\IK I. K SMITH ON THE NORMAL WK.STOX CADMIUM CELL. 395 



anode in dilute sulphuric acid, the latter, in the experiments made at the National 

 Physical Laboratory, consisting of 1 volume of strong sulphuric acid (density 1'84) to 

 5 volumes of water ; its strength was, therefore, about 3'0 molecular. The anode 

 surface was kept well exposed by a glass stirrer, and the current density was from 



1 to 5 amperes per 100 sq. centims. of anode surface. In the second method 

 purchased protonitrate of mercury was sometimes used, but more often it was made 

 from mercury and nitric acid. About 1 5 cub. centims. of concentrated nitric acid 

 was added to 100 grammes of mercury, and when the action was over, or nearly over, 

 the resulting solution was added to 200 cub. centims. of dilute nitric acid (1 of acid 

 to 40 of water). The acid solution of mercurous nitrate thus formed was run as a 

 very fine stream from the narrow orifice of a pipette into 1000 cub. centims. of hot 

 dilute sulphuric acid (1 to 3), the liquid being well stirred during the mixing. 

 Mercurous sulphate was precipitated. It was washed two or three times by 

 decantation with dilute sulphuric acid (1 to 6) and filtered. The third method of 

 manufacture is more costly. A purchased sample of the salt is heated with 

 concentrated sulphuric acid to a temperature of about 150 C. and the hot clear acid 

 carefully poured into dilute sulphuric acid (1 to 6), when precipitation of pure 

 mercurous sulphate results. The fourth method, as originally employed, is trouble- 

 some. Fuming sulphuric acid is added to pure distilled mercury and stirred well 

 until the action between the two is practically at an end. Mercurous sulphate is 

 thus formed in the cold and appears in the crystalline form after a few minutes. 

 Equally satisfactory results are obtained, however, if sufficient mercury is placed in a 

 crystallising dish to cover the base and the fuming sulphuric acid added to a depth of 



2 or 3 millims. The dish is covered with a clock glass and placed in a dark room for 

 one or two weeks. 



In all the methods of production the resulting mercurous sulphate was washed two 

 or three times by decantation with dilute sulphuric acid (1 to 6) and afterwards 

 introduced into a Buchener funnel for the removal of the acid as completely as 

 possible by exhaustion with a filter pump. The sides of the funnel were washed 

 down with neutral saturated cadmium sulphate solution and the salt washed 

 5 or 6 times with more of the same solution. About 5 cub. centims. was needed 

 for each washing. In a few instances the sulphate was straightway employed for 

 the manufacture of the depolarising paste, but in the majority of cases it was 

 transferred together with a little of the cadmium sulphate solution to a small stock 

 bottle. After a week or ten days the solution was always slightly acid to congo red 

 ji;i[KM' and the mercurous sulphate was therefore washed once more before using. 



In much of the earlier work absolute alcohol which had been specially distilled was 

 employed for washing the mercurous sulphate, and the salt was stored in contact with 

 more of the same liquid. We cannot, however, recommend this procedure, as we 

 l>elieve slight hydrolysis results, owing to the absorption of moisture by the 

 alcohol. 



3 E 2 



