Milton Kerker Clarkson College of Technology Potsdam, New York 13676
Brownian Movement and Molecular Reality Prior to 1900
Your Majesty, Your Royal Highnesses, Ledies and Gentlemen. (TJhe Academy of Sciences [has decided] to award the Chemistry Nobel Prize far 1925 to Dr. Richard Zsigmondy, Messor of Chemistry at the University of GLittingen, for proving the hetemeeneous nature of colloidal solutions and for the methods used which have laid the foundation of modem colloid chemistry. Pmfeamr H. G. Soderbaum, Secretary of the Royal Swedish Academy of Sciences ( I )
Your Majesty, Your Royal Highnesses, Ladies and Gentlemen. The Academy of Scienceshas decided to award the Nohel Prize in Chemistry for 1926 to The Svedherg, Pmfessor of Physical Chemistry at the University of Uppsala, for his work on disperse systems. Almost a hundred years ago, or more accurately in 1821, the English botanist Robert Brown discovered with the aid of the ordinary microscope that small parts of plants, e.g., pollen seeds, which are slurried in a liquid are in a state of continuous, though fairlv slow movement in different directions . . . . As we have recenriy heard, Einstein evolved a theory for this so-called Brownian movement which was then developed to a high degree by the now late Smoluchowski . . . . The theory in question has been confirmed convincingly by experimental investigations of several colloid scientists among whom especially two of today's prize winners, Pemn and Svedherg, have occupied and still occupy a leading position. Professor H. G. Soderbaum, Secretary ofthe Royal Swedish Academy of Sciences ( I ) Your Majesty, Your Royal Highnesses, Ladies and Gentlemen. The object of the researches of Professor Pemn which have gained for him the Nobel Prize in Physics for 1926 was to put a definite end to the long struggle regarding the real existence of molecules . . . .The . . . experimental p m f was taken up by two scientists simultanwusly. One of them was Perrin; the other Svedherg. I have to speak of Perrin only. His measurements on the Bmwnian movement showed that Einstein's theory was in perfect agreement with reality. Pmfessor C. W. Oseen, Royal Swedish Academy of Sciences (2) Three Nobel prizes in two years.' What a triumph for Colloid Chemistry which had only emerged a t the turn of the century in association with the new Physical Chemistry. Thomas Graham .(3) had designated as colloids those substances which were characterized by slow diffusion and by an inahility to penetrate certain membranes, and he had formulated a s the central problem "whether the colloid molecule may not be constituted by the grouping together of a number of smaller crvstalloid molecules. and whether the basis of colloidality &ay not really be this composite character of the molecules." Colloidality does, indeed, arise from the disperse condition of materials although in most cases the particles are considerably larger than huge molecules. Thus, the fledgling science preempted the domain of particles which lay between the molecular and the macroscopic. Wolfgang Ostwald (4) was to call it "the world of neglected dimensions." Until Zsigmondy, together with Siedentopf (5), invented the ultramicroscope in 1902, the graininess of moat colloids could only be made "visible" by a Tyndall cone.2 Now, systems which had appeared completely homogeneous under a n ordinary microscope could actually be 764
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seen to consist of a dispersion of particles, and their sizes could be determined. Thus, Zsigmondy demonstrated the reality of colloidal particles. The ultramicroscope made more vivid than ever the Bmwnian movement which was believed t o have its origin in the thermal motions of the molecules of the fluid medium in which the particles were suspended, and i t made auantitative measurements of this movement feasible. particularly for the smaller particles which heretofore had been invisible. D e s ~ i t ethe success of the kinetic molecular theory in accounting for the properties of gases, no direct evidence for the existence of atoms and molecules had been obtained, and the theory suffered attacks a t the hands of Ernst Mach and ~articularlvof Wilhelm Ostwald (9). Indeed, the reality of kolecules &as one of the pivotal questions whose resolution a t the turn of this century comprised the watershed leading into the present epoch of science, and i t was generally recognized that Brownian movement provided a definitive test.. Albert Einstein's theoretical analysis in 1905 (10) set the stage for a crucial experiment to verify the reality of molecules. Svedberg ( l l ) , using t w l s developed by Zsigmondy, was the first to claim to have consummated that experiment. Penin's (12) demonstration of the relation between Brownian movement and molecular movement was so convincing that after his work the question of