi-
tion, to be again  at-
tracted  and repelled,
 as before.   By  this
means  the  ball  will
 continue in action  as  long as any electricity is
 generated, and this may continue for years.
 
44                               Philosophical Experiments.
■*r^
BODIES SIMILARLY ELECTRIFIED REPEL EACH OTHER.
  23. If two pith balls be  suspended by pieces
of silk thread,  and electrified, by being touched
with  excited sealing-wax,  or the  flannel with
which it has been  rubbed, the balls will then fly
apart from each  other;  but if  one of them be
touched with the wax and the other with the flan-
nel, they will then mutually attract  each other,
and adhere together.   This experiment illustrates,
exceedingly  well,  one of  the  principal laws of
electricity mentioned in the Introduction ;  name-
ly,  that  bodies similarly electrified  repel each
other, but that when dissimilarly electrified, they
attract each other.
       IDENTITY OF THE ELECTRIC FLUID.

  If one of the pith  balls
mentioned in  the last ex-
periment be electrified with
sealing-wax that has  been
rubbed  with  flannel, and
the other  ball by  silk rub-
bed with glass, these  balls
will repel each other (as
seen in the figure) which
proves that the electricity
of the silk is the same as
that of the  sealing-wax;
both substances being non-
conductors,   and,  conse-
quently, electrics.

  TO SHOW POSITIVE AND NEGATIVE ELECTRICITY.
  25. To show what Franklin termed " positive
and negative electricity," and Du-
fay, " resinous and vitreous elec-
tricity, and that the one is produ-
ced from a  conductor,  and the
other from a non-conductor, let
one of two balls be electrified by
sealing-wax,  and the  other  by
glass ; they  will then  mutually
attract each  other, showing that
they are oppositely electri6cd.

             THE  DISCHARGING ROD.
   26. When a Leyden jar  is charged with elec-
tricity, the inside and the outside are in different
states.  An equilibrium  may be restored, by ap-
plying the thumb to the outside, and the fore-fin-
ger to the knob communicating with the inside,
when the charge will pass through the body, and
occasion a shock.  As this  is sometimes unplea-
sant, and when  very powerful,
even  dangerous,  the annexed (J^        fd
piece of apparatus is  used to dis-
charge the jar. a, is a glass han-
dle ;  c c, two  balls placed  upon
two wires, d d, which should be
upon a hinge at b, so that  they
may be be opened wide, if neces-
sary.  For  cheapness, the dis-
charger may be merely a piece of
bent wire, with a handle of  dry  wood, wliich is a
non-conductor, like  glass.   When a jar is to be
discharged, place one knob  on  the outside of it,
and the other on the knob at the top.  Be sure
and touch the outside first; for if the knob on
the jar is touched first, a  severe shock will some-
times be given to the experimenter.
       WORKING POWER OF ELECTRICITY.
  27. Electricity may be made to give motion to
bodies in the following way :—a, is  a wooden
board, into which are  fixed  four  glass  pillars,
b b b b ;  the  two which stand opposite d, arc to
be shorter than  those  placed at the back of the
board; from  the top of these stretch  fine wires,
c c ; at D have a chain attached to your conduct-
or ; place on the wire a wheel made of four pieces
of wire,,  two  to be bent round at the ends; fix
them in pieces of wood, f ; the other two to ter-
minate in a point, and to be fixed in the piece of
wood the contrary way to the others;  the points
to be bent, the one up and the other down, as seen
in the engraving.  The  electricity passes  from
the prime conductor up the chain at d, over the
chains b, up the wire  c, on to the wheel, and off
at the points, wliich causes it to turn  round, and
wind itself  up the inclined plane.   Electricity
flies off very quickly from points ; indeed, a can-
die may sometimes be  blown out from a sharp
point on the prime conductor, when charged.
  28. A rotary motion may be obtained by the
instrument represented in the an-
nexed cut.   a, is a wire, to-be pla-
ced on the conductor  of an elec-
trical machine ; and the four wires,
bbbb, are to be fixed on the top,
and bent so as to  turn  freely on -g,
their axis  at c.  When  the ma-
chine is put in action, the electri-
city flies off from the points, and
by  the re-action of  the air, the
wires are forced quickly round.

         ELECTRICITY  FROM A CAT'S BACK.
   29.  Hair, as previously mentioned, is a  non-
conductor, and, therefore, may be employed to ob-
tain electricity from.  Some  amusing experiments
may be performed with  a living cat, by making
the hair on  her back  act as a portion of an elec-
trical machine. f Make friends with Pussey—if a
black  one, so much  the  better—aAl  warm her
back well by the fire ; put  her <p your lap, and
place  your left hand round her throat, so that
your thumb and finger  may nearly meet at her
shoulders.   Then  pass your  right hand up her
back, and a slight electrical shock will be felt.—
If the experiment is performed  in the dark, the
electric spark will be  distinctly visible.   The ra-
tionale of the experiment is, that the  hand, act-
ing  as the  rubber of  an electrical macliine, in
passing  along the cat's back, gives out a portion
of the electric fluid, which the left hand, perform-
ing  the  part of a prime  conductor,  carries off!
and when the right  hand  approaches the left
again, the  electricity flies from one to the other,
and restores  the equiUbrimn,  at the same mo-
ment giving out the spark, and thus causing the
shock. 
Philosophical Experiments.
45
               ELECTROMETERS.
  30. An electrometer is an  instrument used for
the purpose of detecting the presence of electricity
in any substance.   The  woodcut  represents a
common glass bottle, which will form
a cheap piece of apparatus; on each
side of it is  placed a small piece of
gold leaf.  In the centre  a, are two
pieces of gold leaf, suspended  by a
piece of wire passing  through the
the cork.  By means of this instru-
ment,  very slight portions of elect-
tricity may be discovered ; for imme-
diately the knob which rises through  |
the cork  comes  in contact with a
body containing electricity, the cen-
tre leaves fly apart.
  31.  The most common form of electrometer is
one  usually fitted to the  prime conductor  of a
             machine,  and represented in  the
             following engraving a, is a piece of
             wood, terminating in the knob p, in
             order  to prevent the  electric  fluid
             passing away, which  it would do
              were the top pointed,   c is a wire,
              by which the  instrument  is fixed
              in  the conductor ; d is a piece of
              ivory,  divided into  regular  por-
              tions ; and  e is  a piece of wood,
              terminating in  a  pith ball.  Ac-
              cording to  the power of the elect-
              tricity, so  will the ball rise; and
              as the piece of ivory is graduated,
this may be easily ascertained.
   32.  The form in which the electrometer first
                  described is sold by opticians,
                  is represented by the subjoined
                  cut.    It  is  called Bennett's
                  electrometer,  a, is a glass jar ;
                  b, the  sides, coated part of the
                  way  up with gold leaf, or tin
                  foil;  c   c, two pieces  of gold
                  leaf,  a  ttached  to  the ball
                  d, to which  the  body contain-
                  ing electricity is to be applied.
  33.  One of a  simple kind,  that may  be more
easily  made than  those  previ-
ously mentioned, is constructed
on Coulomb's principle,  a, is a
glass jar ; b, a silk thread, sup-
porting a piece  of wood, hav-
ing a fine ball of pith at each
end ; c is a wire, passing through
one side side of the jar.  When
an  electrified body  is brought
into contact  with  the  knob on
the end of this  wire, the pith balls will be at-
tracted or repelled, according to the state of elec-
tricity they are in.

   TO  DRAW  SPARKS OF FIRE FROM THE BODY.
  34. If a person  is insulated (that is,  placed
upon some non-conducting body,) and then made
to receive a quantity of electricity, on presenting
any conductor to him, a portion will pass  off, giv-
ing out the usual electric  spark.
  35. The best  means of insulating a person is,
to place him on  an electrical stool, taking care
that his clothes do not touch any of the furniture
of the room, or it will convey the electricity back
to the earth as fast as it is obtained by the ma-
chine.   The stool is  to
be made as represented
in the cut.   a, is a piece
of  deal  board,  at the
four corners of  which
are to be glued four pie-
ces of the same kind of
wood,  very dry  (b  b.)
These are to rest on the
legs, made of the necks
of wine bottles (c c,) or other pieces of glass, which,
being non-conductors, will not allow any electri-
tricity to pass down to the  earth; and thus, a
person standing on this stool is completely insu-
lated.
   Let a person stand on this stool,  and with one
hand take hold of the conductor of an electrical
machine while in action.  Sparks  may now,  by
presenting the knuckle, be drawn from any part
of his body as easily as from the conductor itself.

    TO IGNITE ETHER BY MERELY TOUCHING IT.
  36. While a person is on the stool, charged
with electricity, let some one approach him with a
warm spoon, containing a few drops of ether.  If
he now brings the knuckle of his hand not con-
nected with the conductor within a short distance
of the spoon, a brilliant spark will be given off,
ami ignite the ether.

        THE ELECTRIFIED HEAD OF HAIR.
  37. While a per-
son  is on  the elec-
trical stool, if he is
charged with much
electricity,  " each
individual and par-
ticular   hair   will
stand on end."  The
electrical head is an
amusing toy  to il-
lustrate  the  same
fact.   Get a wood-
en  head (the more
frightful the better,)
and fix  some  long
hair on it, then place
a strong wire in the neck to
fix  it in the conductor of an
When the machine is put in
rise up, and make the figure
appearance.
!K
 support  it by, and
 electrical machine.
action, the hair will
 present a frightful
             THE ELECTRICAL KISS.

  38. An amusing experiment may be performed
with the electrical stool.   Let any lady challenge
a gentleman, not acquainted with the experiment,
that he  will not be able to kiss her, although she
may offer no opposition.   If he accept the chal-
lenge, let her stand upon the stool, and with one
hand take hold of a chain in connection with the
prime conductor of an electrical machine.   If it
is then  put in operation, and the gentleman ap-
proaches the lady, directly he attempts to kiss
her, a spark will fly in his face, which will almost
certainly startle him, and  prevent  him effecting
his object.   The clothes of the parties must not
come in contact, or the spark will not be given off. 
46                              Philosophical Experiments.
              THE RLNGING BELLS.
  39. A small peal of bells may be made to ring
by electricity, in the following way :—a is a glass
pillar, fixed in a stand of wood, from  the top of
which  proceed the  wires d d, terminating in
brass bells.  The electrical apparatus is then at-
 tached to the ball  (e), and little metal  tongues
 are to be  hung  between the bells by silk, as at
 c c.   The  middle  bell is to have a hole at the
 top, for  the glass pillar to pa?s through.   AVhen
 the machine is charged, a very pretty effect will
 be produced.  The  electricity passes down  the
 wires d d to  the bells, which are then positively
 electrified, and attract the clappers, c c, that are ne-
 gatively, in consequence of being insulated by the
 silk strings, which are non-conductors. The bells,
 therefore,  attract the  clappers, just  as the pith
 balls were in a former experiment, until  they are
 charged,  when they strike against the centre bell
 to discharge  themselves ;  and  thus continue fly-
 ing between the bells until all the electricity is
 taken from the side bells.  The bottom of the pil-
 lar should be coated  with tin  foil, from  the bell
 downward.

                JUMPING BALLS.
   40. Get  two pieces of wood, and  coat  them
 with  tin foil, or two pieces of metal plate; attach
 one of them to the prime  conductor by  a chain,
 and  let  it  hang  about two or
 three inches from  a table. Then       ft
 place some  pith {balls  upon the       M  .
 bottom piece of wood  (b), and    ^g^y--g.
 bring it under the other (a). Im-   v^^'fj)
 mediately  this is  done, and the
 upper piece is charged with elec-
 tricity, the pith balls will begin       *.** • b
 to jump  from one to the other   .gc^zb^Nss
 with  great rapidity.   The cause   f   J   "~^/
 of this is, that the upper piece of       '
 wood being charged with  positive electricity, at-
 tracts the balls till they  become  positive  also,
 when they  are repelled; but parting  with their
 electricity, are again  attracted, and so they keep
 flying from  one to the other, until an equilibrium
 is restored.  Some paper figures may be made to
 dance on the bottom plate, instead of the pith
 balls,  if it is wished.

               THE SPORTSMAN.
  41.  The  piece  of apparatus  that is called by
this name is capable of affording  much  amuse-
ment,  a is a  stand of wood; b is a common ley-
den jar, out of which proceed the  wires n n, one
terminating in the ball f,  the other in  the ball d,
to which are attached a number of pith birds by
silk strings; e is a shelf for the birds to rest on;
c, a sportsman ; g, his gun. To put the appara-
tus in action, charge the leyden jar with electri-
city, by affixing a chain  to the bottom part of it,
and  connecting it with an electrical machine, or
 by applying  it to a prime  conductor,  when the
 birds will fly off from the knob to  which they are
 fixed, in consequence of being repelled.   If the
 sportsman and  gun  be then  turned, so that the
 end of his gun  shall touch the knob f, an elec-
 tric  spark will pass from one end to the other, a
 report will be heard, and the birds will fall down
 as if  shot, in consequence of  the electricity hav-
 ing  been  taken from the  leyden  jar.   There
 should  be a communication  between the sports-
 man  and the jar, formed of tin foil, or  some me-
 tal, as shown by the dotted  line on the  stand.

       LUMINOUS FIGURES BY ELECTRICITY.
   42. While the electric fluid passes along a con-
 ductor,  without  interruption, it is  not perceived;
 but  whenever the connection is broken, a spark
 is produced.  If, therefore, a figure be formed on
 glass with tin  foil,  as represented  in the wood
 cut, by  separa-_______________________




 ed when the in-
 strument is connected with an electrical machine.
 If the tin foil is  separated, so that the vacant
 spaces form the letters of a name, or any particu-
 lar figure,  it will increase the interest of the ap-
 pearance.  It is merely necessary to hold the in-
 strument to  the prime conductor of a machine,
 and the electricity will follow the windings of the
 tin foil,  till it reaches the other end.   The exper-
 iment should be  made in the dark.

      TO IMITATE  THE SOUND OF THUNDER.
  43. Take a piece of sheet iron, about three feet
 long,  and one foot wide, and holding it downward
 at one  corner, move the hand gently, so  as to
 shake it in the  direction of its length.  By this
 means, a great variety of sounds will  be produced,
 varying from the deep growl  of distant thunder
 to those loud and explosive bursts which rattle, in
quick succession, from clouds  immediately above
our heads.  The same effect may be produced by
Bheets of tin plate, but, on account of their small
size,  the sound is shorter  and more acute.

      TO MAKE IMITATION THUNDER CLOUDS.
  44. The annexed  wood cut will afford a clear
idea of the manner in which thunder is supposed
to be produced,  a a is a wooden stand  on which
 are erected two  uprights, b b ;  c c are two small 
Philosophical Experiments.
tf
pulleys, over  which  a silken cord (d) can pass
easily; E  is another silken fine, stretched across
   from one  upright  to  the other; on these silken
   cords, two pieces of thin card-board, covered with
   tin foil, and so cut out as to represent clouds, are
   to be fixed horizontally, and  made to communi-
   cate  by means of the thin wires,  / and g, one
   with the inside, and the  other with the outside,
   of a charged jar (i).  Now by pulling the loop of
   the silk line d, the cloud 1 will be brought near
   the cloud 2 ; continue this action slowly, until the
   clouds,  which are furnished with two small brass
   balls, are  within an inch of each  other, when  a
   beautiful  flush, strongly resembling lightning in
   miniature, will pass from one cloud to the other,
   restoring an electrical equilibrium.
    45. If the jar (i) be put behind, and the cloud
   2 removed, a vessel, communicating by means of
   a wire with the outside of the jar, may be  swam
   in water  under the remaining cloud ; the mast
   being made  of separate pieces, and but slightly
  joined together.  When the  cloud is passed over
  the vessel, the mast will be struck, and shattered
  to pieces.
    These experiments will explain  the manner in
  which accidents  occur  from the  discharge of
  electric  clouds, and show the  cause of thunder
  and lightning.
             THUNDER AND LIGHTNING.
    46. The following interesting explanation is
  given by Brand of the cause of thunder continu-
  ing to be heard for a long time-after the discharge
  of the electric fluid:—" The discharge of elec-
  tricity in a thunder storm is sometimes only from
  cloud to  cloud; sometimes from  the earth to the
  clouds; and sometimes  from the clouds to the
 earth, as one or other may be positive or negative.
 When aqueous  vapor is condensed, the clouds
 formed are  usually more  or  less electrical; and
 the earth below them being brought  into an oppo-
 site state, by induction, a discharge takes place
 when the clouds  approach within a certain dis-
 tance, constituting lightning ;  and the undulation
 of the air, produced by the discharge, is the cause
 of thunder, which is more or less intense, and of
 longer or  shorter duration, according to the quan-
 tity of air • acted upon, and the distance of the
 place where the report is heard from  the point of
 the discharge."  It may not be uninteresting to
 give a further illustration of this idea. The elec-
 tric fluid  travels so  amazingly rapid, that it is
 found impossible to calculate its rapidity.   It has
 been made  to  travel along a wire, four miles in
 length, one end of which being  brought near the
 same  spot from which the  other proceeded, the
exact  time  of the discharge at both ends could
be seen at  once, and not the slightest difference
has been  perceived.   The spark passed from the
end of the wire, after traversing a distance of
   four miles, at the same instant that it was given
   off by a  leyden jar to the  other  end!  Sound
   does not travel so quickly, but moves at the rate
   of twelve miles in a minute.   Therefore, suppos-
   ing the lightning to pass through a space of some
   miles instantaneously, the explosion will be heard
   first from that part of the air agitated nearest to
   the spectator; it will gradually  come  from the
   more distant parts of the course of tjie electricity,
   and, last  of all, it will be hf ard from the  remote
   extremity.  The different degrees of the agitation
   of the  air, and likewise the difference of the dis-
   tance,  will account for the different intensities of
   the sound, and  its  apparent reverberations  and
   changes.
    47. A person may calculate the distance of the
  point from wluch the  lightning is discharged, by
  counting the number of seconds which elapse be-
  tween the moment when the flash of lightning is
  first perceived, and  when  the thunder is  heard,
  allowing five seconds for every mile.

            SAFETY IN A  THUNDER STORM.
    48.  Sir  Humphrey Davy states in his  " Ele-
  ments," that in a violent  thunder storm, when
  the sound  instantly succeeds the flash, the persons
  who witness  the circumstance are in some dan-
  ger ; when the interval is a quarter of a minute,
  they are secure.  The safest situation is the mid-
  dle of a room,  at a distance from  the chimney,
  and standing upon a woolen rug, which is a non-
  conductor. Blankets  and  feathers  being  non-
  conductors, a bed is a place of comparative safety,
  provided the bell-wires are not too near, as they
  are  almost always  melted  in houses struck  by
  lightning.  When out  of doors, it is dangerous to
  take  shelter  under  trees, as the lightnmg often
  rends off the  branches ; the  safest situation is
  within  some  yards of the trunks, and upon the
  dryest spot that can  be found.

          ELECTRIFIED  SHEET OF PAPER.
   49. Take a sheet of  letter paper, and divide it
 into two portions; dry them both at the fire, and
 laying one  over the other, rub them in one direc-
 tion with a piece of Indian rubber.   They will,
 by this means, become  oppositely electrified, and
 attract each other ; if  separated in a dark room,
 a spark  will be perceived.   This is a simple elec-
 trophorus,  and  well illustrates how easily the
 electric  fluid  can be excited.   Unless the paper
 be dry, the experiment will  not succeed; and  it
 should, therefore, be dried just  before it is used.

             THE ELECTROPHORUS.
   50. This instrument, which may be easily con-
 structed, will illustrate  many of the experiments
 mentioned in this work, and is exceedingly useful
 in the laboratory,  where an electrical machine
 would  be in danger  of being  broken.  To con-
 struct  an  electrophorus,  the following method
 is adopted:—Mix together equal parts of  Venice
 turpentine, resin, and shell-lac, and expose  the
 whole in a  crucible,  or pipkin, to  the fire,  until
 they are  thoroughly melted and combined.  Pour
 the mixture  into a hoop, about a foot in diameter,
 placed upon a slab of stone, so that when it cools
 it can easily be removed in a cake; let it be about
half an inch thick, and  cover the upper part with
 tin foil.   Then, turn this  side underneath, in or- 
48                               Philosophical  Experiments.
der that the upper side may be smooth, and place
on  the resin a polished plate of metal, or disk of
wood, covered with tin foil, supported by a glass
handle, rising from the centre; the
metal, or disk of tin  foil, should
be  circular,  but rather  smaller
than  the resin, and. both should
be  rounded at the edges.  This
constitutes an exceedingly useful
machine.   The annexed diagram
is the representation of an clec-
trophorus,  about  twenty sparks
from the upper conductor of which
is sufficient to charge a small Ley-
den phial.  1. Glass handle; 2, disk of brass, or
of  wood, covered with tin foil; 3, cake of resin.

        TO CHARGE THE ELECTROPHORUS.
   51. When it is wished to charge the electro-
phorus, in  order to electrify any  substance, the
following plan is to be pursued:—Rub  the  sur-
face of  the resin with a piece of dry fur  (such as
a portion  of rabbit-skin,) and place the  metal
plate on the  disk of resin.   Upon  raising it,  it
will be found very feebly electrical;  replace it,
touch  it with the finger, and again lift it off by
its handle, and it will give a  spark of positive
electricity.  This  process may  be repeated very
often, without fresh excitation, and the electricity
may be given off*  to  any substance by the  disk
(2) represented in  the wood cut.   It may also be
accumulated in a Leyden jar,  when a powerful
charge  is  required; and thus  the electrophorus
may, to a certain extent, be  made to supersede the
electrical machine.   It can be used for all the ex-
periments described in this work.

              THE ELECTRIC SPARK.
   52. When the knuckle is presented to the prime
conductor  of an electrical machine, or to the
edge of the metal plate of  the electrophorus, af-
ter they have been  charged with electricity, a
spark  is given  out,  and a sharp cracking  noise
heard.  The spark is of a blueish color in the at-
mosphere, in its ordinary state, but in a glass ves-
sel, containing condensed air, it is white; in rar-
ified air, it is of a reddish  tinge; and in a good
vacuum, under a  receiver,  it is of a purple  hue,
very faintly visible.   The  electric spark is  sup-
posed to be occasioned by the sudden compression
of  the air, which occasions a degree of heat, suf-
ficient to  inflame  spirits of wine; but some sub-
stances cannot be ignited by it, until it has pass-
ed through  water, when the effect can be pro-
duced.
 melted, it may be cast in  a mould, like lead, or it
 may be  procured in a sheet, and  cut, similar to
 the copper.   Then provide the same number of
 pieces of cloth, which must be soaked in a solu-
 tion of common salt water; or, what is better, a
 liquid composed of one  part  of sulphuric acid,
 two of nitric  acid, and  sixty of water.   After
 this is done, place one of the pieces of zinc in a
 tea-saucer, and on  it put one of  the pennies, or
 pieces of sheet-copper ;  on this place a piece of
 cloth, and so continue making the  pile—zinc,
 copper, cloth—zinc, copper, cloth—until they are
 all piled on one another; taking care to  observe
 the same arrangement throughout.  The piece
 on the top, which will  be a  penny, should have
 a copper  wire,  which,  for  some  experiments,
 should be tipped with platinum wire, soldered to
 it, and the lower piece, which will be zinc, should
 be treated in tho same  manner.   From the ends
 of these wires a stream
 of the Galvanic fluid
 will  constantly  issue,
 until  all the acid is ab-
 sorbed from the pieces
•of cloth; and although
 the apparatus is on a
 very  small  scale,  a va-
 riety of  exceedingly in-
 teresting   experiments
 may  be  performed with
 it.  When  completed, the pile will appear as rep-
 resented in the accompanying cut.
   As it is necessary to  have at  least twenty or
 thirty pieces of copper and zinc, in order to produc
 a good  stream of Galvanism, and as the  pile is
 inconvenient when made too  high, it is advisable
 to form a  number of small  piles in a dish, and
 connect them at the top and  bottom by wires, or
 plates  of  metal, as
 represented in  the
 figure. A strong Gal-
 vanic power may be
 thus obtained,  with-
 out  the inconveni-
 ence just mentioned.
    54.  Where  the
 Galvanic  pile is not
 divided,  as was before  mentioned, in the  last
 experiment, the  method  usually adopted to sup-
 port  it steadily is, to fix  into  the piece of wood,
 on which the pile is formed,  three rods of wood
 or glass, at three equi-distant  points. Down these
 rods  may slide a piece  of wood, for
 the purpose of keeping by its pres-
 sure  the different parts  of the pile
 in contact.   The figure is  intended
 to represent  this top; the circle  in
  The fight and noise which follow the discharge
of an electrified body is an illustration of thun-
der and lightning.  When, however, electricity is !  the centre shows the situation of the
given off from sharp points, it is not accompanied  pile'  the three small holes in the
by noise.                                        rim are for the reception of the rods.
                                                  55. The zinc plates in the voltaic pile become
     to construct a cheap galvanic pile.        oxydized,  or  rusted, after  being  used a certain
  53. To exhibit  experiments in Galvanism, on ,  time, and  then the Galvanic  action ceases; but
a small scale, a pile may be formed at a very tri
fling  expense, as follows:—Procure about twen-
ty  penny-pieces  (if worn  smooth, so much the
better), or get some sheet copper, cut circular,
and of a large diameter, and the same number of
similar pieces of zinc.  The latter may be formed
by the experimenter  himself;  being very easily
they may be  cleaned  by being  put into  dilute
muriatic acid,  by which the oxide  is dissolved.
It is advisable, when the voltaic apparatus is not
to be used  for some time, to take it down, by
which  means  the zinc plates will be prevented
wearing out too fast.   The  larger  the zinc  and
copper plates are, the more  powerful will be the 
Philosophical Experiments.
49
Galvanic action; and it is better, therefore, to ob.
tain sheet zinc and copper, and «ut it, than to use
penny-pieces, and zinc of a smaller size.
    EXPERIMENTS IN GALVANISM.
To  prove water inflammabl*—To make a cheap Galvanic
  battery—Chemical decomposition  by Galvanism—Galva-
  nic (hock described and illustrated—Change of color by
  Galvanism.
             WATER  INFLAMMABLE.
  56. Water is a compound of two gases, and by
means of the Galvanic pile just  described, may
be resolved  into its elements."  In the following
figure is represented the arrangement that may
be employed for the purpose.  A glass tube must
be filled with water, having the  ends closed with
corks; d  and  c are  wires, one  of them coming
from  the top of the  pile, and the other  from the
bottom of it.  Tho wires  pass through the corks,
and in the tube their ends, c, c, approach to within
a quarter of an inch of each other.  The pile for
this purpose must be  formed of at least fifty or
sixty plates.  After some time, the end  of the
wire connected  with the zinc, or positive end of
the pile will become oxidized, if formed of iron,
or other oxidable metal;  while  from the point of
the other wire, coming from the copper,  a stream
of small bubbles of gas will rise, which is hydro-
gen, and is one of the elements of water.   The
water  displaced by the gas will ooze out round
the corks, and  after a time the tube will be billed
with gas.
   57. The gas obtained by the last  experiment
 may be proved to be inflammable, by pulling out
 the cork at one end of the tube, after holding  it
 upright, and applying a match, when the gas will
 ignite; it must be done quickly, and it is advisa-
 ble to wrap a handkerchief round the  tube, as the
 explosion  often breaks it.   Thus  it wdl be seen
 that  water, which is the great antagonist to fire,
 is really formed  principally of an inflammable
 gas ; and if, instead of using iron wires,  platinum
 is employed, oxygen, the other element of water,
 may  be obtained.  It will, however, be mixed
 with the hydrogen if thus procured, and the mix-
 ture is very explosive.

