Chemical Affinity in terms of Electromotive Force. 31 
formed. On taking a long series of readings of these cells, it 
was found that whilst the E.M.F. was, when the cell was 
newly set up, close to that of an average mercurous-sulphate 
cell, on standing a few days a distinct fall was observable, 
which went on progressively until, after some weeks, a dimi- 
nution in the E.M.F. of between 3 and 4 per cent, was 
brought about, after which the fall ceased or became very 
languid. Thus the following average readings were obtained 
as before, the average E.M.F. of the hot-prepared cells con- 
taining saturated zinc-sulphate solution being taken as 100 
when at the same temperature as the cells examined: cells A, 
B, C, and D were set up simultaneously with turpeth mineral 
prepared by ourselves ; cells E and F simultaneously with 
the purchased substance. The zinc, zinc sulphate, and mer- 
cury used were the same as those used for the hot-prepared 
cells. Notwithstanding, however, that all the cells were as 
alike as possible, yet the rate of fall during the first few weeks 
was by no means identical. 
Age of cell... 
1 day. 
2 to 6 
days. 
lto2 
weeks. 
6 weeks. 
2 to 4 
months. 
6 to 20 
months. 
Cell A 
„ B 
■„ C 
„ D 
100-6 
100-3 
100-4 
100-6 
100-12 
10031 
100-22 
100-46 
99-95 
100-11 
99-90 
99-80 
98-35 
99-65 
99-19 
97-88 
97-04 
98-18 
97-38 
97-39 
97-00 
97-64 
96-96 
97-44 
Average 
100-5 
100-28 
99-93 
98-77 
97-50 
97-26 
CellE 
„ F 
99-4 
99-5 
98-85 
99-41 
97-27 
98-13 
96-78 
9711 
95-80 
96-00 
Average 
99-45 
99-13 
97-70 
96-95 
95-90 
It is evident from these figures that the effect of the pre- 
sence of turpeth mineral in the mercurous sulphate used for 
Clark's cells is in the direction of decreasing the value ; but 
inasmuch as the decrease is progressive, whilst no such altera- 
tion was observed in the Clark's cells examined, at any rate 
during several months after construction, it appears doubtful 
whether the variations in the E.M.F. of different Clark's cells 
set up at various times can be attributed to this cause. 
Permanence of Claries Cells. 
138. A number of cells prepared in various ways (paste 
boiled and cells sealed with paraffin-wax; paste boiled in vacuo 
and cells hermetically sealed ; set up with saturated zinc-sul- 
phate solution, or with weaker solutions) were kept for periods 
of time ranging from a few months to two or three years, and 
