40 ART. 10. K. IKEDA : STUDIES ON THE 



As long as the variation of pressure is not very large we may 

 assume Za. and 2;ß to be functions of temperature alone. 

 In the case of equilibrium 



so that for a constant temperature 



3ft — V 2 



= e 



-ß^' = R = const (2.5) 



9. in this equation is what is called the equilibrium coefficient. 



The ratio between the mean molecular weight of the liquid 

 and the molecular Aveight of ©« has been called by Ramsay and 

 Shields the degree of association. If we express this ratio 

 by Ç, then 



e = v-(v-l) C, = l + (v-l) C'p (26) 



The experimental determination of ç presents considerable diffi- 

 culties. Ramsay and Shields attempted to evaluate it from the 

 temperature coefficient of the molecular surface energy, but with 

 only partial success. Ramsay afterward modified the mode of 

 calculation and his results are widely accepted. But there is a 

 serious doubt about the entire method, because it is based upon 

 the assumption that the composition of the surface film is the 

 same as that of the liquid mass. For mixtures of chemical 

 species of nearly equal volatility the assumption may hold. But 

 in the case of an associated liquid the component chemical species 

 must have a very different volatility. The more volatile species 

 will tend to accumulate in the surface film, because its molecular 

 surface energy will be considerably less than that of the less 

 volatile species, whose critical temperature must be assumed to 

 be much higher than that of the other. 



