286 Messrs. Wright and Thompson on the Determination of 



or weak; and, further, is precisely the same in zinc- cadmium- 

 sulphate, or in zinc-silver-sulphate cells set up with amalga- 

 mated zinc plates. Similarly, the numerical value of the effect 

 of a given alteration in the strength of the copper-sulphate 

 solution is the same whether the zinc-sulphate solution be 

 strong or weak ; and, further, is still the same in copper- 

 cadmium-sulphate or copper-silver-sulphate cells set up with 

 electro-copper plates *. 



(4) The values of a, b, c, d, vary perceptibly with the 



nature of the plate-surface of the metal immersed in the varying 

 solution; i. e. they are not constant for a given difference in 

 strength if the plate-surface vary. In the case of the metals 

 copper, zinc, and cadmium the highest values are observed, 

 cceteris paribus, with plates superficially amalgamated, the 

 amalgam being fluid ; plates freshly electro-coated give lower 

 values, and bright polished plates of fused metal lower values 

 still. 



(5) With the majority of kinds of plate-surface examined, 

 the values of a and b in the expression 



E = + a— b 



are such that a and b are unequal in magnitude for equal 

 amounts of variation in the strength of the two solutions in a 

 two-fluid cell. Sometimes a increases more rapidly than b in 

 numerical value as the solution-strength increases ; in which 

 case the E.M.F. of the combination rises with the strength of 

 the solutions when both are always of equal molecular strength: 

 such a case is presented by a zinc-copper-sulphate cell with 

 electro-zinc and amalgamated copper plates (§ 154). Some- 

 times the opposite is the case, e. g. a zinc-cadmium-chloride 

 cell with amalgamated zinc and electro-cadmium plates (§ 156). 

 Zinc-copper-sulphate cells set up with amalgamated zinc and 

 electro-copper plates show the peculiarity that a and b are 

 nearly equal numerically for all solution-strengths; so that 

 the E.M.F. of such a Daniell cell is practically invariable no 

 matter what the strength of the solutions of zinc and copper 

 sulphate used, provided that these are of the same molecular 

 strength. 



(6) On contrasting the values of a, b, c, d, with the 



E.M.F. 's corresponding to the amounts of heat developed on 

 intermixture of gramme equivalents of the two strengths of 

 the varying solution compared together, the former are usually 



* In copper-silver-sulphate cells the effect of a given increment in the 

 strength of copper-sulphate solution is, of course, to diminish the E.M.F. 

 of the cell, and not to increase it as in zinc-copper-sulphate cells. 