      TO MAKE A CHEAP GALVANIC BATTERY.
   58. To produce any powerful  effect  by Galvan-
 ism, it  is  necessary to have a battery,  which is
 usually employed in
 tliis  form,  where a      ZC  2C  ZC
 plate of zinc and a      u\\  if ||  (T f
 plato of copper are             III
 attached to each oth-      U LI  Li
 er, and immersed in      ___________
 a weak acid solution,    /   /   /  /   2jj&
 in  an   earthen or   i                  || s
 wooen trough (a b),                      v)
 divided into regular   L-----—------—^
 compartments, as in
the figure.   When a number of these troughs arc
 connected  together, so that a stream of Galvan-
 ism from the whole of them may be  made to is-
 sue from the points of two wires in the  usual way,
the most powerful effects  can be produced ; such
as melting metals, and decomposing  compound
substances.
  59. Galvanism  may also be excited, by filling
a number of glasses, a b c d, with dilute acid and
placing into each  a plate of zinc and copper, z c,
not in contact, but connecting the adjoining pie-
ces of the different glasses by wires, following the
same  arrangement  throughout of zinc, copper,
zinc, copper, and so  on ; the zinc of one glass be-
ing joined to the copper of the adjacent one.  As
both sides of the metal in this apparatus are ex-
posed to the acid, a stream of Galvanism is  pro-
duccd, which is very  powerful, considering the
small number of plates.   This form of battery is,
however, not much in use now.
     CHEMICAL DECOMPOSITION BY GALVANISM.
   60.  Place a drop of the following solution upon
a  small piece of glass, or on a  piece of writing
paper, and the platinum extremites of the wires
of the Galvanic  pile must then be placed in the
drop, but not allowed  to touch each other.  The
electricity, or Galvanic fluid, is developed as the
acid fluid in the cloth acts upon the zinc, accu-
mulating in one of the wires, and passing to the
other; the materials in the solution being affected
as it passes through them, and decomposed.
     MATERIALS USED.
  61. Sulphate of soda, com-
posed of  sulphuric acid and
soda resting on paper, dipped
in  solution of  cabbage, and
dried.
  Hi. Iodine of potassium,
composed of iodine and potas-
sium,  mixed  with  a  little
starch.
 63. Acetate of lead (com-
mon sugar of lead.)
 64. Sulphate of copper.
                               PRODUCTS.
                         61.  The acid accumulates
                        at one wire, and reddens the
                        paper, the. alkali at the other,
                        ana turns it green.

                         62. The iodine is immedi-
                        ately separated, and forms a
                        blue compound with the starch.

                         63. Metallic lead is deposited
                        in small crystals.
                         64.  Metallic copper is de-
                        posited in the same manner as
                        the lead.
               GALVANIC SHOCK.
  65. Moisten the hands with  salt and water,
and hold one wire of the pile in  each hand; the
Galvanic fluid will  then be felt to enter the hand
and pass through the body, from one wire to the
other.  When the  apparatus is  large, the  shock
is  exceedingly powerful  and painful ; but  per-
formed with a small pile, it  produces  rather  a
pleasant sensation than otherwise, which does not
pass beyond the fingers or wrists.  It is necessary
to moisten the hands, because the skin, in  a dry
state, is scarcely pervious to  electricity of Gal-
vanism, of such weak intensity  as that procured
from the pile.
  66. If a large battery is employed, the wires
may be placed in separate basins of water; and
then, on dipping the  fingers of each hand  into
the basins, a smart shock is experienced;  a pe-
culiar  aching, accompanied with  trembling,  is
felt up the arms, and, with  a strong battery, the
shock is experienced as high as the shoulders.
  67. A peculiar  taste is perceived, if a  slight
shock be given to the tongue.  To perform this
experiment, place a shilling, or  piece  of  silver,
61 
50                               Philosophical  Experiments.
under the tongue, and place a piece of zinc upon
it, so that the metals do not touch each other. On
bringing the edges of  the  two metals into con-
tact, or making a  connection  between them by
means of a piece of wire, a slight flash will often
be perceived, even if the eyes arc open, and a pe-
culiar  taste will be experienced.  If, instead of
silver,  a more oxidable metal, such as copper, is
employed,  the  taste will be acid; but with the
silver,  it will be rather  alkaline, something  like
the taste of a weak solution of soda.
        CHANGE OF COLOR BY GALVANISM.
   68.  Put  a teasoonful of  sulphate of soda  into
a  cup, and dissolve it  in hot water; pour a little
cabbage blue into  the solution (see  experiment
130,) and put a portion into two glasses, connect-
ing them (as represented in the figure below,) by
a piece of  linen or  cotton  cloth, previously mois-
tened  in the same solution.   On putting one of
the wires of the Galvanic pile into each glass, the
acid accumulates in one, turning the blue to a red,
and the alkali in the other, rendering it green. If
the wires be now reversed,  the  acid accumulates
eventually in the glass where the alkali appeared,
while the alkali passes to the glass where the acid
was.
               MAGNETISM.
Experiments is Magnetism—To make artificial magnets
  —Ditto from a poker—Magnetic attraction and repulsion—
  Effect of a magnet on a  watch—Peculiarity of magneti-
  cal attraction—Polarity of the magnet—To  make a mag-
  net by Galvanism—Effect of a current  of electricity on
  a magnet.
         TO MAKE ART1FCIAL MAGNETS.
   69. The  following  process may be  adopted
to magnetise small bars of steel, which may be
thus  mude  to  become magnets of considerable
power without the aid of a magnet :—Take a
                    i poker, and  support it up-
                     right  between  the knees,
                     having  previously  tied to
                     it, with a  piece of silk, as
                     in the annexed figure, at
                     a, the piece  of  steel  to be
                     magnetised. Hold this part
                     with  the  left  hand, and
                     grasp the tongs  with the
                     right, a  little  below the
                     middle, holding  them in a
                     vertical position;  then  let
                     the steel  bar  be  rubbed
                     with the lower end of the
                     tongs,  from  the   bottom
                     to the top, about a dozen
                     times on  each  side.   By
                     this means, sufficient mag-
                     netic power may be given
                     to the bar to enable it to
                     lift small pieces of iron and
steel.   The  lower part of the bar a,  should be
marked before it is tied to the poker, in order to
distinguish its poles when it is taken off; the end
pointing to  the earth being the north pole, and
the upper end the south.
  70. A magnet may be made by rubbing a piece
of hard steel with  a natural or artificial magnet.
Take a common sewing needle, and pass the north
pole of a magnet from tho eye to the point, press-
ing it gently in so doing.  After reaching the end
of the needle, the  magnet must not  be passed
back  again towards the eye,  but must  be lifted
up, and applied again to that end, the friction
being always  in the same direction.   After re.
peating this a few times, the  needle will become
magnetised, and attract iron-filing the same as
the magnet from which it has derived its power.
  71. A poker may be made magnctical, by sup-
porting-it in a  position slightly inclined  to the
perpendicular, the lower end pointing to the north,
and striking it sharply a few times with a ham-
mer.  This is a simple and  expeditious way of
procuring an  artificial magnet, and may readily
be adopted.  Bars of tempered steel may be mag-
netised  in  this way, and the better  the steel the
more permanent will be the magnet.

  TO SHOW MAGNETIC REPULSION AND ATTRACTION.
  72. The repulsion of both poles of magnetised
bars of iron is well illustrated by  the following
experiment.  If we suspend two short pieces of
iron wire (n s, n s) by threads, they will hang in
contact in a vertical position.  If the north  pole
of a magnet (n) be now brought to a moderate
distance from  the  wires, they will recede from
each  other, as in  Fig. 1.  The ends s s, being
made south poles  by indication  from  the north
pole (n,) will  repel each  other, and so will the
north poles, n  n\  This separation of the wires
will increase as the magnet a approaches nearer
them ; but there will be a particular distance at
which the  attractive force of n overcomes the
repulsive force of  the poles s s, and  causes the
wires to  converge, as in Fig.  2 ;  the north poles,
n n, still exhibit their mutual repulsion.
          Fig.I.                . Fig. 2.
       EFFECT OF MAGNETISM ON A WATCH.
  73.  The following experiment,  if well  per-
formed, is calculated to excite a good deal  of as-
tonishment  amongst  persons who  are  not  ac-
quainted with the powers of the magnet.  Re-
quest the loan of a watch from some person, and
ask if it will  go when laid  on the table.  The
answer, of course, will be in the affirmative, and
the experimenter cun  then inform the  company 
Philosophical Experiments.
51
that he possesses the power of commanding the
watch  to  stop, or  go, at his pleasure.  For this
purpose, a magnet must be fixed underneath the
top of the table, and a mark made on the  table
immediately above it; then if the watch be laid
on  this spot, the magnet will attract the  steel
balance-wheel, and stop the watch.   Directly the
magnet is  moved, the watch will again  go on,
and thus  it may be made to  stop or go, at the
pleasure of the experimenter.  More surprise will
be  created if, after having removed the  magnet
from under the table, another person be requested
to try whether his placing  it on the  table will
have any effect, which, of course, will not be the
case while the magnet is removed.
   This experiment is a very old one, but it illus-
trates how easily chronometers may be affected
by accidental causes ;  and  sometimes the mari-
ner's  compass  itself loses  its power, in conse-
quence of superior magnetic influence, occasion-
ed  by having particular substances  on  board a
ship.
        TO SHOW MAGNETIC ATTRACTION.
   74. If a magnet be  brought into contact with
some iron-filings, they will adhere to it. The filings
are strongly attracted  to each pole,  but  none of
them to the centre of the  bar.  It appears, from
this circumstance, that  the  magnetic power re-
sides chiefly in the poles, and that there is a part
of  the magnet, generally midway between them,
where little or no attractive power is exerted.
   75. The north or south pole of one magnet re-
pels the north or south  pole of another, just as
bodies similarly  electrified repel each other.   If
a magnet be dipped in iron-filings, they will im-
mediately become  attached to one end.   Suppo-
sing this to be the north pole,  each of  the ends
of  the filings,  not in contact  with the  magnet,
will become north poles; while the ends in con-
tact will, by induction, become south poles.  Both
of these will have a  tendency to repel each other,
and the filings will,  therefore, stand on the mag.
net.  The ends  of the filings in contact cannot
repel each other, because they are so strongly at-
tracted by the pole  of the  magnet;  but at their
other ends, where this force is not exerted, they
do repel each other, showing that they are in the
same magnetic state.
            POLARITY OF THE MAGNET.
   76.  If a piece of steel, that has  been rendered
magnetic, be supported so  that it can turn freely,
it is found to point with one end to the north, and
the other  to the south (see Introduction.)  This
properly may be pleasingly illustrated, by tying a
magnetised piece of steel round its  centre with a
piece of sewing silk,  and  supporting it from a
stand, as the pith, bulls are supported in a pre-
vious experiment.   Another method is, to place a
magnet on a piece of cork  swimming in  a  basin
of water.  If the  cork be placed in the centre of
the basin, so that it is  not attracted by the  sides,
the magnet will be found  to turn due north and
south, the same as the mariner's compass, which
in case of accident at sea, it might be made to
supply the place of.

    HOW  TO MAKE A MAGNET BY GALVANISM.
   77.  As mentioned in the Introduction, Galvan-
ism is capable of  forming  magnes out  of com-
mon steel.  To effect  this,  make a  connection
between the poles of an excited battery with the
two ends of a wire formed  into  a spiral coil, by
bending common bonnet wire closely round a cyl-
inder, or tube, of about an inch in diameter;  in-
to this coil introduce  a needle, or piece of steel
wire, laying it lengthways down the circles of the
coil.  In a few minutes after the electric fluid has
passed through the spiral wire, and, consequently,
round the needle or wire, the latter will be found
to be strongly magnetised,  and to possess all the
properties of a strong magnet.
   78.  The same effect may be produced by pass-
ing a charge through this spiral coil from an elec
trical battery.

EFFECT OF  A  CURRENT  OF  ELECTRICITY  ON A
                    MAGNET.
   79.  If a current of electricity be made to pass
along a wire,  under  which, in a line with it, a
compass is placed, it will  be found that the needle
will no longer point north and south, but will take
 a direction  nearly across  the current, and  point
almost east and west. This fact has led philos-
ophers to believe, that there  are constant currents
of electricity  passing  east  and  west across the
earth,  and which, therefore,  cause the needle uni-
formly to point to the north.
               CHEMISTRY.
Introduction to the science of Chemistry—Chemical affinity
  explained and  illustrated—The effects it produces__Dif-
  ference between an "element" and a " compound"—The
  laws of chemical  combination explained—Wonderful ef-
  fects of these laws—Production of poisons from harmless
  elements—change of color  from combination—Change of
  bulk from ditto—•Crystallization—The atomic theory ex-
  plained.
  80. There are  no experiments more pleasing,
more instructive, or more easily performed, than
those which illustrate the leading principles of the
science of Chemistry ;  and as  a knowledge of
them may frequently  be of eminent  service in
preventing dangerous accidents, and of great use
in many little domestic operations, we have made
a numerous collection of the more  pleasing  and
entertaining, and arranged them so that they il-
lustrate all the leading facts in a simple manner.
To perform them, it is  not necessary to purchase
expensive apparatus; for indeed, the  whole of
them may be illustrated for a  few shillings,  and
the more simple the apparatus,  the less danger
there is of the  experiments failing.  Sir Hum-
phrey Davey taught himself chemistry with ap-
paratus,  that did not cost him  more than a few
shillings! and the student who wishes to perform
the experiments which follow need not, therefore,
fear  that  the means of  doing so  are beyond his
reach. The best way to begin it is to purchase
the smallest quantity of the chemicals that can
be obtained ;  for an experiment may, in general,
be tried just as well with a small,  as with a large
quantity  ; and if  it is obtained pure, from a good
operative chemist's, the less that is used the bet-
ter.   Then for apparatus, a few  slips of common
windoxo glass,  which  may be  obtained  at any
glazier's  for a few pence, will answer every pur-
pose.  Chemical mixtures, of all kinds, may be
made on a slip,  by adding two or three drops
of  the  liquids  together,  and   the  effect  will
be seen  quite  as  well aa if  they were mixed 
52                             Philosophical Experiments.
in greater  quantities, in  a large vessel.  Crys-
talization  may  also  be  beautifully  illustrated
in this way;  and, indeed, nearly  all the  ex-
periments  may  be thus   performed,  so  that
the young  student, with a little ingenuity, may
fit up a  laboratory for a few  pence!   This
economical plan  of proceeding has  been acted
upon with  great  success  in Edinburgh,  where
large classes of young gentlemen and ladies have
been taught  chemistry  on this method, by  Dr.
Reid.
   Having  thus stated the manner in which the
following experiments may be performed, we shall
proceed to describe the principal facts they are in-
tended to illustrate.  These are,  the effects  that
result from the different degrees of affinity which
different substances have for each other, and the
changes produced by combination.
   81. Chemical Affinity is attraction of a pecu-
liar kind.  The attraction  of gravitation exerts
its influence on all bodies ; but chemical attrac-
tion, or affinity,  exists  only'between particular
Bubstances.  It is described as " that tendency to
unite, which many bodies, possessing  different
qualities, exert towards each other."  The prin-
ciple will be  better understood  by  the experi-
ments which follow, than by a mere  description,
and  to them therefore,  the student  is  referred;
but there are several interesting  particulars  con-
nected with the subject,  which it is necessary
should be understood before the experiments are
performed.
   82. The earth, and the various substances, an-
imate and  inanimate, that are  found  upon it,
however diversified in their appearance, and dif-
ferent in their  sensible  qualities, are all formed
out of a few  simple elements, which, combining
together in different proportions,  produce the  vast
variety of animals, vegetables, and minerals, with
which wc  are  acquainted. Thus, two or more
elements, in consequence of the affinity they
have for each  other, will  combine and form a
particular compound; and  this  compound, hav-
ing an affinity  or liking   for  some  other  com-
pound body,  or for an element, unites with  it,
and  produces another substance ; by this means
it will  be  seen,  that everything on  the earth
can  be  formed out of a few  simple elementary
substances,  when they  enter  into combination.
   82. The difference between an element and a
compound is this: an element is something  that
is not formed by  the combination of other  sub-
stances, while a  compound, as the word implies,
is a compound of two or more simple substances ;
a something that is formed by the combination of
some of the elements.   Thus  for  instance,  lead
is called a simple substance, because no means
have as yet been discovered by  which we  can
show that it  is  formed  ouf of  any  thing else;
but we can prove that the preparation called  red
lead is a compound, because, on subjecting it to
a certain process, we can  separate it into pure
lead and oxygen,  which are both  simple  substan-
ces.   We call lead,  therefore, in a state of purity,
an element, and its oxide, or red.lcad a compound.
There are only  fifty-four elements in nature, but
the number of  compounds is innumerable.
   84. The cause which produces these combina-
tions is  termed  chemical affinity.  The word
" affinity"  does not convey the exact meaning it
implies; the word  "attraction"  will better ex-
plain how  the power operates.   For instance,
ammonia (an alkali)  has a  strong  affinity, or is
strongly attracted to unite  with od; but it has a
a stronger affinity for any  of  the acids,  or, in
other words, they attract it more  powerfully than
the  oil.   It  will  happen, consequently, that
if oil and ammonia are  brought  into close con-
tact with each other, by  being mixed togeth-
er,  that  they  will  chemically  combine, and
form a compound body,  which in fact, is a kind
of soap; but as the  ammonia has  a stronger af-
finity for an acid  than for the oil, it will happen,
that if we mix a little sulphuric  acid  with the
soapy mixture, that the ammonia will be attracted
fr«m the oil, and chemically combine with  the
acid.  (See experiment 106.)  In this  case, we
have an illustration of what is meant by the term
affinity, and the manner in wliich it operates.  It
is, indeed, merely a word used to express the de-
gree of attraction, or as it has been called the " li-
king," which one substance has for  another ; and
by virtue of which, when allowed to mix togeth-
er, they will combine chemically.   According to
the intensity of the attraction or  affinity, so will
be the force with which the bodies will combine,
and with  which they will   draw the  substance
they are most strongly attracted to from any oth-
er substance with which it  may  be in combina-
tion.
   85.  Chemistry is the  science  which  teaches
the laws that regulate this peculiar kind  of at-
traction ; and the best chemist is he who knows
most perfectly the  different degrees of affinity
which bodies have  for each other.  As there is
no kind of knowledge more practically  useful
than chemistry, it is hoped that the illustrations of
some of the leading  principles  now  given may
he the means of directing the young student's at-
tention more particularly to  the science.
   Chemical attraction differs from  general at-
traction, or  gravity, in a most important particu-
lar.   It is an effect which  takes place only be-
tween the particles of which all bodies arc com-
posed ;  it does not act upon masses, and, conse-
quently, before its  influence can be excited, the
particles must be brought into close contact with
each other.   Some  bodies do not  show the  affini-
ty they have for each other,  unless  they are even
mixed as liquids, or have some  liquid  added  to
them.   If we mix  what  forms a  very pleasant
kind of drink in the summer time, bicarbonate of
of soda and Tartaric acid,  together, in the dry
state, they will remain as a  mechanical mixture
only, the same as if we were to mix a  quantity
of bran and flour together; but if  we add a lit-
tle water, a violent effervescence takes place, the
particles have then  been brought  close enough
for their affinities  to come into action,  and a
chemical compound  is  the result.  The  same
principle may be illustrated,  by a simple experi-
ment, with quicksilver; though the attraction, in
this  case, is different to chemical  affinity.   If we
place two globules  on the  table, a little distance
apart, they will not  attract  each other with suffi-
cient force to be drawn together; but if they are
gradually pushed closer to each other, when they
have passed a certain limit,  they suddenly fly to-
gether  and form one  globule.   It is  necessary,
therefore, in order to produce a combination, that
<•*"»»;*. 
Philosophical Experiments.
53
 the two should  be  brought  close to each other;
 the attraction will not show itself at a distance ;
 and this is the case with chemical attraction.   It
 may be regarded, therefore, as a law of chemical
 combination, that as affinity  is a power exerted
 only by particles of matter upon each other, they
 must be brought into  immediate  contact, before
 any effect can be produced.
   86. Another  rule to  be remembered  is, that
 the affinity of  a  body  for different substances
 varies in intensity.   If the affinity of ammonia
 for oil be represented by the figure 5,  its affinity
 for the acids  will be  equal to  10 ; and, conse.
 qucntly, its tendency to  combine with them will
 be twice that with which it is urged to unite with
 the oil.  Therefore, a  substance, for which a bo-
 dy has the strongest affinity,  will combine with
 it in preference to combining  with any other.
 Many examples might be given of this fact. Po-
 tassium, for instance, has so powerful an affinity
 for the clement called oxygen, that it  will separate
 it from any other  element with which it may be
 united, and will burst into flame when thrown
 upon water.
   87. In most works on  chemistry, tables of the
 degrees of affinity  of a  body for different sub-
 stances are given, showing  what compounds it
 will decompose,  by abstracting the  substance to
 which it  is particularly  attracted.   It  may  be
 stated as a general rule,  that  a body which has
 the strongest  affinity for another substance will
 separate it from any combination it may  have
 formed: this, however,  will  not hold true in  all
 cases.  It was  formerly  supposed,  that a com-
 pound body could  never  be  composed by a sub-
 stance having a  weaker affinity for  either of the
 constituents than  they  had for each other; but
 it has since been found  that sueh an event can
 take place, and it may be  proved by experiment
 99 where a body,  having  a weak affinity for  a
 substance, separates  it  from  another body, for
 which it has a more  powerful affinity.  The cir-
 cumstance may  be explained, by supposing that
 the intensity of  the action between any two bo-
 dies depends on their quantity, as well as on the
 peculiar  affinity of  the atoms individually for
 each other;  and consequently, that a large quan-
 tity of any substance, of weak affinity, will over-
 come  the  affinity  of a  smaller quantity,  even
 though it has generally  a  stronger affinity for a
 substance than that by which it is subdued.
   We must now proceed to  describe the effects
 that are  produced when bodies combine together
 chemically.    When  this takes  place,  certain
 changes are  produced  in them, by wliich  their
 appearance, and sensible  qualities,  are  entirely
 altered.   The" laws of   combination,"  which
 cause these changes are exceedingly simple, and
 few in number;. yet they  are  capable of produ-
 cing  an extraordinary variety of effects.   The
 experiments  which follow will illustrate  these
 laws; and the following particulars  will, there-
 fore, be useful in enabling  the  student to under-
 stand the manner in which they operate.
  88. The first  important  fact to be noticed is,
 that when two substances combine, the compound
 they form is always different  in its nature  to
 themselves.  Two bodies,  decidedly poisonous,
 when combined chemically, may produce a com-
pound, not merely uninjurious, but  even uecossd.
  ry to our existence!   This fact is strikingly illus-
  trated in the combinations of the two  elements
  called oxygen and nitrogen.  For example:—
  The .Itmosphtre is a compound of. .Nitrogen4..Oxygen 1.
  SHrovt Oxide (laughiiic; gas).....Nitrogen 2. -Oxygen 1.
  Nitric  Jicid (aquafortis)........Nitrogen 2. .Oxygen 5.
  Thus it  will be seen, that  the  same  elements
  which, when mixed  together in  the proportions
  first mentioned, produce the air we breathe, form
  one of the  most active  and destructive poisons,
  when  combined  in  the  quantities  necessary to
  produce nitric acid ;  for this  acid and the air, it
  will  be seen, are both formed from the same ele-
  ments, only the proportions in which they are com-
  bined are different.   In the combinations of the
 element called carbon, or charcoal, we have an-
 other striking example of the  different forms one
 substance can assume.  Who would  believe that
 a brilliant  diamond, and  a piece  of common
 charcoal, are  the same  material, only in  dif-
 ferent  forms?   Yet  such  is the case; and the
 chemist has the power, by exposing the  diamond
 to a great heat in oxygen gas, of reducing it to the
 state of charcoal.  This  circumstance   may ap-
 pear very extraordinary, but it is not more won-
 derful than that a piece of lump sugar may be
 converted into carbon.  We have shewn, in the
 experiments which follow (experiment  115), the
 means by which  this  can be accomplished ; and
 it is, therefore unnecessary to allude to it more
 particularly.
   A familiar example of the  fact  that  two bo.
 dies, actively  poisonous  in their  natural state,
 may produce a substance, when  combined,  that
 shall be perfectly innoxious, is seen  in  our com-
 mon table salt.   This is  composed of muriatic
 acid  and soda.  The  muriatic acid, taken inter-
 nally, causes much agony,  and ultimate death ;
 and the caustic alkali (the soda), would  produce
 effects very similar ;  yet when combined together,
 they produce a substance  ranking amongst the
 first  necessaries  of life;  for, without  common
 salt, it would be  almost impossible to maintain
 health.  As an example of  poisons being produ-
 ced from  the combination of substances, wliich,
 in their natural state, are not injurious, we may
 instance the poisons  which are formed  by  ani-
 mals and vegetables.   The dreaded worali—the
 poison used by the Indians—and the pestiferous
 and  destructive upas,  which is produced  from the
 tree of that name, and to the influence of either
 of which animals cannot be exposed without the
 loss of fife, are formed from  the same elements as
 those which produce the luxurious fruits, and the
 wonderful variety of  beautiful flowers that exist
 in the countries where these poisons are  found.—
 In like manner,  the elementary  substances that
 form  the flesh of  the  deer and oxen, upon-which
 man  finds subsistence, are the same  as those
 from  which the deadly poison  of the rattlesnake
 is produced, or the no less dreaded virus of canine
 animals in a state of  hydrophobia.  Thus it will
 be seen how nature, out of a few simple elements,
 is able to  produce such a  wonderful variety of
 substances, whether the result of organization, or
 producce from the mineral kingdom.
  69.  Change of Color is another circumstance
 that  frequently attends  chemical .combination.
 The  most beautiful colors  may be formed, and
 destroyed again,  by means  of a drop or two of
I"' 
54                              Philosophical Experiments.
some liquids, when 'added to others; and  few of
the experiments will probably be more interesting
than those which  are given to illustrate  this phe-
nomenon.
  90. Change of bulk  is another event,  of fre-
quent occurrence,  when  bodies combine.  Two
liquids,  on being  mixed together, may become
solid; and two solids, under simdar circumstan-
ces, may form a liquid-   These facts have been
called " chemical  miracles;" but,  indeed, there is
nothing more wonderful in the circumstance than
in the other beautiful illustrations  of chemical af-
finity that we have given.   All  the  curious in-
stances of likings and  dislikings  which substan-
ces  appear to exhibit towards each other,  are
equally entertaining ;  it is only in consequence of
some effects not being produced so often as others
that we deem them more wonderful.
   91. Crystallization is another  beautiful effect
which frequently attends chemical action.  Eve-
rybody is familiar with the  appearance of crys-
tals, and the different forms they exhibit.   Thus
we have crystals  of sugar, in the form of sugar-
candy, and crystals of Epsom salts, which are as
well known  for their different appearance, as for
their disagreeable qualities.  Both  these  kind of
crystals are as different  in  form as  they are in
taste"; and many others may be  easily  called to
recollection: yet all these particular forms are oc-
 casioned by one  simple law of nature, which is
 another kind of affinity, and causes the particles
 of various liquids, in cooling, to adhere together,
 and assume a crystalline shape.   In the great op-
 erations of nature, crystallization takes place on a
 grand scale.  The Giant's Causeway, in Ireland,
 and Fingal's Cave, in  Scotland, might be men-
 tioned as illustrations;  and the  crystallization of
 water, in the form of ice,  every  one is familiar
 with.   The student  may easily imitate  some of
 the phenomena,  and, in doing  so, will  observe
 how beautifully one general law operates alike on
 the smallest particles, as well as on masses of im-
 mense  magnitude.  It  is merely necessary, in or-
 der to procure good crystals, that the liquid should
 be  set on one side  to  cool gradually,  in  some
 place free from  dust;  and  the  process  may be
 quickened by dropping  a crystal or two into the
 liquid.  From the examples that are given, some
 knowledge of the variety  of  forms crystals as-
 sume may be derived.
    92. Another remarkable fact relating to chem-
 ical affinity is, that the quantity of any substan-
 ces required to form a particular compound is al-
 ways the same; and so long  as a body retains
 its general characteristics, it wdl  always consist
 of the  same elements,  united  together  in  the
 same proportions. For instance, sulphuric  acid
  (oil of vitriol) is always composed of 16 parts, by
 weight, of sulphur, and 24 of oxygen.  No other
  substances can  form sulphuric  acid, nor can its
  own elements produce it, if combined in  any oth-
 er proportions than those just stated.  Water, in
  like manner, is  formed of one part, by weight, of
  hydrogen, and eight of oxygen; and were these
  elements to unite in any  other proportions, some
  new  substance,  different  from  water would be
  produced.  When two or more  elements unite to
  form a compound, the addition or diminution of a
  small quantity of one, often produces an effect re-
  markably different to what would have resulted,
had the proportions been different.  For instance,
there is great dissimilarity,  both in taste and ap.
pearance, between starch  and  sugar; and  yet
they are composed of the same  elements, and ve.
ry nearly in the  same proportions, as will be seen
by the following analysis :—
                Oxygen.      Hydrogen.     Carbon.
Sufjar is composed of. .40..........5............36
Starch.............48.........13.............4}

