Chemical Affinity in terms of Electromotive Force. 25 
for Ey of -075 volt, cadmium acquiring the higher potential ; 
but upon this chemical source of potential-difference there is 
superposed the thermovoltaic action 
ky —k, = +'001—"114= —'118 ; 
hence the net effective H.M.F’. is 
+°075—1138=—:038: | 
that is, instead of cadmium acquiring the higher potential, iron 
does so,and a current flows with an H.M.F. of —:038 volt 
(mean observed value = —‘035 volt). 
As regards iron-lead-sulphate cells, it is to be remarked that, 
since lead acquires the higher potential and since the thermo- 
voltaic constant of lead is a considerable negative quantity 
whilst that of iron is also a considerable positive quantity, the 
value of k; —k, is largely negative, viz. for solutions of strengths 
1:0 MSO, 100 H,O (zinc-sulphate solution being used to 
suspend lead sulphate in), —-200—-114=—-314. That is to 
say, whilst the value of Ey corresponding to this displacement 
of lead from lead sulphate (solid) by iron is ‘428 volt (corre- 
sponding with 93200—73800=19400 gramme- -degrees per 
gramme-molecule), the effective H.M.F. of the cell is little 
more than one fourth of this, viz. -428—°314=:114 (mean 
observed value =‘112 volt). 
The same kind of remark applies to iron-copper-sulphate 
cells in a less degree, the value of k,—/, being here (for 1:0 
MSO, 100 H,O) +:009—-114=—-105 ; that is, whilst the 
value of Hy is ‘821 (corresponding with 93200—55960= 
37240 gramme-degrees per gramme-molecule), the effective 
H.M.F. of the cell is only °821—:105=-716 (mean observed 
value =°715 volt). 
It should consequently result that, if a cell be set up with 
an electro-silver plate immersed in saturated silver-sulphate 
solution opposed to bright iron in an equally strong ferrous- 
sulphate solution, viz. ‘042 FeSO, 100 HO, the value of k,—ky 
will be largely negative, approximating to —°351—"148= 
—499 ; so that as Hq is, in this case, 1-605 (corresponding 
with 93200—20390= 72810 gramme-degrees per gramme- 
molecule), the effective H.M.I’. will be near to 1:605—-499= 
1:106 volt. In point of fact, a number of such cells gave on 
examination the average value 1:103 with a probable error of 
+°0038 volt. 
Lifect of varying the Plate- surface in Lron-copper-sulphate Cells ; 
and Late of Depreciation of L.M.L’. of such Cells with increasing 
Current-density. 
176. Some experiments were carried out precisely as de- 
scribed in §§ 115, 124, and 181, with cells set up with polished 
