The deposition of copper from its solution, when under the action of voltaic electricity, is not produced by the decomposition of the sulphate of copper, as might be supposed, but by the decomposition of the water that acts as the solvent of the metallic salt. Thus, when two platinum wires from the poles of a voltaic battery are introduced into acidulated water, hydrogen gas is disengaged at the wire connected with the negative pole, and oxygen at the other; but when a solution of sulphate of copper is substituted for water, the hydrogen that is disengaged combines with the oxygen that held the copper in solution, and the metal is liberated. The copper thus liberated from its combination with the oxygen is deposited, in a pure metallic state, on the surface connected with the negative pole of the battery. The simplest illustration of electro-metallic deposition is obtained by immersing a silver spoon and a strip of zinc into a solution of sulphate of copper. So long as the two metals are kept apart, no change takes place on the silver, but on bringing them into contact, voltaic action immediately commences, and a coating of copper is deposited upon the spoon, and adheres firmly to the metal. If the action be continued, and the supply of copper be maintained by the addition of fresh crystals of the sulphate, the coat of copper may be increased in thickness to almost any extent. The first applications of the discovery were directed to the copying of medals and coins. An impression of the metal was obtained in fusible metal, which is an alloy composed of tin, lead, and bismuth, melted together in the proportions of two of the latter to one each of the former. This alloy expands on cooling, and thus affords a very sharp impression of the medals; and as it melts at a low temperature, it may be easily removed after the copper coating has been deposited upon it. An electrotype mould, obtained directly from the medal, is, however, more sharp in its definition than an impression, and is therefore preferable, when circumstances admit of its being so taken. For that purpose, the surface whereon the deposition is to be made is smeared over with sweet oil, or with black lead. It is then carefully wiped with cotton wool, but a minute quantity of the oil will still remain, sufficient to prevent the metal from adhering. A simple form of apparatus for the electrotype process is shown in the accompanying diagram.