MR. F. K. SMITH ON THE NORMAL WFSTON CADMIUM CELL. 403 



The mean E.M.F. of the 12 " National Bureau of Standards" cells is less than the 

 mean E.M.F. of the 12 N.P.L. cells by 3 microvolts. 



P 52, P 53, P 54, and P 55 were set up in November, 1906, 

 C 12, C 17, C 19, and C 117 June 



H 26, H 28, and H 29 February, 1907, 



P210 March 



The mean E.M.F. of the 12 N.P.L. cells is not quite the mean of all the N.P.L. 

 cadmium cells, which are believed to be normal ; the latter cells have the higher mean 

 by about 2 parts in 100,000. 



Eleven of the twelve cadmium cells from Washington contain electrolytic mercurous 

 sulphate; the twelfth, W 105, contains mercurous sulphate prepared by the Lunge 

 reaction. The depth of the pastes in these cells is about 1'5 centims. The twelve cells 

 of the National Physical Laboratory contain mercurous sulphate prepared chemically 

 (Method II.). and the depth of the paste is about 0'5 centim. The nett result of 

 these comparisons with other observers is that mercurous sulphate of sufficiently 

 uniform properties can be prepared in several ways, provided that certain conditions 

 are observed. The possibility of an approximately constant size of mercurous sulphate 

 crystal resulting from all the methods must not, however, be overlooked, and as the 

 size of the crystal has not been stipulated by us, it is necessary to examine the 

 evidence on this point. 



H. v. STEINWTSHK* was the first to call attention to the part played by the size 

 of the crystal, and our method of investigation is very similar to that employed by him. 



Effect of the Size of the Crystals of Mercurous Sulphate. 



Twenty samples of mercurous sulphate have been examined under the microscope, 

 and in twelve cases microphotographs have been taken, the magnification being 250. 

 Of these, eleven are reproduced on Plate 5. It will be seen from these micro- 

 photographs that not only do the crystals vary considerably in size, but that each 

 method of preparation produces its own particular type of crystal. The electrolytic 

 method, as employed by us, gives small crystals, somewhat imperfect in shape, and 

 varying in size from 2 to 8 microns (0'002 to O'OOS millim.). The crystals resulting 

 from the chemical precipitation method are very much rounded, like pebbles ; they 

 have the appearance of being formed from perfect crystals, the edges of which have 

 been rounded by the solvent action of the hot dilute sulphuric acid. The uniformity 

 in the size of the crystals is more marked in the samples produced by this method 

 than in the specimens made in any other way tried by us. The size of most of the 

 crystals in figs. 2 to 4 varies from 5 to 30 microns. The mercurous sulphate produced 



* H. v. STKINWEHB, Zeitschr. f. Instrumentenk.,' 25, pp. 205-208, July, 1905. 



3 F 2 



