292 PEOCEEDINGS OF THE AMERICAN ACADEMY. 



force of concentration cells a remarkably useful means of determining, 

 in the case of imperfect solutions, how the activity of a given molecular 

 species varies with the concentration. 



Summary. 



It has been shown that a quantity named the activity, and closely 

 related to the fugacity of the preceding paper, may be so defined that 

 it serves as an ideal measure of the tendency of a given molecular 

 species to escape from the condition in which it is. With the aid of 

 this quantity a series of equations has been obtained, which have the 

 same form as the approximate equations now in common use, but 

 which are perfectly exact and general. The utility of these equations 

 has been illustrated by their application to a number of special prob- 

 lems. From each equation two approximate equations can be immedi- 

 ately obtained, one for the vapor pressure of a substance, the other for 

 its solubility. From equations XXIII, and following, important approxi- 

 mate equations are obtained by substituting concentrations for activi- 

 ties. The most general of the equations are collected for reference in 

 the following list : 



For a pure substance, 



RT 



V 



/gin A _j; 



V dF Jt~ It' 



mnt\ _ Y-Pv 



For one constituent of a mixture, 



/a]nj\ _ J^ 



V dP Jt^n" Rf ^^ 



a In A _Y- Pv 

 dT )p,N~ RT' ' 



XII 



For all the constituents of a mixture, 



f X,d\n^,+K ,d In c<, + • • • \ _ V 



\ dP Jr^y-Rf' ^^^ 



Xl\nl,+N,d\nt^+ . . .\ _Y - Pv ^_,^ 



^^ JP,N RJ- 



