CLARK CELL AS A STANDARD OF ELECTROMOTIVE FORCE. 
593 
§ 16. Treatment of the Mercurous Sulphate. 
In studying the effect of variations in the materials, the impurities usually found 
in the mercurous sulphate appeared to be of primary importance. It is not easy to 
purchase this substance in a state of purity. The common preparations usually met 
with are either grey or very white. The grey powder contains mercury in excess ; 
this does not appear to be hurtful to the cell; the white contains a considerable 
quantity of mercuric sulphate. This is shown by the substance turning yellow on 
adding water, with the formation of turpeth mineral or basic mercuric sulphate. 
This also is not harmful in small quantities, but its presence shews that the original 
powder has contained mercuric sulphate. This has been resolved by the water with 
the formation of the yellow substance and a soluble acid mercuric sulphate, and 
unless this process is complete and all traces of the original mercuric salt have been 
removed, error will be introduced.* Thus good cells may be set up with paste which 
is yellow at starting, provided the change is complete, but if the paste of a cell turns 
yellow after the cell has been made, it means that the mercuric salt is present, and 
this affects the E.M.F. On treating a sample containing mercuric sulphate with 
mercury, the mercuric sulphate is converted into mercurous, and may be used 
with safety. 
The fact that the mercury mixes shows, as Mr. Swinburne has remarked, that the 
sulphate is bad, but the result of the mixing is that the evil is cured. This, then, is 
the rationale of the method of preparing the paste given in the memorandum. The 
washing with water resolves most, if not all, of the mercuric salt present into the 
yellow basic sulphate 3HgO.SO s , and the soluble sulphate HgS0 4 .2S0 3 ; the latter is 
dissolved and removed. The shaking with mercury at a temperature near 30° removes 
any further traces of the acid mercuric salt. The temperature should not exceed 30°, 
for, when, cooling from a temperature in excess of this, crystals may form which are 
not of the proper composition ZnS0 4 .7H 2 0. 
§ 17. Cells set up by Mr. L. It. Wilberforce, March , 1891. 
This set of cells was made by Mr. L. It. Wilberforce during March, 1891 (probably 
on March 19), and are in short test tubes with platinum wires fused through 
their bases for terminals. They cannot, therefore, be directly immersed in water. 
They were constructed with a view of finding the alterations in E.M.F. which 
would result from slight modifications in the materials and in the modes of pre¬ 
paring the cells. The last tests show them to be all in very close agreement 
amongst themselves and with cell No. 1. 
* According to Watts’ ‘ Dictionary ’ the mercuric sulphate in presence of water becomes 
/HgS0 4 .2S0 3 soluble. 
\3HgO.SCL turpeth mineral. 
4 U 
MDCCCXCII.—A. 
