Chemical Affinity in terms of Electromotive Force. 207 
aqueous solutions of that salt. Accordingly a magma of lead- 
iodide and zinc-iodide solution of strength 25 ZnI, 100 H,O 
was employed to surround the electro-lead plates used, opposed 
in one set of experiments to amalgamated zinc in the same 
zinc-iodide solution, and in another set to mercury immersed 
in a magma of mercurous iodide suspended in the same zinc- 
iodide solution. The following values were obtained, the degree 
of concordance being distinctly less than with most of the other 
combinations examined :— 
Zinc-Lead. Lead-Mercury. 
[Sc “489 — 317 
| Minimum ............... ‘418 —'379 
| Average .........ccceccses A455 — 347 
Probable error ......... +:009 +:008 
The lead-mercury cells gave a current in the direction 
opposite to that deducible from the relative heats of formation 
of zinc and mercurous iodides. 
- From these numbers the following valuations of the voltaic 
constant result :— 
_ 1 2:1. S21 eine i eae one eran aera igy 594) 
Zinc-mercury + Mercury-lead { ee == "454 
Mica ons. 6a. oye ie OD 
Julius Thomsen finds Pb, l,=39800 ; whence Ey=°457. 
Hence the thermovoltaic constant for electro-lead immersed 
in a magma of lead iodide suspended in ‘25 ZnI, 100 H,O is 
a minute negative quantity, viz. —‘002. 
This value and those above found for electro-lead in contact 
with saturated-chloride and bromide-of-lead solutions respec- 
tively, viz. —°222 and —-°235, are not strictly comparable, 
since the solution-strength was not the same throughout ; 
whilst a magma containing zinc-iodide solution was used for 
the iodide cells and pure lead-salts for the others, which, 
moreover, had respectively the strengths ‘05 PbCl, 100 H,O, 
and ‘037 PbBr, 100 H,O. Probably from these causes the 
bromide value is not intermediate between the chloride and 
iodide values, as might have been anticipated ; but the last 
