CLARK CELL AS A STANDARD OF ELECTROMOTIVE FORCE. 
5.95 
until the washings no longer give any reaction for a nitrate. It is useless to test the 
wash water for sulphate as mercurous sulphate is slightly soluble in water. The 
moist precipitate, after proper washing, is dried in a current of air at about 17° C. 
Prepared in this way, the powder, which is dull white, will not become yellow when 
moistened with water, nor will the water give an acid reaction. There is no mercuric 
sulphate present. It will slowly become grey if, whilst moist, it be exposed to light. 
£ is a paste containing mercurous sulphate prepared according to this method. In the 
preparation of the mercurous sulphate for 8, however, very dilute solutions were used 
so that no immediate precipitate of sulphate came down. After four or five days 
white crystals about | to 1 millim. in diameter separated. This is the purest 
mercurous sulphate we have used.'* 
rj. The sample of mercurous sulphate in these cells was prepared from mercuric 
sulphate by reducing it with an excess of pure mercury. The white mercuric salt is 
placed in a mortar, and some pure mercury added, with sufficient water to convert 
the whole into a paste ; it is then ground until the whole becomes grey. The end of 
the reaction is easily recognised by the appearance of the surface of the globules of 
mercury, for, during the time that chemical action is going on, their surfaces will 
appear tarnished. The sulphate prepared in this way is grey with excess of mercury. 
The various samples of mercury are denoted by 6, i, and k. 
9. Pure mercury distilled under reduced pressure in the laboratory. 
l. Mercury which had been used for general purposes, and was only filtered before 
being used for the cells. 
k. Mercury distilled in a small glass retort at atmospheric pressure. 
* A very useful suggestion is made by Mi’. Swinburne in his paper before the British Association, 1891, 
in which he recommends the use of zinc sulphate solution for the purpose of precipitating the mercurous 
sulphate, and washing with saturated zinc sulphate to remove zinc nitrate. The sulphate then does not 
require drying. 
4 G 2 
