«Need of the Study of the Quantitative Relations.»—The interpretation of such actions from the point of view of the theory [p256] of ionization offers, then, no particular difficulties. But, as far as we have developed the theory, that is, essentially from its qualitative side, difficulty would be encountered in understanding why certain other reactions, involving a similar simultaneous discharge of positive and negative electricity by ions, which might be expected to take place, do not seem to take place. Thus, solutions of ferric sulphate do not appear to be reduced appreciably by the hydroxide and oxide ions of the water present. Although the possibility of such a reduction exists through the simultaneous discharge of the positive electricity of the ferric ions and the negative charge on the oxide ions (or hydroxide ions) of water (4 Fe^{3+} + O^{2−} → 4 Fe^{2+} + O_{2} or 4 Fe^{3+} + 4 HO^{−} → 4 Fe^{2+} + O_{2} + 2 H_{2}O), comparable with the reduction of ferric ions by sulphide ions, such a reduction does not take place appreciably.[520] And, similarly, whereas the iodide-ion, as we have seen, reduces the ferric-ion very readily, the analogous chloride-ion does not appear to do so. Sodium chloride may be added to ferric sulphate solution and potassium ferricyanide fails to show that any ferrous salt is produced (‹exp.›). The mere possibility of a transfer of charges, or electrons, is therefore apparently[521] not sufficient to induce an oxidation and reduction reaction—much in the same way as, for instance, the mere presence, simultaneously, of the barium-ion and the carbonate-ion, in itself, does not necessarily lead to the precipitation of barium carbonate (p. 90), although the latter is difficultly soluble. In order to understand the problem of precipitation or nonprecipitation of salts, it was found necessary to examine the question from its ‹quantitative› side (p. 91), and, similarly, the solution of the difficulty concerning the occurrence or nonoccurrence of oxidation and reduction reactions, where the possibility of a transfer of electrons is given, will be found in a study of the problem from its quantitative side. «Oxidation and Reduction Reactions as Reversible Reactions.»—In order to reduce the development of the quantitative [p257] relations to the simplest possible terms, we may turn to still simpler oxidation and reduction reactions than those studied thus far. If a rod of zinc is placed in a solution of copper sulphate, copper is deposited and zinc sulphate is formed. If we consider the action to be an ionic one, we have: Cu^{2+} + SO_{4}^{2−} + Zn ↓ → Zn^{2+} + SO_{4}^{2−} + Cu ↓, or, since the sulphate-ion is not directly concerned in the action, we have more simply: Cu^{2+} + Zn ↓ → Cu ↓ + Zn^{2+}. Cupric-ion has been reduced, therefore, to metallic copper, the metallic zinc oxidized to zinc-ion, each zinc atom transferring two electrons to a cupric ion.