MR. F. E. SMITH ON THK NORMAL \VKSTON CADMIUM CELL. 



40D 



wore broken, with the result that the mean size of the crystals was diminished, hut 

 the fragments were still much larger than the crystals prepared by any of the other 

 methods. Figs. 1 and 8 (Plate 5) are microphotographs of some of the crystals of 

 mercurous sulphate after their removal from the cells Si, S 2, and S 3. It was 

 olwerved that in all cases there was a small number of comparatively small crystals, 

 or fragments of crystals, associated with the large ones, but we found this unavoidable. 

 In one instance we took very great pains to eliminate small particles, and succeeded 

 in doing so to a considerable extent, but subsequent examination of the crystals after 

 the manufacture of the depolarising paste and its insertion in a cell showed that their 

 size had been appreciably reduced. 



T.I hie IV. enables a comparison to lx> made Iwtween cells set up with crystals of 

 various sizes. 



TABLE IV. 



The higher E.M.F. of the cells S 1, S 2, S 3 is probably due to slight impurities in 

 the mercurous sulphate, and not to the large size of the crystals. During the 

 preparation of these crystals the sulphuric acid was not stirred, and the only agitation 

 of the liquid was that produced by the mixing and by convection currents. It is 

 possible for slight hydrolysis to result under such circumstances, and also possible for 

 mercurous nitrate to be imprisoned in some of the large crystals which are formed. 

 The latter appears to be not improbable in our case. 



If we exclude from Table IV. those cells set up with the very large crystals, we 

 have as the limits of the dimensions of the others 3 to 30 microns, and we conclude 

 that if the size of the crystals be within these limits they will have very nearly the 

 same solubility and practically give the same E.M.F. in a standard cell. 



This conclusion is not in accordance with observations made by STEIXWEHR, who 

 first called attention to the possible effect of the size of the crystals, and claims that 

 it is a principal cause of the variations observed in standard cells. Lord KELVIN has 

 shown that the saturation pressure of small drops of water is greater than that of 



