1894.] 
163 
[Bache. 
The Secret of the Brownian Movements. 
By R. Meade Bache. 
{Read before the American Philosophical Society, April 20, 1894..) 
We first hear systematically of molecular movement, as a thing 
directly visible, from the writings of the distinguished botanist, Dr. 
Robert Brown, embodied in a paper entitled, "A. Brief Account of 
Microscopical Observations, Made in the Months of June, July, 
and August, 1827, On the Particles Contained in the Pollen of 
Plants ; and On the General Existence of Active Molecules in Or- 
ganic and Inorganic Bodies." 
Of course we know inferentially, through physics and chemistry, 
that the particles of bodies move under the influence of various 
extraneous forces, and we know, too, that, in the case of matter in 
a gaseous form, they move also under the influence of force inherent 
in their own constitution. But the movements of which I am about 
to speak relate to the visible behavior of what should be termed 
inert matter, if that expression were not begging the question of 
whether or not it has in itself power of motion, and thus deciding 
it in favor of the view that it has not, but is moved by extraneous 
force. We must begin, therefore, as is but right, by assuming that 
it is an open question as to whether or not certain particles, in 
aqueous suspension, call them molecules or otherwise, as one pleases, 
organic or inorganic, but not endowed with animal life, are of them- 
selves capable of movement, or are moved by some extraneous force 
or forces at present unknown. The solid ground of fact from 
which we start is that, under the conditions mentioned, they do 
move, for that is undeniable and admitted. 
Dr. Brown soon discarded, through disproving in the course of 
his investigations the surmise which he had made, that the particles 
of pollen indicated by their motion a mode of function analogous 
to that of spermatozoids, and rested eventually in the conclusion 
that particles moving in aqueous solutions are not confined to 
organic bodies or to their products. He had conducted a series of 
experiments on finely crushed glass, on simple earths and metals, 
with many of their combinations, and rocks of all ages, includ- 
ing those in which no organic remains have ever been found. 
Moving particles presented themselves in each of the constituent Bache.J 
164 
[April 20, 
minerals of granite. Even a crushed fragment of the Sphynx gave 
the same results. He tried substances of both aqueous and igneous 
origin, travertine, stalactites, lava, obsidian, volcanic ashes, meteoric 
matter, manganese, nickel, plumbago, bismuth, antimony, arsenic, 
asbestos, actinolite, tremolite, zeolite, and steatite. He tried par- 
ticles of wood, living and dead, linen, paper, cotton, silk, wool, 
hair, and muscular fibre that had been exposed to fire under the 
blowpipe, douched with water, and submitted to immediate exami- 
nation. The particles from all these substances exhibited as vivid 
movement after as before they had been so treated. 
Dr. Brown, about twelve months later, went on further to declare, 
under the head of "Additional Remarks on Active Molecules," 
that:-" The result of the inquiry at present essentially agrees with 
that which may be collected from my printed account, and may be 
here briefly stated in the following terms; namely, that extremely 
minute particles of solid matter, whether obtained from organic or 
inorganic substances, when suspended in pure water, or in some 
other aqueous fluids, exhibit motions for which I am unable to 
account, and which, from their irregularity and seeming independ- 
ence, resemble in a remarkable degree the less rapid motions of 
some of the simplest animalcules of infusions. That the smallest 
moving particles observed, and which I have termed Active Mole- 
cules, appear to be spherical, or nearly so, and to be between 
1-20,000 and 1-30,000 of an inch in diameter; and that other par- 
ticles of considerably greater and various size, and either of similar 
or of very different figure, also present analogous motions in like 
circumstances. I have formerly stated my belief that these motions 
of the particles neither arose from currents in the fluid containing 
them, nor depended on the intestine motion which may be supposed 
to accompany its evaporation. These causes of motion, however, 
either singly or combined with ethers,-as, the attractions and repul- 
sions among the particles themselves, their unstable equilibrium in 
the fluid in which they are suspended, their hygrometrical or capil- 
lary action, and in some cases the disengagement of volatile matter, 
or of minute air bubbles,-have been considered by several writers 
as sufficiently accounting for the appearance. Some of the alleged 
causes here stated, with others which I have considered it unneces- 
sary to mention, are not likely to be overlooked or to deceive 
observers of any experience in microscopical research; and the 
insufficiency of the most important of those enumerated, may, I 1894.] 
