30 Messrs. Wright and Thompson on the Determination of 
Batch of four cells made from mercurous i 1 °* ' qq.qa 
sulphate purchased from Messrs. Hop-/ " a " q Q q - 
kin and Williams (4 99*99 
Batch of four cell made from mercurous \ " ' oq.q- 
sulphate prepared specially by ourselves <[ " ' qq.qq 
for this purpose [» g ; mm 
Batch of four cells made from another ( » J -.^I 
specimen of mercurous sulphate pre- 1 » ' J: 1 Q 
pared by ourselves { J £ Mwf 
General average .... 99*995 
Precisely analogous figures were obtained with several 
vacuum-prepared cells, no one of which gave a value outside 
of the limits 99*7 and 100*3, i. e. outside of the limits of fluc- 
tuation of the ordinary Clark's cells compared with them. 
On opening one of these vacuum-cells so as to admit air, a 
distinct fall in E.M.F., amounting to 0'25 volt, was observed ; 
this behaviour, however, was not shown by other similar cells 
on opening. 
Influence of Mercuric Sulphate in the Mercurous Sulphate. 
137. However carefully the mercurous sulphate may be 
prepared, it is almost impossible to obtain it without some 
admixture of mercuric sulphate. During the boiling and wash- 
ing by decantation this latter becomes a basic salt, the so-called 
" turpeth mineral," which possesses a bright yellow tint, and 
communicates to the mercurous sulphate a more or less pro- 
nounced yellowish tinge. In order to see how far the presence 
of varying quantities of this compound might possibly affect 
the E.M.F. of Clark's cell, several cells were set up m the 
same way as the hot-prepared cells described above, but using 
turpeth mineral only instead of mercurous sulphate. Two 
samples of turpeth mineral were employed: — one purchased 
(from Messrs. Hopkin and Williams), and well washed by 
boiling up many times with water and decantation before use; 
the other prepared by boiling mercury with a large excess of 
pure sulphuric acid, evaporating off most of the acid (which 
process converts all mercurous sulphate present into mercuric), 
adding to a large bulk of boiling water and washing many 
times the yellow heavy powder formed, by boiling up with 
water and decanting, so as to remove all traces of free sul- 
phuric acid, and of the soluble acid mercuric sulphate also 
