Chemical Affinity in terms of Electromotive Force. 121 
_ These two sets of figures lead to practically identical valua- 
tions of the voltaic constant:— 
m= *25, m=1'0. m=2'0. 
202] SE ee 580 "588 ‘b91 
Zinc-Copper — Copper-Lead { ee } 580 { "B01 588 { se, } ‘590 
Mean ......:.. ‘580 588 591 
Taking the valuations of Julius Thomsen, including his 
determinations of the heats of dilution of zinc- and lead-nitrate 
solution, h, and h, respectively, the following values result 
for Hy, the value of Pb, O, NO; aq. being 68070 for 
25 Pb(NO;). 100 H,O:— 
m h, he H Eq Bye |S ee 
25 0 o | 34440 | -759 | 580 | —-179 
10 | — 40 | —1274 | 39206 | +782 | -588 | —-144 
90 | —132 | —2092 | 32462 | -716 | ‘591 | —-125 
ee a eee eee 
Hence the thermovoltaic constant for electro-lead in nitrate 
solution is always negative (as with sulphate and chloride); 
the numerical value, however, decreases with increasing solu- 
tion-strength, whilst the opposite isthe case with lead sulphate, 
whether suspended in zinc- or in cadmium-sulphate solution. 
Lead-copper-nitrate cells resemble lead-copper-sulphate 
cells in that they give an E.M.F’. notably above that calculable 
from the net heat-evolution taking place in the cell due to 
the chemical change. Similarly zinc-lead-nitrate cells re- 
semble zinc-lead-sulphate cells in giving values below those 
thus calculable. 
| IV. Silver. 
191. The following mean values were obtained with cells 
set up with electro-silver plates opposed to amalgamated zinc, 
electro-copper, and electro-lead in nitrate solutions of strength 
mM(NO3)2, 100 HzO (the probable error ranged from +:002 
to +:004 throughout) :— 
Ms Zinc-Silver. Copper-Silver. Lead-Silver. 
25 1:495 ‘429 ‘914 
1:0 1540 ‘450 "951 
2:0 1:556 "446 "965 
Phil. Mag. 8. 5. Vol. 19. No. 117. Feb. 1885. K 