165 
[Bache. 
think, be satisfactorily shown by means of a very simple experi- 
ment." [Dr. Brown here alludes towhat he details at considerable 
length, as to the trituration together of oil and water, so as to 
secure, in one case, by a large proportion of water to oil, lacunse of 
water of various sizes, filled with particles, protected from evapora- 
tion by the oil, and in the converse case, by a small proportion of 
oil to water, to secure minute oil-drops on the surface of the water, 
some of which drops were not larger than the solid particles in the 
water.] 
These passages from the writings on this subject of so skillful, 
careful, and conscientious an observer as Dr. Brown was, excite 
regret that he had not pursued the subject further. He would have 
found, among other things, a much simpler and more lasting 
method of excluding air from the drop under observation, than 
that which he adopted in forming lacunae of aqueous fluid pro- 
tected from evaporation by immersion in oil. He would in all 
probability have discovered the real cause of the motion of or- 
ganic and inorganic particles in aqueous fluids. But he dropped 
the subject, perhaps because he was obliged so to do on account of 
the pressure of other research, when he had obtained some valuable 
results, leaving his investigations negative as to conclusions. He 
doubtless began with the supposition, which seems common at first 
to all investigators of the subject, that evaporation might represent 
a shock that would move the particles suspended in water; that 
vibration from mechanical sources might account for the move- 
ments; that currents set up in the drop of water by differences of 
temperature in it or slight differences in the temperature of the air 
surrounding it, might account for them ; or that mutual attractions, 
derived from gravitation, and inherent in the relative density of the 
particles themselves, might do so ; and he found, as every one else 
will find who experiments in these directions, that the movements 
go on independently of currentsand independently of heat, generat- 
ing or not generating currents, independently of light, and in the 
case where the particles represent an extremely fine division of 
matter, and are at the same time of low specific gravity, even inde- 
pendently of terrestrial gravitation ; and without relation to their 
specific gravity, with the force of their mutual gravitation entirely 
masked. 
Some of these things Dr. Brown implies that he saw in the course 
of his investigations; others he could hardly have failed incidentally Bache.] 
166 
[April 20, 
to see, but one that I have mentioned, he did not rightly interpret. 
It was a distinct lapse in his acuteness of observation, that involved 
in his mention, without perception of its deepest significance, of 
minute drops of oil, some of them not exceeding the solid particles 
themselves in size, standing nearly or altogether at rest on the sur- 
face of water ; the pointing of which fact is very clear, one which, 
had it been known to me, would have induced me to try oil as well as 
water among the first instead of among the last of my own experi- 
ments, as actually happened in their sequence. 
To afford facility for the fullest comprehension of the subject, it 
becomes proper here to resume the historical tenor of our way with 
a brief account of some of the views of Herren Wiener, Exner, and 
Schultze, more recent than those of Dr. Brown. In order to avoid 
the responsibility for the necessary condensation, I prefer to quote, 
as follows, from the summary of their views in the Jahresbericht of 
1867 : 
" Then Chr. Wiener, from whose account the preceding historical 
remarks are quoted, instituted microscopic observations of these 
movements, and came to the conclusion that this trembling, irregu- 
lar, unsteady motion of solid molecules, which alter their direction 
in the briefest fraction of time in their zigzag course, has for its 
basis the continual movements which, by virtue of their molecular 
constitution, belong to fluids. He learned through his investiga- 
tions (1) that the movements are not those of infusoria; (2) that the 
movement is not communicated to the fluid ; (3) that the trembling 
movement is not in any way derived from the varying attraction 
and collision of the various oscillating molecules with one another; 
(4) that the movement is not derived from changes of temperature; 
(5) that, also, the movement is not derived from evaporation. 