The figures  represent the parts of each element,
by weight, that form the two substances; so that
it will be seen, it is only in  consequence of the
starch containing a few more particles of its ele-
ments than the sugar does, that it differs so mate-
rially in its  sensible  qualities.   If we could ab-
stract a few atoms only of the oxygen, hydrogen,
and carbon, from the starch, wc should convert it
into sugar !  and in some  chemical processes this
is really effected.  It is in  consequence of the
beautiful law of  nature we have been describing,
that chemists are able  to tell exactly how much
of any substance is  contained in  any particular
compound ; for the quantity is always the same,
and when it has  been once  ascertained, it is
known  always.  For instance,  sulphate  of mag-
nesia (Epsom salts) is  formed  of sulphuric acid
and magnesia.   If the latter be added to the acid
till effervescence ceases,  it will be found, that any
magnesia thrown into the solution afterwards will
not combine with  the  acid, but will fall  to the
bottom of the vessel; thereby showing, that only
a certain quantity  of  magnesia will  combine
with the acid, to form Epsom salts.
   93.  Thus it will  be seen, that the same great
laws which the  Creator has established are exem-
plified as clearly in the  most simple experiments,
as they are in those  grand phenomena  of  nature
 that must ever excite the admiration  and wonder
of every thoughtful observer : and the young stu-
 dent, with a few old glasses for apparatus, and a
 few pennyworths of chemicals for material, may
 make  himself familiar  with  the works of  the
 Creator,  that,   in former times, confoimded  the
 profound philosophers.  Surely, such pleasure is
 desirable?   It  is not  merely a  gratification of
 curiosity, but a rational way  of exercising those
 faculties which have been given to us to improve.
 A person who so employs his time will have the
 pleasure  of knowiug, that  he is not merely pro-
 viding himself with a means of endless  amuse-
 ment, but storing his mind with information of a
 valuable kind.
   94.  In  describing the different chemical  pre-
 parations that are to be used in the experiments,
 we have employed the terms by which they are
 known to chemists, and added, in a parenthesis,
 the popular names; thus,  "Sulphuric  acid  (oil
 of vitriol.")  All the chemicals may be obtauied
 at an  operative chemist's, by asking for them in
 the former  names; and we again advise those,
 who perform the experiments only to purchase a
 small  quantity, as a few pennyworths of  most of
 the substances will be quite sufficient.
    95.  Chemical solution  is  very different from
 mere mixture.  Solution is a chemical combina-
  tion between a fluid and any substance that  may
  be dissolved in it; whereas mixture is simply a
  division of  the particles of a body by a mechani-
  caj power.  Portions of the substance float about
  in the liquid  it is  mixed  with, but they do not 
Philosophical Experiments.                                55
combine with it;  and these portions will, if the
mixture remains at rest, fall to the bottom, or rise
to the surface, according to their relative specific
gravity as compared with that of the fluid.  This
may be shown by the following experiments :
    EXPERIMENTS IN CHEMISTRY.
Combination of sugar of lead in Chemical  solution—Appa-
  rent anomaly in chemical affinity—How to make soap—To
  analyse soap—Divisibility of sulphate of  iron—Repulsion
  illustrated—Attraction illustrated—Transformation of su-
  gar into charcoal—Charcoal formed without fire—Lime
  formed by the  breath—Laughing gas described—How to
  make it—To produce a solid by  mixing  two liquids—To
  produce a liquid from  two solids—To make  infusion of
  cibbage, and litmus and tumeric papers, for testing—To
  make Time-water for testing—Change of color by chemical
  action—To change and destroy the  color of flowers—Ef-
  fect of alkalies and acids on colors—How to  make 12
  kinds of invisible  ink—To make a paper  that  will not
  light—Fire in water—Deliquescent  salts—Efflorescent
  salts—To make Epsom salts—To test the purity of water
  Experiments on crystallization—Influence of light on ditto
  —How to make  mineral baskets—Rapid crystallization—
  Artificial Quartz—Apparent transformation  of iron to
  copper—Beautiful appearance of hoar frost—To make fu-
  sible spoons—Curious property of burning  camphor—To
  tent the purity of steel.

         EXAMPLES OF CHEMICAL SOLUTION.
   96.  Put into a glass vessel, containing water, a
 few grains of sugar  of lead, and  stir  them to-
 gether with a glass  rod ;  the water will soon be-
 come turbid, in consequence of the sugar of lead
 being insoluble in that fluid, and simply a mixture
 of  the particles with  the water will  take  place.
 If  the  water be  minutely examined, these par-
 ticles may be seen floating in it ; and they will
 ultimately, if left to themselves, fall  to the bottom.
   97. If to this milky fluid be now added a few
 drops of nitric acid, it will instantly become clear
 and transparent; and now not the most minute
 portion of the lead will be perceived in it.   In the
 first  instance, there  was only mixture; in  the
 latter, a perfect solution, because the  combination
 of lead  and nitric  acid is soluble in  water, while
 the sugar of lead is not.
   98. If chalk and water be mixed together, the
 fluid will be turbid; but if a  few drops  of  mu-
 riatic acid be  added, it will become quite trans-
 parent.

    APPARENT ANOMALY IN CHEMICAL AFFINITY.
   99. It is a general law in chemistry, that one
 body, having a strong affinity for  another, will
 combine with it, in preference to uniting with any
 substance  of weaker affinity.   In the following
 instances,  just  the  reverse  takes  place ;  sub-
 stances havmg a weak affinity combme together,
 in preference to uniting with those  lor which their
 affinity is  stronger.  In  the following table,  the
 body first mentioned decomposes  a  compound of
 the second and  third, named in the same fine, al.
 though its attraction for the second is  inferior to
 that of the third.
   100. Potash separates sulphuric acid from  barytcs.
   101. Lime separates sulphuric acid from pota-.li.
   10-2. Nitric acid separates lime from oxalic acid.
   103. Potash separates phospho.ic acid from lime.
   101. Potash separates carbonic acid from lime.
   103. Soda sepirates sulphuric acid  from potash.
    It is necessary, in order that the experiments
  should  fully succeed, that a much  larger quantity
  of the  first-mentioned  substance should  be  used
  than the second or third; and the student must
not be  surprised if the experiment should not be
successful.

     TO MAKE SOAP—EXAMPLE OF AFFINITY.
   106.  Pour a little oil into a phial, and add some
water to it, when it will be found, in consequence
of the  oil having  no affinity for the water, that
they will not combme together,  however  much
they may be shaken ; for directly  the  bottle  is
still, the oil will rise to the surface.  If a small
quantity  of liquid  ammonia  (hartshorn) be now
introduced gradually into the bottle, and the con-
tents shaken together, they will  chemically com-
bine ; for the oil and the ammonia have a strong
affinity for each other, and when mixed they pro-
duce a soapy liquid.  This is soluble  in water,
and, therefore,  when the bottle  is  shaken,   the
three liquids unite  together.  This is a very sim-
ple, but a very  striking experiment, as  it clearly
illustrates the manner in which chemical affinity
separates, and  affords  a  curious instance  of  the
" likings and dislikings " of different bodies.
              TO DECOMPOSE SOAP.
   107. The oil and the ammonia in the last  ex-
periment combine, because  there  is a  strong af-
finity between  them; but if another substance be
introduc d into the bottle, which has a stronger
affinity for one of them  than they have for each
other, the compound will be decomposed.  This
may be  effected  by adding, very carefully,  a
small  quantity  of sulphuric acid (oil of vitriol) to
the mixture.   The  acid  has a stronger affinity
for the ammonia  than the ammonia has  for the
oil, and it will, therefore, leave  the oil, and com-
 bine with the  acid.   The oil will then swim  on
 the top.
         DIVISIBILITY OP SULPHATE OF IRON'.
   108. Dissolve two grains of sulphate of iron in
 a quart of water,  and add a few drops of this so-
 lution  to a wine-glassfull of water, into which a
 few drops  of tincture of galls have been placed.
 The dilute infusion of galls will immediately as-
 sume  a purplish  hue.  This shows that every
 drop of the quart of water, in which the sulphate
 of iron was dissolved, contains  a portion of the
 salt.
          REPULSION.-—STEEL AND WATER.
    109. If the  blade of a well-polished knife be
 dipped into a basin of cold water, the particles of
 each of these ; wo bodies do not  seem to come in
 contact with each other ;  for when the blado  is
 taken  out, the  water blides off, leaving the blade
 quite  dry, as  if it had  previously been smeared
 with any greasy substance.
    110. In the same way, if a common  sewing
 needle be laid  horizontally on a glass of water, it
 will not sink,  but form a kind  oi  trench  on the
 surface on which  it lies, and floats about.   Thia
 proceeds from the  little attraction which exists
  between the cold water and the  polished steel.  It
 is necessary that both the knife, in the last expe-
 riment, and also  'he needle, should be dry  and
 clean ; otherwise, the effect will not be produced.

         REPULSION.--MERCURY AND GLASS.
    111. If a quantity of quicksilver (mercury) be
  poured into a  wine-glass, its upper surface will
  be convex; that is, a kind of trench will be formed
  all round the mercury, between it and the sides of 
56
Philosophical Experiments.
 the glass.   This is in consequence of there being
 no attraction between the glass and the mercury.
   112. An  opposite effect may be produced, by
 pouring a small quantity into a metal cup.   In
 this case, the mercury will  appear concave;  for
 the attraction  of the sides of the vessel for  the
 metal is sufficient to cause it to rise above  its
 level at the edges.
     ATTRACTION BETWEEN' MERCURY AND GOLD.
   113. If a  sovereign be rubbed with mercury, it
 will lose its  usual  appearance, and appear as if
 silvered over; the attraction of the gold for  the
 mercury being sufficient to cause a coating of it
 to remain.
   114. When it is wished to remove the silvery
 appearance,  dip the sovereign in a dilute solution
 of  nitric  acid, which  will entirely take  it  off.
 Some rather laughable circumstances have  oc-
 curred, where persons, having  a little quicksilver
 get loose  in  their  pockets, have been surprised
 to find their sovereigns, apparently changed  to
 shillings.
   The six preceding experiments are not strictly
 chemical, but they are  introduced here  for  the
 purpose of illustrating attraction and repulsion.

 TO TRANSFORM A PIECE OF LOAF SUGAR INTO A LUMP
                 OF CHARCOAL.
   115. It has been  previously  mentioned, that
 the diamond has been proved to be only crysta-
 lizcd carbon; it is  not generally known that su-
 gar is composed  almost entirely of the  same
 substance.   Sugar iR a vegetable production, and
 consists principally of charcoal in a peculiar state
 of combination with water.  This may be proved
 by pouring a little sulphuric acid (oil of vitriol)
 over a piece  of lump sugar, in  a saucer, or other
 vessel.   The acid  will combine with the water
 of the sugar, wliich will, in a few minutes, turn
 black, and appear precisely like  a lump of char-
 coal.  The affinity of sulphuric acid for water is
 so great, that it attracts it from its chemical union
 with the sugar.
        CHARCOAL FCRMED WITHOUT FIRE.
   116.  If a few small cuttings  of wood be placed
 in a glass, and a 1 ttle sulphuric acid poured over
 them, they  will become  black, like  charcoal,
 from a similar cause to that which produced the
 effect described in the last experiment.
           LIME FORMED BY BREATHING.
   117.  Pour  a  little lime water into a tumbler,
 and breathe into it through a small pipe.   Flakes
 of lime  will  be immediately  formed, and the
 water will  become  turbid, in consequence of the
 breath forced out of the lungs, which  contains a
 great portion of carbonic acid,  combining with
 the lime held in solution in the water.

      SINGULAR EFFECTS OF LAUGHING GAS.
   118.  Protoxide of nitrogen, nitrous oxide, or, as
it is more generally termed, laughing gas, is a
 compound that the young chemist generally de-
Bires to procure as  soon  as possible; and  we are
induced, therefore, to give  the following descrip-
tion of its properties, and of the method  to  be
adopted  for obtaining it in a state of purity,  al-
 though  he must not expect to do so without con-
 siderable trouble, and some disappointment.   Ni-
 trous oxide is a compound of the same elements


                                       4
                           4
 as those which constitute the atmosphere;  but, in
 consequence of containing a greater quantity of
 oxygen, its effects upon the human frame, when
 breathed for a short period, are very surprising. It
 is not a gas that can be breathed with impunity
 for  any great length  of time, yet it can be re
 ceived into the lungs for a short period without
 injury.  It is termed  laughing  gas, because its
 general effect  upon persons who respire it is to
 induce a very strong  desire to give way to vio-
 lent fits of laughter.   It does not, however, pro-
 duce this  effect on every individual.   Some are
 made exceedingly melancholy, and others appear
 desirous of  annihilating  everything  on  which
 they can lay their hands.   In general, however,
 the gas only excites the person who breathes it to
 laughter.  It acts as a powerful stimulant for a
 time, but, unlike other stimulants, it  is not  fol-
 lowed  by lassitude,  or  lowness of  spirits,  unless,
 while under its influence, the person is excited to
 excessive muscular exertion. Sir Humphrey Davy
 made a variety of experiments with this gas.  He
 administered it  to various persons,' and, indeed,
 was the first to investigate its properties with any
 degree  of accuracy.   Previous to  his  time, the
 gas was considered to be unfit for the purpose of
 respiration,  but  Davy found  that  it  could  be
 breathed with safety; and in  his further experi-
 ments on it discovered the singular effects it pro-
 duces.  After a few inspirations of it have been
 made, it causes a sense of lightness and expan-
 sion of the chest, and a pleasurable sensation  be-
 gins to extend   over  the whole body; this  in.
 creases, and is accompanied with a desire to inhale
 the  gas ; respiration becomes, therefore,  fuller,
 and is performed with more energy.  Exhilation
 is soon produced; and if the  respiration is con-
 tinued  sufficiently long, a crowd  of indistinct
 ideas, often in  very singular  combinations, pass
 through the mind; there is an irresistible propen-
 sity to  laughter,  and to muscular  exertion and
 violent efforts are made with  alacrity and ease.
 These  effects,  after the  inspiration has ceased,
 continue for four or five minutes,  or sometimes
 longer; they gradually subside, and what  is not
 the least singular, the state of the system returns
 almost  immediately to its usual  standard. We
 have frequently administered the gas  to others,
 and have breathed it ourselves; and when  this is
 done in a proper manner, we have never failed to
 observe or feel the effects above described.  There
 is, however, some difficulty in  administering the
 gas  properly to  a person who has never taken it
 before.   It must be enclosed in a bladder, fitted
 with a stopcock; and unless the person inhales it
 from the bladder without  allowing any of the  at-
 mosphere to enter his lungs at the same time, the
 experiment will  not  succeed.   The  best way is,
 to close the nostrils with the left hand, and then,
 forcing all the  air possible from the lungs by a
 strong respiration, to place the stopcock in your
 mouth, and so breathe in and out of the bladder,
 at the same time keeping the nostrils quite closed.
 If this  be done  properly, the  gas is sure to pro-
 duce its usual effects.  When it is administered
 to a person, unless he has taken it previously, and
is aware of the manner in which it affects him, it
is desirable to have some one near to prevent his
doing any mischief,  in case he should feel so  in-
clined.  Self-command is in general entirely lost 
Philosophical Experiments.
5?
   for a few minutes, although the individual is per-
   fectly  sensible all the time in what a ridiculous
   manner  he  is behaving.   A bladder capable of
   holding a few quarts of gas will be large enough,
   and it is advisable to test the gas by holding a
   light in some of it before it is taken.
             HOW TO MAKE LAUGHING GA8.
      119. There are various  methods of procuring
   this gas,  but we think our readers will find it best
   to obtain it  from nitrate of ammonia. This Bhould
   be  placed in a glass  retort, and exposed to the
   flame  of a  spirit lamp.   It  will soon(melt, and
   Bhortly afterwards the gas will be evolved.  It
   should be  collected in a receiver,  placed  in a
   pneumatic  trough, and allowed to stand a  short
   time over water, in order to remove any impuri-
   ties with which it  may be contaminated.   The
   nitrate of ammonia, when melted, should only be
   kept simmering ; for  if the heat  be increased too
   much, it will cause a slight explosion, and nitric
   oxide and nitrogen gas will be produced.  If it
   be wished to make a considerable quantity of gas,
   it will be advisable, on the ground of cheapness,
   for the operator to prepare the nitrate of ammonia
   himself.   This  may be done by pouring diluted
   nitric acid on carbonate of ammonia, and evapo-
   rating the solution till the greater portion of the
   water  is  gone.
        TO PRODUCE A SOLID FROM  TWO LIQUIDS.
      120. This surprising effect may be produced by
   mixing sulphate of magnesia (Epsom salts) with
   water, until it will dissolve no more, and then
   pouring  into  it a saturated  solution  of  caustic
   potass.  In  this case, the sulphate of magnesia is
   decomposed ;  the  sulphuric acid leaves the  mag-
   nesia,  which then combines with the water, and
   is precipitated in the form  of a white  powder,
   while the acid unites with the potass.
      121. If a saturated solution of chloride of cal-
   cium be  mixed with a saturated solution of  car-
   bonate of potash, both  of which are transparent
   liquids, the  result will be,  the formation of an
   opaque and almost solid  mass.  Mutual decom-
   position  of  the  salts  takes place ; chloride of
   potassium and carbonate of lime are formed ; the
   latter absorbs the whole of the  water of solution,
   and thus a degree of solidity is produced.
     121. If a  little nitric acid be  added to the mass,
   it will  be converted into a transparent liquid ; the
   insoluble carbonate of  lime being converted into
   the soluble nitrate of lime.
     122. If a small quantity of sulphuric  acid be
   dropped  into  a  saturated solution of chloride of
   calcium, an  opaque and nearly  solid mass will be
   produced; as  the chloride  is  decomposed, and
   sulphate  of  lime, a very insoluble  salt, formed.
     123. Dissolve a small quantity of acetate of
   lead (sugar  of lead) in  water, about half  filling a
   beer tumbler;  mix in another glass the like quan-
'   tity of bichromate of potass.   If  the contents of
   one glass be then poured into  the other,  a  solid
   compound will be formed, which  falls to  the bot-
   tom of the glass.
     124.   M;ike a strong solution  of sulphate of
   magnesia (Epsom salts,) by melting as much as
   possible  in warm water.  If a  small quantity be
   poured into  a glass, and a little ammonia (harts-
   horn) added to it, the ammonia wUl combine  with
   the sulphuric acid,  and liberate the magnesia,
I which will then appear in the glass in  a nearly
 solid state.  As both the solution of salts and the
 ammonia are transparent till mixed, this is a very
 striking experiment.

       TO MAKE TWO SOLIDS FORM A LIQUID.
   125. Triturate together, in a wedgewood-warc
 mortar, half an  ounce of sulphate of soda, with
I the same quantity of acetate of lead, and  they
 will combine together  and form a liquid, in con-
 sequence of their giving out their waters of crys-
 tallization.
   126. Mix together nearly equal  quantities  of
 carbonate of ammonia and sulphate of copper in
 a mortar;  pulverise them well, and they will
 form a violet-colored liquid.
   127. Triturate together, in a mortar, half an
 ounce of citric acid, in  crystals, with  a similar
 quantity of carbonate  of  potass.   These  sub-
 stances will combme, and become fluid.
   128. Put  an ounce of sulphate of soda, with
 th3 same quantity of nitrate of  ammonia, into a
 mortar, and  rub  them smartly together with the
 pestle, when they will both part  with their water
 of crystallization, combine together, and  become
 liquid.
   129. Triturate half  an ounce of muriate of
 lime with half an ounce of nitrate of soda : these
 two substances will operate upon each other, and
 become liquid like the others.

             TO MAKE TEST PAPERS.
   130. For the purpose  of many of the experi-
 ments  described, it is  necessary to be provided
 with test papers,  for ascertaining when an acid or
 an alkali is present in any solution.   The follow-
 ing directions will enable the experimenter to pre-
 pare them himself:—Boil a few  leaves of a red
 cabbage, cut into small pieces, in  a small quantity
 of water, or  pour boiling  water over  them; then
 strain it  into  a  piece of cloth,  and dip into  it
 some slips of blotting, or other thin paper, which
 must then be allowed to  dry, and afterwards dip.
 ped again two or three times.   These papers are
 turned of a red color when touched by acids, and
 green by alkalies.  The liquid itself may  be used
 in many experiments, but it must not be kept too
 long after it is made.
   131.  Litmus paper, which is turned red when
 dipped into an acid, may be prepared by boiling
 litmus in water,  and afterwards placing the pa-
 pers in it, as just described.
   132.  Turmeric paper is a test for  alkalies, be-
 ing  changed from  a  bright yellow  to a  reddish
 brown.    They may  be prepared by pouring  a
 small  quantity of boiling water upon some tur-
 meric, and afterwards  dipping the papers in it,
 and drying them.   The test papers should be cut
 into slips, and they will be more handy for use.

             TO MAKE LIME WATER.
   133. Lime water may be formed  by putting a
 small quantity of slacked lime  into a  bottle of
 water, and shaking it till dissolved.  It  is a test
 for the presence of carbonic  acid; and, if left
 uncovered, will soon  absorb it from the atmos-
 phere.
     CHANGE  OF COLOR BY CHEMICAL ACTION.
   134.  Infusion of red  cabbage  (see experiment
 130), or  almost any vegetable blue, will  become 
58
Philosophical Experiments.
red by the addition of sulphuric  acid, or any of
the common acids.
  135. Take a  little sulphate of iron, andd is-
solve it in water; add to it some infusion of oak
bark, or infusion of nut-galls, and it will instantly
become black.
  136. To some common writing ink add a little
muriatic acid, and  the  color will immediately be
des'royed.  To  this add a little solution of some
alkali  (ammonia, for instance), and  the black
color will be reproduced.
   137.  Take a  colorless  solution of sulphate of
copper, add to it a little ammonia, and it will be-
come of a beautiful blue color.
   13S.  To this  mixture add some nitric acid, and
the blue color will disappear.
   139. Add another portion of liquid  ammonia,
the blue color will be reproduced.
   140. Take a  little of the solution of red  cab-
bage, which is of a blue color, and add to it some
solution of potass, or soda, or ammonia, when it
will become green.
   141. Add to this  mixture  sulphuric  acid, drop
by drop  ; it will then change to a blue, and ulti-
mately to a red color.
   142. If a little  chlorine gas be now  passed
through the liquid, it will destroy all the color, as
this is its effect  on vegetable matter.

       TO CHANGE THE COLOR OF  FLOWERS.
   143. Get some violets, and  place  them  in a
glass jar inverted  in a dish  of water.  Place a
metallic vessel,  or a common  piece  of tile, in the
jar, and on it put a little sulphur, which is to be
ignited.   If the violets are exposed to the gas,
which is thus formed for a short time, their color
will be destroyed, and they will be blanched. The
same  effect may be produced on  a variety of
other flowers.
   144. Hold some  of the violets, after the  last
experiment, in  the vapor (muriatic gas),  which
arises  on pouring a little  dilute sulphuric acid on
common salt; they will then assume a red color.
   145. Pour a  little ammonia (hartshorn)  in a
bottle, and drop into  it another portion of the
flowers  blanched by the first experiment; they
will then be turned to a bright green.
   146. Put a  number of flowers, of any  color,
and a  few blades of grass, or some  green  leaves,
into a bottle containing  some  chlorine gas, and
their color will  be immediately destroyed.   This
 is very prettily illustrated, by  placing in the bottle
a spng or two of parsley, which, by the action of
the chlorine, is  rendered quite white.
   147. Chloride of lime dissolved in water, with
a little of  any of the acids added to it, may be
employed for this purpose instead of chlorine gas;
or even  the dry chloride of lime may  be used for
the same purpose.

PURPLE, GREEN,  AND  SCARLET, PRODUCED FROM A
                  BLUE COLOR.
   148. Place a small quantity of the  blue  tine.
ture of cabbage in three wine-glasses ; to the first,
add a  little solution of alum, and the color will be
changed to purple.
   14y. To the second glass, add  a  little solution
of ammonia, which will  render the liquid bright
green.
   150. In  the  third, place a few drops of muri-
atic acid, and this will turn the liquid to a beauti-
ful scarlet.
  These experiments show the effect of a salt,-an
alkali, and an acid, in changing vegetable colors.

TO SHOW THE EFFECT OF ALKAKIES AND ACIDS ON
                    COLORS.
  151.  If a  slip of turmeric paper, which is of a
yellow color, be dipped in ammonia, or any alka-
line solution, it will become of a deep red brown.
If it be dipped in an acid, it will turn quite red.
  152.  A solution of chlorine in water, or a so-
lution of chloride of lime, deprives all vegetables,
and vegetable infusions, of their colors.
  153.  If a  slip of tumeric paper be held  over a
bottle of liquid  ammonia  (hartshorn), its color
will be changed from yellow to brown by the va-
por which rises.
  154.   An  addition of a little of any of tho acida
to the above  mixtures will turn them of a beauti-
ful red color.
               SYMPATHETIC INKS.
  155.  Writing  with inks of this  nature is ille-
gible, unless  a chemical change be subsequently
effected on  them.  Many amusing experiments
may, therefore,  be performed with inks of this
description.  A  letter may be written with cem-
mon  ink, and another with sympathetic ink be-
tween the lines  of the former.   Drawings  may
also  be  made, which shall change their appear-
ance  : thus a picture, representing a winter scene,
may be made to change, and appear like summer,
on  the application of heat and other reagents;
and a variety of  similar effects can easily be pro-
duced.   The following description of the manner
of procuring a variety of these inks will afford the
student a selection on wliich  he can exercise his
ingenuity.

