from Faraday to J . J . Thomson. 387 



motive force E of the cell ; so that in suitable units we have 



-, RTu-v. c, 

 E = -- - log -. 



v u + v Ci 



A typical concentration-cell to which this formula may be 

 applied may be constituted in the following way : Let a 

 quantity of zinc amalgam, in which the concentration of zinc 

 is d, be in contact with a dilute solution of zinc sulphate, and 

 let this in turn be in contact with a quantity of zinc amalgam 

 of concentration c z . When the two masses of amalgam are con- 

 nected by a conducting wire outside the cell, an electric current 

 flows in the wire from the weak to the strong amalgam,* while 

 zinc cations pass through the solution from the strong amalgam 

 to the weak. The electromotive force of such a cell, in which 

 the current may be supposed to be carried solely by cations, is 



RT. c, 



lo- 



V 



Not content with the derivation of the electromotive force 

 from considerations of energy, Nernst proceeded to supply a 

 definite mechanical conception of the process of conduction in 

 electrolytes. The ions are impelled by the electric force asso- 

 ciated with the gradient of potential in the electrolyte. But 

 this is not the only force which acts on them ; for, since their 

 available energy decreases as the concentration decreases, there 

 must be a force assisting every process by which the concentra- 

 tion is decreased. The matter may be illustrated by the analogy 

 of a gas compressed in a cylinder fitted with a piston; the 

 available energy of the gas decreases as its degree of compression 

 decreases; and therefore that movement of the piston which 

 tends to decrease the compression is assisted by a force the 

 "pressure" of the gas on the piston. Similarly, if a solution 

 were contained within a cylinder fitted with a piston which is 

 permeable to the pure solvent but not to the solute, and if the 

 whole were immersed in pure solvent, the available energy of 



* It will hardly be necessary to remark that this supposed direction of the 

 .current is purely conventional. 



2 C 2 



