422 UNPUBLISHED FKAGMENTS. 



ttigQg is the latent heat of so much of the solvent as occurs in the 

 solution. (Or make mg = 1.) 



Raoult makes A oc ^, with exceptions. 



M-Q 



With a coexistent gaseous phase of the solvent (the solutum being 

 not volatile), we have for the solution 



and for the gaseous phase 



dp = y 



Here, on account of the coexistence of the phases, p and fa and dp 

 and djUL 8 have the same values. Hence 



Say 



7s 



, o At 



dp_M s d yi) 



~nf 

 P 7s ^D 



p-P_M 8 TD 



p ys> 



JIT D is the molecular weight [of solutum] in solution ; 

 M s is the molecular weight [of solvent] in vapor. 



But the foregoing equation suggests a generalization which is not 

 confined to cases in which the law of Henry has been proved. The 

 letter M in the equation has been defined as the molecular weight of 

 the substance in the form of gas. Now the molecular weight which 

 figures in the relation between the potential and the density of a 

 substance in a liquid would naturally be the molecular weight of the 

 substance as it exists in the liquid. It is therefore a natural sup- 

 position suggested by the equation that, in the case where Henry's 

 law holds good, and consequently eq. [4], the molecular weight of the 

 solutum is the same in the liquid and in the gaseous phase ; that in 



* [p is the vapor pressure of the pure solvent, P that of the solution.] 

 Assuming that the vapor behaves like an ideal gas, we have y s '=P s . 



L_ At 



