EXP. The migration of the ions throughout the whole solution may be demonstrated by the passage of a current through a large U-tube containing a mixture of a cupric salt and a permanganate,[74] placed under some dilute sulphuric acid. The cupric-ion is blue, all ionized solutions of cupric salts, with a colorless negative ion, being blue, while the permanganate-ion is of an intense purple color. In the limb of the U-tube, in which the cathode is placed, a blue zone, containing cupric ions, is soon seen emerging from the purple liquid and rising toward the cathode (see Fig. 8). It will take some time for any cupric ions actually to reach the electrode and be deposited as metallic copper. On the anode side, purple permanganate ions are seen rising toward the positive electrode.
The movement of the electrically charged particles in opposite directions through the solution constitutes an electric current, and such a current has the properties of a current through a [p046] wire—producing, for instance, heat, or being capable of deflecting a magnet placed in its field of action.[75]
«Electrolysis.»—When ions touch an electrode, they are discharged, and with the discharge, are changed chemically, and, according to the electron theory, also materially. Cupric ions, Cu^{ 2+} or Cu^{ −2 ε}, receiving two electrons at the cathode, are precipitated as metallic copper. When a current is passed through a solution of hydrochloric acid, the hydrogen ions, by the pairing of the discharged ions, yield hydrogen gas consisting of molecules, H_{ 2}; the chloride ions are converted at the anode into chlorine Cl_{ 2}. The formation of these molecules, X_{ 2}, may be variously interpreted, as resulting either from the union of two discharged or neutral atoms, or as consisting, for instance in the case of hydrogen, in the discharge of a hydrogen ion H^{ +}, at the negative electrode, ‹followed by the assumption of a negative charge or electron by the neutral atom›, the new ion H^{ −} combining at once with the positive ion H^{ +} to form the molecule H^{ +}H^{ −}. At the positive pole, by an analogous recharging of a discharged negative chloride-ion Cl^{ −}, we should obtain a positive ion,[76] Cl^{ +}, and immediately the formation of Cl_{ 2} or Cl^{ +}Cl^{ −} would follow. Helmholtz advocated the latter conception of the action at the electrodes, and, more recently, J. J. Thomson[77] brings forward, as a very important argument in favor of it, the fact that iodine, which according to vapor density determinations dissociates into monatomic molecules at high temperatures (I_{ 2} ⇄ 2 I), becomes, simultaneously with this dissociation, also an excellent ‹gaseous conductor of electricity›, as would be anticipated from a dissociation, I^{ +}I^{ −} ⇄ I^{ +} + I^{ −}. This fact is emphasized here, because in it we seem to have a case of ‹conductivity› coinciding with ‹gaseous dissociation›, the existence of which is recognized by the universally accepted laws of gases, and differing in no important respect from the dissociation assumed for conductors in solution.