             FIRST KIND--SILVER LMK.
   156. Write with dilute  nitrate of silver (lunar
caustic), which,  when dry, will be entirely invisi.
ble ;  hold the paper over a  vessel containing sul-
phate of ammonia, and the  writing will  appear
very  distinct.   The  letters .will  shine  with the
metallic brilliancy of  silver.

          SECOND KIND--YELLOW INK.
   157. Write upon  paper with a diluted solution
of muriate of copper; when  dry, it will  not  be
visible, but on being warmed before the fire, the
writing will  become of a beautiful yellow color.

             THIRD KIND--GREEN  INK.
   158. Write with a solution of muriate of cobalt,
and the writing, while dry, will not be perceptible;
but if held before the fire, it will then gradually
become visible, and the letters will appear of  an
elegant green color.

            FOURTH KIND--BLUE  INK.
   159. Write with acetate of cobalt, or  with a
muriate of cobalt,  previously  purified  from the
iron which it generally contains.   When the wri-
ting  has become dry, these letters will also be in-
visible.  Warm  the paper a little, and the writing
will be restored to a beautiful blue.
         FIFTH KIND—ANOTHER  BLUE INK.
   160. Write with a weak solution of sulphate of
iron, and when  dry, yyash the letters with prus- 
Philosophical Experiments.
59
siate of potash, by dipping a feather in it, and
lightly passing it over them :  they will  then ap-
pear of a beautiful blue.
           SIXTH  KIND--BROWN INK.
  161.  If a solution of sulphate of copper be used
instead of  common ink, and allowed to  dry, and
the writing be then washed with prussiate of po-
tash, it will appear of a reddish brown.
           SEVENTH KIND--DARK INK.
  162. Write on paper with a solution of nitrate
of bismuth:  when this is dry, the  writing will be
invisible ; but if the paper be exposed to  sulphur-
etted hydrogen  gas,  the words will be distinctly
legible.
              EIGHTH KIND--BLACK.
   163. A letter written with a dilute solution of
bismuth becomes, when dry, illegible; but a fea-
ther dipped in a solution of sulphuret of potash
will instantly blacken the  oxide, and revive the
writing.
              NINTH KIND--BLACK.
   164. Write with a  solution of nitrate or ace-
tate of lead.   When the writing is dry,  it will be
invisible;  then, having prepared a glass decanter,
with a little  sulphuret  of iron strewed  over the
bottom of it, pour a  little very dilute sulphuric
acid upon the  sulphuret, so as  not to wet the
 mouth of the decanter, and suspend the writing,
 by means of the  glass stopper, within the decan-
 ter.  By  an attention to  the paper, the writing
 will become  visible by degrees, as the gas escapes
 from the  bottom of the vessel.
              TENTH KIND--BLACK.
   165. Write with a weak solution of sulphate of
 iron, let it dry, and it will be invisible.   By dip-
 ping a feather in tincture of galls, and drawing
 the wet feather over the letters, the writing will be
 restored,  and appear black.
                 ELEVENTH KIND.
   166. Mix alum with lemon juice.  The letters
 written with this ink will be invisible  till dipped
 in water.
                  TWELFTH KIND.
   167. A letter written with common milk, or
 with  the juice  of an onion, may  be deciphered
 when held before the  fire, and the paper is singed.
      TO MAKE A PAPER THAT WILL NOT LIGHT.
    168. Pulverise a small quantity of gunpowder
 and alum in a mortar, and dissolve the mixture in
 water.   Paper dipped in this solution, when thor-
 oughly saturated and dried, will not ignite.
                  FIRE IN WATER.
    169. If a few pieces of phosphurct  of lime be
 placed in a tumbler of water, it will be decompo-
 sed, and bright flashes of light will dart from  the
 surface of the water, presenting to those who are
 unacquainted with the cause, a very striking phe-
 nomenon.
               DELIQUESCENT SALTS.
    170.  The  atmosphere  produces two  effects,
 very opposite, on different kinds of salts: some
 absorb water from it and melt,  and  are,  there-
  fore,  termed deliquescent;  others part with  the
  water they may contain, and  become powder,
  they are efflorescent.
     171.  If a httle carbonate of potash be exposed
to the atmosphere,  it melts,  and in a short time
changes to a perfect liquid.
  172. If a few crystals  of  muriate  of lime be
exposed in a similar  manner, they will soon  be-
come liquid.
             EFFLORESCENT SALTS.
  These salts become converted into powder by
the action of the air.   The following are exam-
ples :—
  173. Expose some crystals of phosphate of so-
da, on a sheet of paper, and in a short time they
will be converted into a white powder.
  174. Common Epsom  salts are affected in a
similar  manner, and  they should, therefore, be
kept in a close bottle.
   175. The same effect is produced on the crys-
tals of sulphate of zinc.
             TO MAKE EPSOM  SALTS.
   176. This well-known medicine  (sulphate of
magnesia) may be prepared in the following way :
Put an ounce  of carbonate  of magnesia  into a
tumbler, and  pour over it a quantity of water;
then  add,  carefully, by degrees, sulphuric acid
(oil of vitriol),  until  effervescence  has  ceased,
when the two substances will have saturated each
other, as it is  termed by chemists.   After this,
the liquid must be filtered through blotting pa-
per, and then put  in a warm place, so that a por-
tion may evaporate; crystals will then soon form,
which, on examination, will be found to be com-
mon  Epsom salts.
         TO TEST THE  PURITY OF WATER.
   177. Water, in a state of  purity, can  only be
obtained  by distdlation, or as it falls in the form
of rain.  From its being able to hold, in solution,
so great a variety of substances, it is almost al-
 ways contaminated with some of them.   Spring
 water becomes  impregnated with the  various
 earthy matters through which it runs; and river
 water is still more impure, in consequence of the
 many foreign substances that find their way into
 it.   For chemical  purposes, where it is essential
 that the water should be quite pure, it is necessa-
 ry, therefore, to distil it, by which means the im-
 purities are separated from it.   In order to ascer-
 tain the general properties of any kind of water,
 it may be tested in the following manner :—
    178. Pour a small quantity  of it into a wine-
 glass, and dip into it a slip of litmus paper, when,
 if an acid is contained in the liquid, in any quan-
 tity, the paper will  become red:  if  the water
 contains an  alkali,  the test-paper will  become
 green.
    179. The presence of earthy matter  may  be
 ascertained by mixing a little soap with  water.
 if much earthy matter  is in it,
 the soap will be curded.  This is
 the reason why it is impossible to
 form soap-suds with spring wa-
 ter.
    180.  Evaporate a drop of the     _^S?
  water to be tested from a watch-
  glass.  Small rings will appear if
  it contained onLy a small portion         p
  of impure water ; but a crust  is         Jfii
  seen if it held, in  solution, much         O
  saline or earthy matter, and the      ^f^»
  crust has an ochry tint, if iron be      \ W^r
  present.                          «2*^»*r 
60                              Philosophical
Experiments.
        EXPERIMENTS ON CRYSTALLIZATION.
   181.  Into a vessel, containing a solution of ni-
tric acid in water, drop a few small pieces of car-
bonate of ammonia, as long as any effervescence
continues.  Put  a small  quantity of the  liquid
into a watch-glass, and evaporate a portion of the
water by holding  the  watch-glass over a spirit-
lamp for a few minutes; or place it on the hob of
the  fire-place for a short  time.  If the  glass be
afterward removed, and the liquid allowed to cool,
very beautiful crystals of nitrate of  ammonia will
form.   This  is the salt from which laughing gas
is procured (see experiment 118).
   182.  Moisten  the interior of a glass  tumbler
with muriatic acid, by means of a long  feather ;
and also  moisten the inside of another tumbler,
in the same  way, with liquid ammonia.  If the
mouths of the glasses be now brought together,
the vapors arising from the muriatic acid and the
ammonia will combine, and produce white fumes,
which will deposit themselves in the form of crys-
tals in the interior of the glasses.   This experi-
ment is not always successful.
   183.  If subcarbonate of potash  be added to a
solution of nitric  acid  in water, till effervescence
ceases, or the solutionis saturated,  and a portion
of it  be afterward evaporated in a watch-glass,
or a saucer, very beautiful crystals will immedi-
ately form.
   184.  Melt a httle sulphur in an iron table-spoon,
and pour it into  a wine-glass, of a conical form,
that has been  moistened slightly.   The  sulphur
will immediately  crystallize, and become solid; if
the process be  watched, the crystals may be ob-
served  to  shoot  across the fluid mass in a very
beautiful manner.
   185.  Melt a small  quantity  of sulphur,  cau-
tiously, in a  Florence  flask, and after removing
it from  the spirit-lamp, or flame  by  which the
sulphur has been  melted, pour  away  the liquid
that remains  when the outer portion has become
solid, and the crystallization of the  sulphur  may
then be seen.   The mass  will form a very pleas-
ing object if taken from the flask.
   186.  The following cxperimenl is described by
Dr. Reid, as a pleasing illustration of metallic
crystallization:—Heat a common  plate of sheet
tin, which is merely iron covered with tin, before
the fire, or over a lamp, till it is as warm  as  may
be necessary  to cause  water dropped   upon it to
evaporate quickly, with a  slight hissing  noise.
Let the tin with which the  iron is coated be then
washed with a cloth, well moistened with a mix-
ture composed of water, one ounce, muriatic acid,
one drachm, and  nitric acid, one drachm.  The
cold fluid causes the hot tin suddenly to  assume
the crystalline form ; and as  the acids act upon
the external particles of the tin, and cxjwsc those
below, the crystalline  arrangement is beautifully
seen.
  187.  If a little nitre be dissolved in-boiling wa-
ter, till the water will dissolve no more, and then
allowed to cool, crystals," in six-sided prisms,  will
be formed.
  188.  Dissolve an ouncd.of sulphate  of soda in
two ounces of boiling  wafer.   Pour the  solution
into a wedgewood evaporating dish, or  into a sau-
cer, and put it into  a warm place.  As the water
of  solution evaporates, the  saline matter will
crystallize, resuming the same  form  which the
crys'tals exhibited before being dissolved.
    INFLUENCE OF LIGHT ON CRYSTALLIZATION.
  189. To show  the influence of light  on crys-
tallization, place  a solution of a salt  in a dark
room, and, while it remains uncrystallized, let a
ray be admitted,  and  suffered to fall on one side
of the vessel  containing the solution; crystals
will begin  to form on that side, and  the whole
will soon form about those produced.
  190. Acetate  of lead and fluate of lime as-
sume beautiful appearances, when under the par-
tial influence of light; crystals will grow, as it
were,  towards the light, and assume various pleas.
ing forms.   The camphor jars in druggists' win-
dows show the influence of light.  But, although
light hastens, it is not in all  cases absolutely ne-
cessary to crystallization.
            RAPID CRYSTALLIZATION.
  192. Make a  strong solution of Glauber's salt
in boiling water, in a Florence  flask,  and, while
hot, cork it up ;  a vacuum will thus be formed by
the condensation of the steam above the surface
of the solution  within the flask, when the solu-
tion is perfectly cold.   If the cork  be now  care-
fully taken out, the whole will begin to crystal.
lize.   Should this effect  not be immediately pro-
duced, drop a crystal of the same salt into it, and
it will instantly shoot into crystals,  commencing
with the crystal so introduced.
             ARTIFICIAL QUARTZ.
  193. Any solid substance introduced into a  sa-
line mixture hastens crystallization; and it is for
this reason that the makers of  sugar-candy place
threads in the liquid sugar, in order that regular
masses of crystals may be formed.  This proper-
ty of  crystals to deposite themselves about a nu-
cleus, may be shown in  a  pleasing  manner  by
placing substances of certain forms in solutions of
salts.   Imitations of certain minerals, or natural
crystals,  may be thus produced, of very beautiful
appearance, and so exact in their resemblance to
natural ones, that the difference cannot be discov-
ered without a close examination.   They may be
formed as follows:—Take a hard  cinder (those
from a blacksmith's shop, called  '.* clinkers,"  are
the belts), and place it in a hot solution of alum in
water, sormed of about half a pound  of alum to
a pint of water.   In a short time it  may be taken
out of the solution, and the crystals of alum will
be found to have deposited themselves about it, in
perfect imitation of natural quartz.  Weaker  so-
lutions succeed  fetter,  but  they  take a longer
time to form.
          ARTIFICIAL MINERAL BASKETS.
•   191.  These pleasing chimney ornaments can
easily be  manufactured, and the process beauti-
fully illustrates the manner in which crystalliza-
tion proceeds.  Make  a solution  of sulphate of
copper, alum or copperas, by dissolving  cither of
these substances in hot water, and when it begins
to cool, suspend in it little wire  baskets  for about
ten minutes.  The alum,  &c. will immediately
form crystals on the wire, in the same way  that
sugar does when  formed into sugar-candy; and
baskets, and other ornaments, of the most pleas.
ing and diversified forms, may thus -be easily pro-
duced. 
Philosophical Experiments.                              61
 APPARENT  TRANSFORMATION OF IRON INTO COPPER.
   194. Make  a solution  of sulpeate of copper
 (blue  vitriol),  by dissolving  it  in  water.   If a
 knife, or any other iron instrument, be then dip-
 ped in the solution, it will become covered with a
 coat of copper, and in appearance exactly resem-
 ble that metal.  The iron has a strong affinity for
 the copper, and attracts it from the water.
     BEAUTIFUL  APPEARANCE  OF HOAR  FROST.
   195. Place a sprig of rosemary, or other garden
 herb in a glass jar, so that when it is inverted the
 stem may  be downwards, and the sprig supported
 by the sides of the jar.   Now put benzoic acid on
 a piece of  iron, hot enough to sublime the acid in
 the form of a thick white vapor; invert the jar over
 the iron, and leave the whole untouched until  the
 sprig is covered with the sublimed acid, in the
 form of a beautiful hoar  frost.   This is an excel-
 lent example of sublimation.
            TO MAKE  FUSIBLE  SPOONS.
   196. Melt about four ounces of bismuth, in  a
 crucible, and,  when  fused, tlirow  in  about two
 ounces and a half of lead, and once  and a half of
 tin.  These metals will combine, and form an al-
 loy, which melts at a very low degree  of  temper-
 ature.  If  some of it is formed into  tea-spoons
 (which may easily be done,  by making a mould
 in clay, or plaster of Paris, from another spoon),
 the spoons thus made will  produce much amuse-
 ment ; for  if one of them be placed in hot tea it
 will melt,  and  sink  to the  bottom of the cup,
 much to the surprise  of the  person using them ;
 and even if they do not melt, they will bend con-
 siderably.   They have a bright  appearance, and,
 if made well, will not be easdy distinguished from
 ordinary metal spoons.
     CURIOUS PROPERTY OF BURNING CAMPHOR.
   197. If  a small piece of camphor be ignited at
 a candle, and then placed in a basin of water, it
 will not  only  float and  remain in an inflamed
 state, but will also appear agitated; and in this
 state will move to and fro  on the surface of the
 water, at the same time emitting a very fragrant
 smell.  If,  during the time the camphor is in mo-
 tion, a drop of oU be  let  fall from a feather into
 the basin, the camphor  will  suddenly stop, as if
 arrested by something peculiarly attractive.  A
 drop of any kind of grease produces a like effect.
         TO TEST THE PURITY OF STEEL.
  198. Steel is a compound of iron and charcoal.
 If, therefore, a drop of nitric acid fall on  a piece
 of it, the part wUl immediately become black, in
 consequence of the acid uniting  with the iron,
 and leaving the carbon  free.   If  the acid  bo
 dropped on iron, this effect will not be  produced ;
and the  comparative  goodness  of steel may,
 therefore, be ascertained by this means.
           LIGHT AND OPTICS.
Introduction to the science—General description of the pro-
,  perties of light—Principles of optical  instruments—The
.  beauty of light—Theories respecting light—The cause of
  color—Refraction of light—The human  eye  described—
j. (-Nine of " short-sight"—Ditto of " long sight"—Position
i; of objects on the retina—Description of the human eye—
ti Properties of a convex lens described—Concave mirror—
  Plane mirror—Cause of the rainbow.
   199. The science  of optics is that branch of
natural philosophy which treats  of the nature of
light, the laws by which it is governed, and the ef-
I fects it is capable of producing.  The best sum-
; mary of the leading facts  in the science is given
' in the " Elements of Physics," by Dr. Arnott,
j where  the following analysis  is  illustrated  at
| length ; and the experiments  that follow will de-
 monstrate the leading principles.
   200. Light is an  emanation from the sun, and
 other luminous  bodies, becoming less intense as it
 spreads, and which, by falling on other bodies, and
 being reflected from them to the eye, renders them
 visible.  It moves  with great  velocity, and  in
 straight lines, where there is no obstacle—leaving
 shadows where  it cannot fali.  It  passes readily
 through some bodies, which are, therefore called
 transparent; but when it enters or leaves their sur-
 faces  obliquely, it  suffers  at them  a degree  of
 bending, or refraction, proportioned to the obliqni-
 ty;  and a beam of white light  thus refracted  or
 bent, under certain circumstances, is resolved in-
 to beams of all the elementary colors, which, how-
 ever, on being again blended, become  the white
 light as before.
   201.  Transparent bodies,  as glass,  may be
 made of such form as to cause all the rays which
 pass through them from any given point to  bend
      ■___      and meet again in  another  point
     ^  71      beyond them; the body then, be-
     \\ //     cause usually in form resembling
     \ X /      a flat bean or beantil, being called
      y N      a lens;  and when the light  pro-
     A j\     ceeding from every point of an ob-
    / \ I  \  Ject P'ace^ before a lens is collec-
   /  1/   \   ted in corresponding points behind  .
   /  _j^j, \ it, a perfect image of the object  is
  ^Sll&jH^^I there produced,  to  be seen on a
       l\       white screen placed to receive it,
       / \       or in the air, by  looking  towards
      / \      it in a  certain  direction.   Now
      /  1      the most important optical instru-
      /   \     ments, and  even the living eye,
      /   \     are merely arrangements of parts
     /    I     for producing and viewing  such
     /     \    an image under  a  variety of cir-
     /     \    cumstanccs.  When  this image
     /      I    is received upon a. suitable white
    /      1   surface or screen in  a dark room,
    /       \   the arrangement is called, accord-
   /        1   ing  to  minor circumstances,  a
   I         I   camera obscura a magic  lantern,
   j         \ or solar  microscope.   And  the
   |         \ eye itself is, in fact, but a small
•  I          \ camera  obscura—of  which  the
  /          \ pupil is  the round  opening,  or
  Ls___-=-jj| window,  before  the lens—ena-
              bling the mind to judge of exter-
nal objects' by the size,  brightness,  color, &c, of
the very minute, but most perfect images or pic-
tures formed at the back of the eye, on the smooth
screen offciervc, called the retina.  The art of
painting aims at producing,  on a larger scale, such
a picture^  and which, when afterwards held be-
fore  the eye, and reproducing itself in miniature
upon the retina,  may excite  the same impression
as the original objects.  When the image beyond
a lens, formed as above described, is viewed in the
air, by looking at it in a particular direction, then
there is exhibited the arrangement of parts  con-
stituting the telescope, or common microscope.
   202. Light falling on very smooth  or polished
surfaces, is  reflected so  nearly  in  the order in 
62
Philosophical Experiments.
which it falls, as to  appear to the eye as if com-
ing directly from the  objects originally emitting
it; and  such surfaces  are called mirrors.  Mirrors
may be plane, convex, or concave ;  and certain
forms will produce  images  by reflection, just as
lenses produce them  by refraction ; so that there
are reflecting telescopes, microscopes, &c, as there
arc  refracting  instruments  of  the  same  kind.
Light, again, falling on bodies of rougher or irre-
gular surface, or which have other peculiarities,
is so modified as to produce all those phenomena
of color and varied brightness seen among natural
bodies,  and giving them their distinctive charac
ters and beauty.
   203.  To the presence of light every object in
nature  owes its appearance of form  and color;
and we may estimate the important benefits we
derive from its  presence,  by considering how se-
vere a calamity it is to be blind.   "The  phe-
nomena of light and vision,"  says  Dr. Arnott,
" have  always  been held  to constitute a most in-
teresting branch of natural science;  whether in
regard  to the beauty of light, or its utility.   The
beauty  is seen  spread over a varied landscape,
among  the  beds of the flower  gardens, on the
spangled meads, in the plumage of  birds, in the
clouds  around  the  rising and setting sun, in the
circles  of the rainbow; and the utility may be
judged  of by the reflection, that had man  been
compelled to supply his wants by groping in utter
and  unchangeable darkness,  even  if originally
created with all the knowledge now existing in
the world, he could scarcely have secured his ex-
istence for one  day.  Indeed,  the earth, without
light would have been an  unfit abode even for
grubs,  generated and living always  amidst  their
food.   Eternal night would have been universal
death.  Light, then, while the beauteous garb of
nature, clothing the  garden  and the  meadow,
glowing in the ruby, sparkling in the diamond, is
also the absolutely necessary medium of commu-
nication between living creatures and the universe
around them.   The rising  sun  is what converts
the wilderness of darkness which night covered,
and which, to the young mind not yet aware of
the  regularity of nature's changes,  is so full of
horror,  into a visible and lovely paradise.   No
wonder, then, if in early ages of the world, man
has often been  seen bending the knee before the
glorious luminary, and worshipping it as the god
of nature.   When  a  mariner, who has  been toil-
ing in midnight gloom and  tempest, at last per-
ceives  the dawn of day, or even the rising ol the
moon, the waves seem to him less lofty; the wind
is only  half as  fierce; sweet hope beams on him
with the  light of heaven, and brings gladness to
his  heart.   A man, wherever placed in light, reT
ceives by the eye from every object around—from
hill and tree, and even a  single  leaf—nay,  from
every point, in every object, and  at eijbry moment
of time—a messenger of light, to tell him what is
there, and  in what condition.   Were he omni-
present, or  had he the power  of flitting  from
place to place with  the  speed  of the wind, he
could scarcely be more promptly informed.   And
even in many cases, where distance  intervenes
not, light can impart  at once knowledge, which,
by any  other conceivable means, could come only
tediously, or  not at all.   For example, when the
illuminated  countenance  is revealing the secret
| workings of the heart, the tongue would in vain
 try to  speak, even  in  long  phrases,  what one
I smile of friendship or affection can in an instant
 convey ; and had there boon no light, man never
 could have been aware of the miniature world of
 life and activity which, even  in a drop of water,
 the microscope discovers to  him ; nor could  he
 have formed any idea of  the  admirable structure
 belonging to  many minute objects.   It is light,
 again, which gives the telegraph, by which men
 converse from hill to hill, or  across an extent of
 raging sea; and which,  pouring upon the eye
 through  the  optic tube^ brings intelligence  of
 events passing  in the remotest regions of space."
 (Arnott's Elements of Physics, vol. 2,)
   204. In  the  preceding passage, some  of the
 beneficial  effects  for which  we are indebted  to
 light have been beautifully described; and there
 is  no branch of philosophy  better  calculated  to
 excite our admiration and reverence for the Crea-
 tor than the  science of optics.  But respecting
 the cause of the phenomena,  so  striking and  so
 beautiful—respecting  the nature  of light itself,
 we are left in as dark a state of  ignorance as we
 are of heat and electricity.   Some suppose, and
 the opinion was supported by the great Sir Isaac
 Newton, that light  consists  of particles of solid
 matter, which are constantly projected from the
 Bun,  and other luminous bodies.   Others, on the
 contrary,  maintain that  it is merely a vibration
 of a  very subtle ether,  wliich effects our organs
 of sight in-the same  way as the  vibrations  of
 the air produce sound, and  affect our organs of
 hearing.
    Whatever is its nature,  it moves with a velocity
 that  surpasses  comprehension. (See  experiment
 217.)  " What  mere assertion,"  says Sir John
 Herschel,  " will make any man  believe that, in
 one second of time, in  one beat of the pendulum
 of the clock, a ray of light travels over 192,000
 miles ; and would, therefore,  perform the tour  of
 world  in  less time  than a  swift  runner  would
 make one stride ? "  Yet  such is  the fact; the
 evidence on which it rests is indisputable; and in
 the experiments  which  follow, the means,  by
 which a knowledge of the fact was obtained,  is
 stated  at length.
    205. Light is the cause of color.   It was for-
 merly supposed that light itself was purely white;
 but by means of a prism, it may be proved to con-
 sist of three, or  perhaps  four,  distinct  colors,
 blended together—viz. red, blue, and yellow, and
 perhaps violet ; which, by their union, form white,
 and  every other color  that can present itself to
 the eye.
    Some bodies possess the property of absorbing
 one or more of the elementary colors, and reflect-
 ing  the  others ;  and,  consequently, a body will
 appear to be of the colors that would  be formed
 of those which are reflected.   And where all the
 rays  are absorbed, and none reflected, the body
 will  appear of no color at all, or, in common lan-
 guage, it will be black.  For  instance, the paper
 of this book is white,  because it  reflects  all the
 rays of light  which  fall on it; and as the three
 primitive colors form white when mixed together,
 this is the  impression  they produce on being  re-
 flected to the eye.   The  type appears black,  for
 reasons precisely opposite.  It has  the power of
 absorbing all the rays of fight, and, consequently, 
Philosophical Experiments.                              63
no color is  reflected.  The  fields  appear green,
because the red' elementary ray is absorbed, and
the yellow and blue rays are reflected: these col-
ors, as  every schoolboy knows, when  mixed to-
gether, produce green, and this is the impression
made on the eye.  The color of every object, ex-
cepting  those  which are black, is occasioned in I
the same way; the  elementary rays that arc re-
flected produce, by  their combination, a  third
color, which is the  one impressed  upon our or-
gans of vision.
  206. Light is refracted on passing into a den-
ser medium than that through which it has been
traveling; that is to say, it is bent  out of its
course, and  falls in  a different spot  to what  it
would have done, had it not passed into a denser
medium.  Water is  a denser medium  than the
air, and  cons quently light, in passing  from the
latter into  water, is  bent, or  refracted: the  same
effect is  likewise produced when  it passes  from
air to glass; and according to the density of the
substance, or medium, through which light has to
travel, so will it be refracted.  Sir Isaac  Newton,
when experimenting on this subject,  found that
inflammable bodies refracted light more than oth-
ers ; and finding that  the diamond and water,
neither of which, in  his day, were supposed  to lie
inflammable,  possessed  the  power  of refracting
light in a high degree, he ventured to predict, that
one day they would be found to consist of inflam-
mable materials; and this seeming absurdity has
since proved true!
   The use of lenses in the telescope and micros.
cope is to refract the rays of light, so that they
may meet together at a  particular spot,  and thus
render clearly visible an object that would other-
wise  be indistinct,  in  consequence of  the rays
proceeding  from it being too diffused to aot upon
the organs of vision.
   207.  The human  eye  is a beautiful illustration
of an arrangement to produce this effect.  It is in
in  fact, a natural  camera obscura,  which  is
merely a little  instrument  formed  by placing a
lens in a hole in front of a  small box, through
which the light enters, and, being refracted, gives
a minute, but very clear and beautiful representa-
tion of every object before it (see experiment 227.)
The  human  eye  is  a globular  chamber, of the
size of a large walnut,  formed  externally by a
very tough  membrane, called, from its hardness,
the selerotic coat, or, popularly,  the  white of  the
eye, in the front of which there is one round  open-
ing, or window, named,  because of its horny tex-
ture,  the cornea.  The chamber is lined with a
finer  membrane, or  web—the choroid, wliich, to
ensure the  internal darkness of the  place, is cov-
ered with a black paint.  This lining at the edge
of  the round window is bordered by a folded dra-
pery, called  the  ciliary processes,  hidden from
without  by  being  behind the curious  contractile
window-curtain, the  iris; througli the central
opening of  which, or pupil, the light enters.   Im-
mediately behind the pupil is  suspended, by at-
tachments among the ciliary processes, the crys-
talline lens, a  double  convex  most transparent
body, of considerable hardness, which so influen-
ces the light passing through it  from external ob-
jects, as to  form most perfect images of these ob-
jects, in the same way as the camera obscura, on
the back wall  of the eye, over which  the optic
nerve, then called the retina, is spread as a se-
cond lining.
  The  eye  is maintained in  its globular con-
dition by a watery  liquid, which distends its ex-
ternal coverings,  and which in the compartment
before the lens, or  the anterior chamber of the
eye, being perfectly limpid, is called the aqueous
humor ; and in the remainder, or larger posterior
chamber, being enclosed  in a transparent spongy
structure, so  as to acquire somewhat the appear-
ance of melted glass,  is called the vitreous humor.
The use  of  the  humors of  the eye, and of the
crystalline lens, is for refracting the rays of light
which enter the  pupil, and concentrating  them
on  the retina.  Here every object is represented
in miniature; a landscape of miles in  extent  is
depicted on a little circle, of perhaps less than an
inch in diameter: yet every object is so beautifully
clear, that we are able to perceive  and under-
stand the form and color of the most minute, as
well as of objects  the most extended.
   208.  It is indeed, the little picture  on the reti-
na—not the external objects themselves—that the
mind contemplates; and if, in consequence of the
light being too much bent in  passing through the
lens and. humors of the eye, it is concentrated or
brought to a  focus before it reaches the retina,
we are unable to perceive any object distinctly.
Persons who suffer  from this imperfection are
said to be short-sighted, and they are obliged to
wear spectacles formed of concave lenses, which,
having the property of dispersing light, compen-
sates for its too great refraction by the humors of
the eye, and thus  causes the  focus,  or point at
which the rays meet, to fall exactly on the retina,
where then, of course, a perfect picture is produ-








