Chemical Affinity in terms of Electromotive Force. 275 



Experiments made to determine the maximum Electromotive 

 Forces of variously arranged Daniell Cells containing Zinc- 

 sulphate solution around the zinc, and the maximum pro- 

 portion of the Fall in the E.M.F. of the Cell with gradually 

 increasing currents, that could be due to accumulation round 

 the plates of fluids of different densities through the migra- 

 tions of the ions. 



106. A long series of experiments was next made with the 

 object of determining how far the very considerable diminu- 

 tion in the E.M.F. of a Daniell cell, above shown to exist 

 when moderately strong currents are generated, can be ac- 

 counted for by the strengthening of the solution of zinc sur- 

 rounding the plate, and the weakening of the copper-sulphate 

 solution round the copper plate, which necessarily take place 

 in consequence of the migration of the ions accompanying the 

 passage of the current. Inasmuch as the use of dilute sul- 

 phuric acid introduces complications, these observations were 

 made in the first instance with zinc-sulphate solutions only 

 round the zinc plates ; later on (§ 111), similar experiments 

 with cells containing sulphuric acid are described. 



It results from the experiments of Moser (Annalen der Physik, 

 iii. p. 216) and H. F. Weber (Phil. Mag. [5] viii. pp. 487 & 

 523), that when a stronger solution of zinc (or copper) sul- 

 phate diffuses into a. weaker one of the same salt, plates of 

 zinc (or copper) placed in the two solutions acquire different 

 potentials, that in the stronger solution being at the higher 

 potential: the potential difference reckoned per a [constant 

 difference in specific gravity of solution (e. g. a difference of 

 O'l) is not constant, but depends on the actual values of the 

 specific gravity, being less the stronger the solutions, The 

 maximum value obtained (in the case of zinc-sulphate solu- 

 tions containing respectively 60 and 1 per cent, of crystallized 

 salt) was only '036 Daniell, or about *040 volt ; whence it 

 would seem that if the effect produced by zinc-sulphate solu- 

 tion in diffusing into copper- sulphate solution is of the same 

 order of magnitude as that produced by diffusing into another 

 zinc-sulphate solution of strength equivalent to that of the 

 copper-sulphate solution, the effect on the E.M.F. of a Daniell 

 cell, due to migration of the ions, cannot possibly materially 

 exceed *04 volt; whilst, whatever the magnitude of the effect, 

 it must tend to diminish the E.M.F. of the cell, since it par- 

 tially equalizes the difference of potential between the zinc 

 and copper plates set up by the chemical action alone. In 

 order to find out the actual magnitude of the diminution due 

 to this diffusive action in various cases, a number of determi- 



