THERMODYNAMICS OF THE YOLTAIC CELL. 79 
Now suppose a quantity dq of electricity pass thrcugh the cells, 
during which process an amount ds of the salt is dissolved in the 
weaker solution, and passes out of solution in the stronger ; then 
if « denote the electrochemical equivalent of the salt—z.e., the 
é : ds 
amount dissolved per unit current — then — = dq. 
€ 
Hence in accordance with what has gone before—equation (B)— 
éF oF 
=—e 
aot Saas ive 
+ 
The quantity ee 
cannot be directly calculated, but we may 
e 
determine it indirectly as follows :— 
If a quantity of water dw be evaporated from the weaker 
solution and condensed into the stronger, the work done is 
= = dw. Now oF can be calculated from a knowledge of the 
w w 
maximum pressures of aqueous vapour above the solutions—this 
calculation is given later—if then we can determine the relation 
éF 
and —— we can deduce the value of the former, and 
s bw 
consequently the e. m. f. of the liquid cell. 
between 
~ 
The required relation is obtained as follows :—- 
Let us divide F into two parts, F, and F,, belonging respectively 
to the weaker and stronger solutions. 
ee ee oF, 
bw ow ow 
Let w and s denote the masses of water and salt in the weaker 
solution. 
w ; : 
Put — = i; and let f, denote the work done in separating 
8 
unit mass of the solution into water and salt 
-Fi=Wws)\fi=HesU +f 
If we vary w by evaporation or condensation, keeping s constant, 
we have 
CO ee } oh 
Se [a+)f | ine 
ae, [G4tDA] 