molecular reality was no longer raised? It is the importance with which this topic was viewed earlier in this century 1 Actually the award of the Nohel prize.for 1925 had been postponed so that all three of these awards were made in the same year. ZFaraday (6) had forecast that colloidal gold solutions were comprised of "a mixture of a colorless, transparent liquid, with fine particles of gold . .. . . . . so thin and minute as to come far within the dimensions of an undulation of light . . . if a cone of sun's rays he thrown dcmss the fluid by a lens, the particles are illuminated . . . and become visible not as independent particles but as a cloud . . . . The highest powers of the microscope have not as yet rendered (them) visible." Similarly, Tyndall (7) had proposed that aemsols might he comprised of submicroscopic particles. ". . . this precipitation may be rendered of any degree of fineness, forming particles distinguishable by the naked eye, or particles which are probably far beyond the reach of our highest microscopic powers. I have no reason to doubt that particles may be thus obtained whose diameters constitute but a very small fraction of the length of a wave of violet light. With regard to liquid suspensions, Tyndall (8) added: "At all events it is experimentally demonstrable that there are particles which act similarly upon light and which are entirely ultra-microscopic . . . (a mastic suspension) was of a beautiful cerulean hue, this color arising wholly from the light scattered by the mastic particles . . . . The liquid containing them was examined by a microscope magnifying 1,200 diameters. The mastic particles entirely eluded this power." =Eventhat arch critic of atomism Ostwald (13) proclaimed that "the agreement of Brownian movement with the demands of the kinetic hypothesis . . . which have been proved . . . most completely by 3. Perrin, entitle even the cautious scientist to speak of an experimental proof for the atomistic constitution of space filled matter." Henri Poincarb (14) summed matters up in these words: "The brilliant determination of the number of atoms made hv M. Penin has completed this triumph of atomism . . . .The atom of the chemist is now a reality."
that accounts for its singular recognition by the award of three Nobel nrizes to these colloid chemists. The broad aspects of this subject have been thoroughly treated recently (15) from the perspective of the contributions of Jean Pemn. My own interest was aroused by the role of The Svedberg, and this has been considered a t some length elsewhere (16). In this paper I wish t o sketch the history of the study of Brownian movement from its discovery to that point a t the turn of this century when the stage was set for these momentous discoveries. Robert Brown
The invention of the microscope brought into view a fascinating new world of motile animalcules and bacteria which differed from dancing dust particles made visible in a sunbeam in that the source of motive power lay in the organism itself rather than in turbulent air currents. And so when in 1827 Robert Brown4 turned his lens onto cytoplasmic granules dispersed in water there was no reason to anticipate that their chaotic movements might constitute a momentous discoverv. His recornition that this was a different kind of motioiis a tribute t o his genius (17). Brown's observations were camed out with a simple microscope.5 He was interested in the mode of actionof pollen in the process of impregnation and sought species having irregularly shaped rather than spherical pollen grains in order to trace their movement to the ovum more easily. Granules extracted from the pollen of Clarckia pulcella seemed suited for this purpose, and i t was while examining the shape of these particles immersed in water
'Robert Bmwn (1773-1858), a native of Scotland, educated at the University of Edinburgh and an avid botanist, became a proteg6 of Sir Joseph Banks early in his career. In 1801 he shipped out as naturalist to an expedition whieh surveyed the coast of New Holland. The huge collection of plants which he acquired during the four-year voyage pmvided the hesis over the next several years for many of his contributions to the anatomy and physiology of plants as well as to systematic botany. In 1810 he was appointed librarian to Sir Joseph Banks, and upon Sir Joseph's death in 1820, he inherited the library, collections, and the house of his patron. In 1827 the books and specimens were transferred to the British Museum in which Bmwn assumed the office of keeper of the botanical collections. He succeeded to many honors in the course of a Long and active life, including in 1839 the Copley Medal of the Royal Society "for his discourses on the subject of vegetable impregnation." In addition, special mention should he made of his discovery of the nucleus of the plant cell. In 1866 the Ray Society issued his "Miscellaneous Works" in 2 volumes, edited by J. J. Bennett, which inchdes his Florae Novae Hollondiae, puhlished in London in 1810.Certainly a claim to eminence is established most solidly when one's name is dropped in a literary work. In Chapter 17 of George Eliot's novel "Middlemarch" (1872), the young doctor jokingly proposes a barter for the vicar's glass vase. "I have some sea-mice-fine specimens-in spirits. And I will throw in Robert Brown's new thing-'Miemscopic Ohservations on the Pollen of Plants'-if you don't happen to have it already." 