6 BULLETIN OF THE UNIVERSITY OF WISCONSIN. 



termed ions. These ions migrate through the solution under 

 the influence of the electric current, whence their name. The 

 conduction of electricity by the solution depends upon the pres- 

 ence and the movements of the ions. Those ions that travel 

 toward the negative pole are considered as charged with positive 

 electricity and are termed cathions, while those that move toward 

 the positive pole are called anions and are charged with nega- 

 tive electricity. The number of cathions in a solution is always 

 equivalent to the number of anions present, so that electrical 

 neutrality of the solut : on is preserved. Thus in the case of so- 

 dium chloride the ions are Na and CI, and any solution of this 

 salt contains these ions together with a certain amount of un- 

 dissociated JSTa CI, the relative quantities of dissociated and un- 

 dissociated salt depending on the concentration of the solution. 

 Similarly solutions of hydrochloric acid contain the ions H and 

 CI and undissociated molecules of HC1. 



It would seem at first that matters become more complicated 

 by thus considering the solutions just mentioned; while this is 

 true in some cases, namely when concentrated solutions are under 

 consideration, yet in dilute solutions where dissociation is nearly 

 complete things appear more simple. Thus in very dilute so- 

 lutions hydrochloric acid and sodium chloride are practically 



completely dissociated, and the dissolved substances that these 



+ — 



solutions contain are consequently the ions H and CI in the 



+ - 



former and Ka and CI in the latter. The ions are to be re- 

 garded as distinct and separate substances subject, however, to 

 the law that the solution contains as many positive as negative 

 ions. The ions are furthermore not identical with ordinary hy- 

 drogen, chlorine and metallic sodium respectively, for they differ 

 from these in the amount of energy they contain. It is neces- 

 sary to supply these ions with additional energy in order to 

 change them from the ionic condition to the ordinary state. 

 This can be done, for example, by electrolyzing hydrochloric 

 acid or sodium chloride. 



Two dilute solutions containing chemically equivalent quan- 



