DILUTION OF LITHIUM CHLORIDE 
509 
pass from one phase to the other when the two phases are brought 
together; (2) if a is greater in one phase than in another, the 
substance will tend to pass from the first phase to the second; 
(3) the activity of a perfect gas is equal to its concentration; 
(4) the activity of a solute in a perfect solution is equal to its con- 
centration.” When two phases of the same system, but of 
different concentrations or activities are brought together, the 
material in the phase of high activity will tend to escape over 
into the phase of lower activity. 
This escaping tendency Lewis called by the term fugacity, /. 
It has the dimension of pressure. Activity is defined in terms 
of the fugacity by the equation, 
a=f / RT 
where R is the gas constant and T the absolute temperature. 
In applying this conception of activity to a working cell, let 
us consider first the cell involving transference, e. g. 
A g - AgCl j LiCl - LiCl | AgCl - A g. 
a" a' (a'^a') 
During the passage of one faraday of electricity one equivalent 
of chloride-ion is formed on the dilute side from the silver chlor- 
ide electrode, while on the concentrated side one equivalent of 
chloride-ion is removed from the solution to the electrode. At 
the same time Nc equivalents of lithium-ion migrate into the 
dilute chamber and 1-Nc equivalents of chloride-ion migrate to 
the concentrated side. The total result is the transfer of Nc 
equivalents of lithium chloride from the concentrated to the 
dilute solution, or from the solution of activity a to that of 
activity a'. The free energy accompanying the transfer of one 
mole of the salt is given, therefore, by the relation : 
E\F 
Nc 
= RT loge S-- 
a' 
( 1 ) 
where E is the electromotive force, F the faraday (96,494 
coulombs), Nc the transport number of the cation, R the gas 
constant (8.3160 joules), T the absolute temperature (298.09°), 
and a" and a' are the activities of lithium chloride in the two 
solutions. 
The following cell does not involve transference: 
Ag - AgCl | LiCl - LiCl (LiHgx ) - (LiHg x ) LiCl - LiCl | AgCl - Ag. 
a" a” a' a 1 a n ^>a 
The passage of one faraday of electricity involves on the dilute 
