

Chemical Affinity in terms of Electromotive Force. 285 



tallic salts and plates of the respective metals contained there- 

 in , an increase in strength of the solution surrounding the plate 

 acquiring the higher potential, in virtue of the normal action of 

 the cell (i.e. the plate functioning as the copper plate of a 

 Daniell cell), causes an increment in the potential-difference set 

 up between that plate and the other. The opposite effect is 

 always produced by an increase in the strength of the solution 

 surrounding the other plate. The increment and decrement 

 thus producible are, in certain cases, very considerable: with a 

 Daniell cell a difference of nearly 6 centivolts (or upwards of 

 5 per cent, of the total E.M.F.) is producible, according as 

 strong copper-sulphate and weak zinc-sulphate solutions are 

 taken, or vice versa ; with cells containing zinc and cadmium 

 chlorides analogous differences up to even 30 centivolts are 

 thus producible. 



(2) A " law of summation " holds, expressible thus : — The 

 effect of the sum of a series of changes in the strengths of the 

 solutions in a two-fluid cell is equal to the algebraic sum of the 

 effects of each change severally. Thus, by increasing the strength 

 of the copper-sulphate solution in a Daniell cell (from, say, 

 •25 CuS0 4 100 H 2 * to 2'0 CuS0 4 100 H 2 0), a given incre- 

 ment in E.M.F. ensues, = a: by similarly varying the zinc- 

 sulphate solution (from, say, # 25 ZnSO 4 100 H 2 to 5'0 ZnS0 4 

 100 H 2 0), a decrement ensues, = 5. Then the difference in 

 B.M.F. between two cells, one containing *25 CuS0 4 100 H 2 

 and *25 ZnS0 4 100 H 2 0, and the other containing 2*0 CuS0 4 

 100 H 2 0, and 5'0 ZnS0 4 100 H 2 0, is the sum of these amounts, 

 i. e. is a— b ; so that if e is the E.M.F. of the first and E that 

 of the second, E = 6 + a— b. 



Similarly, if three cells be compared, alike in all respects 

 save that the copper-sulphate solutions have (for example) the 

 strengths '25 CuS0 4 , 1-0 CuS0 4 , and 2-0 CuS0 4 (per 100 H 2 

 in each case), the excess of E.M.F. of the second over the first 

 being experimentally found =c, and that of the third over the 

 second being similarly found = d, then a = c-{-d. 



(3) The values of a, b, c, d, are independent, not only 



of the actual strength of the solution which does not vary in any 

 two cells compared, but also of the nature of that solution and of 

 the meted immersed in it. 



For example, in zinc-copper-sulphate cells with amalga- 

 mated zinc and electro-copper plates, the effect of a given 

 alteration in the strength of the zinc-sulphate solution is sen- 

 sibly the same whether the copper-sulphate solution be strong 



* Instead of expressing molecular strengths in the form MS0 4 wH 2 0, 

 and so on, it is more convenient for certain purposes to express them in 

 the form mMS0 4 100H 2 O, and so on. 



