80 RESEARCH COMMITTEES. 
If on the other hand we vary s by abstraction or addition of 
salt, keeping w constant, we obtain 
Ont 5 
Sha +i) fits (A+ fh], 
= (140) fra > (1 + 2) fil. 
SS Ajay Se 
os G=—h = [d+OA] = hw (8) 
We must now determine oF, as a function of h. In the first 
bw 
place we observe that 
ih 
oF, 
ie 
where p denotes the vapour pressure and v the corresponding 
specific volume. Then, as Helmholtz has shown in the second of 
the memoirs above referred to, we may evaluate this integral as 
follows :— 
First, let the amount dw evaporate from pure water under 
saturation pressure P and specific volume V; this does an 
amount of work =P.V. dw. 
The vapour is then expanded—out of contact with water—till 
it attains the specific volume 7, and saturation pressure p,, of the 
vapour overlying the solution; this does a further amount of 
Vn 
work = dw Wf p. dv. 
V 
Finally the vapour is compressed in contact with the solution 
under constant pressure p, until it condenses. This absorbs 
work = 7,,. %,. dw. 
ony 
ow 
Vn 
=—PYV =i. p.dv + pny 
V 
or, by partial integration 
6 F, A 2 C7; 
ae if .0 ap. 350 vee (7) 
We can now determine an expression for the e. m. f. in terms of 
calculable quantities. 