ccd.   This will  be better understood by the an-
nexed diagram, where the rays are seen to con-
verge, after passing through  the crystalline lens,
and come to a focus before they reach the retina,
at  the back of the eye.
   209. Some persons have a defect the reverse of
the foregoing; in consequence of which, the rays
of  light are not collected into a focus when they
reach the retina.  They are not refracted enough
in  passing through the eye,  and, consequently,
 while objects at a distance are  distinctly visible,
those near the eye  are very indistinct.   Old per-
sons are generally subject to this defect of vision ;
 the cornea, or front of the eye, becoming  flatter
in  old age than it is  in youth; and, therefore, in
order to perceive an object clearly, it must be held
at  some distance from the organ of vision.  Old
gentlemen and ladies like books printed in a large
type, and with wide spaces between, because then
the letters   are more clear; and they hold the
newspaper the other side  of  the  candle, because,
if held closer, the letters are indistinct, from the
light not being  brought  to a point on the retina.
 The diagram annexed will explain  the cause of
i long-sightedness.  The rays of light, proceeding 
64                              Philosophical Experiments.
 from the cross, it will be seen, do not come to a fo-
 cus until they have passed through the retina at
 the back of the eye, and, consequently,  a perfect
 image is not formed un it.  If a convex glass be
 held in front of the eye of a person suffering from
 this imperfection, he will see very  clearly; be-
 cause it is  the  property of convex lenses  to re-
 fract light very powerfully, and spectacles  of this
 kind must be used by such persons.
   210.  It will be perceived,   by the above dia-
 grams, that the images  of all  objects formed on
 the retina must be in an inverted position, and
 this may be  rendered  evident by experiment.
 (Sec experiment 227.)   Yet we  are unconscious
 that this is the  case  with ourselves; everything
 appears upright, and  in  its proper position ; but
 this is because, while we become acquainted with
 the color  of an object by its picture on the retina,
 we judge of its position  by the direction in which
 the light comes from it to the  eye.
   211.  There is, indeed, no organ of sense so lia-
 ble to deceive us as the eye ; and,  unless correc-
 ted by the other senses, we  should  be constantly
 led into error  The axiom, .that " Seeing is believ-
 ing," may frequently  be proved false.   We see,
 for instance, a rocket, ascending into the air, and
 although we know that  it is only  a small  body
 alight at one end, yet, to our eyes,  it appears a
 long line of flame. This is a deception to which
 wc are  liable,  in  consequence of a peculiarity
 of the retina, by which any impression made up-
 on it lasts  for about the  sixth  part of a second.
 If,  therefore, a succession of similar impressions
 be made upon it, with great rapidity, one will not
 have passed away before another is jnade, and
 thus the impression will appear  continuous.  Sir
 David Brewster has  given many remarkable il-
lustrations of this  curious property of the retina ;
 some of which will be found amongst  the exper-
 iments which follow, and other examples are giv-
 en, which, it is hoped, will be sufficient to explain
 many of the absurd stories respecting ghosts and
 apparitions, in which some people so earnestly be-
 lieve ; for it will be evident, that a man may ac-
 tually see an apparition with his eyes shut.'  The
 retina has the power of  recalling  an object that
 has once been impressed  upon it; and, therefore,
 when the mind is internally engaged in  recalling
 to recollection the person of some dear friend, de-
 ceased, it  is easy to believe that the retina may
 have the image of the individual recalled, and
 thus apparently make him, in reality, appear be-
 fore us.   The sensibility  of the  retina, with re-
gard to colors, is  likewise a cause that has fre-
quently been deceptive.   This   is fully explained
in the pages which follow.
  212.  Another source  of  deception  may  be
found in the power which lenses have of chang-
ing the  direction of light; as inlhe case of the
eye, camera obscura, &c.
  213. A  convex  lens possesses the property of
concentrating  the  rays of light, and may  there-
lore be accordingly used  as  a   " burning glass."
The greater the convexity of the lens, the
nearer will  be  its focus; for the curious
fact has been discovered, that the focus of
a double lens of  glass is just  where  the
centre of the sphere would be, of which
the surface of the lens is a portion ; and,
therefore, the greater the convexity of the
lens,  the nearer  the focus will  be, as the
surface,  in  that  case, is  a portion of a
smaller sphere.   We learn from this why
the more powerful the microscope is,  the
nearerthe object to be examined  must be
brought  to  it;  and  many  pleasing illustrations
may  be madewith the convex mirror,  that forms
an  elegantpiece of furniture for the parlor.
   214. A concave lens, instead of converging the
rays of light, disperses them; and, as has
just been mentioned in the case of short-
sighted persons, may be used to compen-
sate  for  too great refraction by  convex
lens.  This and the concave lens are the
two principal forms ;  they are modified in
various ways, and a person who under-
stands their principle of action, will be able
to comprehend the cause of  a great vari-
ety of the phenomena of nature.
  215. Aplain mirror, or common looking-
glass, is a more curious optical instrument
than is generally supposed; it reflects light in a
direction precisely  opposite to that in which  it
falls upon the glass.   The ray falling on the mir-
ror is called the incidental ray, and that reflected
from it, the reflected ray.   If a person stand in
front of  a looking-glass,  his  image will be re-
flected directly back  to himself, and he will be
able to view his person; if he stand a httle on
the right side, his image will be reflected a little
to the left,  and he will no longer see himself; but
a person standing as much to the left of  the glass
as he is to the right of it, will see him, and he
                     will see the person.  Sup-
                     pose a and b a lookingglass,
                     then  all the rays of light
                     coming from c are reflected
                     directly back on the object
                     from which they came, and
                   c a  person   standing  there
                     will see himself; if he moves
                     to d, the light is reflected
                     to e, and, therefore, only
                     a person standing exactly
                     at e  will  be able to  see
                     him.  At the same time,
he  will  be able to  see  any object  at e,  be-
cause its light will  be reflected exactly where he
stands at d.  A knowledge of this principle en-
abled the magicians, as they were  called in an-
 
Philosophical Experiments.
65
cient times to perforin m my surprising cxperi-
ments, by which they imposed  upon tlie people,
who were '^norant of tlie  cause, and made them
believe that they were gifted with extraordinary
powers by the Creator; while, in  fact,  t^y were
only using bis name to beguile and miblead those
who believed them.
  216. The rainbow, that "miracle of beauty,"
depends on the refractive power of  the drops of
rain, in the same way ms the prismatic colors that
we see reflected  from a chandelier, depend on the
refracting property of glass.   A person who  un-
derstands what  has just been said, respecting the
different  qualities  of plain and convex and con-
cave mirrors, will easily comprehend the cause of
the rainbow,  from the following description:—
When the sun shines upon the spherical drops of
rain, and, its light falling  upon the whole central
part of any drop, passes  completely  through  it
(as it would if it entered opposite y in  the cut;)
still that portion which enters near the edge of
the drop, as at a, from the sun  at r, is refracted,
and  reaches the back surface of the  drop at y,
              i" so slantingly, or  at an angle so
                great, that it is entirely reflected,
                instead of   being  transmitted ;
                the  ray, therefore,  is returned to
                b, where it again escapes from
                the drop,  and, as here shown, de-
                scends to the earth, or the  eye,
                at  c.  Thus, every drop of  rain
                on which the sun shines is a lit-
                 tle mirror  supended in the sky,
                 and  returns, at a certain angle
                 all around  it, a  portion  of the
              flight which falls on it; and an
eye placed in the required direction, receives  that
reflected  light, in the same way that an object is
perceived by reflection in  a  mirror, as  illustrated
by a previous diagram.  It may be  asked why,  if
the  rainbow depends  on drops  of falling rain,
does  it  not immediately  disappear as  tlie  rain
reaches the ground ?   This  is really the case; we
do not see the  same rainbow for  two seconds to.
gcther; for immediately the  drop that reflects the
light passes the  spot on which the eye is fixed  we
no longer perceive the rays  coming from it, but
we have a continued impression of the rainbow,
because  as fast  as one drop passes away, another
supplies its place, and we  do not perceive any in-
termission in the colors reaching the eye, because,
as previously  explained, an  impression remains
on the retina for the sixth  part of asecond.and the
reflection from  each  drop comes more quickly
than this.  The light being refracted by the rain-
drops, is resolved  into its  elementary colors;  and
these, with their combinations,  appear  in the fol-
lowing order, commencing from the internal por-
tion of the  bow—viz, red,  orange, yellow, green,
blue, indigo, and violet.  These  are  called  the
prismatic colors,  and it was formerly supposed
that they were  the elements of white light;  but
at the present time it is more generally believed
that  the  elementary colors  are but red, yellow
blue, and violet, and that  these, by uniting, form
the  additional tints ; thus, red.and yellow lorm
orange, and the orange ray is found  between the
red  and  yellow rays; and the same principle is
applied  to explain the appearance  of  the green
and  indigo rays.  Independent of tlie religious
feeling that n turally associate  them selves with
the rainbow,  there  are few nntural phenomena
more  calculated  to impress us with a conviction
of the benevolence  of  the Creator.   It shows,
not merely that our animal wants bavc been pro.
vided lor  but  thjt provision has been also made
for gratifying  our taste for the beautiful.   What
can be more exquisitely lovely than the colors of
the rainbow ?   If light had not been formed of
colored  rays,  we might,  it is true, have  had a
rainbow in which a thousand little images of the
sun would have  been reflected to  us; but they
would not have had  the effect of the present ar-
rangement : they would all have been of one mo-
notonous color—white.   Hid this been the case,
too, what  an unlovely spectacle  would nature
have  presented, compared with  her present  ap-
pearance !  The rose might have retained its
odor,  but how  much of its sweetness would it have
lost had  the  color, which now  adorns it, been
wanting !   The  form and countenance of man,
too, might have been as regular and perfect as it
is,  but what would have  supplied  the  blooming
tint that  indicates both youth and health, and
which, in  the female, sex, bestows a charm that
art can never imitate ?   Indeed, throughout na-
ture, although ncirly every phenomena could take
place as regularly with light of one color only, as
by its consisting  of several, yet, had such been
the case,  how  many of those enjoyments and
pleasures would have been lost to  us, that we now
derive from the  beautiful colors which so  pro-
fusely adorn the works of the Creator 1
 EXPERIMENTS IN LIGHT AND OPTICS.
Velocity of Light—Light travels in straight lines—Light pro-
  duced from quartz stones—To obtain a light from loaf su-
  gar—Transparency of gold—Hefractiou of light—A nat-
  ural camera  obscura—How to maUe a camera obscura—
  Dark spot on the retina—Insensibility of the optic nerve
  —Curious effect of indirect vision—Illusion  of the eye
  with respect to the situation of objects—The magic lan-
  tern—Duration of impressions on the retina—the circle of
  fire—The thaumathrope—Composition  of light—Loiig-
  continued image of the sun on the retina—To  read the in-
  scription of a coin in the dark—To read the inscription
  that has been erased from a coin—Laughable effect of ho-
  mogeneous light—Heat of the colors of the spectrum—To
  show that light and heat are distinct in the sun's rays-
  Prismatic colors—Ocular spectra, or accidental colors-
  Phosphorescence of the  sea illustrated—Phosphorescence
  of the retina—Solar phosphori—Breathing light and dark
  ne>s—Changes of the kaleidoscope.
              VELOCITY OF JLIGHT.
  G17. Light travels  witb the amazing velocity
of 192,000 miles in a second of time; so that it
has been pleasingly remarked, that during a sin-
gle vibration of a  commou  clock  pendulum,  it
would  go from London to Edinburgh, and back,
200  times although  the distance between  these
places  is 4°^  miles.   It may be interesting to
know h"»v philosophers have been able to deter-
mine, with such certainty, that light really travels
with this amazing velocity; for the fact is known
as certainly as  any  phenomenon in nature.  The
method adopted was the following:—The eclipses
of the satellites  or  moons  of the  planet Jupiter
had been carefully observed for some time, and a
rule was  obtained, which foretold the instants, in
all future time, when the satellites were to glide into
 he shadow of the planet, and disappear,  or again
to emerge into view.  Now  it was found that 
€6                             Philosophical Experiments.
these appearances took place sixteen minutes and
a half sooner when Jupiter was near the earth, or
on the same side of the sun with the earth, than
when  it was on the  other side; that is  to  say,
more distant from  the  earth  by one  diameter of
the earth's  orbit,  or path in the heavens which
it takes in revolving round the sun, and at all in-
termediate stations, the difference diminished from
the sixteen minutes and  a half, in  exact  propor-
tion to the  less distance from the earth.  This
proves, then, that light takes sixteen minutes and
a half to travel across the earth's orbit, and eight
minutes and a quarter for half that  distance, or
to come to us from the sun.  This being its amazing
velocity, it  may, for  all  useful purposes  on the
earth, be regarded as passing between bodies in-
stantaneously ;  and it is for this reason that we
perceive the flash from a gun at a distance, for a
perceptible time, before we hear the  report,  and
why we may count several  seconds between the
appearance  of  a flash of lightning, and hearing
the thunder which follows.

        LIGHT TRAVEL3 I.N STRAIGHT LINE8.

   218.  To prove that fight proceeds from lumin-
ous bodies,  in  straight lines  (see introduction,)
procure a straight flexible tube, and look through
it  at  any object, which will  then be distinctly
visible.   If, however,  one end  of the tube be now
bent,  either  upwards  or downwards,  on looking
into the tube, it will be found perfectly dark;  the
light  coming in at the  bent  end will not turn
a corner, and,  consequently, none  of it reaches
the eye.   The  annexed engraving of a beam of
light proceeding from  a candle will  illustrate  the
fact of light proceeding in right lines.
     LIGHT PRODUCED FROM QUARTZ STONES.

   219.  If two of these stones (milk stones,  as
boys sometimes call them) be struck against each
other, they will emit light; and if they are struck
under water, the same effect will be produced.

      TO OBTAIN A LIGHT FROM LOAF SUGAR.

   220.  If two large pieces of loaf sugar (the
larger the better) be struck against each other in
the  dark, a light blue flame, \ike lightning, will
be produced.   The same effect takes place when
a  grocer chops up a lump of sugar i^th his iron
hatchet, only it is not often noticed.

            TRANSPARENCY  OP GOLD.

   221. All opaque substances might become trans
parent it they could be made sufficiently thin;
and what it is in  the constitution of one mass, as
compared with another, which fits the former to
transmit light,  and the  latter to obstruct it, can-
not clearly be explained ; but we perceive that the
arrangement of the particles has more influence
than their peculiar  nature.   Nothing is  more
opaque  than thick  masses of the metals ; but
. nothing is more transparent than equally thick
 masses of the same metals in solution, nor than
 the glasses, of which a metal forms a large pro.
 portion.   The thousand  salts formed  by the
 union of the metals, or earths, with the  diluted
 acids, are all  transparent when, in cooling from
 the fluid to the solid state, their  particles have
 been allowed to arrange  themselves according to
 the laws of their  mutual attraction—that is  to
 say, to form crystals; but the same substances in
 other  states, as  when  reduced to  powder, arc
 opaque.  Even the  metals  themselves when re-
 duced to leaves of great thinness, arc transparent,
 as may be perceived by looking at a lamp through
 a fine  gold leaf.   The  light will  be visible, but
 the flame will appear of a greenish hue.

              REFRACTION OF LIGHT.
   222. A beam of light passing the atmosphere
 into any denser medium, such as water, or glass,
 or even into  a heavier gas,  is bent, or refracted
 (see Introduction, 206;)  and light passing from a
 dense  into  a lighter  medium,  suffers an opposite
 change in  its direction.  Refraction, however,
 does not  take place if the light falls perpendicu-
 larly upon  a surface ; it is only when it falls ob.
 liquely, that it is  bent  out  of its  course.  The
 following experiments will illustrate the refraction
 or bending of light by water.
   223.  Place a coin in a vessel, so that on stand-
 ing  at a certain distance off, it is just hid by the
                                    edge  of the
                                    vessel. Then
                                    get some one
                                    to pour wa-
                                    ter  into  it,
                                    and the coin
                                    will  imme-
                                    diately  be-
                                    comevisible;
 it will appear to have  moved  from the side  to  the
 middle of the vessel as shown by the cut.
   224.  If a basin, or other  vessel, be placed  so
 that a ray of the  sun shall  fall low on one side
 within it, on filling it up with water, the image of
 the sun  will appear shinning on the bottom of the
 vessel;  the  water having refracted ihe beam of
 light which fell on the side so  much, that  it then
 falls on the bottom.
   225.  Place a stick upright in a tub, and after-
 wards fill it up with water; or place a stick in a
 vessel of water, and it will appear bent and crooked.
  226.  A ray of
 light which pasaes B
 through a piece of
 plate glass will suf-
 fer considerable re-
fraction, after the
manner represent-
ed in the annexed
engraving,  where
a is the piece of
glass, c  and d the
parts where the ray enters and emerges, falling on
 e ; and  the dotted line  shows the degree of re-
fraction,
          a natural camera obscura.
  227. The human eye is a camera obscura ; for
on the back of it, on the retina, every object in a
landscape is beautifully  depicted  in miniature
 
Philosophical Experiments.
67
(see  Introduction.)  This may be proved by pro-
curing a bullock's eye, and carefully removing, or
thinning, the outer coat of it behind, taking care
not to cut it, or the vitreous humor will escape,
and the eye be rendered useless.  If the pupil of
it be directed to any bright objects, when it is thus
prepared, they will appear beautifully distinct in
miniature on the back part, precisely the  same as
objects appear in the camera  obscura.   The ef-
fect will be  hightened if the eye is  viewed in  a
dark room, with a small hole in the shutter; but
in every case the  appearance will be very strik-
ing.

        HOW TO MAKE A CAMERA OBSCURA.
   228.  This is a very pleasing and instructive lit-
tle piece of apparatus, and may be purchased for
three or four shillings.  As  some of our  readers,
however, may like to make  one themselves, and
as this may be done at a very trifling  expense,
we give the following directions for the purpose :
Make an oblong box, about  a foot long, six inches
wide, and three or four high ;  at one end make a
round hole, in which fix a small magnifying glass,
which may be obtained at any optician's.  Then,
at the other end, place  a piece of looking-glass,
slantingly, so that when the light falls on it after
passing through the magnifying glass, it may be
reflected to the top of the box, near the end.  On
this part place a piece of ground glass,  or trans-
parent  oiled paper,  and, after covering  the re-
mainder of  the top with wood, and blacking the
inside with ink all  over, the camera obscura is
complete.   On pointing the end of the box to a
landscape, the  light will  pass through the lens,
and  every tiling will appear beautifully distinct,
in miniature, on the ground glass.  If, therefore,
a piece of writing paper be placed on it, a very
pleasing drawing may  be  made with a pencil;
and thus the camera obscura may be  made an
endless source of pleasure and instructive amuse-
ment.  The wood cut represents a section of the
apparatus; a,  is  the lens;  b, the  back, against
which rests the looking-glass;  and c is the ground
glass, on which the paper is laid,

DARK SPOT ON THE RETINA--INSENSIBILITY OF THE
                 OPTIC NERVE.
  229. There  are  few persons aware, that when
they look with one eye, there is some  particular
object  before them to which  they are absolutely
blind.  If we  look with the right eye, this point
is always about   15°  to the right of the object
which we arc viewing, or to the right of the axis
of the eye, or the point of most distinct vision.—
If we look with the left eye, the point is far to the
left.   In order to be  convinced of  this curious
fact, which was discovered by M. Mariotte, place
two  colored wafers upon a sheet of  white paper,
at the distance of  three inches apart, and look at
the left-hand wafer with the right eye, at the dis-
tance of eleven or twelve inches, the experimenter
standing  in front of the wafers, taking  care to
keep the eyes straight above the wafer, and the
line which joins the eyes parallel to the line which
joins the wafers.  When  this is done, and the left
eye closed, the right-hand wafer will no longer be
visible.  The  same effect will be produced if we
close the right eye, and look with the left eye at
the right-hand wafer.  When we examine the re-
tina, to discover to what part of it this  insensi-
bdity to light belongs, we find that the image of
the invisible wafer has fallen on  the base of the
optic nerve, or the place  where this nerve enters
the eye, and  expands itself to  form  the retina,
which is a kind of network-expansion of the op-
tic nerve over the interior coat of the eye, and is
the part on  which  impressions are made, that
render external objects visible to us (see Introduc
tion.)
  230. If  the above  experiment be performed
with candles,  instead of wafers, the candle on
either side wdl not  entirely disappear, but leaves
behind a  faint, cloudy light, without, however,
giving any thing like an image of the object from
which the light  proceeds.  From a  knowledge of
the preceding facts we might,  perhaps, be led to
imagine, that on viewing a landscape, whether we
use one or both eyes, to see a black or dark spot
within  fifteen degrees of the point which  most
particularly attracts our notice.  The Divine Ar-
tificer, however, has not left his work thus imper-
fect.   Though the base of the  optic nerve is in-
sensible to light that falls directly upon it, yet it
has been made susceptible of receiving luminous
impressions from the parts which surround it;
and the consequence of this  is, that when the wa-
fer disappears, the spot wliich it occupied, in place
of being black, has always the same color as the
ground upon which  the wafer is laid—being
white  when  the wafer  is  placed upon a white
ground, and  red when it  is placed  upon a red
ground.   This  curious effect may be  rudely il-
lustrated, by comparing  the retina to a sheet of
blotting-paper, and the base of the optic  nerve to
a circular portion  of it, covered with a piece of
sponge.   If a shower falls upon the paper, the
protected  part will not be wetted  by the rain
which falls upon the  sponge that covers it; but
in a few seconds it will  be  as  effectually wetted
by the moisture wliich it absorbs from the wet pa-
per with  which it is surrounded.   In like man-
ner, the insensible spot on the retina is stimulated
by a borrowed light;  and the apparent defect is
so completely removed, that its existence can be
determined only by the experiment already de-
scribed.
       CURIOUS EFFECT OF INDIRECT VISION.
   231. When the  eye  is  steadily occupied in
viewing any particular object, or when it takes a
fixed  direction, while the  mind is intently en-
gaged on any subject of contemplation, it sud-
denly loses sight of, or becomes blind to, objects
seen indirectly, or upon  which it is not fully di-
rected.  In order to witness this illusion, put a
little  bit  of white  paper on a green cloth, and
within three  or four inches of it place a narrow
strip of white paper.  At the distance of twelve
or eighteen inches,  fix one eye steaddy upon the
little bit of white  paper, and in a short time a
part, or even the whole of the  strip of paper will 
63                              Philosophical Experiments.
vanish, as if it had been removed from the green
cloth.  It will again appear, and again vanish ;
the effect  depending greatly upon the steadiness
with which the eye is kept fixed.  This  illusion
takes place when both the eyes are open, though
it is easier to observe it when one of  them is clo-
sed. Tlie same thing happens when the object is
luminous,  instead of being a bit of paper.  When
a candle is thus  seen  by indirect vision, it never
wholly disappears, but it spreads itself out into a
cloudy mass ; the centre of which is blue, encir-
cled with  a bright ring of  yellow light.

ILLUSION OF THE EYE WITH RESPECT TO THE SITU-
              ATION OF OBJECTS.
  232. Take a ring, of about an inch in diame-
ter, and suspend it from a piece of string, so that
it  may  hang on  a level with the middle of the
face, and  with the edge toward the eyes, so that
the orifice of the ring cannot be seen.   If one
eye be then closed, it will be found nearly impos-
sible to thrust the end  of a stick   through  the
ring, as the eye  will be  deceived with regard to
the situation of the ring; although  this may be
accomplished easily when both eyes are open.

