‹In all these cases the use of the conception of the degree of ionization of the electrolytes› makes possible a much broader and more general application of the principle of the independent migration or mobility of the ions than was possible before the theory of Arrhenius was proposed, and marks a distinct advance in the theory of conductivity, over what was possible on the basis of the theory of Clausius. [p058] «Faraday's Law.»—If a definite quantity of electricity, a faraday,[93] or 96,600 coulombs, is passed through a solution of hydrochloric acid, a definite quantity (36.5 grams, one mole) of the hydrogen chloride is decomposed, and one gram of hydrogen and 35.5 grams of chlorine are liberated by the discharge of one gram (‹i.e.› one gram-ion) of the hydrogen-ion and 35.5 grams or one gram-ion of the chloride-ion. In a solution of cupric chloride, the chloride-ion is identical in every respect with the chloride-ion found in a solution of hydrochloric acid. In the solution of cupric chloride, however, a molecule of the salt, when it is completely ionized, produces two chloride ions for every cupric ion (CuCl_{2} ⇄ Cu^{2+} + 2 Cl^{−}). Since the solution never shows the presence of an excess of either form of electricity, and the negative charge on each chloride ion is the same as on a chloride ion formed by the dissociation of hydrogen chloride, a cupric ion must hold ‹exactly› double the positive charge that a hydrogen ion does. In modern terms, each hydrogen atom, present as an ion, has lost one electron, and each copper atom present in the form of a cupric ion has lost two electrons. Our unit quantity of electricity, 96,600 coulombs, can discharge therefore ‹only half as many of the cupric› as of the hydrogen ions, and since each cupric ion is 63.6 times as heavy as the hydrogen-ion (Cu = 63.6, H = 1), 63.6 / 2 grams of copper, the ‹equivalent weight›, will be deposited in place of one gram of hydrogen. Similarly, from a solution of ferrous chloride FeCl_{2}, 55.9 / 2 grams of iron (Fe = 55.9) will be deposited, the ferrous ion being Fe^{2+}; while from a solution of ferric chloride FeCl_{3}, only 55.9 / 3 grams of iron will be deposited by 96,600 coulombs, the ferric ion, Fe^{3+}, holding three times the charge that a hydrogen ion does. In other words, a given quantity of current will decompose ‹equivalent› quantities of electrolytes and deposit ‹equivalent quantities› of metals. This is the well-known law of Faraday. The theory of Arrhenius agrees with it, as did the theory of Clausius. It cannot be considered as evidence bearing on the question of the preference to be given to either of the theories of ionization, since the degree of ionization of electrolytes is not involved in the relations covered by the law. But any other relation would have been incompatible with the theory of Arrhenius. The law is of particular importance in giving us [p059] the best clew that