RICHARDS. — ENERGY OF rnYSICO-CHEMICAL REACTION. 479 



drate, as well as the physico-chemical potentials and actual pressures of 

 the three molecular species. The equation of equilibrium would then be 



laiHlM^li^i^ = 0. 



This expression is an analysis of the forces which are at work in 

 maintaining equilibrium, p^, the pressure of the uudissociated NH4SIT, 

 occurs first in the numerator as opposing P, the sublimation tension of 

 the solid substance, and then in the denominator as a factor of the chem- 

 ical reacting tendency of the ammonic sulphydrate. These two of course 

 cancel, and since at constant temperature P and all the z values are 

 constant, we obtain for the chemical part of the equilibrium the familiar 

 expression 



p^p'z — k' . 



This is entirely in accord with the results of Isambert, and is undoubtedly 

 true. 



Many other examples of the application of the pressure equations 

 might be given. For example, the dissociation of nitric peroxide is 

 easily treated by the reaction-isobar, while that of hydriodic acid needs 

 merely the simplest form of all, which is both isochor and isobar. 

 Moreover, it is obvious that upon introducing electrolytic solution ten- 

 sions and osmotic pressures into the reaction isobar, it becomes at once 

 the well known expression for the temperature coefficient of electro- 

 motive force. In short, whenever pressures are under consideration, one 

 of the two special forms of the equations above affords a ready means 

 of treatment. 



The results of this paper may be summarized as follows : — 



1. It is pointed out that the study of pressure affords a more direct 

 method of analyzing the progress of a reaction than the study of volume, 

 concentration, or entropy. 



2. An expression called the " reaction-metatherm " has been evolved 

 which represents in terms of pressure the temperature coefficient of the 

 equilibrium ratio of ideal physico-chemical reaction. 



3. Since this equation always contains in its second member the heat 

 actually evolved in the reaction, whether under constant volume or con- 

 stant pressure, it is a mathematical expression of the Theorem of 

 Maupertuis or Le Chatelier. 



