LEWIS. — THE LAW OP PHYSICO-CHEMICAL CHANGE. 



59 



since p is constant at constant temperature. Hence equation (13) 

 becomes 



Subtracting two such equations we obtain an equation for two phases, 



*^ 



ain!^ 



9P 



Vl — ^2 



RT 



(15) 



IV. 



The General Law of Fugacity. 



Equations (11) and (15) show a similarity which may be made more 

 striking by a few simple transformations. In equation (11) ^12, the 

 heat absorbed in any reversible transformation of the substance from 

 the first to the second state is equal to the difference in entropy 

 between the second state and the first, multiplied by the absolute tem- 

 perature ; that is, —^ =^ — (aSi — S^, 



where S^ and So, represent the entropy of the first and second states 

 respectively. 



Substituting in equation (11) and transposing the constant R, we 

 obtain, 



9RTln 



^2 



9 T 



-^ p 



= - (5i - S,). 



(16) 



In equation (15) R T is constant, and may be transposed, bringing 

 the equation into the form, 



9RT\ntl^ 



^2 



9P 



J T 



Vi — V2. 



(17) 



The symmetry of equations (16) and (17) with regard to the quan- 

 tities T and —S on the one hand, and P and v on the other hand, is 

 perfect. This similarity is peculiarly interesting in the light of the 

 brilliant theory of Helm, according to which two quantities are funda- 

 mentally connected with each kind of energy, the one its intensity, the 



