Energy Increment and Trouton s Rule. J 61 



The hypothesis is thus in agreement with Trouton's law 

 and with Perrin's suggestion, but in addition it indicates 

 that the Trouton constant at the critical temperature should 

 be identical with Wien's displacement constant. The value, 



ch 



of this constant is X mas T = — -- ~0'28986, where X is 



4' yuox k, 



measured in cms. The molecular latent heat of vaporization 



L = N7ii/, where v = ^ — ; hence L = n ooaJr - Inserting 

 A max . U'Joyob 



the values e = 2*998 . 10 10 cm. per sec, N = 6*062 . 10 23 , 



Ji = 6 m i)8o . 10~ 27 erg sec, 



L = 9-866 T c , 



if L be measured in calories per gram-molecule. 



This value of Trouton's constant is thus somewhat lower 

 than the experimental value, but greater than that calculated 

 from van der Waals' expression. 



In the following tables are summarized the values of the 

 latent heat of vaporization calculated by this means with 

 the generally accepted values for T c and compared with the 

 corresponding values calculated with the aid of van der 

 Waals' equation from the expression 



_ 27 R 2 T C 2 

 64 pj>' 



with the value derived from Trouton's rule and those 

 deduced from vapour-pressure data. 



The values of X max . vary from 2*14: //, for mercury to 

 579*2 fju for helium. It will be noted that for the simple 

 molecules the Wien displacement constant value for the 

 latent heat of evaporation agrees more closely with the 

 experimentally derived figure than that of Trouton ; with 

 the more complex molecules, on the other hand, the Wien 

 constant values agree remarkably closely with the values 

 of the latent heat calculated from the critical data of van 

 der Waals' equation, but are below those experimentally 

 determined with which Trouton's calculations coincide. 



The divergence between the calculated values and those 

 observed in the cases of the more complex molecules is 

 generally ascribed to association, and the striking agree- 

 ment between the values of L calculated from the Wien 

 constant and from the critical values in these cases lends 



Phil Mag. Ser. 6. Vol. 42. No. 247. July 1921. M 



