Conversely, we frequently have occasion to oxidize ferrous salts to the ferric condition, and among the most convenient reagents for the purpose are chlorine and bromine water (‹exp.›). For instance, we have 2 FeCl_{2} + Cl_{2} → 2 FeCl_{3}, or, considering the action from the point of view of the theory of ionization,[514] 2 Fe^{2+} + Cl_{2} → 2 Fe^{3+} + 2 Cl^{−}. In this case the oxidation of the ferrous to the ferric ion consists in the assumption of an additional positive charge; reduction of chlorine to the chloride-ion consists in the assumption of negative charges by the chlorine atoms. «Definitions of Oxidation and Reduction in Electric Terms.»—The definitions must then be amplified and ‹oxidation be considered to involve ultimately the assumption of positive, or the loss of negative, electrical charges by ions or atoms, reduction to involve ultimately the assumption of negative, or the loss of positive, charges›. According to the electron theory of electricity a unit negative charge is an electron, the unit positive charge, probably, the charge left on an atom when it has lost an electron; and, thus, oxidation may simply be defined, according to the electric theory of oxidation and reduction, as consisting, fundamentally, in the ‹loss of electrons by atoms or ions›, reduction as consisting in a ‹gain of electrons›. For instance, when hydrogen sulphide reduces a ferric salt, 2 Fe^{3+} + S^{2−} → 2 Fe^{2+} + S, ‹the sulphide ions transfer their electrons to the ferric ions›. «Oxidations and Reductions by Electric Currents.»—All of the oxidation and reduction reactions which have been discussed may, in fact, be effected by the use of an ‹electric current› in the place of chemical agents. Ferrous salts, for instance, are ‹oxidized›, at the positive pole, by the current to ferric salts:[515] Fe^{2+} + ⊕ → Fe^{3+}, or Fe^{2+} − ε^{−} → Fe^{3+}. Ferric salts are ‹reduced›, at the negative pole, to ferrous salts: Fe^{3+} + ⊖ → Fe^{2+}, or Fe^{3+} + ε^{−} → Fe^{2+}. [p253]