PRACTICAL STANDARDS FOR ELECTRICAL MEASUREMENTS. 35 
with a basic nitrate, and basic salts are to be avoided, as will afterwards 
be seen. Samples of the salts were prepared by these precipitation 
methods and the results were far from satisfactory. Two samples of the 
sulphate were also obtained from the same manufacturer ; the Clark cells 
prepared with these differed in E.M.F. by 0-0004 volt ; each was sub- 
jected to the same treatment and there was no observable difference in 
colour. 
A more satisfactory specification of the depolariser being desirable, 
other modes of manufacture eliminating the above troublesome elements 
were sought. Concentrated sulphuric acid and mercury react very slowly 
at ordinary temperatures, but much more rapidly at temperatures approxi- 
mating to 300° C. The resulting salt is associated with H.SO,, which 
probably is not very difficult to remove if the salt be ina fine state of 
division. Dr. Muirhead has prepared mercurous sulphate in this way and 
forwarded two Clark cells containing the product to Bushy House. A 
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second method of preparation due to Divers and Shimidzu is reported in 
the ‘Journal of the Chemical Society’ (47, 639). Briefly, pure mercury 
and fuming sulphuric acid saturated with SO, are brought into contact. 
They react in the cold, though there is no visible evidence for some time 
owing to the solubility of the product in the liquid ; the acid also becomes 
saturated with SO,. If the main portion of the liquid be removed from 
the salt, this latter may be freed from SO, by warming ; mercurous 
sulphate associated with H,SO, is thus obtained. The acid is removed 
_ by washing. Dr. Carpenter has prepared five samples of the salt in this 
way ; they were made from five lots of the fuming acid from different 
manufacturers and mercury distilled im vacuo at the laboratory. These 
sulphates were examined in four-limb cells of the cadmium type ; the 
results of the observations are given in Table II. The standard of 
reference is cell No. W 17, a cadmium cell more than two years old and 
known to have changed but little since its manufacture. 
D2 
