82 SCIENCE PROGRESS 



was found to be 40 x io~ 13 dynes. Consider a gram molecule 

 of the substance occupying a volume of 22*4 litres at normal 

 temperature and pressure. If there are N molecules in a gram 

 molecule, the concentration will be N /2"24 x io 4 , and therefore 

 the pressure exerted should be equal to N x 40 x io~ 15 /2"24 x io\ 

 According to Perrin's view this pressure must equal the atmos- 

 pheric pressure, viz. roi3 x io 6 dynes, and hence we have : — 



^ _ volume of i gm. molecule x press, i atmosphere 

 press, due to i molecule 

 2*24 x io 1 X I"OI3 x io G 

 4 X IO _u 



= 5 '66 x io 23 . 



Thus we arrive at the conclusion that the number of molecules 

 in a gram molecule of a substance is of the order 5 - 6 x io 23 . 



This is the result arrived at by Perrin, making the above 

 assumptions. How does it compare with values determined by 

 entirely different methods ? Without dwelling longer on the 

 subject, it may be said that the value agrees very well indeed 

 with these values, which vary from 4*3 to 9/6 x io 23 . 



This agreement between Perrin's value, and the value 

 obtained by entirely different methods is very striking, and to 

 quote Perrin's words : " It would therefore seem that the 

 granules in suspension in a colloidal liquid function like the 

 invisible molecules of a perfect gas with a molecular weight of 

 about 3*3 x io 9 ." 



Under such circumstances, the mean kinetic energy of a 

 colloidal particle would be equal to that of a molecule, and 

 therefore the Brownian movements could be explained as due 

 solely to molecular agitation as was suggested by Gouy. 



There has been some criticism of Perrin's work by Duclaux 

 and others. Among the objections which have been raised 

 are the following : 



Duclaux has urged that the gamboge enters partly into a 

 true solution, and does not form a colloidal liquid at all. Again, 

 it has been objected that the method of Stokes cannot be 

 applied to find the mass of these very small particles. Against 

 this may be placed the fact that Perrin has since used three 

 different methods, and has experimented with particles of very 

 different sizes, with the same result. 



However, as his full paper has not yet appeared, and until 

 the work has been repeated, it is very difficult to criticise it. It 



