396 MR. P. E. SMITH ON THE NORMAL WESTON CADMIUM CELL. 



Cadmium Amalgam. 



The amalgam has been prepared in two ways, (1) by depositing cadmium 

 electrolytically in a weighed quantity of pure mercury, the electrolyte being an 

 acid solution of cadmium sulphate, (2) by heating together pure cadmium and 

 mercury, the resulting mass being washed with dilute sulphuric acid to remove the 

 surface dross. The amalgams used have been one part of cadmium to seven parts of 

 mercury (12^ per cent.) or one part of cadmium to nine parts of mercury (10 per cent.). 

 We have employed amalgams of other concentrations, but not in the cells discussed in 

 this paper. The investigations of DEARLOVE,* KEEP and BOETTGER,! BUL^ 

 PUSCHIN, and JAEGER|| have shown that attention must be paid to the percentage of 

 cadmium m the amalgam. It appears probable that in an amalgam containing 

 between 6 and 14 per cent, of cadmium, there is at normal temperatures a solid and 

 a liquid phase, of which the former is an isomorphous mixture of mercury and 

 cadmium. The E.M.F. of the amalgam towards a cadmium sulphate solution does 

 not depend on the relative amounts of the two phases, and on adding cadmium to the 

 amalgam no change in E.M.F. occurs therefore until the liquid phase disappears. 

 Similarly, if cadmium is extracted from the mixture the E.M.F. is constant until the 

 solid phase disappears. A rise in temperature increases the liquid and diminishes 

 the solid phase : for any particular amalgam there are, therefore, limits of tempera- 

 ture between which the two phases are always present. JAEGER has shown that for 

 all amalgams in which the two phases exist the E.M.F. towards a cadmium sulphate 

 solution is constant for a given temperature. DEARLOVE first proposed a 12^ per cent, 

 amalgam ; this is satisfactory at all ordinary temperatures and has been generally 

 employed. 



The Cadmium Sulphate. 



We have usually ground the purchased crystals and made a saturated or nearly 

 saturated solution by agitation and warmth. This was filtered to clear and placed in 

 crystallising dishes to slowly evaporate. The resulting crystals were well washed 

 several times with water and the final solution tested for acidity with congo red 

 paper. 



A saturated solution of cadmium sulphate yields crystals of the composition 

 CdSO 4 |H 2 O at all temperatures up to 74C., when CdSO 4 H 2 O separates instead. 

 KOHNSTAMM and COHEN! believed that they had discovered a transition point at 



* A. DEARI.OVE, 'The Electrician,' vol. 31, p. 645, 1893. 



t KKRP and BOETTGER, 'Zeitschr. f. anorgan. Chem.,' 25, p. 1, 1900. 



J BIJL, 'Zeitschr. f. Phys. Chem.,' 41, p. 641, 1900. 



PUSCHIN, ' Zeitschr. f. Phys. Chem.,' 34, p. 621, 1901. 



|| W. JAF.GEK, 'WiED. Ann.,' 65, p. 106, 1898; 'Zeitschr. f. Instrumentenk.,' 20, p. 317, 1900. 



f T. KOHNSTAMM and E. COHEN, ' WIED. Ann.,' 65, p. 344, 1898. 