             THE MAGIC  LANTERN.
  233.  This well-known and pleasing philosophi-
cal toy is  so great a favorite, that it requires no
recommendation to introduce  it to the notice of
the student in optics.  Its  principle will be best
understood from the annexed engraving, showing
                         the form of the instru-
                         ment,  and the situa-
                         tion of the lenses em-
                         ployed.  These lenses
                         are set in a tube, which
                         can  be lengthened or
                         shortened  at pleasure,
                         in order to adjust  the
                         image of the object on
                         the wall, or other sur-
                         face on  which  it  is
                         projected.    Probably,
                         by  the assistance of
                         the wood cut, some of
                         our readers may be en
                         abled to   construct a
                         magic  lantern ;  and
                         the pleasure tobe deri-
                   I   ^ve(* **om **w^ ampiy
                   f-_-/ff!'recompensc for a little
                   l,M7 trouble.  To  use it, it
                         is advisable to  hang
                         up  a  table-cloth,  or
                         sheet,  on  a line at a
                         little   distance  from
tlie wall  of a  room, and  for the  exhibitor to
stand behind it, as by this means he can increase
or diminish the size of the objects at  pleasure, by
adjusting  the tube, and standing near, or at a dis-
tance from the  cloth.   Grotesque  figures,  ani-
mals, &c, painted on glass slides,  having  the
other parts covered with black varnish, to pre-
vent the  transmission of light, except through
the figure, are generally used to exhibit the pow-
ers of the magic  lantern,  and may be made with
a  little ingenuity, or purchased at the optician's,
where the lenses may also be obtained.  Their
situation will be seen from the cut.
DURATION OF IMPRESSIONS ON  THE RETINA--THE
               CIRCLE OF FIRE.
  234. It was  formerly  thought that these im-
prcssions disappeared from the retina the moment
the object that produced them  was removed from
before the eye; but  this is found not to be the
case, for an  impression—that is to say, a pic-
ture in  miniature of any thing held  before our
eyes—remains on the retina about the sixth part
of a second after  the  body that produced it has
been withdrawn  (sec Introduction).   This may
appear so very inconsiderable  a portion of time,
as hardly to be of much importance;  but it ena-
bles us to explain the cause  of many striking phe-
nomena connected with vision, that would other-
wise be  inexplicable.  The following pleasing ex-
periments may be performed, to show the effect of
the retina retaining  an image of an  object after
it  is withdrawn  from before  the eye:—Take a
short piece of wood, and, having  burnt one end
of it, so that it shall  remain luminous, tic it to a
piece of string, and swing  it rapidly round in a
circle.   The effect will be, that we shall perceive
a complete  circle of fire.  If a piece of ignited
charcoal be fixed to the rim of a wheel, to wliich
a rapid motion  is then given, the same effect will
be produced,  The cause of this appearance will
be  understood  from  the  preceding observations.
Tlie eye is fixed on a particular part of the wheel
where the charcoal is placed,  and the image of
the burning  charcoal will remain on the retina
the sixth part of a second : it is evident, there-
fore, that if we can turn the wheel so rapidly that
the charcoal shall revolve and  reach the spot on
which our eye is fixed in less time than this, that
we shall not be informed, by our eyes,  of its hav-
ing moved  at  all; for an  image of the burning
charcoal will be constantly on the retina.  As it
does not  matter  on what  part of the wheel we
place our eyes to perceive the  same effect, the
consequence is, that the whole of the rim of the
wheel appears on  fire.

             THE THAUMATHROPE.
  235. Dr. Paris  designed a very pleasing toy to
illustrate the property of the retina", described in
the last  experiment; it is called " the thaumath-
rope, and may be easily  constructed in a variety
of forms, by the exercise of a  little ingenuity.—
The toy may be purchased,  but we  advise the
reader to make it himself;  which may readily  be
done from the following directions:—Cut a piece
of card  round, about the  size of half a crown,
and fix a piece of thread  on opposite sides of the
circumference; then  paint on  one  side  of the
card some little object, such as a man  without a
hat, and on the other side, near the top, a hat by
itself, reversed.   Now,  on  twisting  the  card
quickly round,  by means of the threads, the hat
\vdl appear on the man's  head;  for the impres-
sion of one side of the card on the retina will not
have ceased before the other side is presented to 
Philosophical Experiments.
69
the eye.  A variety of objects may be painted on
the card ; such as a mm and a horse (as in the
above cut), a juggler and a number of balls, two
men with drawn swords, &c.  Indeed, there arc
a few philosophical toys that afford better oppor-
tunities  for the  display of taste and ingenuity
than the  thaumathrope.
            COMPOSITION OF  LIGHT.
  236. A beam of light,  passing through a prism,
is decomposed; and if the rays be received on  a
sheet of  white paper,  they  will appear of the
same number  and colors  as  they  do in  the
rainbow; viz. red, orange,  yellow,  green, blue,
indigo, and violet. It is supposed by philosophers,
in the present day, that  the orange, green, and in-
digo colors are not elementary, like the others, but
arc formed by  the mixture  of the rays from the
red and  yellow,  yellow and  blue, and blue and
violet parts of  the spectrum.  Sir Isaac Newton
first proved that white light was not one color, as
had previously been supposed ; and its composi-
tion may be pleasingly  illustrated in the follow-
ing manner :—Paste a sheet of white paper on a
circular piece of board, about six inches in diam-
eter, and divide it, with a pencil, into fifty parts
(see cut).   Then paint a portion, equal to six
parts, of a red color;  four parts, orange; seven,
yellow ;  eight, green;  eight, blue ;  six, indigo ;
and eleven, violet.   The  colors should be of a
dark tint  in the  centre, and gradually become
fainter at  the edges, till  they mix with the one
adjoining.  If the board be then fixed on an axle,
and made to revolve quickly, the colors will no
longer appear separate and distinct, but becoming
gradually less visible, they will ultimately appear
white, giving this appearance to the whole sur-
face of tlie paper.
LONG-CONTINUED IMAGE OF THE SUN ON  THE RE-
                    TINA.
  237. The  experiments just mentioned will be
sufficient to  illustrate a few of ,the curious effects
that are produced, in  consequence of objects re-
maining on the retina  after  they have  been re-
moved from before the eye; but, occasionally, ef-
fects of a very alarming character have been pro-
duced from the  same cause ;  and as it is certain
that nearly all ghosts and apparitions, which per-
sons have asserted they have seen, existed in their
eyes,  in consequence of circumstances connected
with this  particular property of  the eye, the fol-
lowing account of an experiment, performed by
Sir Isaac Newton, may be both entertaining and
useful.   Any of our readers may perform a simi-
lar experiment by looking, for some time, steadily,
at a burning  body,  such as a candle, for exam-
ple ; but, prob ibly, after reading the following an-
ecdote, they will be  content  without  personally
performing the experiment.
  238.  Mr.  Boyle  having  mentioned,  in his
" Book on Colors," an individual who  continued,
for years, to see the sun whenever he looked upon
bright objects,  in  consequence of having once
steadily contemplated that luminary for a consid-
erable period, Locke mentioned the case to New-
ton, who gave the following account of a similar
occurrence that had happened to himself:—" The
observation  you mention in Mr. Boyle's ' Book
of Colors,'  I once made upon myself, with the
hazard of my  eyes.   The  manner  was this:  I
looked,  a very little  while, upon the sun in the
looking-glass, with my right eye, and  then turn-
ed my eyes into a dark corner of my chamber,
and winked to observe the impression  made, and
the circles of colors wliich encompassed it,  and
how they decayed by degrees, and at last vanish-
ed.  This I repeated a second and a third time.
At the third  time, when the phantasm of light,
and colors about it, were almost vanished, intend-
ing my ftincy upon them to see their last appear-
ance,  I found, to my amazement, that they began
to return ; and, by little and little, to become as
lively and vivid as when I had newly looked upon
the sun.  But when I ceased to intend my fancy
upon  them, they again vanished.  After this,  I
found that as often as I went into the dark, and
intended my  mind upon them, as when a man
looks  earnestly to see any thing that is difficult to
be seen, I could make the phantasm return, with-
out looking any more upon  the sun; and the oft-
ener  I made it return, the more  easily I could
make it return  again : and  at  length,  by repeat-
ing this, without looking any more upon the  sun,
I  made such an impression on my eye, that if I
looked upon the clouds, or a book, or any bright
object, I saw upon it a round, bright spot of fight,
like the sun ; and, which is still stranger, though
I looked upon the sun with my right eye only, and
not my left, yet my fancy began to make an impres-
sion on my left eye, as well as upon my right: for
if I shut my right eye, and looked upon the clouds,
or a book, with my left eye, I could see the spec-
trum  of the sun almost  as plain as with my right
eye, if I did but  intend my  fancy a little time
upon  it; for at first,  if I shut my jight eye, and
looked «vith my left,  the spectrum of the sun did
not appear till I intended my fancy upon it;  and,
by repeating this, appeared every time more ea-
sily.  And  now, in a few  hours time,  I had
brought my  eyes  to  such a pass, that I could
look upon no bright object with either eye, but  I
saw  the sun before me, so that I durst neither
write nor read;  but to recover the  use  of my
eyes, shut myself up in my chamber, made dark,
for three days together, and used all means in my
power to  direct my imagination  from the sun;
for if I thought upon him, I presently saw his
picture,  though  I  was in  the dark.  But, by
keeping in the dark, and  employing my  mind
about other things, I began, in three or four days,
to have more use of my eyes again;  and by for-
bearing  to look upon  bright  objects, recovered
them pretty well—but not so well but that, for
 some months after, the spectrums of the 6un be-
 gan  to  return as  often  as I began  to meditate 
*0
Philosophical
Experiments.
upon the phenomenon, even  though I lay in bed
at midnight, with my curtains drawn.  But now
I have been well for many years; though I am
apt to think, if I durst venture my eyes, I could
still make the phantasm return, by the power of
my fancy."  This experiment proved how easily
the image of a bright  object might be recalled,
and the mind be deceived by an appearance that
was not real.  There  can be no doubt but that
persons have often, in this manner, had the forms
of deceased friends recalled so vividly, that they
have  appeared to be actually  present; and per-
sons not  acquainted with the fact that the eye
can  deceive  the mind  in this way, would never
doubt the reality of what they thought they saw.

TO READ THE INSCRIPTION OF A COIN IN THE DARK.
  239. To do  this, take a silver coin, and after
polishing  the surface as much as possible, make
the parts of it wliich are raised rough, by placing
on them a little oil of vitriol, or some other acid;
the parts not raised are to be left polished.   If the
coin thus  prepared is placed upon a mass of red-
hot iron, and removed into  a dark room, the in-
scription  upon it will become distinctly visible,
and  may be read by a  spectator.  The mass of
red-hot iron should be concealed from the observ-
er's eye, by having something  placed before  it,
both for the  purpose of rendering the eye fitter
for observing the effect, and of removing all doubt
that the  inscription is  really read  in the  dark ;
that is, without receiving any light,  direct or re-
flected, from any other body.  If, instead of rough-
ening the raised parts of the coin by an acid, we
polish  the raised parts and roughen the depressed
parts, we shall still be able to read it, from its be-
ing, as it were, written  in black letters upon  a
white ground—the reverse of the first experiment.

TO READ THE  INSCRIPTION THAT HAS BEEN ERASED
                 FROM  A COIN.
  240. When such a coin is laid upon  the red-
hot iron, the letters  and figures that have been
either wholly obliterated, or obliterated to such a
degree as to be illegible,  become oxidated, or rust-
ed ; and the film of oxide radiating more power-
fully than the rest of the coin,  the illegible in-
scription may be now distinctly read, to the great
surprise of an observer  who had  previously ex-
amined the blank surface of the coin.
  241. In order to explain the cause #f this re-
markable phenomenon,  and that of the previous
experiment, it  is necessary to  notice a method,
that has been long known, of deciphering the in-
scriptions  on worn-out coins.  This  is  done by
merely placing the coin  upon a hot iron; an oxi-
dation takes  place over the whole surface of the
coin, the film of oxide changing its tint with the
intensity or continuance of  the heat.   The parts,
however,  where the letters of the  inscription had
existed, oxidate at a  different rate from the sur-
rounding parts; so that these letters exhibit their
shape, and become legible, in consequence of the
film of oxide  which covers them having a differ-
ent thickness, and therefore reflecting a different
tint  from  that of the surrounding parts.   The
tints thus  developed sometimes pass through ma-
ny orders of brilliant colors—particularly pink
and green, and settle in a bronze, and sometimes
a black tint, resting upon  the inscription alone.
When the  experiment is often repeated, the coin
loses  its property of becoming  luminous in this
way; but regains it  on being exposed for a lime
to the atmosphere.  The reason  that  the parts of
a coin, that hus been roughed by an acid, become
more  luminous than the parts  that  are left in a
polished state, is,  because all black and rough sur-
faces  radiate light more copiously than polished
and smooth surfaces ; and hence the inscription
is luminous when it is rough, and obscure when
it is polished.
    LAUGHABLE  EFFECT OF HOMOGENEOUS LIGHT.
   242.  An experiment,  productive  of  much
amusement, and  at the same time well illustrat-
ing the cause of  colors,  may be performed with
homogeneous light.   If a little  spirits of win«—
or some spirit,  such  as gin—be  ignited in a dish,
and a quantity of common table salt thrown on
it, the lights in the room having been previously
put out, every person present will have a most
ghastly look, and their clothes will assume an ex-
traordinary color; the hue  of health will  disap.
pear from the cheeks, and the different  articles of
clothing will entirely change their complexion.—
The  cause  of  this phenomenon will  be found in
the circumstance, that the color of bodies depends
upon  the rays of  light being partly absorbed, and
partly reflected.  A ray of white light can be
shown to consist of seven  prismatic  colors, as
they are called (see  experiment 246), and differ-
ent bodies possess the power of absorbing one or
more  of these colors, and reflecting all the rest;
and the combination of the rays that are reflect-
ed gives the particular color to  an object.   Red-
colored  bodies, for instance, absorb  all the rays,
except the red  ray ;  and tliis accordingly appears
to be  the color of the body—and  so on of all other
colors.  When, therefore, homogeneous light is
produced  by the  means above mentioned,  as the
articles of dress,  and  the faces of the  company,
do not absorb  all the  yellow rays, but reflect a
portion, the color  of  every thing is changed.
  243. If we  were  to illuminate  scarlet cloth
with pure and unmixed yellow light, it would ap-
pear  yellow, because the scarlet  cloth does not
absorb all  the  yellow rays, but reflect some of
them  ; and  if we illuminate  blue cloth with  yel-
low light, it will  appear nearly  black, because it
absorbs all  the yellow light, and reflects hardly
any of it.   But, whatever be the nature and co-
lor  of the bodies  on which the yellow light falls,
they must appear yellow ; for no other light falls
upon  them, and  those which are not capable of
reflecting yellow fight must appear absolutely black,
however brilliant be their color in the light of day.
This  is the cause of  the curious appearance of
the company  when  homogeneous light is pro-
duced, as above mentioned.
     HEAT OF THE COLORS OF THE SPECTRUM.
  244. The heating  powers of the  rays of the
solar  spectrum arc  found  to be of  different  de-
grees. If  the  bulb  of an  air  thermometer be
moved in  succession through the differently-col-
ored rays, it wdl be found to indicate the greatest
heat in the  red rays;  next  in the green ; and so
on, in a diminishing progression, to the violet.
TO  SHOW THAT  LIGHT AND  HEAT ARE DISTLNCT IN
                THE SIN'S RAYS.
  245. If the thermpmeter be  removed entirely 
Philosophical Experiments.
71
out of the confines of the red rays,  but with the
bulb still in the line of the spectrum, it rises even
higher than in the red rays, and continues to rise
till  removed  half  an inch  beyond the extremity
of the red ray.  In this  situation, quite out of the
visible light, the thermometer has been known to
rise, in two minutes  and a half, from 61 degrees
to 70 degrees.  The ball of  the thermometer em-
ployed should be extremely small, and be black.
cned over with  India Ink.
               PRISMATIC COLORS.
                           246. Cut a small slit
                         in a window-shutter (a,)
                         on wliich the sun shines
                         at some period of the
                         day;  and directly op.
                         posite the hole,  place a
                         prism (p).  A beam of
                         light,in passing through
                         it, will be decomposed;
                         and if the light fall on
                         a sheet of paper, or
                         against  a white  wall,
                         the seven colors of the
                         rainbow  will  be ob-
                         served, as  in the en-
                         graving (see Introduc-
                         tion.)
                           247. If the split in
                         the window-shutter be
                         very  small,  and no
                         prism is placed oppo-
                         site to it, only four col-
                         ors  will appear—viz.
red, green, yellow, and violet.

    OCULAR SPECTRA, OR ACCIDENTAL COLORS.
  248. If we cut a figure out of red paper, and,
placing it on a sheet of white paper, view  it  stea-
dily for some seconds, with one or both eyes fixed
on a particular  part of it, we  shall observe the
red color to become less brilliant.   If  we  then
turn the eye  from the red figure  upon the white
paper, we shall see a distinct green figure, which
is the spectrum, or accidental color of the red fig-
ure.  With differently-colored figures,  we  shall
observe differently-colored spectra, as in the fol-
lowing table:—
Color of the originalfigurc:  Color of the spectral figure.
       Red..................Blueish Green.
       Orange................Blue.
       Yellow..................Indigo.
       Green................... Reddish Violet.
       Blue.................Orange Red.
       Indigo............... Orange Yellow.
       Violet......• • •......Yellow.
       Whit.-...............Black.
       Black................White.
  249. The two last  of these experiments, viz.,
white and  black figures, may  be satisfactorily
made by using a white medallion on a dark ground,
and a black profile figure.  The spectrum of the
former will be found to be black, and that of the
latter white.
  250. These ocular spectra often show them-
selves without any effort on our part, and  even
without  our knowledge.   In a highly-painted
room, illuminated by  the sun, those parts of the
furniture on which the sun does not directly fall,
have always the opposite, or accidental color.  If
the sun shines  through the chintz in a  red  win-
dow-curtain, its light will appear green, varying,
as in the  foregoing  table, with  the color of the
curtain ; and  if we look at  the  image of a can-
dle, reflected from the water in a blue finger glass,
it will  appear yellow.   Whenever, in short, the
eye is affected  with  one prevailing color, it sees,
at the same time, the spectral or accidental color,
just  as when  a musical string  is vibrating, the
ear hears, at the same time, its fundamental and
its harmonic sounds.

   PHOSPHORESCENCE OF THE SEA ILLUSTRATED.
  251. The sea frequently presents a very beau-
tiful phosphorescent appearance, which  arises, in
most cases,  from the presence of innumerable
quantities of exceedingly minute living creatures,
like glow-worms, furnished with the means of be-
coming phosphorescent.  The effect they pro-
duce may be imitated with a basin of water, in
the following manner:—Pour a little  phospho-
retted ether on a lump of sugar, and drop it into
the water, which should  be made lukewarm.   In
a dark place, the surface of the water will  soon
become luminous; and  if it be  moved, by blow-
ing gently with the mouth, beautiful and brilliant
undulations of the surface will be  visible, exhib-
iting the appearance of liquid combustion. Those
who cannot see the ocean in a flame, may adopt
this  method of feebly imitating it;  and it will
serve to give them a  faint idea of a phenomenon,
which has called forth the admiration of all who
have ever seen it.

       PHOSPHORESCENCE OF THE RETINA.
  252. A  class of ocular deceptions have  their
origin  in  a property of  the eye, and they have
been very imperfectly  examined.   The fine ner-
vous fabric  which  constitutes  the retina, and
which extends to tlie brain, has the singular prop-
erty of being phosphorescent by pressure.  When
we press  the eye-ball  outward, by applying the
point of the finger between it and the nose, a cir-
cle of light will be seen, which Sir  Isaac  Newton
describes as " a circle of colors,  like those in the
feather of a peacock's  tail."   He  adds, that " if
the eye and the finger remain quiet, these colors
vanish in a  second of time; but if the finger  be
moved, with a quavering motion, they appear
again."  The luminous  circles  always continue
while the pressure is applied, and  they may be pro-
duced as readily after the eye has been long in
darkness, as when it has  been recently exposed to
light.  When the pressure is very gently applied,
so as to compress the fine pulpy substance of the
retina, light is  immediately created, when  the
eye  is' in total darkness; and when in this state,
light is allowed  to  fall  upon it: the part com-
pressed is more sensible to light than any other
part, and, consequently,  appears more luminous.
If we  increase the pressure, the eye-ball, bemg
filled with  incomprehensible fluids, will protrude
all round the point of pressure; and, consequent-
ly, the retina, at the protracted part, will be com-
pressed by the outwurd pressure of the contained
fluid, while  the retina on each side—namely, un-
der the  point of pressure, and beyond the protru-
ded  part—will be drawn toward the protruded
part, or dilated.  Hence, the part  under the fin-
ger, which was originally compressed, is  now di-
lated;  tie adjacent parts compressed,  and the 
72
Philosophical Experiments.
more  remote parts immediately without this, di-
lated also.
  253. Now we have observed, thrrt when the eye
is, under these circumstances, exposed to  light,
there is a bright,  luminous circle shading off, ex-
ternalby and  internally, into total darkness.  We
are led, therefore, to the  important  conclusions,
that when the retina is compressed in total dark-
ness,  it gives  out  light;  that  when  it is com-
pressed while exposed to  light, its sensibility to
light is increased ;  and that when it  is dilated,
under  exposure to  light,  it  becomes  absolutely
blind, or insensible, to all luminous impressions!
  254.  When the body is in  a state of perfect
health, this phosphorescence of the eye shows it-
self on many occasions.  When the eye-ball, or
the eye, receives a sudden blow, a bright flash of
light shoots  from the eye-ball.   In the act of
sneezing, gleams  of light are emitted from each
eye, both during the  inhalation of the  a>r, and
during its subsequent  expulsion ; and in blowing
air  violently through the nostrils, two patches of
light  appear abeve the axis of the eye, and  in
front of it, while  other two luminous spots unite
into one, and appear,  as it were, about  the point
of the  nose  when the eyes are directed to it.—
When  we turn the eye-ball, by the action of its
own muscles, the retina is affected at  the  place
where  the   muscles  are  inserted ; and  there
may be seen, opposite each eye, and toward the
nose, two semicircles of light, and other two, ex-
tremely faint, toward the temples.  At  particular
times, when  the  retina is more  phosphorescent
than at  others, these semicircles are  expanded
into complete circles  of fight.   In a state of in-
disposition, the phosphorescence of the  retina ap-
pears in a new and alarming form (see  Introduc-
tion).  When we consider the variety of distinct
forms which, in a state of perfect health, the im-
agination can conjure up when  looking into a
burning fire, it is  easy to conceive how  the mass-
es of colored light,  which  often float before the
eyes in illness, may be moulded, by the same pow-
er,  into those  fantastic shapes that  often haunt
the sick man's couch.   Sir D. Brewster has ably
illustrated these  phenomena  in  his  " Natural
Magic."

               SOLAR PHOSPHORI.
  255. There  are many  substances in nature,
which, when heated to a.certain  degree, acquire
the property of becoming luminous at  low de-
grees of temperature,  and when  merely exposed
for  a  time  to the sun.   Canton's phosphorus,
which is obtained  from  calcined oyster-shells,
possesses this  property;  and  common oyster-
shells maybe rendered phosphorescent, by attend-
ing strictly to the following directions, which are
given by the discoverer for the purpose:—Take
the most flaming  coals off a brisk fire, and throw
in some thick oyster-shells; then replace the coals
and calcine  the shells  for an hour. Remove them
carefully, and,  when cold, it will be found that,
after exposing them for a few minutes to  the sun,
they will glow, when taken into a dark room,
with most of the  prismatic colors.
  256. Fluor spar, several varieties of  phosphate
of  lime, and  marble,  become  luminous, when
heated to a certain  point, without undergoing
combustion.   Their  luminous  property  may  be
best exhibited, by scattering them, in coarse pow.
der, upon an iron plate, heated to redness.
   287.  Many animal substances are nuturall)
phosphorescent.   This property in the glow-worm
is well  known; and it  appears that  salt-water
fish become luminous  in about twelve hours after
death, the brilliancy increasing till putrefaction is
evident, when it decreases.  This  effect,  how-
ever, docs not take place in fresh water.
        BREATHING LIGHT AND DARKNESS.
   258.  The  following experiment,  if performed
with care, is exceedingly  striking, and beauti-
fully exhibits one of the many optical delusions
to  which we are  liable.  Let s be a  candle,
whose  light  falls   at an
upon  two   glass   plates,
(a  b),  placed  close  tov
each other, and  let the re-
flected  rays  (a c b d) fall
at  the  same angle  upon
two similar  plates  (c n),
but  so  placed,  that  the
plane of reflection from the.
latter is at right angles to
the plane of reflection from
the former.  An eye placed
at e,  and looking,  at the
same time,  into the  two
plates,  c  and d, will see
very  faint images  of the
candle  (s),  which,  by  a
slight  adjustment of  the
plates,  may  be made to
disappear  almost  wholly,
allowing the  plate c to re-
main as it is; change the
position of d, till its incli-
nation   :«the ray  b d,
d is diminished about 3^°,
or made nearly 53°  11'.—
This  distance may  easily
be  found  by a "little prac-
tice.  When this is done,
the image that had  disap-
peared  on looking into  d
will be restored;  so that-
the spectator at e, upon looking into the two mir-
rors, c d,  will see no light in c, because the can-
dle has nearly disappeared; while the candle  is
distinctly  seen in d.   If, while  the spectator is
looking into  two mirrors, either he, or another
person,  breathes upon  them gently, and quickly,
the breath will revive the extinguished image in
c, and will extinguish the visible image  in d.—
The  following is the  cause of  this singular  re-
suit:—The light a c b d is polarized by reflection
from the plates a b,  because it is incident at the
polarizing angle, 56° 45', for  glass.   When we
breathe  upon the  plates c d, we form  upon their
surface  a thin film of water, whose polarizing an-
gle is 53° 11';  so that if the polarized rays, a c
b d, fell upon the plates c d, at an angle of 53°
11', the candle from  which they proceeded would
not be visible, or they would not  suffer reflection
from  the  plates c d.  At  all  other angles, the
light would  be reflected, and the candle visible.
Now the plate d is placed at an angle of 53° 11',
and c at an angle of 56° 45', so  that when a film
of water is breathed upon them, the light will be
 
Philosophical Experiments.                               73
reflected from the latter, and none  from the  for-
mer;  that is, the act of breathing upon the glass
plates will  restore the invisible,  and extinguish
the visablc image.  This is an experiment of
Brewster's.

        CHANGES OF THE KALEIDOSCOPE.
  259.  The following curious  calculation  has
been made of the number of changes this won-
derful instrument wdl admit:—Supposing the in-
strument to contain 20 small pieces of glass, &.c,
and that you make 10 changes in each minute, it
will  then  take the inconceivable space of 462,-
880,899,576 years, and 360 days, to  go through
the immense variety of changes  it is capable of
producing;  amounting (according  to  our  frail
idea of the nature of things) to an eternity.  Or,
if we  take only  12  small pieces of glass, and
make  10 changes in each  minute, it will then
take 33,264 days, or 91 years and 49 days, to ex-
haust its variations.
           HEAT OR CALORIC.
Introduction to the science of heat—Theories of heat—Gen-
  eral effects of heat  in nature—Heat the antagonist to at-
  traction—Latent heat—Sources of heat—Combustion—
  Haat of the sun—Heat produced by friction—Heat by per-
  cussion—Heatby chemical mixture—Heathy electricity and
  Galvanism—Heatby animals—Spontaneous combustion—
  Properties of heat—Expansion of bodies—Heat—Tendency
  of Hea to equilibrium—Summary of the properties of heat.

