5C4 X W. Gibbs — Eqiiilibrmm of Heterogeneous Substances. 



zinc) and an electrolytic fluid containing a salt of zinc, but not capa- 

 ble of dissolving the mercury.* We may regard 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 solid- 

 ity of the electrodes and the consequent resistance to the diflfusion 

 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 dififerent from its condi- 

 tion when acting on an ion, in which case its quantity is not inde- 

 pendently 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 llY.) 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 sub- 

 stance thus dissolved would of course be uniform throxighout 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 resist- 

 ances. 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 



* If the electrolytic fluid dissolved the mercury as well as the zinc, equilibrium 

 could only subsist when the electromotive force is zero, and the composition of the 

 electrodes identical. For when the electrodes are formed of the two metals in differ- 

 ent proportions, that which has the greater potential for zinc will have the less poten- 

 tial for mercury. [See equation (98).] This is inconsistent with equilibrium, accord- 

 ing to the principle mentioned above, if both metals can act as cations. 



