38 REPORT—1904. 
The first set of observations with each cell was made about five 
minutes after adding the solution ; the second set of observations about 
twenty minutes afterwards ; and the third set three hours afterwards. 
For the first two sets of observations the temperature of the four-limb 
cells was unsteady ; for the remaining observations they and W 17 were at 
the same steady temperature. Other cells of the Rayleigh H form have 
been constructed, and the comparisons are equally satisfactory. 
An alternative method of preparing the salt was next sought. This 
second method is very simple. Any purchased sample of mercurous 
sulphate is heated together with mercury and concentrated H,SO, on a 
water-bath for half an hour, the mixture being stirred occasionally. At 
the end of that time the remaining solid is allowed to settle and the hot 
clear acid carefully poured into a large volume of distilled water. Mercu- 
rous sulphate is soluble to a considerable extent in hot concentrated 
H,SO, ; the result of the dilution is, in consequence, to precipitate the 
salt. As thus produced the mercurous sulphate is in a finely divided 
state and of a pure white colour. It is well to at once admix with a 
little mercury and filter. The washing is performed as before. Portions 
of three purchased samples of Hg,SO, were dealt with in this way, 
and after treatment gave identical results with the cells dealt with in 
Table III. The three original samples prepared in the ordinary way 
produced cells differing in E.M.F. from the standard by 40, 160, and 
10 hundredths of a millivolt. 
A third method devised by Professor Carhart does not necessitate the 
use of concentrated acid. In order to hasten the reaction between 
mercury and dilute sulphuric acid (one to six) an electric current is 
passed from the mercury to a sheet of platinum foil suspended in the 
liquid. It is essential that the liquid be kept well stirred so as to keep 
the mercury surface exposed. Professor Carhart has employed a beaker 
or crystallising dish to contain the liquids, and used a current of about 
0-3 ampére ; the current density, however, is not stated. At Bushy 
House the salt so produced has been compared with those prepared 
by the two previous methods. Under ordinary circumstances about three 
grams of the salt—very grey owing to the presence of mercury in a fine 
state of division—is obtained per hour. The current density at Bushy 
House has been about ‘01 ampére. It was gratifying to find that the 
product (washed as before) gave identical results with the other methods. 
Very violent agitation was maintained during the preparation. When 
the liquid is not stirred a yellow compound (apparently turpeth mineral 
HgSO,.2Hg0) is also produced, and cells the pastes of which are prepared 
with the product have an E.M.F. when first set up more than a millivolt 
higher than the normal. Particular stress must therefore be laid on the 
instruction to keep the mercury surface well exposed. The same thing 
was found to happen when attempting to form mercurous sulphate by the 
electrolysis of a saturated cadmium sulphate solution in an ]{-form 
vessel, the electrodes being pure mercury. 
It will be observed that the remarks on the depolariser apply equally 
to Clark and to cadmium cells. Cadmium cells alone were made up in 
the final tests because of their small temperature coefficients ; but Clark 
cells have also been set up and similar results obtained. It is also neces- 
sary to add that all purchased samples of Hg,SO, are not so abnormal as 
those dealt with in Table I., nor does the E.M.F, of an abnormal cell 
always fall so rapidly as is indicated there. (The rate of fall is probably 