  260. Heat,  or  caloric, as it is philosophically
termed, is the most active principle in nature ;
there is not a chemical change that takes place,
in which it is not concerned, and, without it, vi-
tality would become extinct.   Wc  are all famil-
iar with the sensation it causes, and philosophers
have been able to determine with exactness, the
various phenomena it is capable of producing;
but this is  all  that has been done, for we are as
ignorant, in the present day,  of  what heat really
is, as the first phUosopher who turned his atten-
tion  to  the subject.  The reason of this is, that
heat has never been exhibited by itself, apart from
material  bodies ;  it is always found in combina-
tion with them, and we can, therefore, only judge
of its effects.  Many theories  have, however,
been  proposed, from time to  time, to explain the
principle of heat; but, for the reason  above sta-
ted, they have been necessarily unsupported by
facts, and were  merely suggestions of the au-
thors.
  By some, heat has been thought to be particles
of matter that are  shut off from burning bodies
with  amazing velocity; but wliich, being infi-
nitely  small, are  incapable of injuring our  most
delicate organs.   Others, again, suppose that it is
no material substance, but  only the effect pro-
duced by certain vibrations among  the particles
of which all  bodies are composed.  Both  these
theories are, however, incapable of proof;  and as
all the  phenomena of heat can be perfectly un-
derstood, without any thing of itself as a princi-
ple, the student need pay little regard to either of
the theories just mentioned, or puzzle himself  in
endeavoring to understand its essence.
  261.  Dr. Arnott beautifully describes the gen-
eral  effects of heat in his treatise  on that sub-
ject.   " In the winter of  climates,"  says he,
" where the temperature is, for a time, below the
freezing point of water, the earth, with its wa-
ters, is bound up in snow and ice ; the trees and
shrubs are leafless,  appearing  every where like
withered skeletons; countless multitudes  of liv-
ing creatures, owing either to the bitter cold, or
deficiency of food, are perishing  in the snows—
nature seems dying, or dead: but what a change
when spring or heat returns ! The earth is again
uncovered and soft; the rivers flow ; the lakes are
again liquid mirrors ;  the warm showers come to
foster vegetation, which soon covers the ground
with  beauty and plenty.   Man,  lately inactive,
is  recalled to many duties:  his water-wheels are
every where at work; his boats are again on the
canals and streams ; his busy fleets of industry
are along  the shores.   Winged life, in new mul-
titudes, fills the sky;  finny fife similarly fills the
waters; and every spot of  earth  teems with vi-
tality and joy.   Many  persons  regard these
changes of season as if they came like the suc-
cessive  positions of a  turning-wheel, of which
one  necessarily brings the  next; not adverting
that it is the single circumstance of change of
temperature which docs all.  But if the cold of
winter arrive too early, they unfailingly produce
the wintry scene;  and if warmth come before its
time in spring, it expands the bud and the blos-
som,  which a return of frost will surely destroy.
A seed sown in an ice-house never  awakens to
life."
   262.  Heat is the  great antagonistic force in
nature  opposed  to attraction.   While the effect
of the latter is to draw the particles of matter in
closer contact with each other, the effect of heat
is to  make them  separate;  and  hence, on the
power of heat depends the different  states in
which bodies  are  found, either as solids, liquids,
or gases.   It is well known that, if we apply heat
to a lump of ice,  it melts and  becomes  water;
and if we  continue the heat, this water becomes
a gas, called steam.  In this instance, therefore,
heat  is capable of producing  all the different
states in which matter is found  existing in na-
ture ; and the experiment proves that it is heat
which  causes matter to assume  these forms.—
Water, in  its natural state, is liquid; yet the ab.
straction of the heat peculiar  to it, as water, at
once reduces it to  the solid form; and, in  some
parts of the world, this is the only state in which
it is found. Within the tropics, on the  contrary,
ice, and  its different modifications, snow, hail,
&c, are never seen ; so that a person, wishing to
send a friend within this latitude the greatest cu-
riosity he  could from  England, packed up  a box
of snow !   He was unable, however, to make the
box as impervious to heat  as an ice-house; and
the only curiosity  the friend found, therefore, on
opening the box, was a cupful of  water!   When
natives of India, and similar hot countries, ar-
rive  in England, nothing occasions them greater
surprise than to see water in a solid state; and it
is said  that an eastern monarch, having called a
traveller before him to relate what he had seen in
 foreign  countries, forgave him all the falsehoods
he thought he had uttered, until the traveler told
him  that, in some  places, water becomes natu-
rally hard, and could be cut like  blocks of mar-
 ble for several months of the year; upon which,
 the monarch ordered him instantly to be put to
 death, for endeavoring to deceive him by the re-
.*<* 
46                              Philosophical  Experiments.
   lation of what  it was impossible could ever hap-
   pen.
     263. The sensation we experience on approach-
   ing a burning body, at once informs us of the ex-
   istence of heat;  but it exists  also in all bodies,
   in a state in which it is insensible to the touch,
   and in which condition  it has no effect upon the
   thermometer.  In this  state,  it is called  latent
   heat.  In the following  pages, numerous instan-
   ces are given of its existence in this form, which
   will enable the  reader easily to understand the dif-
   ference between this  and sensible heat; but, in
   order to  explain it briefly here, we  may allude
   again to the changes  that take place in water, as
   illustrations. If an uncovered vessel, containing
   water, be placed upon the fire, the heat will soon
   make it boil;  and if a thermometer be  then pla-
   ced in  it, it will indicate 212 degrees.  Now,
   if the heat be ever so much  increased, while the
   vessel  remains uncovered, the water wdl never
   become hotter  than this ; but  it will soon evapo-
   rate in the form  of stearn, and all the additional
   heat applied, after the water has reached a tem-
   perature  of  212 degrees, is-.absorbed,  to  form
   steam;  or, in other words,  becomes the latent
   heat of steam.  We might suppose that the steam
   would be hotter than the water, as it receives all
   this additional heat;  but if the thermometer be
   held in the steam it will not rise higher than 212
   degrees:  this is because  all the heat  absorbed by
.   the  steam  has become hMent;  and if we  cool
A steam, so that it shall retuR to its  original state,
   as water, it will  give out* the exact quantity of
   heat that it required in  order to assume  the form
   of steam.  The more solid a' body becomes, the
   more latent heat it gives out; so water, when it
   changes to  ice, allows its latent heat to escape:
   and before ice  can again become water, it must
   absorb a sirmlar quantity to that previously evol-
   ved. The temperature of ice is 32 degrees, and
   water will  not freeze  until  it is cooled to  that
   point;  it will not freeze at  33  degrees.  We
   might, therefore, suppose that, by adding one  de-
   gree of heat to ice, we could change it  into wa-
   ter ; but, in fact, it requires one hundred and for-
   ty degrees,  as  one hundred and thirty-nine de-
   grees become latent, and we cannot  ascertain its
   presence by the thermometer, or by the touch.
      Latent heat exists in all bodies, however cold
   they may feel; and whenever  they are compress-
   ed, a portion of this heat will be squeezed out, in
   the same way that we may squeeze out water
   from a sponge, although, to the sight, it may ap.
   pear nearly  dry.   It will be found,  therefore, in
   the following experiments, that whenever a body
   is subjected to  pressure, heat is forced  out; and
   this will be  found the case, not merely with solid
    bodies, but  with liquids, and even with the air it-
    self. (See experiment 284.)  The principal sour-
    ces of heat are,  combustion, the  sun, friction,
    percussion, chemical mixture, and Galvanism.—
    Animals, also,  have the power of generating heat
    during respiration; and there  are other means by
    which it is  occasionally produced, such as com-
    pression, and  what is termed spontaneous com-
    bustion ; by which animal  bodies are consumed
    without any apparent cause.
      264. Combustion is  the artificial means more
    generally adopted  to produce heat.   Certain bo.
    dies in nature—such as wood and coals—are call-
ed combustibles, and  when  ignited  in  any gas
which supports combustion—such as the atmos.
phere or oxygen—chemically combine with this
supporter of combustion, and give out much heat
that  had previously been latent.   Sometimes the
combustion  is instantaneous;  sometimes  it  is
slow : this depends on the affinity of the burning
body for oxygen; and in  the following experi.
ments, many instances are  given of the intense
chemical action that  sometimes  takes place.—
The  laws  which regulate  combustion are also
fully explained, so that it is  unnecessary to refer
to them here.  (See experiment 311.)
  265.  The sun is the principal source of heat,
and  it is calculated that heat comes to us from
that luminary at the amazing velocity of 200,000
miles in a second of time—a speed which it i9
impossible for us rightly to comprehend.  The
heat of the sun docs not depend on the distance
it is from us ;  for, although we are many millions
of miles nearer  to  it in winter  than we  are  in
summer, yet winter is coldest; because the rays
from the sun fall more obliquely on this part  of
the globe, at that  season of the  year, than they
do in  summer.  When the rays fall directly on
the earth, there is great heat evolved; and tins is
principally the reason why the climate is always
so hot  at the  Equator.   The heat derived from
the sun differs, in many respects, from that pro.
cured by other means.  It passes readfiy through
glass, while the heat from a  fire does not.  Some
writers have even stated, that artificial heat will
not pass through glass at all; but this is  a mis-
take, which any one  may  ascertain by holding
his hand near a gas-glass, while the gas is burn-
ing : the heat, however, does not  pass through
readily, and advantage  has been taken of this
circumstance in the manufacture of elegant fire-
screens, by which persons may enjoy " the Eng-
lishman's comfort" of looking at the fire, without
experiencing any unpleasant degree of heat. The
sun's rays have been  found, by experiment, to be
divisible into  two portions,  the  one conveying
light, the other heat:  and it has been ascertained
these rays are distinct from  each other (sec 245);
hence  it happens, that the light of the  sun may
be reflected from a body, while its heat is absorb-
ed.  This is really the case  with the moon.  The
rays of light proceeding from that luminary  are
cold, and not  the slightest degree of heat can be
derived from  them,  even  when a number  are
made  to fall on one spot, by means of  a " burn-
ing-glass."  A very different effect is produced
when  the  sun's rays are condensed in a  similar
manner: the most intense heat may be produced
by this means; and, by arranging a number of
plain mirrors (looking-glasses) in such a way that
the  image of  the sun from  all of them shall fall
on one spot, a sufficient  degree  of heat may be
produced, even to melt the metals, gold and pla-
tinum.  It was by such an arrangement of mir-
rors, that Archimedes was  enabled to set on fire
the  shipping  of the  enemy who besieged Syra-
cuse.
   266. Friction is another means of procuring
heat. When two substances are rubbed together,
they become  warmer;  and in machinery, great
care is obliged to  be taken, in order to prevent
any part catching fire from  this cause.  If a
piece  of glass, which is incombustible, be held 
Philosophical Experiments.                              ' 5
against the edge of a grit-stone, while revolving,
the glass  will become too hot to hold : this is in
consequence  of the  friction  between  the  glass
and the stone forcing out a portion of  their latent
heat; for it is  only in this way we can account
for the heat that is produced.   In the experiments
which  follow, many instances are  given, which
will more fully explain this phenomena. (See ex-
periment 283.)
   267.  Percussion is the method adopted by
house-wives, with a tinder-box, to procure light;
and its  effects are, therefore,  pretty well  known.
When  two bodies are struck together, heat is
given out; and if, as is the  case of  a flint and
steel, one of the substances happens to be harder
than  the other, a portion of the softer substance
will often be melted.  In the  instance alluded to,
the flint, being  harder than the steel, melts a por-
tion of  it, which, while red-hot, falls upon the tin-
der, and ignites it; thereby producing sufficient
heat to  melt and light the sulphur on the end of a
match.   The cause of the heat is supposed to be
the sudden compression which the bodies under-
go, and which forces out a  portion of the heat
that had previously been latent.  One striking
instance of the effect of percussion is given in
experiment 285.                               '
   268.  Chemical mixture is a constant source of
heat, since all experiments in chemistry, in which
the bulk of a body is reduced, produce it; and in
other cases, when the affinity of two bodies for
each  other is  suddenly  developed by chemical
mixture, intense and  instantaneous heat is pro-
duced.  In some cases, however, intense cold is
produced  by mixture : this is in consequence of
the bodies expanding, when,  of course, an effect
precisely opposite is produced to that which takes
place when a body is  condensed.  Chemical ex- j
periments, explaining these facts, are generally |
very striking and beautiful; and  numerous  ex-
amples will be  found  in the following pages: it
is necessary,  however, to caution the student to !
attend particularly to  the directions that are giv- ,
en, and to experiment with very small quantities
of the  chemicals  to be used, by which  means, j
very often, the  process will be far more satisfac-
tory, and  illustrate the fact much better, than if,
a large quantity were employed.
   269.  Galvanism and Electricity are the last
means  that have been alluded to, when  describ-
ing the principal sources of heat; and for pro-
curing  an intense  degree of artificial heat, Gal-
vanism  is the best method that can be adopted.
By its  means, all the  metals  may be fused with
the greatest facility; thereby showing, that  the
temperature produced must be aqual to that which
can be derived from the sun.  Sir  Humphrey
Davy had a Galvanic battery erected for him at
the Royal  Institution, by wliich ho was  enabled
to fuse  every substance exposed to its influence,
and by which he decomposed the various earths,
that had formerly been considered to be simple
bodies,  and determined  many other important
chemical facts.
  270.  The temperature of animal bodies is in-
dependent of  the surrounding atmosphere, or
other  medium in which they live;  for it is found
that the heat of the  human  body is nearly the
same all over the world.  Living bodies exhibit a
remarkable difference from unorganized matter in
this respect: the latter soon acquire a tempera-
ture similar to that of the bodies by which they
are surrounded ; for instance, a candle, if brought
into a room, of which the temperature is verv
high, begins to melt;  if some water is exposed to
the air, when  it is  below 30 degrees, the water
is frozen:  thus, in  both instances,  we see how
readily  unorganized  matter  acquires  the same
temperature as that of the medium in which it is
placed.   This, however, is not the case with liv-
ing bodies ; they maintain an equal temperature,
with very slight variation in summer and winter,
at the Poles, and at the Equator.  To do this it
is necessary that they should be enabled, in a
cold climate, to generate a great quantity of heat,
and in  a hot climate, to dispose of  it readily.—
At present, we have merely to describe the man-
ner in which  animals generate heat; but in the
experiments which follow, will be found  several
illustrations of the  mode in which  the  supera-
bundant heat is disposed of..  (See 289.)
   271. The phenomenon of respiration is analo-
gous, in many respects, to combustion.  The
blood circulating in the veins contain a conside-
rable  quantity  of carbon; and before the blood
can circulate through  the body, to perforin its va-
rious functions, it is  necessary that this carbon
should be removed.  This is effected in the lungs.
The blood is conveyed there, in an impure state,
in very small blood vessels, which are permeable
to the air, and are placed over small globules of
thin cellular tissue, being the terminations of the
windpipe.  When we inspire the air, it is con-
veyed to these globules, or air-cells; and, pass-
ing through them, enters  the blood: the oxygen
of the  air then combines with  the carbon, and
forms carbonic  acid, just  the same as it is form-
ed when we burn a candle, in a glass, under wa-
ter.  (See experiment 306).  This carbonic acid,
mixed with the nitrogen of the  air,  is given out
when we expire our breath ; and its qualities may
be proved by experiment 117.   Now we know, in
the case  of the candle, that the union of its car-
bon with oxygen  occasions heat; and the same
effect is produced in the lungs.   The formation
of carbonic acid  there is attended with the evo-
lution of heat, and this is conveyed by the blood
to every part of the body.  (See experiment 309.(
   The means by which the same temperature  is
maintained by the body, in cold weather  as  in
hot, are vital;  for when we are exposed to cold,
an impulse is given to the function of respiration,
by means of which, the blood is more frequently
brought into  contust  with the air,  and, conse-
quently, a greater degree of heat is generated.
   272. Spontaneous  combustion  can hardly  be
enumerated among  the general  sources of heat,
since it occurs so seldom.   Instances are record-
ed, however, in the scientific journals, of several
well-authenticated instances,  in   which persons
have been discovered burning slowly away, some-
what in the way that phosphorus burns, at a low
temperature, in the atmosphere.   It appears, in-
deed, as if the body underwent some change, by
which a considerable  portion  of it was changed
to phosphorus, or some substance very nearly re-
sembling it.  Phosphorus  is  principally formed
from animal  mutter; and the  supposition has,
therefore, some support.  But, as the instances
when this peculiar mode of generating heat have
*\ 
76                                Philosophical Experiments.
occurred but seldom, philosophers have not had
sufficient opportunities of investigating the phe-
nomenon satisfactorily.
  Having thus explained the distinction between
latent and sensible heat, and  described the prin-
cipal means by which it is obtained, it only re-
mains for us now to describe generally its effects.
This may be done briefly, because, in the follow-
ing experiments, these are fully illustrated.
  273. Heat expands bodies.  This is a univer-
sal law, and there are but one or two apparent ex-
ceptions.  Were there no such thing as heat, li-
quids and gases could not exist; all matter would
be solid.   Heat is the cause of bodies becoming
fluid ; it insinuates itself between  the particles of
which they are composed, and forces  them fur-
ther apart:  if a great degree of heat is applied,
the particles  are separated so  far, that  they then
assume the form of gas.  Steam is a familiar ex-
ample ; and the thermometer acts solely on this
principle.  There are only two or three excep-
tions  to this law, and they are only so in appear-
ance.  The principal one is water, which, instead
of contracting when  cooled down below 32 de-
grees, expands, when it assumes the form of ice.
This  is a beautiful provision, since the  ice, float-
ing on the water, prevents it parting readily with
its heat,  and thus  does not allow our rivers, &c,
to become a solid mass of ice, as they  otherwise
would.   The cause of the water expanding, and
becoming lighter when it freezes, is because the
crystals  of ice  have interstices between them,
which are filled with air.
   274. Heat  has a tendency to equilibrium; it
endeavors to produce, in all bodies, the same de-
gree of temperature: it  is,  therefore, constantly
given off from objects  which  are  hotter than
those by which they are surrounded—1st, by ra-
diation (see experiment 298): 2dly, by conduc-
tion (see experiments 290,292, 294); 3dly, by re.
flection  (see experiment 297).  The wonderful
effects produced  by these  means are  fully  de-
scribed  in the following experiments; and it is
unnecessary, therefore, to do more than allude to
them in  this introduction.
        EXPERIMENTS  IN  HEAT.
Sensations of heat—Various theories respecting—Latent heat
  —In Iron—In water—In a common button—Developed by
  friction—In the air—Production of heat by percussion—
  Expansion of bodies by heat—Cold by evaporation—Con-
  ducting powers of different substances—To  break glass in
  any direction by heat—Cooling power of wire gauze—Dr.
  Franklin's experiment on heat of colored cloths—To break
  a roll  of sulphurw y the heat of the hand—Prince Rupert's
  drops—Reflection  on  heat—Formation  of dew—Water
  boiled by being cooled—Production of freezing mixtures—
  Condensation of steam—Brightness of flame—To prove
  that flame is hollow—How a candle burns—Oxygen ne-
  cessary for combustion—Requisites to produce  a flame—
  Intense and rapid  combustion—Detonation  with sulphur
  and chlorate of potass—Vivid combustion under water—
  Detonation of chlorate of potass.
  To sum up what has been stated above, the stu-
dent will bear in mind the  following principles,
which  the  experiments  will  illustrate:—That
heat exists in two distinct states—as sensible heat
and as  latent heat; that it may be procured, both
naturally and artificially, by the methods which
have been described;  that it has a tendency to  in-
crease  the bulk  of all bodies it combines with,
and diminish the bulk of those it separates from ;
and that it is constantly passing from one body to
another, endeavoring to restore an equilibrium by
the means  just enumerated.
         SENSATIONS OF HEAT DECEPTIVE.
  275.  If we place  one of our hands in a  basin
of hot water, and the other in a basin of cold wa-
ter, and  then immerse both of them in another
basin containing luke-warm water, we shall feel
it to be cold to the hand that had been placed  in
the hot water, and  hot to  that which had been
immersed  in the cold.   Our sensations of heat
are very apt to deceive us.
  276. Two travelers arriving at a spot half way up
a mountain, will feel the temperature of the  place
to be hot or cold, according to the warmth of the
place they have each of them left.  The man as-
cending the mountain will feel cold, because the
higher we ascend in the air the colder it is;  while
the  traveler  descending  will  feel  warmer, for
the same reason.   Thus, any particular part of a
mountain  may  appear agreeably warm to one
man, and intensely cold to another.

                 LATENT HEAT.
  277.  A  quantity of heat exists in all bodies  in
what is termed a latent state; that is, it cannot
be felt by the touch (see Introduction, 263). The
following experiments will, however, illustrate the
fact  better  than any description can.
  278.  Hammer a piece of cold iron for a short
time, and  it will become  exceedingly hot;  if
hammered briskly on an anvil, it may be  made
red-hot.
  279.  Rub  a common brass  coat-button, for a
short time  on a piece of wood ; on applying it  to
the  hand,  it will be  found to have  acquired a
high degree of temperature.

              LATENT HEAT IN IRON.
  280.  The quantity of heat which  exists, in  a
latent state, in iron, may easily be shown by the
common flint and steel of a tinder-box. On  strik-
ing  the steel  sharply with the flint, a portion  of
the  latent  heat of the former is given out, suffi-
cient to melt a particle of the iron, and render it
red-hot.  If  the flint and  steel be struck sharply
together, several  times, over a clean sheet of writ-
ing-paper,  a number of black spots will be  found
on it, which are  the particles  of steel that have
been melted, and fallen on the paper.

             LATENT HEAT IN WATER.
  281.  Water contains much heat, in a  latent
state; and if we could compress it with the same
ease that we can many other, bodies, we might
squeeze this heat out, and render it sensible  to our
touch.   The  same effect, however, may be pro-
duced in another way.   Pour about a  wine-glass-
ful of sulphuric acid (common oil of vitriol) into
a Florence flask, or other thin glass  bottle, that
will, therefore,  bear a  sudden  degree of'heat
without breaking ;   into  the  flask  pour  about
twice or three times the quantity of  wa-   a
ter, and in a  few minutes,  although  both   B
the liquids were cold when  mixed, a very  B
great degree -of heat will become sensible.  I\
Indeed,  if the flask be immersed  in  wa./  \
ter, a sufficient degree of heat may be ob-i.  j§
tained, by  this means, to make it boil__J^f
Phosphorus may  also be very easily ignited  by  Us 
Philosophical Experiments.                              77
The cause of this effect is believed to be as fol-
lows :—The sulphuric acid has a very strong af-
finity for water,  and combines with it  so  inti-
mately, that the liquids, when mixed together, do
not measure so much as the sum of their meas-
ure when  separate.  Thus, a pint of sulphuric
acid,  mixed with a pint of water, would not fill
a quart measure.   It is  evident, therefore, as the
liquids have decreased in bulk, a portion of the
latent heat that belonged to them must be given
out in their changing to a more solid state; and
this is the result that is produced.   Care  must be
taken,  in  making  the  experiment,  to hold  the
Florence flask so  that, if it breaks with the heat,
the mixture within it shall not fall on the clothes
or furniture, as, otherwise, they will be much in-
jured.   Thin  glass vessels bear a sudden degree
of heat much better than thick ones, and a Flor-
ence  flask is, therefore, well adapted for the ex-
periment.
       LATENT HEAT IN A COMMON BUTTON.
   282.  If  we take a common  brass button, and
rub it smartly for a few minutes on the  floor, or
against a piece of wood, it will become hot enough
to ignite phosphorus.  At school, boys are prac-
tical philosophers  when  they perform this experi-
ment, and  illustrate its effects, by placing the hot
button against the hands of  their companions.
who may  not be aware of the effects of friction
in developing latent heat.

      LATENT HEAT DEVELOPED BY FRICTION.
   283.  Friction is one of the modes of procuring
artificial heat; and savages are said  to  procure
fire through its means  alone (see Introduction).
If two pieces of dry wood be rubbed together for
for some time, they become heated; and if the
friction is continued, they may  be made to in
flame.   Large  tracts of forests have frequently
been set on fire  by these means in winter time,
when the boughs, being dry, the agitation of them
against each other,  by high winds, produce suf-
ficient heat to ignite them.  Sir Humphrey Da-
vy even  obtained heat from ice, by friction : he
rubbed two pieces together in vacuo, and the la-
tent heat developed  by this means was sufficient
to melt a portion of the ice.   The effect of fric-
tion, in producing heat, is seen when a coach is
going down a hill, with the " skid" on one of the
wheels:  sometimes the heat produced is so great,
that it has been known  to set the wheel on fire.—
Great care is taken in constructing machinery to
provide against friction; not merely because it
impedes the action of  the  machine, but also to
prevent  the  heat that would  otherwise be pro-
duced, which might prove of serious injury.  The
experiment (282) with a common  button is  a
good example of the effects of friction in produc
ing heat.
            LATENT HEAT IN THE AIR.
   281.  To exhibi' this, an instrument is sold at
the philosophical  instrument-makers, called an
air syringe, or " the philosopher's tinder-box;"
but the same effect may be produced by a com-
mon  popgun, which  it very much resembles.—
Stop up the end of a popgun with  wax,  or some
other substance,  so securely, that no air can es-
cape, and  make  the rammer to fit the tube as
tightly as possible,  by greasing it.  If a piece of
tinder be then placed in a little hole, which must be
made at the end of the rammer, and it is driven
down quickly, and withdrawn again into the air,
the tinder will be ignited.  This is caused by the
heat that is given out on  the sudden compression
of the air in the  tube ; and if the  apparatus is
made air-tight, a light may be produced by this
means much easier than by a flint and steel (see
Introduction).  Although a popgun  will do, a
metal tube,  stopped at one end with a piston, to
fit air-tight,  is better.

      PPRODUCTION OF HEAT BY PERCUSSION.
  285. The experiment (280)  with the flint and
steel is an example of the development of heat,
by  what  is termed percussion, or the  sudden
striking together  of  two bodies.  Blacksmiths
frequently employ another plan to light their fires.
If a piece of iron be laid upon an anvil, and
Btruck sharply with  a hammer for some time, it
may be made red-hot;  the hammering causing
the iron to  give out nearly all its latent heat.—
The effect may be  shown by merely beating a
common nail with a hammer for some time, when
it will become warm enough to ignite a piece  of
phosphorus; but if we wish to light a match  by
this  means,  we must use a large piece of iron,
and beat it upon an anvil, or something similar.
The use of the latent heat in the iron, which is
forced out by this means, was to make it mallea-
ble and ductile, both of which  properties it loBes
when its latent heat is forced out.

         EXPANSION OF BODIES  BY HEAT.
  286. Measure a  bar  of iron,  and afterward
place it in a fire till  it becomes red-hot, when it
                    will be found, on again  be-
                    ing measured while in that
                    state, to  have increased  in
                    length.  When it  coolr, it
                    will be of the same length
                    that it was before being ex-
                    posed to the fire (see Intro-
                    duction).  The annexed cut
                    represents the  iron rod and
                    measure  by which this  ex-
periment is generally performed.
   287. Fill a phial with spirits of wine, and then
immerse it, nearly to the mouth, in hot water.__
The heat will expand the alcohol so much, that
it will run over the sides of the phial.  When it
is taken  out  of the hot water, and  cooled, the
quantity that has been forced out wdl be seen  by
noticing how much would then be required to fill
the bottle.

