Vol. 6, 1920 CHEMISTRY: NOYES AND MacINNES 
19 
This simple assumption has been justified in the case of sHghtly ionized 
acids and bases, where the ion-concentration in the solution is small, by 
the fact that the so-determined ionization values change with the concen- 
tration just as the mass-action law requires. This, however, is not true 
even approximately in the case of salts and of the largely ionized acids 
and bases (such as hydrochloric acid and sodium hydroxide) ; and we are 
forced to conclude, either that owing to the change in the ion-mobilities 
the conductance-ratio is not a correct measure of ionization, or that the 
chemical activity or mass-action effect of ions, and perhaps also of the un- 
ionized molecules present with them, is not proportional to their concen- 
tration, as the ordinary mass-action law assumes. 
In either case the conductance-ratio affords us no reliable information 
as to the chemical activity of ions; and we must turn to other properties 
for a quantitative measure of this important factor, which determines the 
equilibrium of all chemical reactions between salts, acids, and bases in 
solution, and also the magnitude of many physical properties thermo- 
dynamically related to the activities. 
The term activity must, in the first place, be defined in a precise way; 
and from a chemical standpoint the most practical method is to define it, 
as was proposed by G. N. Lewis, ^ as the quantity which when substituted 
for the concentration of the substance in mass-action expressions will 
express its effect in determining the equilibrium. Thus the activity of 
the substance is its ''effective concentration" from this mass-action 
view-point. 
Correspondingly, the most obvious method of determining the relative 
activities of a substance in solutions of different concentrations is to find 
its concentrations in a gaseous phase in equilibrium with the solutions; 
for in gases at low pressure the concentration and activity can ordinarily 
be assumed to be proportional. For example, we know that the ratio of 
the activities of un-ionized hydrochloric acid in its 11-molal and 8-molal 
aqueous solution at 30° is 12.0, since the partial vapor-pressures of the 
hydrochloric acid in those solutions have been found to be 11.3 and 0.94 
mm. of mercury. This quantity is also the ratio of the products of the 
activities of the hydrogen-ion and chloride-ion in the two solutions, since 
these products are by definition the quantities that must be substituted 
in the mass-action expression for the equilibrium of the reaction HCl = 
H+ + Cl~. Thus denoting the pressures in the two solutions by pi 
and p2y the activities of the un-ionized molecules by ai and a^, and those 
of the ions by ai+, ai~, and a^'^, a2~, we get: 
pi _ Cil _ <^i'^ 
p2 a2 a2'^ a2~ 
This direct method is, however,^of very limited applicability in the case 
of largely ionized substances,^since they seldom have appreciable vapor- 
