Chemical Affinity in terms of Electromotive Force. 103 
Value of m. E. 16590—(h, —h,). i: E-—Eg. 
Oi ‘330 16590 "366 — 036 
5 329 16190 357 — 028 
10 ‘324 15340 338 —‘014 
2:0 "315 13890 "306 + 009 
4:0 ‘307 12290 ‘271 +036 
6:0 "296 11390 "251 +045 
8-0 "262 10890 ‘240 + 022 
It is here evident that whilst the voltaic constant for cad- 
mium-zine-chloride cells continually diminishes as the solu- 
tion-strength increases, the value of Hy diminishes still more 
rapidly; so that the thermovoltaic constant is negative for 
weaker solutions and positive for stronger ones, the actual 
values of this constant being somewhat larger than those 
observed with zinc-cadmium-sulphate cells (§ 172), but still 
always under 5 centivolts. With the sulphate ceils the values 
of the constant were opposite in sign, viz. positive for weaker, 
and negative for stronger solutions. 
II. Copper. 
181. On immersing a plate of clean metallic copper in a 
strong solution of cupric chloride, the plate speedily becomes 
visibly coated witha film of sparingly soluble cuprous chloride 
formed in virtue of the reaction 
Cu + CuCl,=Cu,Cly. 
With weaker solution the actual deposition of cuprous chlo- 
rides in the solid state is not always noticeable, although its 
formation and presence are readily detected analytically. It 
is hence evident that any cell containing as one half a solution 
of cupric chloride and a copper plate must really contain as 
the electrolyte on this side a mixture of cuprous chloride with 
more or less cupric chloride, the film of fluid in immediate 
contact with the copper plate containing but little if any of 
the latter salt in solution. In point of fact, on examining a 
number of similar cells set up with electro-copper plates 
immersed in +25 CuCl, 100 H;O on the one side and amalga- 
mated-zine plates immersed in *25 ZnCl, 100-H,O on the 
other, values were obtained fluctuating between 1:083 and 
1:106 volt; whilst another series of analogous values fluc- 
tuating between almost exactly the same limits, and averaging 
practically the same value, was obtained with a corresponding 
set of cells in which the copper plate was immersed, not in 
cupric-chloride solution alone to begin with, but in a magma 
