EQUILIBRIUM OF HETEROGENEOUS SUBSTANCES. 335 



electrical current. But the case in which the ion being a non- 

 conductor 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 fluid meet, so that the electrolytic 

 process 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 

 may take place before the deposit will have the properties of matter 

 in mass. Even when the ion appears in mass, or is absorbed by the 

 electrode or electrolytic fluid, the non-homogeneous film between the 

 electrolytic fluid and the electrode may contain an additional portion 

 of it. Whether the ion is confined to the surface of the electrode or 

 not, we may regard this as one of the cases in which we have to 

 recognize a certain superficial density of substances at surfaces of 

 discontinuity, the general theory of which we have already considered. 



The deposit of the ion will affect the superficial tension of the 

 electrode if it is liquid, or the closely related quantity which we have 

 denoted by the same symbol a- (see pages 314-331) if the electrode is 

 solid. The effect can of course be best observed in the case of a liquid 



