Chemical Affinity in terms of Electromotive Force. 29 
one side, and of pure mercury with a magma of mercurous- 
sulphate and zinc-sulphate solution on the other, the zinc 
sulphate being of the same strength on each side : with these 
cells the H.M.F. continually fell as the solution-strength rose*. 
It is evident that these cells differ but little from a cold- 
prepared Clark’s cell, the essential difference being that in the 
latter the zinc rod is not coated with fluid mercury to begin 
with, and is immersed, not in pure zinc-sulphate solution, but 
in the same magma of mercurous-sulphate and zinc-sulphate 
solution as surrounds the mercury. This difference in con- 
struction gives rise to a slight difference in H.M.F. with 
weaker solutions of zinc sulphate, the Clark’s cell uniformly 
reading lowest ; with saturated or very strong solutions the 
difference between the two kinds of cell is less than the expe- 
rimental errors. Thus the following average values were 
obtained as the voltaic constants of mercury in contactwith mer- 
curous sulphate and m ZnSO, 100 H,0, and as the E.M.F.’s 
of cells like Clark’s but set up with m ZnSO, 100 H,O instead 
of saturated zinc-sulphate solution :— 
M. Voltaic constant. E.M.F. of Clark’s cell. 
Volt. Volt. 
5 pls. 1-457 1-457 
4:89 1-462 1-462 
4°33 1-466 1:465 
3°58 1-471 1-469 | 
27 1477 1-475 
‘1 1°514 1:510 | 
The heat of formation of mercurous sulphate being unknown, 
the corresponding thermovoltaic constants cannot be exactly 
calculated ; in all probability, however, the value of Eg is near 
to 1°202 volt, the value for mercurous-zinc-nitrate cells, when 
m="25 (§ 192), so that the thermovoltaic constant is a large 
positive quantity, near to +°25 volt at least. 
[To be continued. | 
* It may be noticed that mercurous sulphate is about three times as 
soluble in saturated zinc-sulphate solution as in distilled water. Three 
saturated solutions gave, on analysis, numbers represented by the following 
formulee :— 
Mestllcd water. 3.06 cites cee ene ‘0017 Hg,SO, 100 H,0. 
Zinc-sulphate solution, about one- 
BeREEREAAUTALCG, . 32s 5 oie scene ‘0042 Hg,SO,, 2:0 ZnSO,, 100 H.0. 
Saturated zinc-sulphate solution .. ‘0054 Hg,SO,, 5°75 ZaSO,, 100 H,O. 
