210 Dr. C. R. A. Wright on the Determination of 



plates, immersed in solutions of the sulphates of these metals 

 respectively, are closely analogous to ordinary Daniell cells 

 (containing zinc sulphate solution). Slight variations in the 

 E.M.F. generated are introduced by varying the condition of 

 the plate-surfaces; but in all cases the maximum E.M.F. 

 actually generated with clean pure plate-surfaces and with 

 solutions of equal molecular strengths is close to that calculable 

 from the net chemical action taking place in the cell when 

 generating a current. When the cadmium plates are not 

 amalgamated, or are covered with crystalline solid amalgam, 

 the electromotive forces are close to "75 and *36 volt for 

 cadmium-copper and zinc-cadmium cells respectively, the 

 values corresponding to the net chemical actions as deduced 

 from Thomsen's therm o-chemical results being substantially 

 the same. When the cadmium plates are covered with fluid 

 amalgam, the electromotive forces are lower in the first case 

 and higher in the second by upwards of "04 volt — a quantity 

 distinctly exceeding in magnitude the E.M.F. corresponding 

 with the heat of solution of cadmium in mercury, although of 

 the same sign. 



(2) The electromotive forces of zinc-silver, cadmium-silver, 

 and copper-silver cells containing the respective sulphates of 

 these metals differ from those of zinc-cadmium, zinc-copper 

 (Daniell), and cadmium-copper cells in this respect, that the 

 maximum electromotive forces generated (the fluids being of 

 equal molecular strength) are not sensibly the same as those 

 calculated from Julius Thomsen's thermal data, but in every 

 case fall short by an amount not far from *35 volt. When 

 the silver plates are not mercurialized, or are coated with crys- 

 talline amalgam, the electromotive forces (which vary slightly 

 with the precise nature of the plate-surfaces) are, in the three 

 cases, near to 1*53, 1*17, and 0*42 volt respectively, the 

 metallic solutions being of equal molecular strength. When 

 the silver plates are covered with fluid amalgam, the electro- 

 motive forces are in each case about '09 volt higher than the 

 values obtained with electro-deposited crystalline metal, this 

 increase almost exactly coinciding with the increment corre- 

 sponding with the heat of solution of silver in mercury. 



(3) As long as the cadmium and zinc [or copper] solutions 

 employed are of the same molecular strength within the limits 

 indicated respectively by MSO 4 50H 2 O and MS0 4 1250 H 2 0, 

 the E.M.F. developed with a given pair of cadmium and 

 zinc [or copper] plates is sensibly independent of the actual 

 strength of the solutions, these cells behaving precisely like 

 Daniell cells in this respect. With Daniell cells the solutions 

 are practically of the same molecular strength when they 