Consequently, there remained to him nothing to ascribe as the 
cause of the peculiar movements but the property of the fluid itself. 
l'his explanation received direct confirmation from Wiener's obser- 
vation, that the amount of the movement has a certain relation to 
the size of the molecule. Lately S. Exner has extended the investi- 
gations of Wiener. Among the various influences which Exner 
sought to test with reference to the molecular motion was whether 
either chemical causes or mechanical ones, such as pressure, 
vibration, and so forth, could in any way produce an acceleration or 
a retardation of the effect. Only by exposure to light and heat 
did the motion become accelerated, and then in such a manner as, 1891.] 
167 
[Bache. 
in the case of glycerin, the molecules of which under ordinary con- 
ditions show scarcely any, if any movement at all, to exhibit it 
clearly when warmed up to fifty degrees centigrade of temperature. 
Exner also examined into the properties of fluids in which solid 
molecules remain suspended. The results of his investigation re- 
solve themselves into the following points: (i) The liveliness .of 
the molecular movement is heightened by light and heat, and by 
radiant as well as by conducted heat; (2) one of the consequences 
of the molecular movement is, that the molecules, in a specifically 
lighter fluid, not only do not sink to the bottom, but overcome the 
force of gravitation to such a degree as to spread themselves equally 
throughout the fluid ; (3) the velocity of this scattering is as the 
intensity of the molecular movements influenced by light and heat. 
It should be mentioned here that Fr. Schultze had already stated 
that substances, when most finely divided, especially such as seemed 
under the microscope to be amorphous, and exhibited the brownian 
movements, remain suspended in pure water and in many other 
fluids for days, weeks, and months at a time, so that the fluid con- 
taining them presents a cloudy, or at least an opalescent appear- 
ance." 
The account of the views of Herr Wiener in the Jahresbericht 
makes an important omission. It devotes itself chiefly to reciting 
what, in his view, does not cause the brownian movements, but 
does not mention precisely to what he does ascribe them. Herr 
Wiener says, in the last paragraph of his article, in Poggendorf's 
Annalen, 1863:-"The weight of the preceding conclusion, that 
one cannot ascribe the trembling movement to any exterior cause, 
is very greatly added to by the ascertained fact, that the diameter 
of the similarly moved water masses is so small that it nearly corre- 
sponds with the wave-length of red light, and still more closely with 
that of radiant heat." This passage gives the keynote to his views 
on the brownian movements. After a most elaborate series of 
experiments, in which he measured on a micrometrically divided 
glass slide, with the addition of diagonal lines, and by watch, the 
range and the time of the movements, he reached his most impor- 
tant conclusion, that, because the dimensions of the aforesaid wave- 
lengths of light and heat have a certain close correspondence with 
the diameters of the minutest particles and water masses, they form 
the moving impulse of the motions of the particles. He pictured 
to himself that the aether surrounding the particles, being continu- Bache.] 
168 
[April 20, 
ous with the aether of space, acts, through the rays of light and heat 
on the particles and minute water masses, generating in their inter- 
action, as the visible resultant of the forces in play, the movement 
of the particles in suspension in aqueous solutions. But, if the 
cause of the movements assigned by Herr Wiener were the true 
one, the same cause ought to be operative in the case of alcohol 
and in that of the fixed and volatile oils: but it is not. This con- 
clusion of Herr Wiener's seems to me to be derived from the un- 
satisfactory fact of a coincidence, of which kind of proof we habitu- 
ally perceive more than enough to obscure, bewilder, and often to 
baffle our feeble efforts to penetrate beyond the veil of phenomena, 
of things as they seern, to the everlasting noumena, things as they 
are, near the inscrutable throne of nature. 
