1902.] on the Ions of Electrolysis. 27 



mechanism of electrolysis, or perhaps I should say the progress that 

 has been made towards an explanation of the phenomena. 



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 decomposition can occur aud no current 

 can pass. 



And indeed at one time it was supposed that tliis was so. But 

 Faraday, in a series of ingeniously devised and carefully executed 

 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-electrolytic conduction by the electrolyte. 

 But the study of the phenomena of the polarisation of the electrodes 

 led ultimately to the complete explanation by Helmholtz | in 1873 of 

 this apparently metallic conduction 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 proportional to 

 the amount of electricity transmitted. 



The phenomena of the polarisation of the electrodes may be 

 described shortly as follows. 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 observation was first made by 

 Gautherot J 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. Eitter, appar- 

 ently without knowing anything of Gautherot's work, made a great 

 many observations on the same subject. I cannot refrain from reading 

 to you a passage from a letter from Christoph Bernoulli to van Mens. 

 I take it from the translation published in Nicholson's Journal, 

 October 1805. "As Mr. Bitter at present resides in a village near 

 Jena, I have not been able to see his experiments with his grand 

 battery of two thousand pieces, or with his battery of fifty pieces, 

 each thirty-six inches square, the action of which continues very per- 

 ceptible for a fortnight. Neither have I seen his experiments with 

 the new battery of his invention, consisting of a single metal, and 

 which he calls the charging pile. 



* Grotthuss, Anuales de Chimie, Iviii. p. 54 (1806). 



t Helniholtz. Fogg. 150, p. 483 (1873) ; Faraday Lecture, Chem. Soc. Trans. 

 39, p. 287 (1881) ; Wied. 34, p. 737 (1888). 



X Gautherot, Annales de Chimie, xxxix. p. 203 (1801). 



