LEWIS. — A NEW SYSTEM OF THERMODYNAMIC CHEMISTRY'. 287 



Combining this equation with the above gives, 



Nid In ^1 + N^d In ii = 0, or d hi ^i = 0. 



That is, the activity of the CaCL • 6 HgO in the liquid phase is not 

 changed by the addition of a small quantity of water, and it will there- 

 fore remain in equilibrium with the solid CaCl2 • 6 H2O without change 

 in the fi-eezing point. 



This example illustrates the general manner of treatment of systems 

 in which molecular species may change through dissociation, association, 

 or through the mutual reaction of two or more species. 



A little consideration of the simultaneous use of equations XXIII and 

 XXIV with the preceding equations shows why it is that such equations 

 as V and VIII hold for the activity of a molecular species such as 

 H2O, in a given pure phase, regardless of whether this phase is really 

 composed entirely of the species H2O or in part also of others such as 

 (H20)2, (H20)3, (H"^ + 0H~), etc., provided always that these other 

 species can be formed from, and are in equilibrium with, the molecular 

 species HgO. 



It may seem, at first sight, that equations XXIII and XXIV, as well 

 as the preceding equations, while entirely exact and general in their 

 scope, may not be readily applied to certain concrete problems where 

 the value of the activity cannot be obtained from existing data. As a 

 matter of fact, however, it is seldom important to know the numerical 

 value of the activity in any one state, but rather the ratio between the 

 activity of a substance in one state and that in another, and this ratio 

 may be obtained in a variety of ways. 



In fact one of the most important problems to which the equations 

 derived in this paper may be applied, concerns the dissociation of salts 

 in aqueous solutions into their ions, although from the nature of the 

 ions we are never able to determine the numerical values of their 

 activities. Let us consider the dissociation of such a substance as 

 hydrochloric acid in aqueous solution, according to the reaction, 



HCl - H+ + Cl- 



According to the ordinary mass law, 



ChCci 



C 



= K. 



HCl 



Now this equation has been shown to be false, if we calculate the 

 concentration of the ions from conductivity data. In all probability 



