THEORIES OE ELECTROL YSIS. 3 or 



in solution to pass through it, but is quite impermeable to 

 barium chloride. Now, on the theory of free ions, some of 

 the chlorine will still pass, since it could do so in the first 

 case, but the electric forces will prevent any considerable 

 separation from taking place. But, if we place some sub- 

 stance like copper nitrate on the other side of the mem- 

 brane, the chlorine ions, which diffuse in one direction, are 

 replaced by nitric acid ions, which diffuse in the other, and 

 this process will continue till we soon find nitrate mixed 

 with the barium chloride, and chloride mixed with the 

 copper nitrate. The salts cannot have directly reacted 

 with each other, for neither alone can pass through the 

 membrane, but the phenomenon is readily explained on the 

 hypothesis of free ions. 



It has been shown by Ostwald and others that the 

 properties of salt solutions {i.e., of electrolytes) are largely 

 additive, that is to say, can be found by superposing the 

 properties of the ions which they contain. Such relations 

 have been traced in the densities, colours, refraction 

 coefficients, optical rotatory powers, surface tensions, 

 viscosities, and thermal capacities. 



Even stronger evidence, the weight of which seems not 



to have been fully recognised, is given by the results of 



Kohlrausch's measurements of conductivity. He proved 



that, in very dilute solution, the molecular conductivity was 



constant, that is, that the conductivity was proportional to 



the concentration. If we suppose that the ions are free 



from one another, and move forward under the influence of 



the electromotive force, this relation naturally follows. But 



if we reject this hypothesis, we have to suppose that an ion, 



which is then imagined to be free from an opposite ion only 



at the instants of collision, can take a step forward only when 



the molecule of which it forms part meets another molecule. 



Now the frequency with which such collisions occur is 



proportional to the square of the concentration, so that 



the average velocity with which an ion can work its way 



through the solution must also vary as the square of the 



concentration. But the conductivity is proportional to the 



number of ions multiplied by their average velocity, hence 



21 



