12 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



One of the greatest difficulties in the dissociation theory of 

 electrolysis is to account for the breaking up of such a molecule 

 as sodium chloride, which is ordinarily very stable, into its ions. 

 The strength of the dissociation theory, however, lies in the 

 extent to which it correlates a wide range of experimental fact, 

 and in this respect the dissociation theory is incomparably more 

 useful than any other theory that has been hitherto proposed.* 



6. The voltaic cell. The chemical action that is caused by 

 the flow of current through an electrolytic cell is usually forced, 

 that is, work has to be done to bring the chemical action about 

 or, in other words, an electric generator such as a dynamo or a 

 battery must be used to push the current through the electrolytic 

 cell. When, however, secondary chemical actions take place at 

 one or both electrodes, it frequently happens that the total chem- 

 ical action that is brought about by the flow of current through 

 an electrolytic cell is a source of energy. In such a case the 

 electrolytic cell itself can maintain its own current through the 

 electrolyte from electrode to electrode and through an outside 

 circuit of wire which connects the electrodes. Such an electro- 

 lytic cell is called a voltaic cell. 



Example. When a strip of clean zinc and a strip of copper or 

 carbon are dipped into dilute sulphuric acid, no appreciable chem- 

 ical action takes place. When the plates are connected together 

 by a wire, a current immediately starts to flow through the circuit, 

 leaving the cell at the copper or carbon electrode (the cathode) and 



* Several simple applications of the dissociation theory to the interpretation of ex- 

 perimental results are given in Practical Physics, Franklin, Crawford and Mac- 

 Nutt, Vol. II, page 108, page 144 and pages 146 and 147. A splendid example of 

 the application of the dissociation theory to the rationalization of a very complicated 

 experimental result is given by E. C. Franklin and H. D. Gibbs in the Jcttrnal of 

 the American Chemical Society, Vol. 29, pages 1389-1396, October, 1907. 



Any student who wishes to become acquainted with the facts of electrolysis must 

 familiarize himself with the details of the dissociation theory, and, since no other 

 theory has ever been proposed which is to be compared in effectiveness with the dis- 

 sociation theory, the student's efforts should be directed first of all to a thorough 

 understanding of the theory. After he has mastered the theory its imperfections may 

 properly be pointed out. 