5Although many writers have coupled Bmwn's observations with the invention only a short time earlier of the first achromatic objectives (la), the instrument used by Brown was merely a "double convex lens, which has been several years in my possession . . . . . . . to give greater consistency to my statements, and to bring the subject as much as possible within the reach of general observation, I continued to employ throughout the whole of the inquiry the same lens with which it was commenced.': In the work reported in the second of his papers (17). he utilized a number of additional simple lenses of varying quality and powers "and also with the best achromatic compound microscopes either in my own possession, or belonging to my friends!' The referee of this paper has called to our attention an article by David Layton which notes the common error of attrihuting Brownian movement to the pollen rather than to the cytoplasmic granules extracted from the pollens (J. CHEM. EDUC., 42,367-8(1965)).
that Brown noted their rapid irregular movements. He observed that these "arise neither from currents in the fluid, nor from its gradual evaporation, but belonged to the particle itself." T h e ensuing steps were inexorable. First, Brown showed that the pollen grains of all the living plants which he examined exhibited this motion. Then he showed that the same phenomenon existed for dead plants which had been immersed in spirit or which had been preserved dried for a s long as a century. Next, he was surprised t o find that particles formed by crushing other parts of plants also exhibited the motion, and this led him to abandon the no~ r on e r-t vof the male tion that this mobiltv- mieht be a . sexual organ. Now Brown wondered whether he was dealing with the elementary molecules of organic bodies ~ r o ~ o s eby d Buffon and others. These were supposed to he capable of motion, yet distinguished from real animalcules. Indeed, he did find that all varieties of organic material, including coal as well a s the atmospheric soot and dust of London, exhibited what would eventually be known a s Brownian movement, provided the materials were appropriately pulverized. But he continued still a step further, and tried a siliceous fossilized wwd, then ground glass, and finally a huge variety of minerals. The "active molecules" were universally present. "In a word, in every mineral which I could reduce to a powder, sufficiently fine to be temporarily suspended in water, I found these molecules more or less copiously." Experiments and Hypotheses
Although Brown stated that he had demonstrated his observations on the motion of pollen particles to numerous colleaeues6 and the suhiect was taken UD from time to time, it cardly entered the mainstream of 19th century science. Some of the earlv investigators proposed that the effect was essentially thermal in nature, others that it was electrical (19), but none of these studies displayed any particular perspicacity. The first glimmerings of a correct physical explanation were suggested by Christian Wiener' in 1863 (24). He en'He cites "Messrs. Bauer and Bicheno, Dr. Bostock, Dr. Fitton, Mr. E. Forster, Dr. Henderson, Sir Everard Home, Captain Home, Dr. Horsfield, Mr. Koenig, M. Lagasca, Mr. Lindley, Dr. Maton, Mr. Menzies, Dr. Pmut, Mr. Renourd. Dr. Roget, Mr. Stokes, and Dr. Wollaston." He also wrote to Dr. Wollaston and Mr. Stokes. lone must take care not to assume that Christian Wiener (1826-1896)is the namesake for the so-called "Wiener ~mhlem." This was Norbert Wiener (1894-1964). A new story begins in mathematics in 1921 when Wiener wrote his first paper on Brownian movement (20). In working on a problem of integration in function space, he sought for a simple model, and Brownian movement, which he had studied in Einstein's paper (10). seemed appropriate. "In the study of Bmwnian movement, our attention is first attracted by the enormous discrepancy between the apparent velocity of the particles and that whieh must animate them if, as seems probable, the mean kinetic energy of each particle is the same as that of a molecule of the gas. This discrepancy is of course due to the fact that the actual path of each particle is of the most extreme sinuosity, so that the observed velocity is almost in no relation to the true velocity . . . we may regard the Bmwnian movement as made up (1) of a large number of very brief, independent impulses acting an each particle, and (2) of a continual damping action of the resulting velocity in accordance with Stokes law." Wiener's idea was to study the ensemble of paths of a single particle performing Bmwnian motion. This is "Wiener's pmblem," or the random walk problem, of which he and a host of successors have attempted to make precise mathematical sense. Thus, the study of the analytical structure of Brownian motion has led to much of the modem work in stochastic processes and has brought pmhahility theory into the mainstream of mathematics (21-23). Volume 51, Number 12, December 1974
