Chemical Affinity in terms of Electromotive Force. 13 
always greater than the former, so that in this case the H.M.F. 
continuously increases in value as zinc sulphate is added to 
the copper sulphate, as indicated in curve no. 1. With in- 
termediate strengths of copper-sulphate solution, the former 
tendency at first predominates over the latter one, the 
H.M.F. of the cell falling as more zinc sulphate is inter- 
mixed with the copper sulphate ; but by and by the second 
tendency becomes stronger relatively to the first, and finally 
overpowers it, thus causing the H.M.I’. of the cell first to cease 
diminishing, and then to rise continuously untilit reaches and 
even surpasses the first value. 
Mutatis mutandis, evidently the same kind of explanation 
applies in the case of cells where the mixed electrolyte is a 
magma of a sparingly soluble salt suspended in a solution of a 
more soluble one. 
kt should hence evidently result that the effect of “ polari- 
zation’ on cells set up with such magmas (2. e. the depre- 
ciation in H.M.F’. observed with increasing current-density) 
should be much more marked than with cells containing only 
readily soluble salts in moderately concentrated solution; since 
the film of fluid surrounding the plate on which metal is 
deposited must, with such a magma, become very rapidly de- 
preciated, as regards the amount in solution of the sparingly 
soluble salt, on account of the decomposition of that salt 
caused by the passage of the current. This is, in fact, the case, 
depreciation of some tenths of a millivolt being readily ob- 
servable even with currents of density too small to produce 
any measurable effect on such diffusion-cells as those described 
in Part VIII. In order to avoid errors from this cause ex- 
ceeding 0°5 millivolt in magnitude, it was found indispensable 
to work with currents of density not exceeding from ‘01 to 
‘02 microampere per square centimetre when cells containing 
such magmas were employed ; so that with plates exposing 
some 3 or 4 square centimetres of surface, and cells of near to 
1 volt in H.M.F., a total resistance of at least a megohm, and 
usually of several megohms, was employed ; and proportion- 
ately in other cases. . 
It should further result, if the above explanation be correct, 
that if two cells be compared, alike in all respects save that 
in one the copper plate is surrounded by a mixed solution 
m CuSQ,, n ZnSO, 100 H,0, and in the other by a solution 
of copper sulphate only, of equal strength, viz. (m +n)CuSO, 
100 H,O, the difference between the H.M.F.’s of the two for 
a given value of the ratio = will not be constant, but will 
