325 



reduced" etc., which occur under many of the figures, might use- 

 fully be replaced by the approximate number of times the object 

 is magnified or reduced. 



D. J. S. 



Brownian Movement and Molecular Keality. By M. Jean 

 Perrin. Translated by F. Soddy, M.A., F.R.S. 8| x 5^ in. 

 93 pages, with 7 figures. London, 1910. Taylor & Francis. 

 Price, 3s. 



It was in the yeai- 1827 that the botanist, Robert Brown, first 

 directed attention to the fact that small particles suspended in. 

 liquids were in a constant state of movement. Most microscopists 

 are familiar with this " Brownian movement," as it is styled from 

 its first observer, in the apical vacuoles of Desmids belonging to 

 the genus Closteriimi. Under the above title we have a trans- 

 lation by Mr. F. Soddy of Prof. J. Perrin's memoir dealing with 

 his experimental researches on the behaviour of small particles 

 suspended in fluids and the bearing these results have upon the 

 question of molecular reality. The experiments were made upon 

 uniform emulsions of gamboge and of mastic, and by a process of 

 fractional centrifuging Prof. Perrin was able to obtain emulsions 

 in which the nearly equal granules suspended in the liquid varied 

 in diameter in different experiments between one-tenth of a 

 micron and one micron. From the data obtained during his 

 experiments the author is able to show that the law of gases 

 first enunciated by Avogadro, already extended by Van 't Hoff 

 to dilute solutions, extends also to uniform emulsions. In other 

 words, the suspended particles behave in all respects like the 

 molecules of a perfect gas, their movements, visible under the 

 microscope, being due to the molecular impact of the surrounding 

 molecules. The " Brownian movement " thus offers us, on a 

 different scale, the faithful picture of the movements possessed, 

 for example, by the molecules of oxygen dissolved in the water of 

 a lake, which, encountering one another only rarely, change their 

 direction and speed by virtue of the impacts with the molecules 

 of the solvent. We are much indebted to Mr. F. Soddy for 

 putting this interesting memoir in an English dress. 