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visaged Bmwnian movement as arising from molecular vibrations, and, although his molecular model hardly resembles the modem one, Wiener is the first to see the importance of Brownian movement as a probe for the ultimate structure of matter.8 He also reported the first measurements of the speed of the particles and its dependence upon the particle size. These measurements were actually of the rate of displacement of the particle on a crosshatched field of view rather than the instantaneous velocity. Sigmund Exner (26) followed t h e same technique a s Wiener in measurine the rate of displacement of the Darticles. He also envisaged the ~ r o w n i a nmotion as a minifestation of movements in the liquid but proposed that these consisted of streaming on a- microscopic scale. He did not offer a theory for the origin of this streaming but did observe that i t is livelier the lower the viscosity of the liquid. Indeed, he attrihuted the enhancement of the Brownian motion which he ohaerved upon heating or irradiation to the concomitant lowering of the viscosity. Thus, although both Wiener and Exner localized the source of the Brownian motion in the properties of the liquid itself, there was still no picture of it a s arising from molecular activity in the modem sense. Giovanni Cantoni attempted to develop a physical theory of Brownian motion based on t h e mechanical theory of heat (27). He assumed that a colloidal particle can be treated as a giant molecule in thermal equilibrium with the simple molecules comprising the fluid in which it is immersed.9 Although Cantoni published his model after the appearance of Wiener's paper, he seems to have werlooked that paper. Indeed, his own work was overlooked, and years later, upon reading Gouy's kinetic molecular model for Brownian movement, Cantoni brought attention to his earlier work (28). A clear and explicit description of how Brownian mwement might arise a s a result of molecular impacts was first given by Carbonnelle, Delsaux, and Thirion from 1874-1880, and shortly thereafter by Nageli in 1879 (29). Let us consider Nageli fmt because, although he developed some of the main features of the model, he rejected i t in favor of what even he considered to be a rather vague and qualitative description. As a botanist concerned with the transport of fungi, bacteria, and other small particles through the air, Nageli recognized that the smallest of these "dancing" particles might not fall out even in the stillest air. He pointed out that Since the idea that gas molecules fly about each other at great speeds has entered into physics and because there is general agreement that this is irrefutable, the conjecture may also be made that the 'dancing movements' of motes observed in sunlight arise through impacts of the gas molecules acting in all directions. And one can go even further and conjecture that these smallest motes are tossed about in this way as elastic balls, which act as the air molecules themselves and remain lastingly suspended. Nageli calculated the speed that would be imparted to a typical dust particle if it were bombarded directly and elastically by a single gas molecule and concluded that this cannot be the cause of the motion because the narticle moves many orders of magnitude more rapidl;. Althoueh he realized that one is dealine with laree numbers of molecules, he claimed that because of the randomness of the molecular motion the cooperative effect would still be insufficient to propel the particle a t the observed veloeitv. The areument was then extended to Brownian mot i o n i n liquid: Here, he assumed that, despite the higher densitv of molecules. both the frictional drag of the liauid on the moving and the intermoleklar cohesive forces among the molecules would conspire to make the kinetic molecular hypothesis no more likely a n explana766