I will not weary my hearers with the recital of the numerous 
details of my own experiments, the names of the substances that I 
tried, the modes in which they were treated, the manipulations of 
various sorts necessary to the prosecution of the work. Every one 
knows the difficulties that will arise in new investigations, which 
will themselves suggest the means of countervailing them as the 
work proceeds. In this particular case one difficulty was to obtain 
finely enough divided matter in other liquids besides water. It 
may be interesting to mention that I did not read anything on the 
subject until my own experiments were nearly finished. By this 
course I avoided any possible bias expressed or implied in the 
directions to be pursued and the conclusions to be drawn, and I 
had ultimately the satisfaction to perceive, as I had often before 
observed, how, owing to the constitution of the mind, men neces- 
sarily follow the same general and often particular track in their 
procedures. It is not in the course they follow, that they differ 
much, but in the conclusions which they reach in pursuing what is 
virtually the same way. Fortunately for me, constrained to be 
absent for months in the field on geodetic duty, and at all times 
constantly engaged at my profession, night still lent itself to my 
slowly accumulated results. That the investigation was most inter- 
esting, I need hardly say. 
As electric currents have been demonstrated in the human body, 
I naturally thought that all slight differences of tension between the 
liquid and particles, or in the liquid itself, might set up electric cur- 
rents. Therefore I passed the galvanic current through liquids filled 
with particles, watching them carefully. There was not the slightest 1891] 
169 
[Bache. 
visible effect thereby produced on the movements of the particles. 
If the movements had been produced by electric currents, then so 
strong a current as I often passed through a drop of water ought to 
have left no manifestation of movement possible from the neces- 
sarily weak, if actually existing, currents supposed to be actuating 
the particles. The molecular movement, so-called, is, as described 
by Herr Wiener, a zigzag one, but that term does not exactly con- 
vey the peculiarity of the motion. It is a combination of a jerky, 
wobbling movement, performed within determinate bounds, entirely 
irrespective of the sweep of currents in the liquid, or even of the 
effects in some cases of terrestrial gravity, and in no case seemingly 
affected by the influence of local gravitation of particle to particle. 
Taking the vermilion of the sulphide of mercury, as finely divided 
as it can be made, and turning the microscope at even a slight 
angle from the vertical, the effect of terrestrial gravity on the par- 
ticles becomes at once apparent, but taking the carmine, reputedly 
made of cochineal, the particles are not affected in the slightest 
degree by terrestrial gravity. Of course it is hardly necessary to 
say that any solution should be weak, in order to allow the sub- 
stances under examination to receive the finest division of which 
each is susceptible as dissolved. The specific gravity of sulphide 
of mercury is not only much greater than that of cochineal, but 
additionally it is not susceptible of nearly so minute division as 
cochineal is. Of all substances that I experimented with, cochineal 
seemed to be that which is capable of the finest division, and at the 
same time of the most brilliant illumination. Gamboge, which 
appears to be the substance of predilection among many persons to 
experiment with for the brownian movements, offers nothing com- 
parable to the brilliancy and the fineness of particles of carmine 
derived from cochineal. With a weak aqueous solution of carmine 
one may see by daylight, on a background of faint blue, and by 
ordinary artificial light, on a golden one, thousands of tiny parti- 
cles, bright as sparks of ruby, shimmering and performing their 
independent evolutions over the field of view. 
Just as one sees a boat managed by an unskillful helmsman pursue 
its erratic way in going about, being taken aback, or heeled over by 
a flaw of wind, without for a moment attributing its movements to 
currents or any other cause but the true one, so the constant observer 
of the brownian movements knows full well that the particles them- 
selves are moving, not being moved by currents or by gravitation Bache.] 
170 
[April 20, 
towards the earth or among themselves. He, from the first, recog- 
nizes the fact that the smaller the particles are, the more vivid is their 
movement. He recognizes another, that, although many large par- 
ticles do not, as masses, move at all, yet the larger masses are all 
alive, as it were, with smaller ones, seen clearly around their periph- 
ery, on the silhouette of which they are seen plying like banks of 
oars in an ancient trireme. He is struck with and convinced of 
still another thing, that whereas one might expect to find that all 
particles would manifest an attraction for each other through gravi- 
tation, and that the larger and largest, but all in proportion to their 
relative size, would attract and absorb the relatively smaller and 
smallest ones, nothing of the kind occurs, but the smaller, down to the 
smallest, go their own way, sometimes even touching the largest 
and bounding off and away as if they do not, as indeed they do not 
visibly, submit to the force of gravitation. Of course they cannot 
escape the influence of gravitation, whether terrestrial or among 
themselves, but the effect of gravitation upon them is masked, in 
what manner will appear later. 
