March 27,, 191 1] 



NATURE 



lO' 



MOVEMENT OF MOLECULES. 

 Brownian Movement and Molecular Reality. By Prof. 

 M. Jean Perrin. Translated from the "Annales de 

 Chimie et de Physique," 8me Series, September, 

 1909, by F. Soddy, F.R.S. Pp. 93. (London : 

 Taylor and Francis, 1910.) 

 'T^HIS small volume of ninety-three pages is a trans- 

 -L lation by Mr. F. Soddy, F.R.S., of a memoir pub- 

 lished by Prof. J. Perrin in the Annales de Chimie et 

 de Physique in September, 1909. In this paper, Prof. 

 Perrin gave a detailed account of his work upon the 

 distribution in fluids of small particles, which show 

 the Brownian movement, and the bearing of these 

 lesults on the kinetic theory of matter. An interesting 

 survey is first given of the historical development of 

 this subject. The English naturahst. Brown, directed 

 attention in 1827 to the fact that small particles sus- 

 ••nded in liquids were always in a state of rapid 

 but irregular movement. Comparatively little notice 

 was paid to this observation until 1889, when Gouv 

 showed that the Brownian movement could not be 

 ascribed to temperature difTerences, but was a per- 

 -^istent effect which was probably a necessary conse- 

 quence of the kinetic theory of matter. Since that 

 time a large amount of interesting work has been 

 done, especially by Continental workers, to throw 

 further light on the magnitude and nature of the 

 Brownian movement. 



After a brief account of the kinetic theory of matter 

 and its application to the determination of the dimen- 

 sions of atoms, Perrin gives a full description of his 

 own experiments. Using an emulsion of gamboge 

 and of mastic, he was able to determine the distribu- 

 tion in liquids of nearly equal granules, the diameter 

 of which in different experiments varied between 

 oi/i and M (m = 1/1000 millimetre). By counting the 

 number of granules at different levels by means of a 

 high-power microscope, he found that the concentra- 

 tion of uniform granules decreased in an exponential 

 manner with the height, in the same way as baro- 

 metric pressure due to our atmosphere decreases with 

 the altitude. The diameter of the granules was deter- 

 mined by direct weighing, and also by means of the 

 formula of Stokes. From the data thus obtained, he 

 has shown clearly that these granules distribute them- 

 selves exactly like a gas of very high molecular 

 weight. It follows from this that each of these 

 granules has the same kinetic energy of movement as 

 a molecule of any gas or liquid at the same tempera- 

 ture. This is a very important deduction, for it shows 

 that the law of equipartition of energy in all proba- 

 bility holds, not only for single molecules, but for 

 granules containing many millions of molecules. 

 From the experimental data, it is not diflRcult to 

 deduce directly the numerical value of Avogadro's 

 constant, i.e. the number of molecules in one cubic 

 centimetre of any gas at standard pressure and tem- 

 perature. The determination of this constant allows 

 us at once to deduce the mass of any molecule, and 

 also the value of the fundamental unit of charge 

 iried by the hj'drogen atom. 



The experiments of Perrin are highly ingenious and 

 interesting, and throw much further light on the 

 behaviour of these granules. By examining some 

 NO. 2160, VOL, 86] 



large granules of diameter, about 13^ which contained 

 an inclusion visible in the microscope, he has, been 

 able to show that a particle, in addition to a slow 

 Brownian movemeni:, exhibits a spontaneous irre- 

 gular rotation, and that the mean energy of this 

 rotation is about equal to the mean energy of trans- 

 lation, thus establishing another deduction from the 

 law of equipartition of energy. 



In the last few years a number of methods have 

 been developed for the determination of atomic con- 

 stants, and a brief account of these is given at the 

 end of the volume. It is of interest to compare the 

 values obtained by Perrin with those obtained by 

 other observers by entirely distinct methods. It is 

 simplest to compare the results in terms of the value 

 deduced for the charge e carried by a hydrogen atom. 

 Perrin finds 6 = 41 x 10-'" electrostatic units, while 

 Rutherford and Geiger, from their counting experi- 

 ments, found a value of 465 x 10-^". Recently 

 Millikan, as a result of an admirable series of experi- 

 ments on small drops of oil, has found a value equal to 

 490 X 10-'°. In the course of his work, Perrin made 

 special experiments in order to test the validity of the 

 application of Stokes's formula for determining the 

 diameter of a globule by its rate of fall, and con- 

 cluded that it held accurately over the range of dia- 

 meters of granules employed in his experiment. On 

 the other hand, Millikan finds that Stokes's rule is not 

 valid for small particles, and that the error becomes 

 considerable for particles of diameter about 2m, which 

 is about the diameter of the granules used by Perrin 

 in his most accurate series of experiments. The devia- 

 tions from Stokes's law observed by Millikan are in' 

 general agreement with the conclusions recently de- 

 duced by Cunningham as a result of a mathematical 

 investigation. If Millikan is correct, the size of the 

 granules deduced by Stokes's method requires correc- 

 tion, and this would tend to bring the value of e 

 found by Perrin in closer accord with that of Millikan. 

 It seems possible, also, that the apparently large 

 variations observed by Ehrenhaft in the value of the 

 fundamental charge carried by very small particles, 

 visible in the ultra-microscope and showing large 

 Brownian movement, may be due to the failure of 

 Stokes's formula for very small particles. 



The questions discussed in this volume are of great 

 interest and importance in molecular physics, and the 

 reader cannot fail to be impressed by the remarkable 

 advances which have been made in recent years in 

 showing the validity and essential reality of the kinetic 

 theory as an explanation of the properties of matter. 

 The work of translation is on the whole well done, 

 and the vigorous style of the author is retained. The 

 translation is in a few cases somewhat peculiar. For 

 example, on p. 5, " Le mouvement brownien . . . 

 persiste la nuit, dans un sous-sol, -k la campagne," is 

 translated, "The Brownian movement . . . persists 

 equally, for example, at night on a subsoil in the 

 country." Again, on p. 8, "nous constatons un 

 dquilibre " is given "we establish an equilibrium." 



Mr. Soddy has done a valuable service in bringing 

 to the attention of the general and scientific reader the 

 very interesting and clearly written account of this 

 subject given by Perrin. E. R. 