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tion in liquids than in gases. And so Nageli abandoned what we now know is the correct hyjmthesis and assumed instead that each particle was surrounded by a sheath of molecules adsorbed from the fluid and that the Brownian motion somehow resulted from intermolecular forces of attraction and repulsion. The apparent dilemma posed by Nageli was resolved by Carbonelle, Delsaux, and Thirion.10 T h e crucial clue is that because there is a distribution of molecular velocities and because the motions are random, there will be fluctuations of density on a microscopic scale and hence of pressure in any small region of a liquid or gas. These irregularities average out on a macroscopic scale, but one may
% Wiener's view "matter is broken down into corporeal atoms which mutually attract each other, and ether atomr which repel each other. The latter are found in the empty space between the corporeal atoms or gmuped with them to form molecules and act through their repulsive force so that these touch neither one another nor the ether atoms . . . . The heat of a body consists in a state of vibration of its ether atoms and corporeal molecules." The liquid is thought of as comprised of randomly moving aggregates about 0.0012 mm in diameter. Their mobility is derived from the vibrations of the ultimate atoms of which they, in turn, are comprised, and the Bmwnian particles are buffeted about in this turbulent sea as long as their dimensions are not t w much larger than the aggregates of liquid. The motion slows down eonsiderably for particles larger than about 0.0023 mm; the impacts presumably have little effect on these. The motion speeds up to a maximum value as the size of the particle in Brownian motion decreases to 0.0012 mm. The motion is not enhanced for smaller particles since a greater activity than that of the aggregates themselves cannot prevail. According to Wiener, there is nothing mysterious about the magnitude 0.0012 mm. This is just double the wavelength in water of infrared radiation. Accordingly, the diameter of the "zusammenhbngenden" masses of liquid and the leneth of standinn infrared waves are related to each other. These co~~luaions are b k d on Wiener's observation of the s m dependence of the rapidity of the Bmwnian movement. Although thi* hardly resembles the present view of things, a number oi more modem writers have attributed such a view to Wiener, e.g., H. Freundlieh (25) writes: "He therefore arrived at the view that Brownian movement is a phenomenon such as would be demanded by the kinetic theory of heat; the irregular impacts of the surmunding liquid molecules against the suspended particles he considered to be the cause of the motion." T h e motion of the particle relative to the fluid arises from "the osmotic action and reaction between the vibrating solid and the enveloping liquid . . . . So, I think that the dancing motion of these extremely small solid particles in a liquid e m be attributed to the different velocities that will be present at the same temperature, be it that of the solid particles or of the molecules of liauid that are eallidine " on all sides . . . . With the same eircumstances, this movement is greater. the greater is the magnitude of the difference betwcen the molecular velocities of the hquid and of the wlid or t h m r resp~erives p m f i r heotr. And, far extremely small solid particles, whose surface is Large compared to the voiume, the result of the impulsions of the large number of liquid molecules will be effectivefor their constant thermal vihration." "'he respective roles of these three Belgian Jesuit collaborators are unravelled in a paper by Thirion (30), whose purpose was to establish their priority over G. Gouy to whom hoth P. Langevin (31) and J. Perrin (12) had attributed the hypothesis that in Bmwnian motion one sees an echo of the molecular agitation. Father Joseph Delsaux read a paper before the Royal Miemscopical Society an June 6, 1877. Delsaux referred, in this paper, to the earlier investieations hv "a friend of mine." Thirion stated that thrs was Father Carbonelle, the founder of Ibr,ue.s dce Questions Srtentifiqu~s,who had formulated these rdeaa in connertmn with his observation of liquid inrluaioni in quartz. Carbunelk had submitted an unsigned note destined to accompany a mCmoire published by two geologist friends, and the proofs were sent to Delsaux. For reasons which Thirion failed to explain, these disapoeared and the note was never ouhlisbed. Later. Carbonelle asked hirion to collaborate with him and they puhl&hed a paper (33) based upon unpublished notes and experiments of Carbonelle, dated 1874. ~
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isolate smaller and smaller surfaces of evaporation or pressure in order to smve at dimensions incapable of assuring the compensation of the irregularities, and then the effects of the discontinuity and of the thermodynamic agitation of the liquid must make themselves felt . . . Thus, according to the mechanical theory of heat, any particle of matter freely suspended in a liquid must ascillate incessantly, if it is sufficientlysmall.
