J. W. Gibhs — Eqnilihr'mm of Heterogeneous Substances. 505 



relating to the diffusion of the dissolved ion is not satisfied. This 

 may be the case with hydrogen and oxygen as ions (oi' apparent ions) 

 between electrodes of platinum in some of its forms. 



(III.) The ion may ai)pear in mass at the electrode. If it be a 

 conductor of electricity, it may be regarded as forming an electrode, 

 as soon as tlie deposit has l»ecome thick enough to have the proper- 

 ties of matter in mass. The case therefore will not be diiferent from 

 that first considered. When tlie ion is a non-conductor, a continuous 

 thick deposit on the electrode would of course prevent the possibility 

 of an electrical cnrrent. But the case in which the ion being a non- 

 conductoi' is disengaged in masses contiguous to the electrode but 

 not entirely covering it, is an important one. It may be illustrated 

 by hydrogen appearing in bubbles at a cathode. In case of perfect 

 equilibrium, independent of passive resistances, the potential of the 

 ion in (687) or (688) may be determined in such a mass. Yet the 

 circumstances are quite unfavorable for the establishment of perfect 

 equilibrium, unless the ion is to some extent absorbed by the electrode 

 or electrolytic fluid, or the electrode is fluid. For if the ion must pass 

 immediately into the non-conducting mass, while the electricity passes 

 into the electrode, it is evident that the only possible terminus of an 

 electrolytic current is at the line where the electrode, the non-conduct- 

 ing mass, and the electrolytic fluif\ meet, so that the electrolytic pro- 

 cess is necessarily greatly retarded, and an approximate ceasing of the 

 current cannot be regarded as evidence that a state of approximate 

 equilibrium has been reached. But even a slight degree of solubility 

 of the ion in the electrolytic fluid or in the electrode may greatly 

 diminish the resistance to the electrolytic process, and help toward 

 producing that state of complete equilibrium which is supposed in the 

 theorem we are discussing. And the mobility of the surface of a 

 liquid electrode may act in the same way. When the ion is absorbed 

 by the electrode, or by the electrolytic fluid, the case of course comes 

 under the heads which we have already considered, yet the fact that 

 the ion is set free in mass is important, since it is in such a mass that 

 the determination of the value of the potential will generally be 

 most easily made. 



(IV.) When the ion is not absorbed either by the electrode or by 

 the electrolytic fluid, and is not set free in mass, it may still be 

 deposited on the surface of the electrode. Although this can take 

 place only to a limited extent (without forming a body having the 

 properties of matter in mass), yet the electro-chemical equivalents of 

 all substances are so small that a very considerable flux of electricity 



