104 PROCEEDINGS OF THE AMERICAN ACADExMY. 



tween the metals, as this term is negligible so far as our present 

 knowledge goes. The electromotive force of this type of cell will be 



n e \ ® PMi ° Pi) 



+ 



where z represents the difference of potential between the solutions, 

 and the valency of the metals M^ and Mg is the same. If the wander- 

 ing velocities of the ions Mi and Mg are nearly equal, and pi and j^a 



be made so, the value of z approaches zero, while the term log — 

 drops out entirely. The electromotive force of the cell 



MiljsMiXl/jM^XIMa 



is given very nearly by the expression 



TT = log ~y^ X 10-^ volts, 



ne ® Pmi 



and is independent of the absolute concentration of the salts MjX and 

 MoX. Let us take as a concrete case the cell Zn|ZiiS04lCuS04|Cu, 

 and let the concentrations of the ZnS04 ^.nd CUSO4 always be equal. 

 It has been found experimentally that the electromotive force of this 

 cell is independent of the absolute concentration.* Suppose that, in- 

 stead of diluting the two solutions with pure water, we add a solution 

 of potassium sulphate. According to Nei-nst's theory, this will have 

 no influence on the electromotive force except in so far as it affects the 

 dissociation of the two sulphates, and thereby the concentrations of the 

 Zn and Cu ions. If the dilution be carried far enough, we shall come 

 at last, without change of electromotive force, to the cell with neither 

 zinc nor copper sulphate, to the cell 



ZnIxZnS04 + xK2S04lxCuS04 + xK2S04|Cu, 



which is the same as the cell Zn|xK„S04lCu. In other words, the 

 one-liquid non-reversible cells are the limiting cases of the two-liquid 

 reversible cells, in which the concentrations and wandering veloci- 

 ties of the reversible ions are equal, the dissociation being supposed 

 to be complete. This last clause is necessary, for if the percentage 

 dissociations of the zinc sulphate and cojiper sulphate were different, 



* Wright, Phil. Mag., [5.], XIII. 265, 1882. 



