46 Dr. H. J. S. Sand on the Concentration 



value was roughly independent of size, distance, &c. o£ the 

 electrodes, and only a function of the composition of the 

 electrolyte. 



About the same time Hittorf was engaged in his funda- 

 mental researches on the processes which take place within 

 the electrolyte. His experiments, as is well known, proved 

 that in the interior of the solution of a mixture of two salts, 

 both take part in the conduction of the current at all current- 

 densities. Regarding the process which takes place at the 

 electrode, he at that time assumed that the ions of both salts 

 were primarily deposited in the same proportion in which 

 they had taken part in the conduction of the current within 

 the liquid. In most cases, however, one of the components 

 would act chemically upon the solution with a definite reaction- 

 velocity; and it was only when this velocity was exceeded by 

 the rate at which it was being primarily deposited at the 

 electrode that its liberation would become perceptible*. 

 Thus in a mixture of copper sulphate and sulphuric acid, 

 hydrogen would be primarily liberated at all current-den- 

 sities ; but it would only become visible when the velocity 

 with which it was supposed to decompose copper sulphate in 

 the nascent state was exceeded by the rate at which it was 

 being primarily set free. 



It appears that it was Le Blanc who first insisted sufficiently 

 on the fact that the mode of conduction of the current within 

 any part of the interior of the electrolyte does not necessitate 

 the same mode at the electrodes. His investigations having 

 proved that the same minimum electromotive force must be 

 applied to various acids and bases in order to cause the con- 

 tinuous passage of a current of appreciable magnitude through 

 them, Le Blanc framed his well-known theory, that whereas 

 conduction through the interior of an acid or a base is due 

 in almost exclusive preponderance to its own ions, the trans- 

 mission from the solution to the electrolyte must be ascribed 

 primarily to the ions of water. Generalizing this statement, 

 he expressed the view f that in a mixture of ions at an elec- 

 trode those exclusively are set free which require the smallest 

 E.M.F. for their liberation; and it is only when . their con- 

 centration at the electrode has gone down to zero, that those 

 requiring a higher E.M.F. for their liberation also appear. 

 Thus in a mixture of copper sulphate and sulphuric acid, 

 copper only should be deposited till its concentration at the 

 electrode has gone down to zero, after which hydrogen also 

 would be liberated. 



* Pogg. Ann. ciii. p. 46. 



f ZeiUchr. phys. Chem.sm. p. 172. 