It seemed to me that magnetic earth-waves might affect particles 
in such delicate suspension as those of which we are speaking, some 
of which are no greater in diameter than 1-100,000 of an inch, seen 
under various powers capable of magnifying from 650 to 1300 
diameters. Accordingly, I have placed the particular fluid under 
examination in the lines of force of a permanent magnet, with the 
magnet on one side and the keeper on the other of the drop of fluid. 
Concentrating the gaze on individual particles, to observe if their 
movement were modified, and then on others in succession, and 
often repeating the experiment, nothing could be observed other 
than the movements existing before the magnet had been brought 
into requisition. The only kind of particles susceptible to the influ- 
ence of the magnet were those of precipitated iron, but iron is 
always obedient to the magnet. 
Heat I applied in various ways, either irregularly or in an en- 
deavor to distribute it as equably as possible on the glass slide on 
which the particular experiment was made. Mere currents are set 
up during the adjustment of temperature from radiation. At the 
same time one can observe and differentiate the motions due to the 
brownian movements, the motions along currents, and also the 
motions from terrestrial gravity, if any, exhibited by particles, if 1894.] 
171 
[Bache. 
the specific gravity of the substance be great, and the microscope 
be set at an angle with the vertical. 
Cold I also applied, putting the slides with their cover-glasses in 
a freezing mixture of broken ice and water, and reducing them to a 
very low temperature. Still the movements went on as apparently 
unmodified as ever. Herr Exner says, it will be remembered, that 
glycerin, which under ordinary conditions shows absolutely none, 
or almost no molecular movement, shows it clearly when warmed up 
to the temperature of fifty degrees centigrade. In all the finely 
divided bodies, however, which I examined, there seemed to be 
no increase or diminution in the intensity of the movements, cor- 
responding with their alternate subjection to heat and cold. There 
were occasions in which I thdught that I observed acceleration from 
light* but I always ended by imputing it to the force of imagina- 
tion, and if it were not justly ascribable to that cause, the fact that 
it could be so ascribed, is proof positive that if, through the influence 
of light and heat, any intensification of the movements of the parti- 
cles took place, it must have been very small. Moreover, the evi- 
dence is certainly here, to show that even if the movement were 
intensified by light or heat, that was the only influence that could 
be ascribed to them, that light and heat could not be deemed the 
cause of the movement. And lastly, Herr Wiener's micrometric 
measurements of the range of movement at different temperatures 
completely bore out this conclusion. 
The theory of Herr Wiener, that the movements are due to the 
action of the red-wave of light and heat is refuted by the single 
fact that, as I have proved by experiment, one may interpose at 
pleasure between the source of light or heat and the particles, either 
a violet glass or a red glass, without being able to observe the 
slightest alteration in the movements, either as to their range or 
their velocity. That is to say, red rays may be either partially 
excluded or selectively admitted, without diminishing or increasing 
the liveliness of movement. Hence light can have nothing to do 
with the phenomenon under discussion, and I have just shown, 
through the citation of the freezing mixture experiment, that heat 
can have nothing to do with it. 
I have reserved to the very last the discussion of the question as 
to whether or not the shock, if any there be, from evaporation can 
have anything to do with the movements, although this was a point 
that entered into my first investigations. I have reserved it to the Bache.] 
172 
[April 20, 
last, because its discussion requires more than the brief space which 
I have devoted to the previous individual results, and because it 
leads directly to the conclusion that I have finally reached as to the 
true cause of the movements. I started out with the conception, 
which it seems is common to every one, that evaporation might be 
accompanied with a series of minute explosions, which produce 
shocks that manifest themselves through the mass of an aqueous 
solution, in the form of minute movements of finely divided matter 
held in unstable equilibrium by suspension in the fluid, and that 
these, escaping cognizance from any ordinary observation, might 
be visible as such, or in their effects, through the instrumentality of 
high powers of the microscope. I had come to believe long before, 
from observation and experiment, that no tremors from mechanical 
agency or any other, except perhaps from evaporation, could pro- 
duce the peculiar movements known as brownian, and finally it 
remained to discover if this or any other intrinsic cause were at 
work that would account for them. 