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Moreover, the inequalities will become more and more apparent as one imagines the body to be smaller, and the oscillation will become a t the same time more and more lively, changing continually in intensity and direction. Similar arguments were extended t o small bubbles in liquid'l and to liquid or solid particles suspended in gases.'= Carbonelle clearlv saw tbat "The dimensions alone influence the phenomenon. T h e physical state, the chemical nature, the shape, even the temperature hardly introduce a variation or a t least the variations are secondary and more often imperceptible." However, he leaves the impression that tbe movement is primarily oscillatory. Indeed, the major contribution of Delsaux was to recognize that the motion was rather a random displacement He cites Clausius' description of the molecular motions a s consisting of both rotation and translation and points out: "That which distinguishes the heat motion of liquids from the thermal motion 111solid bodies is. amone other thines. the movement of translation with which tG liquid more: cules are animated." T h a t these men did not develop a mathematical theory and that thev failed to c o m ~ r e h e n dthe imnlications of their description of the fluctukions of the th&odynamic ~ r o w r t i e son a microsco~iclevel for the second law of iheAodynamics is hardly surprising. Boltzmann and Gihbs were yet t o develop statistical mechanics and to wed the statistical and thermodynamic points of view. Indeed, the remaining step in the elucidation of the theow of Brownian motion would he its mathematization nea& 30 years later by Einstein and Smoluchowski based upon statistical mechanics. This would thrust Brownian motion into the forefront of research in physics hecause it would provide a touchstone for testing the fundamental basis of statistical mechanics .about which there was to he so much controversy. Remarkable a s they were, the reflections of Carbonelle and his collaborators remained little known. On the other hand, several papers published some years later by G. Gouy (36) did have considerable influence. P e m n (12), among others, attributed to him the proper explanation tbat Brownian motion originates in the molecular impacts.13 Gouy also camed out numerous observations, which verified the experience of past investigators, including attempts to observe the effects of external vibrations, thermal currents, light, magnetic fields, electric current, and the properties of both the liquid and the particle media. Gouy's description of the molecular action is hardly a s perspicacious a s that of Carbonelle and Thirion, particularly since the notion of fluctuations eludes him. Indeed, he is a t a loss t o account for the resultant movement of the particles in terms of random molecular motions and suggests that somehow these are coordinated on a microscopic scale. I do not mean to say that Brownian motion is produced directly by the non-coordinated movements of the molecules which one often regards as the constituent of thermal motion. It seems, indeed, that on that hypothesis it would be produced only for much smaller particles-comparable to the molecular dimensions. But one can conceive that the molecular movements in the liquid may be partly coordinated, for distances comparable to s micron, without ceasing to be entirely independent for larger distances which are still much smaller than the dimensions our apparatus is able to realize. The existence of Brownian motion appears to show that something analogous exists in reality.
