We may apply the conclusions, reached, to the action of metallic zinc when it is introduced into the solution of a cupric salt. The oxidation of zinc to the zinc-ion and the reduction of the cupric-ion to copper must be ‹reversible› reactions, Zn ↓ + Cu^{2+} ⇄ Zn^{2+} + Cu ↓, which will come to a condition of equilibrium, according to the laws of equilibrium, when [Zn^{2+}] : [Cu^{2+}] = K = 10^{38}. The value of this ratio shows that the cupric-ion will be ‹practically› completely reduced, and precipitated as copper, by a sufficient quantity of zinc, the trace of cupric-ion, required to maintain the equilibrium ratio, being too minute to be detected. By the study of this oxidation and reduction reaction with the aid of potential differences, as just described, the validity of the relation is subject to demonstration, and the value of the equilibrium constant is brought into definite relation to the solution-tension constants of the metals. Each element has its own characteristic solution-tension constant (see the table at the end of Chapter XV), and the relation just established for the reduction of cupric-ion, at the expense of the oxidation of metallic zinc, may be applied to any pair of metals and their ions.[540] «General Principles Concerning Equilibrium in Reversible Oxidation and Reduction Reactions.»—We may now extend the conclusions, reached in the study of these particularly simple oxidations and reductions, to oxidation and reduction reactions in general. We must expect that, ‹when such an action is reversible› and subject to the laws of equilibrium, its course will, as in all [p269] previous applications of the equilibrium laws, depend, at a given temperature, in the first place, ‹on the values of constants›. The (solution-tension) constants, involved in this class of actions, measure what we may call the affinity of atoms and ions for electric charges, or electrons. In the second place, the course of the action will depend, in each case, on the concentrations of the ions, concentrations which are, to a considerable extent, ‹variable› at will, as we go from case to case. In the third place, all such reversible reactions will come ultimately to a ‹condition of equilibrium›, in which neither action is absolutely completed, and the course of the action, in any given system not in equilibrium, will always ‹proceed toward› this condition of equilibrium. The oxidation and reduction reactions, such as Zn ↓ + Cu^{2+} ⇄ Cu ↓ + Zn^{2+}, to which we have heretofore limited the discussion of the quantitative relations, are particularly simple actions, involving only ‹two› variables (in this case [Cu^{2+}] and [Zn^{2+}]). But the knowledge of the general principles of the quantitative relations will now enable us to answer questions, in connection with more complicated cases, which the qualitative relations alone did not put us into the position of answering (see p. 256).