At this point I encountered an obstacle. My high powers of the 
microscope were both water-immersion lenses. It seemed, there- 
fore, that even when I had had the drop of liquid under observation, 
sealed beneath a cover-glass, I might have included, by the use of 
the water-immersion lens itself, an evaporating surface which might 
have produced the optical illusion of the movement of the particles 
in suspension. I proceeded, however, with my experiments, upon 
the assumption that this, as the event proved to be the case, was 
not true, and meanwhile procured from Vienna a one-fifteenth dry- 
lens by Reichert, the highest power of dry-lens that he makes. 
I had already obtained for high-power lenses a film of liquid thin 
enough to be observed through all its strata, free of air within the 
cell, and protected from evaporation by being hermetically sealed. 
Any ordinary manufactured cell is too deep, and with all precau- 
tions taken contains a little air. On the other hand, the mere 
cover-glass superposed on a glass slide contains too slight a 
depth of fluid. I made a cell by using gum-shellac traced in a 
circlet on a glass slide, which cell, when partially dried, is filled to 
the brim with the liquid to be observed upon, whereupon the cover- 
glass is pressed into the yielding gum, thereby expressing the con- 
tained air with the superfluous liquid, when the product, dried over 
night, is fit for use on the following evening. One slide, prepared 
in this manner and filled with a slightly tinted solution of carmine 1894.] 
173 
[Bache. 
from cochineal, had been observed upon by me for weeks, with a 
one-tenth water-immersion lens, and afterwards, upon the arrival of 
the one-fifteenth dry-lens, was observed upon without showing any 
variation in the range and vividness of movement of the particles 
subjected to examination. I have even covered the whole micro- 
scope with a pall of thick, black, woolen cloth, so that not a ray 
of light could enter it, either through the cover-glass or the eye- 
piece, and then carefully placing the eye close to the eye-piece, 
have suddenly thrown light upon the cover-glass, when the brownian 
movement among the particles was perceived in as active play as 
ever. I have therefore concluded, from all these experiments, that 
neither heat nor light, nor electricity, nor magnetism, nor mechan- 
ical shock, nor finally evaporation, is operative in producing the 
movements; in a word, that the particles move uninfluenced by 
these forces. I am therefore constrained to believe, upon the basis 
of the information that I have obtained in the manner described, 
that it is not the particles which are moved by their own energy, 
or moved by any energy directly imparted to them from outside 
sources, but that it is the fluid that moves them. 
If their own energy moves the particles, we should see them at 
the same time obedient also to the law of gravitation among them- 
selves, manifested as the resultant of whatever, forces are in play, 
whereas, although they must be obedient to the law of gravitation 
among themselves, its effects, and generally, as well, those of terres- 
trial gravity, are so masked as not to be at all perceptible. Now, 
when we consider how minute all of these particles are, and yet 
that they move apparently unhindered with such constancy and 
force, it ought to be apparent, I should think, that they have no 
self-motive power. However erratic the paths of individual parti- 
cles may be, the likeness among the movements is extraordinary, so 
almost identical in every case, varying in greatness of range and 
rapidity only in inverse ratio to the size of the particle, that we can- 
not conceive of selfactuated particles so behaving; for relative 
greatness of size in self-actuated particles ought to coincide with 
relatively greater, not relatively less, energy of movement; whereas, 
here the case is reversed. But there are other facts that I have ob- 
served through experiment, which also prove what I say. In 
alcohol, and as far as my experiments go, in fixed and volatile oils, 
the brownian movements are not observable, and yet the micro- 
scope plainly reveals that the movements of foreign bodies in all Bache.] 
