404 



MR. F. E. SMITH ON THE NORMAL WESTON CADMIUM CELL. 



by the action of fuming sulphuric acid on mercury is evidently in the form of large 

 crystals, but most of these appear to get broken in the washing processes, with the 

 result that the size of the grain is considerably diminished. Figs. 9 and 10 show 

 these fragments of crystals, which vary in size from 5 to 25 microns. 



We have not taken any microphotographs of crystals produced by Method III., as 

 this method is only of interest as an independent means of obtaining mercurous 

 sulphate ; it is far too troublesome to be generally employed. We have, however, 

 prepared some very large crystals by a method suggested by H. v. STEIN WEHR.* A 

 dilute acid solution of mercurous nitrate was allowed to flow very slowly indeed from 

 a burette into dilute sulphuric acid (1 to 4) heated to 100 C. over a water-bath. 

 Small crystals of mercurous sulphate were at first formed, which grew, however, in 

 the slightly super-saturated solution of mercurous sulphate until many of them 

 attained a length of a few millimetres. The crystals were washed with dilute 

 sulphuric acid (1 to 6) and the finer particles were removed by agitation with the 

 acid and rapid decantation. The resulting product was undoubtedly of large grain, 

 and there were numerous particles present of the size shown in fig. 5. The length of 

 this crystal is about 400 microns. 



In September, 1906, three cells were set up which contained these large crystals 

 of mercurous sulphate as a depolariser. Table III. gives the results of observations 

 on these cells from October, 1906, to May, 1907. 



TABLE III. 



The cells are not in very good agreement and the mean E.M.F. is about 8 in 100,000 

 higher than the E.M.F. of the -normal cells in Table I. Recently we have set up 

 more cells with a second sample of large-grained mercurous sulphate, and they also 

 have E.M.F.'s higher than T01830 volts. In order that there should be no doubt 

 about the size of the crystals in S 1, S 2, and S 3, they were unsealed in June, 1907, 

 the pastes washed with dilute sulphuric acid to remove the cadmium sulphate crystals 

 and the residue examined microscopically. It was apparent that during the prepara- 

 tion of the depolarising paste and its insertion in the cell many of the large crystals 



* H. V. STEINWEHR, ' Zeitschr. f. Instrumentenk.,' 25, pp. 205-208, July, 1905 ; also ' Zeitschr. f, 

 Electrochem.,' pp. 578-581, 1906. 



