Chemical Affinity in terms of Electromotive Force. 19 
cadmium immersed in mCdSO, 100H,0O (referred to amalga- 
mated zinc immersed in equally strong zinc-sulphate solutions) 
are postive for weak solution-strengths and negative for con- 
siderably strong solutions, being in no case numerically great, 
viz. as tollows :— 
| at | ne ba ae Thermovoltaic 
H. constant. 
2 25, bode es 15 +:005 
‘5 “S01 | 2 °7358 +003 
1:0 “360 359 +001 
20 *359 *362 — .003 
30 398 *365 — ‘C07 
4-0 "3079 | *367 — “0095 
| 5:0 OO. | *367 —°010 
Til. Silver. 
173. The experiments already described in Part VI. show 
that the H.M.F.’s of zinc-silver-sulphate cells and the corre- 
sponding cadmium- and copper-silver cells are respectively 
1°536, 1°1805, and -4235, when amalgamated-zine and electro- 
silver, -copper, and -cadmium plates are used with solutions 
of (042 MSO, 100 H,O throughout. The probable error 
attaching to these values was in each case between +°001 and 
+0015. Hence, referred to amalgamated zine in zinc- 
sulphate solution of this strength as zero, the following values 
result for the voltaic constant of electro-silver in saturated 
silver-sulphate solution :— 
Semen iver cells... 2. we et ee eee 1:536 
From Cadmium -Silver + Zince-Cadmium eells 1:1805+ °362=1°5425 
From Copper-Silver + Zinc-Coppercells.. 1°114 4+°4285=1-:5375 
These three valuations do not accord so well as those obtained 
in most of the experiments subsequently detailed and made 
by means of the galvanometer ; the chief causes of the dis- 
erepancies lie in the greater want of permanence and wider 
range of fluctuation noticed with these cells (vide $§ 125-128, 
Part VI.), probably due to the extremely weak solutions 
necessarily employed owing to the sparing degree of solu- 
bility of silver sulphate. 
Taking the mean value 1:539 volt as the voltaic constant 
for electro-silver in saturated silver-sulphate solution, it 
results that the corresponding thermovoltaic constant has a 
large negative value ; for J rae Thomsen finds that Ag,, O 
2 
p] 
