LEWIS. — A NEW SYSTEM OF THERMODYNAMIC CHEMISTRY. 291 



This value of Ajy may now be substituted in equations XXV and 

 XXVI. The former gives a formula for the electromotive force when 

 only one substance takes part in the electrolytic process, as in certain 

 concentration cells. The latter gives a general equation for any 

 reversible cell whatever. These are, 



E=^\nl XXVII 



E 



^lniT-^ilnf^f XXVIII 



mF mF &A 



In XXVII, m is the number of Faraday equivalents accompanying the 

 passage of one mol ; in XXVIII, it is the number accompanying the 

 disappearance of a mols of A, b mols of B, etc. 



One application of equation XXVII is of special interest, "We may 

 take it for granted that whenever two phases are in contact and a given 

 molecular species is present in one of them, it will be present to some 

 extent in the other. For example, if a rod of metallic silver dips into 

 a solution of silver nitrate, we may suppose that silver ions are present 

 not only in the solution, but also in the metal. The process which 

 takes place at this electrode during the passage of a current may 

 therefore be regarded as consisting in the passage of silver ions out of 

 the electrode into the solution, or vice versa, Equation XXVII gives 

 us, therefore, an expression for the single potential difference between an 

 electrode and an electrolyte. If the ion in question is an elementary 

 one (and monatomic) m is equal to v, the valence of the ion, and we 

 may write equation XXVII in the following form, 



E = ~\n^ XXIX 



VF $ 3 



where E is the single potential difference, &/ is the activity of the ion 

 in question in the electrode, and £ s is the activity of the same ion in 

 the electrolyte. It is obvious that the quantity £ M is very similar 

 to the electrolytic solution pressure of Nernst, but while the latter 

 depends at a given temperature, not only upon the character of the 

 electrode but also upon the nature of the medium in which the elec- 

 trolyte is dissolved, £ M depends solely upon the character of the elec- 

 trode. Moreover, while equation XXIX is universally true, the 

 equation of Nernst is obviously only true when the activity of the ion 

 in the electrolyte is proportional to its concentration. We have in 

 the application of equations XXIX (or XXVII) to the electromotive 



