On the Determination of Chemical Affinity. 25 
same clearness of tone which distinguishes the same arrange- 
ment in air at the ordinary pressure from that in a vacuum. 
The degree of vacuum existing in the bulbs is that of the usual 
incandescence lamp, or about one millionth of an atmosphere. 
It might be thought from M. Edlund's researches, recently 
referred to in the Philosophical Magazine, that polarization of 
the metal contacts would prevent the microphonic action in 
vacuo; but this is evidently not the case. 
V. On the Determination of Chemical Affinity in terms of 
Electromotive Force. — Part VII. By C. E. Alder Weight, 
D.Sc. (Lone!.), F.R.S., Lecturer on Chemistry and Physics, 
and C. Thompson, Demonstrator of Chemistry, in St. Mary's 
Hospital Medical School*. 
On the Electromotive Force of Clark's Mercurous- Sidphate 
Cell ; and on the Work done during Electrolysis. 
On the E.M.F. of Clark's Cell. 
133. 
IN the course of the series of experiments partly d< 
scribed in Parts V. and VI. a larp-e number < 
le- 
scribed in Parts V. and VI. a large number of 
observations have been made with various cells after Clark's 
construction (Proc. Roy. Soc. xx. p. 444), in all cases com- 
pared with one another and with other cells by means of the 
quadrant-electrometer only, so that they never generated any 
current other than the minute leakage current through the 
not mathematically absolutely insulating materials between 
their poles. 
In some instances the mercurous sulphate was purchased 
(from Messrs. Hopkin and Williams), and was well washed 
before use by numerous boilings with distilled water and de- 
cantations. In other cases the mercurous sulphate was pre- 
pared by heating twice-distilled mercury (previously purified 
by nitric acid) with pure sulphuric acid at as low a tempera- 
ture as possible consistent with any action taking place, and 
thoroughly washing the resulting sulphate by repeatedly boil- 
ing with distilled water and decantation. The action was 
never allowed to go on until more than a fraction of the mer- 
cury used was converted into sulphate, in order to reduce the 
amount of mercuric sulphate formed to a minimum. 
The cells were made out of pieces of ordinary combustion- 
tubing (selected on account of the absence of lead in the glass) 
* Communicated bv the Physical Society, having been read May 12. 
1883. 
