Chemical Affinity in terms of Electromotive Force. 17 
unique peculiarity, that the value of its voltaic constant in 
sulphate solution, as above defined, is practically uniform for 
all solution-strengths ; for, as already shown, the H.M.F’. of a 
zinc-copper-sulphate cell is sensibly the same whatever the 
solution-strength when amalgamated-zine and electro-copper 
plates are used, viz. (on the assumptions above discussed, 
§ 166) 1:114+-:0005 volt; so that the voltaic constant for 
electro-copper in sulphate solution is sensibly +1:114 for all 
solution-strengths (i. e. when referred to amalgamated zinc 
in zinc-sulphate solution of the same strength as zero). 
The thermovoltaic constant of electro-copper also shows the 
same independence ; for the experiments described in § 160 
show that the heats of dilution of zinc- and copper-sulphate 
solutions are sensibly identical for equal dilution-ranges; hence 
the heats of formation of zinc- and copper-sulphate solutions 
both of strength m MSO, 100H,0 (where m is the same) 
must differ by a constant amount whatever the value of m: 
this amount for m=°25 (and therefore for all other values of 
m) is, from Thomsen’s results :— 
Zn, O, SO; aq.=106090 
Cu, O, SO3aq.= 55960 
~50130=1:105 volt. 
Hence, finally, since H=1:114 and H,=1°105 for all solu- 
tion-strengths, the thermovoltaic constant for electro-copper 
in sulphate solution is constantly 1°114—1:105=+:009 for 
all solution-strengths. 
Il. Cadmium. 
172. The experiments with the quadrant-electrometer 
described in Part VI. show that the electromotive forces of all 
zine-cadmium-sulphate cells set up with solutions of zine and 
cadmium sulphate of the same strength, m MSO, 100H,0, are 
comprised within the limits °360+°0015, when m is between 
"08 and 2:0, and that those of the analogous copper-cadmium- 
sulphate cells are similarly comprised within the limits 
1525 +0015. 
The experiments described in Part VIII., however, enable 
still more exact figures to be deduced. Applying the formula 
-H=e+a—2, and taking the values of a and 6 from the tables 
quoted, the following figures result as the most probable ones, 
the probable error in no case being so great as +002 volt:— 
Phil. Mag. 8. 5. Vol. i9. No. 116. Jan. 1885. C 
