12 Messrs. Wright and Thompson on the Determination of 
Hence it appears that cells containing very sparingly so- 
luble compounds like lead sulphate, silver chloride, &c., sus- 
pended in solutions of more soluble salts, are comparable with 
Daniell cells in which the copper plates are surrounded with 
solutions containing only small quantities of copper sulphate, 
together with varying quantities of zinc sulphate, increasing 
the strength of the solution of the more soluble salt, in the 
first case producing the same general effect on the H.M.F. of 
the cell as addition of more zinc sulphate to the copper- 
sulphate solution in the second. 
169. A probable explanation of these results is the fol- 
lowing :—The passage of a current, however feeble, decom- 
poses both electrolytes in the mixed fluid simultaneously ; 
so that, for example, in the Daniell cell containing mixed 
zinc- and copper-sulphate solution round the copper plate, 
both zinc and copper would be thrown down simultaneously 
in the metallic state, were it not that whilst still nascent the 
zine acts on the copper-sulphate solution by a secondary 
action, precipitating the equivalent amount of copper, and 
becoming itself zinc-sulphate solution. A portion of the 
energy gained by this secondary action is nonadjuvant, the 
amount of nonadjuvant energy depending, inter alia, on the 
number of molecules of zinc sulphate thus reproduced relatively 
to that of copper sulphate in solution, in such a fashion that the 
stronger the copper-sulphate solution, ceteris paribus, the less is 
the amount of nonadjuvant energy. Hence, from this cause the 
E.M.F. of a Daniell cell would be necessarily diminished by 
addition of zinc sulphate to the copper-sulphate solution. But 
this addition produces also an effect of another kind ; for 
addition of zine sulphate to the fluid surrounding the plate 
acquiring the higher potential, making that fluid stronger so 
far as zinc sulphate is concerned, must set up an interdiffusion 
effect which will be superposed upon the other sources of 
difference of potential, tending to raise the potential of the 
late immersed in the fluid thus strengthened relatively to 
that of the other plate. Hence there are two causes, acting in 
opposite directions, thus brought into play, tending to affect 
the potential-difference set up between the two plates of the 
cell: if the one predominate, the E.M.F. of the cell as a 
whole is lowered ; if the other predominate, the E.M.F. is 
raised. With weak solutions of copper sulphate the former 
tendency is always greater than the latter, and the H.M.F. 
falls continuously as zinc sulphate is added to the copper- 
sulphate solution, as indicated in curves nos. 5 and 6, fig. 2. 
With strong solutions of copper sulphate the latter tendency is 