174 
[April 20, 
these is as free as in aqueous solutions; and I think more so. So 
molecular movements of solid particles in suspension in aqueous 
fluids must take place perforce of the constant repulsions of the con- 
stituent molecules of the particular liquid present-water. The 
minute drops of oils supernatant on water, some of them no larger 
than the particles in the water below, observed by Dr. Brown, as he 
says, to be almost or wholly motionless, so behave because the 
molecules of water glide by the molecules of a substance for which 
they have not even the affinity that would compass opposition. Be 
the globules of oil on water never so small or large, the molecules of 
the aqueous fluid glide by them. Whether a small or a large 
globule of oil be the particle itself on water, there is no movement 
of the particle. Dr. Brown says none, or almost none. I think 
that he was mistaken, that there is no molecular movement whatever. 
Fixed oils have not the same molecular constitution as volatile 
oils, nor these the same as alcohol, nor either the same as water. 
Whatever these differences may signify in various behaviors, under 
varying conditions, one, among the rest, distinguishes water from the 
rest and all other liquids. Despite its apparent perfect fluidity, the 
reluctance of its molecules to move among themselves as smoothly 
as do those of other liquids among themselves is one of its most 
evident characteristics. We see this exemplified by the way, long 
since ably demonstrated, in which a wave is built up from ripples, 
by the way in which the surf breaks along the shore, and in the 
ease with which a small proportion of oil in contact with water 
modifies or subdues its energy. Only recently I steered a boat in 
Boston harbor between two headlands, between which, and far 
beyond, white-caps covered the surface of the water, surrounding a 
placid lakelet of a square mile in area, black by contrast to its 
white-capped margin, over the surface of which lakelet I was soon 
smoothly gliding; and this change from turbulent to placid waters 
was wholly due to the merest film of oil from Boston's great sewer 
discharging its contents three miles away on the lowering tide from 
the head-house on Moon Island into the current running towards 
the sea. I am aware, of course, that part of the calmness described 
was owing to the fact that the oil lessened the friction of the wind 
on the water. But that was not the only cause of the calming 
effect produced by the oil. Oil prevents the friction of parts on 
the surface of water already in agitation, and thereby quiets the 
wave already risen. The area which I have just described as a 1894.] 
175 
[Bache. 
smooth lakelet had been only a short time before my arrival'at the 
place in precisely the same state of agitation as the surrounding 
waters. The surface is the part where the wave begins to form, 
and where it receives constant increments, the wind propagating 
these, and by impact on the growing wave or billow as a whole, 
forming and propelling it as a mass, despite its tendency in deep 
water to oscillate freely in the vertical without translation horizon- 
tally. It is easily conceivable that, although particles of oil may, 
as I have stated, experience no sensible friction when in contact 
with the molecular movements of water, so almost infinitesimal are 
they in range, yet that oil forming a film over a large surface of 
water may, through friction, as an enclosing sheath, tend to quiet 
the water, and thus impair and gradually destroy its ability to con- 
tinue the massed effect known as a wave, at the place, the surface, 
where not only is it generated, but where it most effectively tends 
to preserve its energy of movement. 
Thus, it is not only through its weight that water, when set in 
active motion, becomes so formidable as we know it to be when in 
angry mood. It is because, besides the momentum with which it 
can be endowed through its great weight, it lends itself, through its 
molecular constitution, to the storage of enormous energy and to the 
yielding up of that energy reluctantly. Assuming the existence of 
a sea of oil or one of alcohol, and either in a state of turbulence, 
and moreover eliminating in imagination the difference in weight 
between these and water, either in comparison with water equally 
turbulent would gently come to rest. 
The difference between Herr Wiener's view and mine is radical. 
