26 Profestor A. Crum Brown [Jan. 31, 



WEEKLY EVENING MEETING. 

 Friday, January 31, 1902. 



The Eight Hon. Lord Kblvin, G.C.V.O. D.C.L. LL.D. F.R.S., 

 Vice-President, in the Chair. 



Professor A. Crum Brown, M.D. D.Sc. LL.D. F.R.S. 



The Ions of Electrolysis. 



The subject of Electrolysis must always have a special interest for 

 the Royal Institution. It was here that Davy showed its practical 

 value by his brilliant discovery of the metals of the alkalies and 

 alkaline earths ; and it was here that Faraday laid the foundation of 

 the scientific discussion of electrolysis; it was here that with his 

 singular experimental skill and clearness of insight he discovered and 

 expounded the laws of electrolysis which will always be known by 

 his name. It is therefore with a good deal of diffidence that I stand 

 here to continue the story. And there is much to be said : for, like all 

 good work, Faraday's work has been fruitful, and in consequence of it, 

 as well as of the genius and skill of subsequent investigators, we now 

 know much about electrolysis which Faraday did not and could not 

 know. 



The great difficulty left was that of the mechanism of electrolysis. 

 That the cation and the jDOsitive electricity travel together towards the 

 cathode, and that the negative electricity similarly travels with the 

 anion towards the anode, and that on their arrival at the electrodes 

 the electricity is delivered to the metallic conductor and the matter is 

 set free to appear as the ion itself, or to break up, or to act on the 

 electrode, or on the solvent, or on 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 l)y Faraday. He had made some way in finding out how the 

 liberated ions act, when they do act, on the things in the presence 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 

 depending on electrolysis. On this large field I do not now purpose 

 to enter. What I wish to call your attention to this evening is the 



