274 Dr. S. A. Shorter on the Constitution 



Equation (10) then becomes 



r 1 _To = (Pl_el\t^). . . . {21) 



Pi Po \Po P\'^ d P' 9 



r -=(^-"')($), • • • < 22 > 



I£ we suppose the vapour phase to consist of an ideal mixture 

 of gases of molecular weights m and m 1 respectively, the 

 above equation reduces to 



r _ Pi ( m oPo ™ lPl \/dr\ 



where R is the " gas constant," and p and p x the partial 

 pressures of the vapours of C and C x respectively. If we 

 suppose the state of the system to be specified by the con- 

 centration c of the solution of G 1 in C we may write 



R0\ Po Pl )(dp\ 

 \dcJt 





The concentration c may be defined in any of the three 

 usual ways 



c = Pn 



Po 



and pi 



po + Pi 



Let us now consider the case of an ideal dilute solution of 

 Ci in C . In such a solution the partial pressure of the 

 solute is proportional to its concentration, and the relative 

 lowering of the pressure of the solvent is equal to the 

 " molar fraction " of the solute. We have therefore 



Pi=Kpi, (25) 



where K is some constant * about the magnitude of which 



* Or more precisely, some function of the temperature. 



