288 PROCEEDINGS OF THE AMEEICAN ACADEMY. 



this calculation is correct for solutions more dilute than tenth normal,^^ 

 at least we may say that the conductivity data furnish the only means 

 ive have at iwesent for calculating the ion concentrations. Every other 

 method ivhich has been employed measures not the concentrations, but the 

 activities of the ions. 



According to equation XXIII the activities of the undissociated acid 

 and the ions are connected by the equation, 



Cuci 



If therefore the mass law is false, it must be because the activity is not 

 simply proportional to the concentration for one or more of these three 

 substances. The problem, therefore, is to determine how the activity of 

 the undissociated substance and the activity of the ions vary with the 

 concentrations of both. It seems that all the facts which are at present 

 known concerning electrolytic dissociation can be explained by the 

 assumption that the ions are normal in their behavior ; in other words, 

 that the activity of each ion is simply proportional to its concentration, 

 but that the undissociated portion of a strong electrolyte is abnormal in 

 its behavior, the activity being proportional to the concentration of the 

 nndissociated substance multiplied by a quantity which depends solely 

 on the total ion concentration, and increases with the latter. ^^ 



This simple statement suffices to explain qualitatively all the known 

 anomalies of strong electrolytes. The exact quantitative formulation 

 of this principle can hardly be made until still more experimental 

 work has been done. 



However, these considerations illustrate the method of treating 

 chemical equilibrium when the ordinary mass law fails ; in other words, 

 when for one or more of the reacting substances the activity is not 

 proportional to its concentration. For 'a complete analysis of such a 

 case it is necessary to know how the activity of each of the reacting 

 substances changes with its concentration and with the concentration 

 of the other substances present. 



" The data upon wliicli this paragraph is based are chiefly those contained 

 in tlie very complete and instructive summary by A. A. Noyes, entitled, " The 

 Physical Properties of Aqueous Salt Solutions in Relation to the Ionic Theory." 

 (Technology Quarterly, 17, 293, 1004). 



^8 Probably, strictly speaking, the activity of the ions is likewise a function 

 of the concentration of the undissociated substance, decreasing as the latter in- 

 creases ; but since the concentration of the undissociated substance always is very 

 small in dilute solutions of strong electrolytes, its influence on the activity of the 

 ions is therefore of minor importance. 