             COLD BY EVAPORATION,
   288.  Evaporation  produces cold,  because a
fluid, when it assumes  the form of vapor, ab-
stracts a  portion  of heat from the body  from
which it evaporates (see Introduction). To prove
this, place a small quantity of ether on the back
of the hand,  and as it  evaporates, considerable
cold will be felt.   When persons catch cold after
getting their clothes wet, it is  in consequence of
the  evaporation of water cooling the  body so
much, that the blood-vessels of the skin are una-
ble to perform their accustomed functions.   This
is the  reason why it is always  important to put
off wet clothing as soon as possible, and to keep
 
78                              Philosophical Experiments.
up the  heat of the body  by some  stimulating
drink, or, what is better, by exercise.
 CONDUCTING POWERS OF DIFFERENT SUBSTANCES.
  289.  Some substances will  allow heat to pass
through them much easier than others.   The for-
mer are called  good conductors; the latter, bad.
If we place a piece of thick iron wire in the fire,
it will soon become so hot that we cannot hold it;
but if we use a piece of glass, we shall find that
we can handle it  without inconvenience, even
when one end is intensely  heated.  In this case,
the iron is a good conductor, while the glass is a
very bad one.  Advantage is  taken of this cir-
cumstance in manufacturing many domestic ar-
ticles—for instance, the handle of metal teapots
is made of wood, because, otherwise,  the hand
would not be able to bear the heat that would be
conducted to it.

   TO BREAK GLASS IN ANY DIRECTION BY HEAT.
  290.  Glass being a  bad conductor of heat, ad-
vantage may be taken of the circumstance to
break a piece in any  direction, as follows:—Dip
a piece  of worsted  thread  into spirit of turpen-
tine, and put it round the glass, in  the direction
required to be broken; then set fire to the thread,
and the glass will crack exactly in a  line with it.
A piece of red-hot iron ma}' be used for the same
purpose, but the effect can be produced more easi-
ly with  the worsted.
        COOLING POWER OF WIRE GAUZE.
  291.  If a  piece of wire gauze be held in the
flame of a  candle, it will be seen that the flame
will not pass through it. This is in consequence
        Tof the metal conducting away the heat
         so rapidly from the gaseous tallow, in
        passing through, that when it reaches
        the other side, it is not sufficiently hot to
         combine oxygen,  and therefore passes
         away in the form of smoke.  That such
         is the case, may be proved by applying
         a light to the smoke, which will then
ignite, and burn like the flame beneath.
  292.  The  safety-lamp of Sir Humphrey Davy
was the result  of his discovery of the cooling
powers  of wire gauze.  Before this lamp was in-
vented,  the  men employed  in coal mines were
exposed to  great danger, in consequence of the
frequent explosions that took place when a stream
of explosive  gas, called fire-damp, came in con-
tact with  their candles.  This gas, which very
much resembles the  common gas used in our
shops, is very explosive when mixed with air, and
is formed in fissures in coal in the mines.   It fre-
quently happened, therefore, that when a miner
was at  work, he liberated a quantity of this gas,
which,  mixing with the atmosphere around, be-
came as explosive and destructive as gunpowder;
and when it came in contact with a  light, its ef-
fects  were  as  destructive.  Accidents  were so
frequently occurring, and so many men were kill-
ed, that at last a society was formed, to endeavor
to find out some means of preventing them ; but
their  efforts were unsuccessful.  At length they
applied  to Sir Humphrey Davey, who, on experi-
menting on the gas, discovered that it would not
explode unless it came it contact with flame; and
he thought that  if, therefore, a light  could be en-
closed, so  that while a sufficient quantity of air
 should  be supplied to it to keep it burning, the
 flame should be prevented communicating with
 the  air  without, that a  perfect  safety  lamp
 would be provided for the miner.   After a varie-
         J3^    ty of experiments, he constructed
         iJL    one similar to that  represented in
         |ijij7  the accompanying wood-cut.  It
         lip   represents a  common lamp, sur-
         H   rounded with  a ehicld of iron
         IB   wire gauze, which  has  the same
         \W\   effect  in  extinguishing   flame,
         |§|    when it endeavors to pass through,
         |3    as a piece of gauze  has with  a
         |H    candle.  When, therefore, the mi-
         ifij |   ner, with his lamp, is  suddenly
         IB    surrounded with a quantity of ex-
         |nl   plosive  gas,  he  is no  longer in
         3&  danger; because the flame of the
         =jffij[glamp  cannot pass through  the
         a|H§? gauze  to  explode  the  gas.  So
         ^0*  effectual, indeed, is this  simple
 piece of apparatus, that it is found,  even when
 the gauze  becomes red-hot from the quantity of
 explosive gas  burning within  the  lamp, that it
 will not explode that on the outside.  And from
 a knowledge of the simple property of wire gauze,
 which any one may illustrate with a  candle, the
 philosopher has been enabled to construct  an in-
 strument which has  been the means of saving
 the lives of thousands.

 DR. FRANKLIN'S EXPERIMENT ON HEAT OF COLORED
                   CLOTHS.
   293.  On a winter's  day, when the ground is
 covered with snow, take four  pieces of woolen
 cloth, of equal dimensions, but  of  different col-
 ors—viz.  black, blue, brown, and white, and lay
 them on the surface of the  snow, in  the imme-
 diate neighborhood of each other. In a few hours
 the black will have sunk considerably below the
 surface; the blue almost as much;  the  brown
 evidently less; while the white will remain near-
 ly even on the surface.   Thus, it appears that the
 sun's rays are absorbed, and conducted through
 them to the snow, much more rapidly than by the
 other colors.  And  it is, perhaps, for this reason,
 that dark-colored clothes are  preferred in winter,
 while lighter ones  are considered more suitable
 for summer.

 TO BREAK A ROLL OF SULPHUR  BY THE HEAT OF THE
                    HAND.
   294. Sulphur is a very bad conductor of heat.
 If a roll of it be held in the warm hand  for a few
 minutes, it will snap, and break into two or more
 pieces.  If it be held over the flame  of a  lamp,
 it will crumble, and separate still more.   Break-
 ing a roll of sulphur, by the mere  heat of the
 hand, is a very pleasing illustration  of the effects
 of heat.

            prince rupert's drops.
  295. The little glass toys, known by this name,
 may be purchased at any glassblower's, for  about
sixpence a dozen; and  they illustrate, in a very
striking manner, the effects of heat on  bad con-
ductors.  Glass is a bad  conductor,  and this is
a cause  of its being  frequently  broken.  If hot
water, is poured suddenly into a tumbler, the in-
terior expands before the heat can affect the out.
side, and it therefore tears  the  particles on the 
Philosophical. Experiments.
79
 outside from each other ; in other words, it cracks
 the glass.  This is the reason why it is necessary
 to have retorts, and other kinds of chemical ap-
 paratus,  intended to be exposed to much heat,
 made very thin, in order that the heat may pass
 readily through them.   Prince Rupert's drops are
 lumps of glass, let  fall, while melted, into water,
 and are thereby suddenly cooled and solidified on
 the outside, before the  internal part is changed;
 then as this at  last hardens, and would  contract,
 it is kept extended by the arch of  external crust
 to which  it adheres.   Now, if a portion of the
 neck of the lump be broken off, or  if other vio-
 lence be done, which jars its substance, the co-
 hesion is  destroyed, and the whole  crumbles to
 dust, with a kind of explosion.  Any glass  cool-
 ed suddenly when made, remains very brittle, for
 the reason  now stated.  What  is called the Bo-
 logna jar, is a very thick, small bottle, thus pre-
 pared, which bursts by a  grain of  sand falling
 into it.   The  process  of  annealing,  to render
 glass-ware more tough and durable, is merely the
 allowing it to cool very slowly, by placing it in an
 oven,  where the  temperature is caused  to fall
 gradually.
             REFLECTION OF HEAT.
   296. Bright substances reflect heat much bet-
 ter than dark and rough ones, which absorb it
 best.   If a piece of tin be held before the fire, it
 will take  some  time before it becomes hot, be-
 cause  it reflects the light and heat; but if it be
 covered  with lamp-black, it wdl soon become
 heated.  Meat-screens  for our kitchen fires are
 formed on this principle: they reflect the heat on
 to the roasting meat.
              FORMATION OF DEW.
  297. Dew is deposited on the leaves of plants,
 in consequence of their giving out so  great a
 quantity of heat, by  radiation,  on clear, bright
 nights, when there are no clouds, that the tempe-
 rature  of the plant is reduced below that of the
 atmosphere.   When  this takes place, the heat of
 the atmosphere is absorbed  by the  plant, and is,
 consequently, unable to retain so great a quantity
 of moisture; warm air  possessing  the  power of
 holding more vapor in it  than cold air.   The
 manner in  which dew is deposited on plants may
 be seen, by bringing a glass of cold water from a
 well into a  warm room.   The cold water absorbs
 some of the heat of the surrounding air, which,
 in passing through the sides  of the glass, leaves
 behind the  water it contained in a state of vapor :
 tliis will  be seen on the glass.  Water decanters
 show this in summer.
       WATER BOILED BY BEING COOLED.
  298. Water boils, on the  surface of the earth,
 at a temperature of 212 degrees; but on the tops
 of high mountains, where  the weight of the at-
mosphere is somewhat  less, the boding point is
lswer than  212 degrees.  To prove that the pres-
 sure  of the atmosphere has  great influence  over
the  boiling point,  the following striking experi-
ment can be performed :—Fill a Florence flask
about half full of water, and then expose it to the
 heat of a spirit-lamp  until it boils ; when  this
takes place, put a cork in the flask,  and remove it
from the lamp, when the steam wdl  be condensed.
 In a very short  time, however, if the bottle has
been corked tightly, so that the air cannot enter
when the steam is condensed, the water will be-
gin to boil again; and if the flask is placed in cold
water, the boding will be  stdl more violent.  If
the flask be  then  removed from  the cold water,
and placed in hot water, the boiling will cease. ^
   The cause of this, apparent anomaly  is, that
when the flask is placed in cold water, the steam
or vapor in the bottle is condensed; and there be-
ing, therefore, little or no pressure on the surface
of the water, the air having been expelled by the
steam, the heat contained in the water is sufficient
to cause it to boil, and  the additional heat im-
parted to it by the hot water makes it bod rapidly.
When the flask is placed in the hot water, a vapor
is formed, which presses on  the  surface, similar
to the atmosphere; and, consequently, prevents
the liquid boding, except at a higher temperature.
Water will bod at 90 degrees in vacuo, and when
the steam is condensed in the flask, there is a very
good vacuum formed above the water; so that,
indeed, the warmth of the hand  is then  almost
sufficient to make the wa£er boil.
       PRODUCTION OF FREEZING MIXTURES.
   299. Liquids, when they become more dense,
or solid, give out  heat (as in experiment 281;)
solids, when they are rendered fluid, without the
application  of heat, become very cold.   Advan-
tage is taken of this circumstance  to produce
what are termed frigorific mixtures.  The follow-
ing  may be  formed, without much trouble, and
will illustrate the fact very well.
   300. Mix  two parts of snow, or pounded ice,
with one of common salt.  The mixture soon be-
comes liquid, and the temperature falls 41 degrees
below the freezing  point of water.   To produce
this effect, a quantity of the materials must  be
used.  From  a knowledge of the effects  of salt
upon ice, it is frequently used, during a frost, to
melt the  ice upon the pavement, &c.
   301. Mix  sixteen parts of water with five  of
nitre, and five of sal ammoniac, in fine powder,
when the temperature will fall about 40  degrees
below the freezing point.  The former experiment
can only be conveniently performed in the winter;
but this can be produced at any time.
           CONDENSATION OF STEAM.
   302. Water, when exposed to a heat of 212
degrees, is formed into  steam.  In this state, the
particles  that  compose water are so far separated
by heat, that it is found steam occupies the 1,700th
space of an equal weight  of water.  When, how-
ever, the  heat is abstracted, the steam is con-
densed into the original quantity of water from
which it  was  formed.   To prove this, procure a
Florence od-flask, and bod a small quantity of
water in it; when the steam is issuing from the
mouth, put in a  cork, having previously covered
the hand with a towel, to  prevent the steam scald.
ing the hand.  If the flask be now placed in cold
water, and  the cork taken out, the steam will be
condensed, and the water will rush into the bottle,
and completely fill it.  This is sometimes effected
with so  much force, that the flask is  driven out
of the hand, and even broken ; care must, there-
fore, be used in performing the experiment.
             BRIGHTNESS  OF FLAME.
  303. The brightness of a flame depends upon 
SO                              Philosophical Experiments.
the quantity of oxygen  it receives, as may be
shown by a common candle.  If we examine the
flame, we shall perceive, that on the outside it is
bright, while there is a cone, of a dark color, in
the centre : this part,  indeed, is  not a light, as
may be proved  by  holding  a clean card, for  a
minute, in the centre of a flame  ; on withdrawing
it, there will be found  a dark circle—showing
that it was only the outer portion of the flame, in
contact with the air, that was capable of burning
the card.   If all the tallow were consumed, there
would be no smoke; for smoke is  the particles of
tallow, passing off in an unconsumed state.  In
the common lamps, used by butchers,  the waste
in this way is very great; and  before  the intro-
duction of the Argand burner for lamps, this was
an evil that could not be remedied. The inventor
of this burner considered, as the brightness of a
flame, and the complete combustion of the ma-
terial, depended upon the supply of oxygen, that
if, instead  of having the wick of a lamp in a
solid  piece, like the wick of a  candle, it were
made hollow, or so that it might be placed round
a hollow cylinder, the, r»mbustion would be more
perfect.   He  accordingly constructed  a  lamp,
with  a burner of this kind, and the effect he an-
ticipated was produced: at the  same  time, he
found that, by using a glass cylinder,  similar to
the common gas-glasses, that the draft was pro-
moted, and a greater supply of oxygen afforded
to the flame.  This invention has since been uni-
versally adopted; and gas-burners are constructed.
on a similar principle. In lamps, the wick, which
is  hollow, is placed round a brass tube, and can
be raised, or dressed, at pleasure; the  lower end
is  placed in oil, which it  absorbs,  and  brings
gradually in  contact with the flame ; a glass  is
placed round this, and  air is supplied to the inte-
rior of the flame through the tube, and  to the ex-
terior in the ordinary way.

             HOW A CANDLE  BURNS.
   304. The combustion of a candle (see  Intro-
duction) illustrates many natural  laws  in a sim-
ple manner.  When the wick is lighted, it melts
a portion of the tallow immediately beneath, and
forms a little cup, in which  a quantity of the li-
quid tallow continues.   The wick,  by capillary
attraction, draws up a  portion of this tallow,
which enters the flame.  Here  it becomes a gas,
and combines with the oxygen of the atmosphere,
                 j.    forming carbonic acid. A
           -^£    portion of the gas formed
         «Mg^    from   the  melted  tallow
 '^k^^l&k^ )l    may be ignited away from
 ^|%      ^^feinfiL    the candle, by  placing a
   '        ^S&l^ sma"  time> rather wider
             ^a£r    than the bore of a piece of
                     tobacco-pipe, in the dark
                     part of the flame: the gas
                     fwill pass through this, and
                     if a light be applied at the
                     other end, it may be igni-
                     ted.  The existence of the
carbonic  acid may also be shown by  holding  a
lighted match a little above the candle, when the
former will be extinguished.
   305. The benefit of  the Argand burner wdl be
seen immediately, if we place a card at the bot
torn of the tube, so  as to stop the  supply of airt^
the interior of the flame.  The consequence wil
be, that much of the oil will be unconsumed, and
the lamp will give off a great deal of smoke.

       TO PROVE THAT FLAME IS HOLLOW.
   306. Drop,  carefully, some  spirits of wine on
the surface of water, within a small hoop, float-
ing in a large  basin ; set the spirits on fire, and
then introduce the  finger under the edge  of the
hoop, and up through the water, into the interior
of the flame.  If this be done carefully, no heat
will be felt, except the finger is raised sufficiently
high to touch  the film of flame itself.
        REQUISITES TO PRODUCE FLAME.
  310. There are several requisites for the pro-
duction of a flame, and without  which, it can-
not exist.  The first is, that oxygen must be pre-
      OXYGEN NECESSARY FOR COMBUSTION.
  307. Put a little water in a soup-plate, and on
it place a piece of wax taper,  or  candle  lighted,
so that it may swim on the surface; then take a
common  beer tumbler, and cover the taper over
with it: the consequence wfll  be,  that,  after a
short time,  the light  will  be  extinguished, and
the water from the plate will rise a little way in
the tumbler.   This experiment shows the neces-
sity of oxygen in combustion ; for  directly the
light had absorbed all of  it  that was contained in
the air under the glass, it went out.   The rising
of the water is in colrsequence, principally, of the
cooling  of the  air  again  within the tumbler;
while it was  heated, a  portion was driven out,
and on cooling, the water supplies  its place, be-
ing forced in by  the  pressure  of the air on the
outside.
  308.   Another  experiment,  illustrating  the
same fact,  may  be  performed in the following
way:—Take a large  wide-mouthed  bottle, with
the bottom broken off, and place it a httle way in
a pail of water; then hold  the nostrils close with
one hand, and breathe the air in the bottle sever-
al times.   By this means, the   greater portion of
the oxygen contained in the air m the bottle will
be absorbed, as we all  abstract oxygen from the
air every time  we breathe.  When the  mouth
is withdrawn from the mouth of the bottle, place
the hand over  it, and  get a person to  introduce
into it a lighted match, which, it will  be seen, is
instantly extinguished.   Instead  of using a bot-
tle, a  better piece of apparatus is a common gas-
receiver.
  The rationale  of both these  experiments is
nearly the same.  In  the first,  the burning taper
absorbsMhe oxygen, and forms carbonic acid gas:
in the second,  a similar effect is produced by the
carbonaceous  matter of the blood uniting with
the air in the bottle, and forming the same gas.
This gas  will neither support life nor flame: and
if, therefore, a  living animal, or a light, be intro-
duced, the first should be killed, and the latter ex-
tinguished.
  310. The use of the bellows, in making a fire
burn brighter,  is  simply because they  force a
greater quantity of air, and consequently, more
oxygen,  against  the  burning  material  than it
would otherwise receive.   The damper  of a
chimney  produces an effect precisely opposite, by
lessening the supply of air sent  to the fire. 
Philosophical  Experiments.                             81
  sent, in some form, to  support  combustion; and
  the second, that the  body  must  be heated to a
  high degree of temperature.   There is one excep-
  tion to the first of  these requisites,  since bodies
  will burn in chlorine gas, which it has been pro-
  ved does not contain oxygen.
   311. Flame is the intensely heated particles of
  a burning body, in the act of umting with oxygen.
  Thus, in the  case of a common lamp, the oil  is
  absorbed by the wick, and the particles, when
  they enter the flame, become  white-hot, and then
  pass off as a  gas.  Experiment 308  will show
  the necessity for oxygen to support combustion ;
  and it is, therefore, only necessary  to prove, that
  whatever  lowers  the temperature of a burning
  body destroys-flame.
   312. Get a piece of  wax candle,  and pull
  all   the wick  out except  one  thread,  so that,
  when this is lighted, the flame  shall be very
  small.  Then make a little ring  of iron wire,
  about  the eighth  of an  inch  in   diameter,
  which affix  to a handle.   It  will be  found
  that if, when the wick is lighted, this wire ring  is
  passed over, so as  to go below  the flame, that it
  will be immediately extinguished.  This is  in
  consequence of the iron wire being a good conduc-
  tor, ahd, therefore, the  heat of the flame is carri-
  ed  awa>j so quickly by  it,  that  the wax cannot
  become white-hot,  and unite  with the oxygen of
  the atmosphere; the  flame is,  therefore,  extin-
  guished.  The was, in a gaseous  state, will not
  combine with  the oxygen, unless at a high degree
  of  temperature ; and it is evident, therefore, that
  anything which cools the wax must prevent the
  combination taking place.
    313. A  similar  effect to  that produced by the
  cold ring of wire may be shown, by merely bring-
  ing a penny piece, or other good metafile conduc-
  tor, in contact with the flame.  When the candle
  has been  extinguished by this means,  ligkt  it
  again, and make the ring of wire hot; then passit
  again over the flame, and it will not put it out.—
  This shows, satisfactorily, that, in the former in-
  stance, the effect was  produced simply in conse-
  quence of the rapid absorption of the heat by the
• metal.
          INTENSE AND RAPID COMBUSTION.
    314. To illustrate the sudden combustion  of
  bodies, through* intense chemical action, the fol-
  lowing experiment may be  performed ?—Mix  a
  little chlorate of potass, about the siz"£ j&a. pea,
  with the  same quantity of  loaf sugar, having
  previously reduced them each to powder.  Place
  the mixture on a piece of tile, or iron, and dip a
  glass rod into a bottle of sulphuric acid ; let the
  acid from the rod drop upon the mixture of chlo-
  rate of potass and  sugar, and they wdl imme-
  diately flame.
    Care must be taken in making this experiment,
  not to use a  large quantity of  the materials em-

                                         T H E
 ployed : the effect will be produced as well with
 the  quantity we  have mentioned, as if it were
 twenty times as great, and there is no danger at-
 tending  the experiment;  but if a  large quantity
 be used, the matter will  perhaps fly  about,  and
 might occasion injury.  The face must be kept
 from the mixture when  the  sulphuric acid is
 dropped into it, as it bursts into flame almost in-
 stantly.
   The  rationale of the experiment  is, that the
 chlorate of potass and the sulphuric  acid have a
 powerful affinity for each other, and when they
 come in  contact, they combine with great in-
 tensity ; -much heat is, therefore, given out—suf-
 ficient to ignite the materials.   The loaf sugar is
 used, becaaa^feircbuiBis readily, and  thus affords
 material for the potass and acid to act  upon: that
 this is its only use, may be seen by mixing some
 of the chlorate of  potass with  sulphuric acid by
 themselves, when the effect, will more resemble an
 explosion than the combustion that ensues in the
 former instance.
   315. If equal parts  of  chlorate of  potass and
 camphor, mixed togethejAe touched with a drop
or two  of sulphuric ac^^the  camphor  will in-
 flame.  The same effect takes place with spirits
of wine, or charcoal, instead of camphor.
   These experiments must be conducted with
care.

 DETONATION  WITH  SULPHUR  AND  CHLORATE OF
                    POTASS.
   316. Into a mortar put two or three grains of
 chlorate of potass; and after having reduced it to
 powder  with the pestle, introduce  some  flour of
 sulphur, or  brimstone, very finely  powdered.  If
 the two be now rubbed together, they will be de-
 tonated, with a smart noise, like the cracking of a
 whip ; and  this may be repeated  a dozen times,
 with the same quantity of materials.  This is a
 very pleasing experiment, quite unattended with
 danger of any kind.

        VIVID COMBUSTION  UNDER WATER.
   317.  Place a small piece of phosphorus, and a
 few grains of chlorate of  potass, in a tumbler, or
 other vessel, and  pour on  them, gently, some hot
 water.  Tins will  inflame the phosphorus, and its
 combustion, being supported by the  chlorate of
 potass, a very pleasing and vivid light will be wit-
 nessed under the water.
   318.  If a little oil be placed on the top of the
 water in the vessel, it will be inflamed.

       DETONATION OF CHLORATE OF POTASS.
   319.   Many substances, when heated to a cer-
 tain degree of heat, change  their form,  with an
 explosion.   If a  little  chlorate of potass be put
 on a fire-shovel,  and  placed over the fire, when
 the  shovel has received a certain heat, the chem-
 ical detonates, with a sharp report.
                                    •
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  This splendid work, which is as eloquently written as it is
lucidly arranged, embraces the following  subjects :—Intro-
ductort  Remares;  Mathematics —Arithmetic,  Geo-
metry, Al««hra; Of Observation and  Experiment Me"
chanics. Astronomy, Optics, Hydrostatics, 8tc.,  Electricity.
Magnetism, Chemistry :  Giving a Historical Sketch andOen-
eral Summary  of  the Principles of each of the  foregoing
branches of Science.      ...,,..     .<
  Price for the whole, including Lardner's Lectures, 25 cents
per single  copy.  Postmasters  and  others will  receive  nv«
copies for $1.

NO.  IV. OF  USEFUL  BOOKS  FOR  THE
                    PEOPLE.
    GRIFFITHS CHEMISTRY AND DALTON'S
                   PHILOSOPHY.
Chemistry of the Four  Ancient Elements,
        FIRE, AIR, EARTH  AND WATER.
Founded upon Lectures delivered  before her Majesty the
       ren, by  Thomas  Griffiths, Lecturer on. Chemistry[of
       Bartholomew's Hospital. Illustrated by upward of
   70 Engravings.
  This Essay on the Chemistry of the Four Ancient Elements
is chiefly intended for those who have not studied the scitnee.
Abstruse details and theories are avoided, useful and interest-
ing information supplied, and instruction united with enter-
tainment.   Explicit  directions  are  given respecting the per-
formance of the Experiments.
 The Book of Philosophical Experiments,
Illustrating the Principal Facts and  Curious Phenomena
   of Electricity, Galvanism,  Magnetism, Chemistry,  Op-
   tics, Heat, <J-c, with Introductory Observations on each
   Science, and upwards of  300 Experiments.  By J. S.,
   DALTON.
  " A person who Performs an Experiment and thoroughly
understands the nature of it, will  hardlv ever forget the gfrin-
ciple it illustrates.  It has been the object oi the wrrtey to in-
troduce only such experiments as may be performed wjfth sim-
ple apparatus, and such as may be  easily and cheaply  pro-
cured." Extracts from the Preface.
  O" The two books above named have met. with a rapid
and extensive sale In England, and continue to" be very pop-
ular and in great demand, notwithstanding each one is sold at
about four times the price of the <.ost of both works together
in the edition printed in the " Series of Useful Books for the
People."
  The above works are neatly printed, on clear new type with
about 150 Engravings, and together are sold at the exceedingly
low price of 25 cents.  Five copies for $1.
                         GREELEY & M'ELRATH.

 NO.  V.  OF  USEFUL  BOOKS  FOR  THE
                     PEOPLE.

   PRINCIPLES  OF  POLITICAL ECONOMY,
 Or the Laws of the Formation of National Wealth,
Developed  by means of the Christian Law of Government;
  being the substance of a case delivered to  the Hand-loom
  Weavers' Commission.
            BY WILLIAM ATKINSON.
With an Introduction, Treating of the present state of
   the Science of Political Economy, and the Adaptation
   of its Principles to the Condition of our own Country
   and the upbuilding of its Prosperity.
               By Horace Greeley.
   The work of Mr. Atkinson  is one of the most instruc-
 tive and--Imirable that  has yet  appeared on the  important
and di^liaft*subject of Political  Economy—difficult, not so
much in'its essence, or in the nature of its laws as in the ap-
plication of those laws to  the dissimilar, complicated, vary-
ing circumstances  of different nations and periods.  The
laws educed by Mr. Atkinson  are so just, so clear, so vital,
and yet so strangely overlooked  by those who arrogate to
themselves the title of the  Political Economists in our day,
and who resolve all Political Economic Science into doing
nothing—of whose universe Somnus is God—and who ele-
vate the maxim of the counter, ' Buy where you can cheapest'
to the summit  of attainable wisdom ar.d statesmanship.  In
their  eyes the cheapest possible production—coin being the
unerring standard—is the great end  of human effort;  and he
who is outstripped in the  headlong race is doomed to' ruin
and starvation  by the law of envious strife and depressing
competition, which  they blasphemously represent as  the or-
der of Providence!  These are the views put forth by the
Political Economists enthroned in our Colleges and Seuates.
as liberal and enlightened .'—at least, these are the inevitable
results from those  views.  It was needed that the specious
fallacies put forth bv these teachers should be scrutinized and
confuted, and this has been done in a  masterly manner by
Mr. Atkinson.  As an elementary treatise on Political Econ-
omy—as a refutation of the doctrines of ' Free Trjde' theo-
rists—nothing equal to his work has ever appeared on either 
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