_If the same current be made to pass through several different electrolytes, the quantity of each ion produced will be proportional to its combining weight divided by its valency, and if the current vary, the quantity of each ion liberated per second will be proportional to the current._ This is the great law of electro-chemical equivalents. The amount of hydrogen liberated per second by a current of one ampere is about .00001038 gramme, or nearly one six-thousandth of a grain. This is the electro-chemical equivalent of hydrogen. That of any other substance may be found by Faraday's law. From Faraday's results it appears that the passage of the same amount of electricity is required in order to decompose one molecule of any compound of the same chemical type, but it does not follow that the same amount of energy is employed in the decomposition. For example, the combining weights of copper and zinc are nearly equal. Hence it will require the passage of about the same amount of electricity to liberate a pound of copper from, say, the copper sulphate as to liberate a pound of zinc from zinc sulphate; but the work to be done is much less in the case of the copper. This is made manifest in the following way:--A battery, which will just decompose the copper salt slowly, liberating copper, oxygen, and sulphuric acid, will not decompose the zinc salt at all so as to liberate metallic zinc, but immediately on sending the current through the electrolyte, polarization will set in, and the opposing electro-motive force thus introduced will become equal to that of the battery, and stop the current before metallic zinc makes its appearance. In the case of the copper, polarization also sets in, but never attains to equality with the electro-motive force of the primary battery. In fact, in all cases of electrolysis, polarization produces an opposing electro-motive force strictly proportional to the work done in the cell by the passage of each unit of electricity. If the strength of the battery be increased, so that it is able to decompose the zinc sulphate, and if this battery be applied to the copper sulphate solution, the latter will be _rapidly_ decomposed, and the excess of energy developed by the battery will be converted into heat in the circuit. One important point in connection with electrolysis which Faraday demonstrated is that the decomposition is the result of the passage of the current, and is not simply due to the attraction of the electrodes. Thus he showed that potassium iodide could be decomposed by a stream of electricity coming from a metallic point on the prime conductor of his electric machine, though the point did not touch the test-paper on which the iodide was placed.