406 



SCIENCR 



[N. S. Vol. Vll. No. 16&. 



Beccari, about the middle of the IStli cen- 

 tury, obtained metals from oxides through 

 which the spark had passed, and in 1778 

 Priestley noted the production of an acid gas 

 when the electric spark was passed through 

 air. Similar experiments were made by 

 Cavendish and Van Marum on decomposed 

 ammonia. It is not, however, until after 

 the discovery of the voltaic cell that the 

 subject of electrolysis really begins. I have 

 already referred to the discovery of ISTichol- 

 son and Carlisle in 1800, and the subse- 

 quent work of Davy and of Faraday. The 

 peculiar phenomenon of the appearance of 

 separated elements only at the end plates in 

 the electrolytic cell led to considerable spec- 

 ulation, and was explained by Grothuss on 

 the supposition that the molecules separated 

 into two parts, one positively and the other 

 negatively electrified, and that these parts 

 formed a chain between the plates along 

 which chemical action traveled by a contin- 

 ual interchange of mates, the end parts 

 going to the plates. This theory was held 

 for many years, and is still to be found in 

 some text-books. Faraday's work is by far 

 the most valuable of the early contributions 

 to the subject. He gave the following laws : 



The amount of chemical decomposition in 

 electrolysis is proportional to the current 

 and the time of its action. 



The mass of an ion liberated by a definite 

 quantity of electricity is directly propor- 

 tional to its chemical equivalent weight. 



The quantity of electricity which is re- 

 quired to decompose a certain amount of 

 an electrolyte is equal to the quantity which 

 would be produced by recombining the sep- 

 arated ions in a battery. 



These laws are all of the greatest impor- 

 tance and the last one clearly points out the 

 reversibility of the electrical process. By 

 forcing a current through an electrolyte it 

 is decomposed and the mutual potential 

 energy of the components consequently in- 

 creased. By allowing the components to 



recombine in a battery the mutual poten- 

 tial energj' is reduced and a current of 

 electricity is the result. An excellent illus- 

 tration of this action is exhibited by the 

 secondary battery. 



In 1857 Clausius gave a theory of electrol- 

 ysis and at the same time reviewed the 

 weaknesses of the hypotheses of Grothuss 

 and others. Clausius assumes that the 

 molecules of the liquid are in continual 

 motion; that impacts frequently occur which 

 produce temporary dissociation, leaving 

 atomic groups charged with opposite elec- 

 tricities, and that during these separations 

 any directive agency, such as an E. M. F., 

 is able to cause a motion of these atoms in 

 opposite directions. This is probably the 

 first indication of the idea of the purely di- 

 rective character of the applied electro- 

 motive force taking advantage of dissocia- 

 tion to produce chemical separation. 



The energy side of the problem now be- 

 gan to attract attention and the develop- 

 ment of what may be called the ther- 

 modynamics of electro-chemistry began. 

 Among the most prominent workers in this 

 field have been Joule, Helmholtz, Gibbs, 

 Kelvin, Boscha and Favre. 



In 1853 Hittorf made quantitative deter- 

 mination of the change of concentration near 

 the electrodes when a current is passed 

 through a solution. This work is of histor- 

 ical interest because it formed practically 

 the starting point for what may be called 

 the modern view of electrolysis. Hittori's 

 experiments extended over several years and 

 served practically to establish the theory of 

 the migration of the ions in the solution. 

 Hittorf communicated the following laws : 



The change in concentration due to cur- 

 rent is determined bj' the motion which the 

 ions have in the unchanged solution. 



The unlike ions must have different ve- 

 locities to produce such change of concen- 

 tration. 



The numbers which express ionic veloci- 



