(6) 



The only difference is (his that l!ie molecules of a salt in solution 

 are neutral!}^ electric, whereas the metal particles in solution are 

 charged with positive electricity, hence the physical meaning of 



+ 

 the equation ft^ = [im is simply this that in absence of a potential 



difference, per second an equal number of metal particles are dissolved 



as there are deposited. 



If we express this in the most current terms, we may say, that 

 when C = K the metal-ions have reached their concentration oj 

 saturation, and that K therefore represents the solubility of the 

 metal-ions. 



To prevent confusion, it will be necessary to point out that the 

 fact that the dissolved metal-particles in equilibrium with the solid 

 metal have an electric charge, is attended by peculiarities which 

 are met with in no other department. 



Thus it will appear presently that in every solution of copper- 

 sulphate which is not extremely diluted, the concentration of the 

 copper-ions is supersaturated with respect to copper. Yet such a 

 copper-sulphate-solution is in a perfectly stable condition, because 

 the copper-ions constitute a part of the following homogeneous 

 equilibrium, 



CuSO,:^Cu" ^so: 



which is perfectly stable as long as the solution is unsaturate or is 

 just saturate with O^/SO^-molecules. 



If we now, however, insert a copper bar into the solution, the 

 condition changes, because the Cw-ions which were at tirst only in 

 equilibrium with the CuSO^-mo\^ and with the aSO/'-Ious, must now 

 also get into equilibrium with the copper bar, and, the concentration 

 of the C'M-ions with respect to copper being strongly supersaturate, 

 tiic (/M-ions will immediately deposit on the copper, till the further 

 depositing is prevented in consequence of the appearance of a double 

 layer. 



We shall further see that in the most concentrated solution of a 

 zinc-salt the concentration of the zinc-ions always remains below the 

 concentration of saturation, which a})pears immediately when we 

 immerge a zinc-bar into such a solution; the zinc emits zinc parti- 

 cles with a positive charge into the solution, till the appearance of 

 the electric double layer puts a stop to the phenomenon of solution. 



In order to find the values of K for different metals we make 

 use of the observed potential difference with a definite value of C. 



