LEWIS. THERMAL PRESSURE. 149 



phase. When two phases in equilibrium are subjected to infinitesimal 

 changes of pressure resulting in a second state of equilibrium, 



d\l/ 1 — d tpz, or in other terms, -pr~ d P 1 = ~~ d P. 



9 ipi p _ 9^ 



9P 1 d ^~9P 2 " 



(3) 



where 9 denotes a partial differential ; finally, 



9P\ _ dj\ __ oj 

 9if/o d P x a. 2 



9P, 



from equation (2). In general, therefore, 



9ilf 



where k is a constant. 



That is, the change in the escaping tendency of any phase with a given 

 change in the external pressure is proportional to the specific volume of the 

 phase. For example, if solid and liquid benzol are in equilibrium at one 

 pressure and this pressure is increased, the escaping tendency of the 

 liquid is increased more than that of the solid in the ratio of the specific 

 volumes, 1.13 to 1.11. The liquid phase, therefore, totally disappears. 

 In the case of water, whose liquid is denser than the solid, the phenome- 

 non is exactly reversed. The above law, therefore, expresses quantita- 

 tively what the principle of Le Chatelier states qualitatively. 



In the preceding discussions we have dealt with different phases of a 

 simple substance, not with a mixture, but the same method of proof and 

 therefore the same law can be shown to be applicable to all cases in 

 which the phases considered are all capable of being converted entirely, 

 under the conditions of equilibrium which exist,* into one substance, 

 whether this be a pure substance or a mixture. 



Equations (1), (2), and (3) apply, therefore, to all cases where there is 

 association, dissociation, polymerization, or isomerization, provided that 

 all these different molecular species are in "true" equilibrium with one 

 another. 



* This qualifying phrase is necessary. The different phases must not merely 

 have the same composition. Thus, a system composed of water and a gaseous 

 phase of hydrogen and oxygen in equivalent proportions differs essentially from a 

 system of solid amnionic sulphydrate and a gaseous phase containing ammonia and 

 hydrogen sulphide in equivalent proportions. The latter is subject to the above 

 treatment, the former is not. 