Gouy was also troubled by the fact that the Brownian movements were considerably slower than molecular velocities. Here he is talking about the speeds of the molecules, presumably in the gaseous state, not that of the colloidal particle, acting a s a molecule, which later also hecomes a pmblem.14 Perhaps the most important aspect of Gouy's deliberations is his emphasis not so much upon effecting a physical explanation of Bmwnian movement as upon utilizing Brownian movement a s a tool with which to explore molecular dynamics. The Brownian movement shows us, therefore, assuredly not the movements of the molecules, but something derived closely from these and furnishes us a direct and visible pmof of the exactness of the hypothesis on the nature of heat. If one adopts these views, then the phenomenon, the study of which is far fmm being terminated, takes on assuredly an importance for molecular physics of the first order. If we take the year 1900 to be the great divide between a period of groping for a proper physical model and the elaboration of a quantitative description and a quantita"Although the motions of bubbles of gas within liquid inclusions in minerals were commonly observed by mineralogists, that this phenomenon was a kind of Brownian motion was not commonly recognized; e.g., in a paper communicated by Professor Stokes, W. N. Hartley describes a number of observations and experiments, but makes no reference to the work of others or to a possible connection with Brownian motion (34). The effect was attributed to constant heat exchange with the surroundings and a highly inhomogeneous and varying temperature field within the crystal. 12The first report of the microscopic observation of the movement of aerosol particles is given in a short note by L. J. Bodaszewski (35). Bodaszewski said nothing about Brownian movement, but he did state that "In the appearance of the independent movement of the vapor and smoke partides one cannot help but notice a picture similar to the hypothetical motion of gas molecules in accordance with the kinetic theory of gases." '=We return now to the question of priority raised by Thirion. In a footnote to the Journal de Physique paper (36). Gouy points out that the movement of gaseous bubbles contained in the liquid inclusions of many minerals had been often observed by mineralogists. He points out that De Lapparent (37) had noted the similarity of this phenomenon to Brownian motion and had described briefly the explanation of Carbonelle and Thirion. This was based upon the fluctuation of vapor density within the small bubble due in turn to fluctuations in the rates of evaporation into the bubble and condensation out of it. Since De Lapparent was interested only in this particular phenomenon, he did not discuss the treatment hy Carhonelle and Thirion of pressure and density fluctuations in a liquid and its relevance to the Brownian movement of solid articles. Gouy could not have read the original paper by Carbonelle and Thirion, for he says, "This explanation dws not appear to be able to be applied to solid or liquid particles." No wonder Thirion felt impelled to establish the true position of his paper. "The notion that Brownian motion could result simply and directly from the random motions of the minute molecules also troubled William Ramsay (38). He was particularly interested in the ability of added electrolytes to promote coagulation and subsequent settling and clarification of turbid disperse media. He noted that the colloidal "particle has one hundred billion times the estimated mass of a water molecule; hence, if its motion be produced by bombardment from water molecules, these must exist in complex gmups of considerable mass, and of some stability. It is very unlikely that pedetic motion is the result of electric charges in the partieles . . . . The fact that pedesis is stopped by the addition on an electrolyte would appear to show that the water complices are disintegrated by the presence of ions; it may be that the individual water molecules are attracted by one or another ion, or by both." Like Jevous, he assumes that the Brownian motion prevents coagulation and that addition of electrolyte promotes coagulation by reducing the Brownian motion. This notion persisted in various forms until it was dissipated by Svedberg (11). Volume 51, Number 12, December 1974
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tive theory of Brownian motion, then the work of Felix M. Exner (39) makes a fitting last chapter t o this first period. Exner prepared the suspensions upon which he made his observations in the same way his father had done a third of a centunr earlier (26). . . He also followed his father's technique in measuring the displacement of the particles in the course of about a minute and demonstrated, as others had already done, that this measure of the velocity decreased with article size and increased with tem~erature. Exner clearliviews the colloidal particle as a huge molecule participating in the thermal agitation of the fluid. If one proceeds with the view that the suspended particles participate in the thermal motions of the molecules of the liquid, then one can assume a proportionality between the kinetic energy and the temperature . . . . . . .it may still be eoneeiuoble, if one starts with the oiew of a relationship between the movement of the liquid molecules and the suspended particles, that the uisible mouements and their corresponding measured quantities will houe future value for w in making clear the inner movements of a liquid.
Einstein's 1905 theory brought this conception to fruition. The experiments initiated by Zsigmondy and Svedberg were definitively consummated by Perrin so that there has been no doubt since then that these "inner movements of a liquid" are molecular. Literature Cited (1) ''Nobel Ladwes in Chemiatw 11922-1911." Elvvier Publishing Co., N w York, 1966. (21 "Nobelkdursa i~Ph9aica1922-1941; ElscvierPublishing Co.,NeaYork. 1965. (3) Graham. Thomas.Phil. Roy. Srr. LoM'on,