He speaks of the motion common to fluids as the cause of the 
brownian movements. But such motion, at least as perceptible 
through the microscope, does not exist, except in water or in some 
other liquid in which water is, as I have proved by experiment, a 
considerable constituent. Then Herr Wiener, although accounting 
for the brownian movements by hypothetical movements common 
to all fluids, really makes their causation the vibratory effect of rays 
of light and heat, to which, he thinks, fluids through their consti- 
tution lend themselves. I, on the contrary, show that the molecu- 
lar motion, called brownian, taking place under all conditions 
imposable, is a property of water and of water only, and that light 
and heat have naught to do with producing it, although, as I have 
admitted, they may possibly act in intensifying it. All that I may Bache.] 
176 
[April 20, 
claim to have detected is a phenomenon which reverts to the 
molecular constitution of water, as to which the moving, solid par- 
ticles in it concerned in the brownian movement have no more to 
do than has a current-metre to do with the flow of the stream the 
swiftness of which it measures. We do not deny that a gas may be 
essentially pure, and therefore homogeneous, a chemical as well as 
physical entity, and that, nevertheless, its molecules may have 
repulsions among themselves: on the contrary, we affirm it. Simi- 
larly water, recognized, as it is, as a chemical condition, not a me- 
chanical mixture, has, as here demonstrated, repulsions among its 
molecules. 
When I take into account all that I have detailed, and remember 
also that these moving particles of which we have been speaking, 
hermetically sealed under glass, as I have them now under cover- 
glasses, move indefinitely in time, unmodified in range and veloc- 
ity, through changes of temperature, through light and darkness, 
through electricity and magnetism, in the presence of every force 
to which I have been able to subject them, I cannot but think, 
when I add that these movements are active in proportion to the 
fineness of subdivision of the particles, that they are caused by the 
mutual repulsions of the molecules of aqueous fluids. Did I see a 
relatively large mass moving as vigorously as the most minute one 
visible to the eye, I should regard this theory as untenable from 
that single fact alone, because it would be impossible that molecu- 
lar action should concentrate effect on a relatively great mass; but 
when I see, as I do, the largest masses remaining unmoved, and 
descending in the scale, smaller ones, showing the effects of a faint 
impulse, and descending further still, others exhibiting sluggish 
movement, until the sight reaches the smallest particle visible, 
finding in that the most eccentric and vehement movement of any 
exhibited, I know then that I am looking at a sea where the little 
waves dash in vain on the impressionless rocks, barely disturb the 
floating ships and hulks, but twist and swirl and make frantically 
dance the little cockle-shells of boats wherever they may happen to 
be upon the surface; and that, in fine, I am witnessing the molec- 
ular movement of this sea in its effort to escape into space. The 
aqueous fluids, finding no release, as under my cover-glasses, the 
movements would go on forever; finding it in freedom from con- 
finement, they go on until the fluid which is the condition of their 
manifestation is in a few minutes dissipated in evaporation. 1894.1 
177 
[Bache. 
I must confess that, although every conclusion reached through 
labor bestowed gives a certain pleasure in legitimate appetite for 
knowledge gratified, yet this is so far beneath what I had thought 
might lie hidden under the mystery of the brownian movements, 
I experience a sense of disappointment. I had thought that this 
investigation might be one of the paths that lead to the solution of 
the question whether or not energy is immanent in matter or a thing 
apart from it. For many years after the beginning of this century 
nothing fundamental in physics was known beyond the fact that 
matter is indestructible. It has been learned since, but no longer 
ago than about fifty years, that energy also is indestructible. It 
still remains perhaps to be shown that energy is but an emanation 
and manifestation of matter, reacting on it. Advanced as our 
knowledge is within a few years as to molecular movement, I had 
hoped that the investigation of the brownian movements might 
yield some contribution to molecular theory, and thence lead to a 
profounder knowledge than we now possess of molecular behavior 
in the abstract. I am able, however, to claim for the demonstra- 
tion here no more than that the brownian movements are not the 
self-movements of finely divided particles in suspension in aqueous 
solutions, which Herr Wiener had also ascertained, but simply that 
which he did not ascertain, movements generated by the molecular 
action of aqueous fluids, instead of being, as he and Herr Exner 
also thought, in differing form, phenomena due to light and heat. 
Perhaps even this moderate conclusion may be disputed, but it 
remains to be disproved.