378 Messrs. Wright and Thompson on the Determination of 



exceeds that of a for all cases where equal molecular strengths 

 of solutions are used in zinc-cadmium-chloride cells; L e. the 

 E.M.F. of a cell set up with zinc and cadmium chlorides, 

 amalgamated zinc, and electro-cadmium decreases as the solu- 

 tion-strength increases, both solutions being equal in strength. 

 A number of observations made with the galvanometer 

 showed that this is the case, and that the value of the E.M.F. 

 of any zinc-cadmium- chloride cell is given in volts by the 

 equation 



E = -330 + a-6, 



where '330 is the E.M.F. of a cell containing amalgamated- 

 zinc and electro-cadmium plates, and solutions of the chlorides 

 of equal strength '25MC1 2 100 H 2 0; whilst a and b are the 

 values deduced from the above tables for the difference of 

 potential set up for solutions of this strength as compared with 

 the particular strengths employed in the cell examined. 



157. A noteworthy point in connexion with the value *330 

 volt for solutions *25MC1 2 100H 2 O is the following. Ac- 

 cording to Julius Thomsen (Journ. Prakt. Chemie, xi. p. 402), 

 the heats of formation of cadmium and zinc sulphates dissolved 

 in400H 2 O ('25MS0 4 100 H 2 0) exhibit exactly the same dif- 

 ference as exists between those of cadmium and zinc chlorides 

 dissolved in 200 H 2 0; thus: — 



Zn, 0, S0 3 , aq= 106090 

 Cd, O, S0 3 , aq 89500 



16590 



Zn, Cl 2 , aq= 112840 

 Cd, Cl 2 , aq 96250 



16590 



Consequently, as the heat of dilution of ZnCl 2 200 H 2 

 to ZnCl 2 400 H 2 exceeds that of CdCl 2 200H 2 O to CdCl 2 

 400 H 2 0, and the excess is to be added to the above difference 

 in the case of the chlorides, the heat evolved in the displace- 

 ment of cadmium by zinc from CdCl 2 400 H 2 is slightly 

 greater than that from CdSO 4 400H 2 O. The electromotive 

 forces of zinc-cadmium-sulphate and zinc-cadmium-chloride 

 cells for solutions of strengths MS0 4 400 H 2 0, and MC1 2 

 400 H 2 (-25MS0 4 and -25MCl 2 per 100 H 2 0) do not ex- 

 hibit this relationship: on the contrary, the E.M.F. of the 

 latter is materially less than that of the former; and, further, 

 is distinctly less than that corresponding to the heat evolved 

 in the net chemical change with which the E.M.F. of the 

 former fairly coincides; thus: — 



E.M.F. corresponding to 16590 gramme-") 



degrees per gramme-molecule, or 8295 > = # 365 volt, 

 per gramme equivalent ) 



