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XXXIII. On the Determination of Chemical Affinity in terms of 

 Electromotive Force. — Part VIII. By C. It. Alder Wright, 

 D.Sc. (Lond.), F.R.S., Lecturer on Chemistry and Physics, 

 and C. Thompson, Demonstrator of Chemistry, in St. Mary's 

 Hospital Medical School*. 



[Plate VI.] 



On the Electromotive Force set up during Inter diffusion. 



147. XT is well known that the E.M.F.'s of cells like those 

 JL of Daniell and of Grove vary with the degree of 

 concentration of the solutions used therein, and many measure- 

 ments of the amounts of such variations have been from time 

 to time published by different experimenters, e. g. by Regnault 

 {Ann. de Chim. et de Phys. [3] xliv. p. 453), Svanberg (Pogg. 

 Annalen, lxxiii. p. 290 [1848]); and more recently by Streintz 

 (Carl. Rep. xv. p. 6 [1879]), Baumgartner {ibid. xv. p. 105 

 [1879] ), and Fromme (AnnalenderPhysik,viii. p. 326 [1879] ). 

 From various causes, the nature of which will be apparent later 

 on, the figures obtained by different observers do not exhibit 

 such a degree of concordance as to lead to the establishment of 

 any fundamental principle coordinating the variation in E.M.F. 

 with the degree of dilution of the solutions used, excepting this 

 general rule, that when the strength of the copper sulphate in 

 a Daniell cell or of the nitric acid in a Grove's cell is decreased, 

 a fall in the E.M.F. results ; and that when the strength of 

 the solution surrounding the zinc is decreased, a rise in E.M.F. 

 ensues, provided the solution is one of zinc sulphate : with 

 other salts, and with dilute sulphuric acid, some observa- 

 tions indicate rise, others fall, in E.M.F. with decrease of 

 strength. 



It has been shown by Moser (Annalen der Physik, iii. p. 216 

 [1878]) that when two solutions of the same metallic salt, 

 but of unequal strengths, are allowed to communicate so as to 

 mix by interdiffusion, a current is set up passing from the 

 weaker to the stronger solution through the fluid when elec- 

 trodes are employed of the metal contained in the salt used 

 (e.g. zinc for solutions of zinc sulphate, copper for solutions of 

 copper sulphate, and so on) ; whilst the experiments described 

 in Part V. (§ 110) indicate that the variations in E.M.F. of a 

 Daniell cell containing zinc-sulphate solution round the zinc, 

 according as the zinc-sulphate or the copper-sulphate solution 

 varies in strength, are approximately equal numerically to the 



* Communicated by the Physical Society. Read January 26. 1884. 



