112 PHYSICAL SCIENCE 



being carried forward by the medium to supply the 

 place of the energy dissipated by the current. 



The mobility of any one ion is, in dilute 

 solutions, independent of the nature of the other 

 ion present, at all events in simple salts, such as 

 the chlorides of sodium, potassium, and lithium. 

 This independence itself indicates that the ions 

 are free from each other, and again suggests 

 some form of dissociation. 



The phenomena of conductivity also point to 

 the same idea. To set free an ion or its products 

 at the electrodes requires the expenditure of a 

 certain amount of electric work, and at the 

 electrodes an equivalent reverse electro-motive 

 force exists. When, however, this reverse force 

 is overcome, the passage of the current through 

 the solution is opposed by no other reversible 

 forces, and it is found that the work expended 

 is that required to force the current against the 

 frictional resistance of the electrolyte alone. The 

 current is proportional to the excess of the electric 

 force applied beyond what is needed to overcome 

 the effect at the electrodes ; this part of the con- 

 duction conforms to Ohm's law, which describes 

 the process in metallic conductors. In the body 

 of the solution, then, as distinct from the transition 

 layer in contact with the electrodes, the electric 

 forces do no reversible work, such as would be 

 needed to separate the ions from each other. 

 Whatever freedom is requisite between the ions 

 for the purpose of conduction, must necessarily 

 exist whether the electric forces act or not ; the 

 function of the electric forces when applied is 

 simply to force the ions, already separated from 



