334 EQUILIBRIUM OF HETEROGENEOUS SUBSTANCES. 



a cell in which hydrogen acts as an ion between electrodes of 

 palladium charged with hydrogen as another illustration of the same 

 principle, but the solidity of the electrodes and the consequent 

 resistance to the diffusion of the hydrogen within them (a process 

 which cannot be assisted by convective currents as in a liquid mass) 

 present considerable obstacles to the experimental verification of the 

 relation. 



(II.) Sometimes the ion is soluble (as an independently variable 

 component) in the electrolytic fluid. Of course its condition in 

 the fluid when thus dissolved must be entirely different from its 

 condition when acting on an ion, in which case its quantity is not 

 independently variable, as we have already seen. Its diffusion in 

 the fluid in this state of solution is not necessarily connected with 

 any electrical current, and in other relations its properties may be 

 entirely changed. In any discussion of the internal properties of 

 the fluid (with respect to its fundamental equation, for example), it 

 would be necessary to treat it as a different substance. (See 

 page 63.) But if the process by which the charge of electricity 

 passes into the electrode, and the ion is dissolved in the electrolyte 

 is reversible, we may evidently regard the potentials for the substance 

 of the ion in (687) or (688) as relating to the substance thus dissolved 

 in the electrolyte. In case of absolute equilibrium, the density of 

 the substance thus dissolved would of course be uniform throughout 

 the fluid (since it can move independently of any electrical current), 

 so that by the strict application of our principle we only obtain the 

 somewhat barren result that if any of the ions are soluble in 

 the fluid without their electrical charges, the electromotive force 

 must vanish in any case of absolute equilibrium not dependent upon 

 passive resistances. Nevertheless, cases in which the ion is thus 

 dissolved in the electrolytic fluid only to a very small extent, and 

 its passage from one electrode to the other by ordinary diffusion is 

 extremely slow, may be regarded as approximating to the case in 

 which it is incapable of diffusion. In such cases, we may regard 

 the relations (687), (688) as approximately valid, although the 

 condition of equilibrium relating to the diffusion of the dissolved 

 ion is not satisfied. This may be the case with hydrogen and oxygen 

 as ions (or apparent ions) between electrodes of platinum in some 

 of its forms. 



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

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

 as soon as the deposit has become thick enough to have the properties 

 of matter in mass. The case therefore will not be different from that 

 first considered. When the ion is a non-conductor, a continuous thick 

 deposit on the electrode would of course prevent the possibility of an 



