882 



SCIENCE. 



[N. S. Vol. XV. No. 388. 



trode, or on the solvent, or ou something 

 present in the solution; that the quantity 

 of each ion so set free is proportional to the 

 quantity of electricity transferred from 

 the one electrode to the other and to the 

 equivalent of the ion— that is, as we would 

 put it now (if purists will allow us to 

 speak of the atomic weight of NH^ or of 

 NO3), to the atomic weight of the ion 

 divided by its valency; all that was made 

 out by Faraday. He had made some way 

 in finding out how the liberated ions act, 

 when they do act, on the things in the pres- 

 ence of which they find themselves; and 

 where he led, others have followed, so that 

 we have now many electrolytic methods of 

 oxidation, of reduction and of synthesis, 

 and great manufacturing industries de- 

 pending on electrolysis. On this large field 

 I do not now pui-pose to enter. "What I 

 wish to call your attention to this evening is 

 the mechanism of electrolysis, or perhaps I 

 should say the progress that has been 

 made towards an explanation of the phe- 

 nomena. 



The earlier theories, from Grotthuss* in 

 1806, all assume that the decomposition is 

 caused by the attraction of the electrodes 

 or by the passage of the current, and that 

 a definite electromotive force, different for 

 each electrolyte, is required in order that 

 decomposition shall take place. According 

 to these theories, if the electromotive force 

 is below that definite minimum no decom- 

 position can occur and no current can pass. 



And indeed at one time it was supposed 

 that this was so. But Faraday, in a series 

 of ingeniously devised and carefully ex- 

 ecuted experiments, showed that with 

 electromotive force below the minimum 

 necessary for the production of bubbles of 

 gas on the electrodes, a perceptible current 

 could pass for many days. He supposed 

 that this small current was due to non- 



* Grotthus.s, Amwles de Ghimie, LVIII., p. 54 

 (1806). 



electrolytic conduction by the electrolyte. 

 But the study of the phenomena of the 

 polai-ization of the electrodes led ultimately 

 to the complete explanation by Helmholtz* 

 in 1873 of this apparently metallic conduc- 

 tion by the electrolyte, and to a proof that 

 any electromotive force, however small, 

 sends a current through an electrolyte and 

 gives rise to separation of the ions propor- 

 tional to the amount of electricity trans- 

 mitted. 



The phenomena of the polarization of the 

 electrodes may be described shortly as fol- 

 lows: In the electrolysis of water (or 

 rather of dilute sulphuric acid) it had been 

 observed so long ago as 1802 that platinum 

 or silver plates which had been used as 

 electrodes acquired peculiar properties, so 

 that for a short time the plate that had been 

 the anode acted like the silver, and the plate 

 that had been the cathode like the zinc of a 

 voltaic cell, producing a short-lived and 

 rapidly diminishing current. This observa- 

 tion was first made by Gautherot,t a teacher 

 of music in Paris, who notes the effect of 

 the current on the tongue and states that 

 he had succeeded in decomposing water 

 by means of his apparatus. Shortly after, 

 J. W. Ritter, apparently without knowing 

 anything of Gautherot 's work, made a great 

 many observations on the same subject. I 

 cannot refrain from reading to you a pas- 

 sage from a letter from Christoph Bernoulli 

 to van Mons. I take it from the translation 

 published in Nicholson's Journal, October, 

 1805 : "As Mr. Ritter at present resides 

 in a village near Jena, I have not been able 

 to see his experiments with liis grand bat- 

 teiy of two thousand pieces, or with his 

 battery of fifty pieces, each thirty-six 

 inches square, the action of which continues 



* Helmholtz, Pogg., 150, p. 483 (1873); Fara- 

 day Lecture, Chem. 80c. Trans., 39, p. 287 (1881)'; 

 Wied., 34, p. 737 (1888). 



t Gautherot, Annates de Ghimie, XXXIX., p. 

 203 (1801). 



