An earthenware jar, _a_, serves to hold the solution of copper, which should be maintained in a saturated state by the addition of crystals of the salt. A porous tube, _b_, holds a rod of amalgamated zinc, to the top of which a binding-screw is soldered. The copper mould or medal, _c_, is suspended in the solution by a wire, which is held tight by the binding-screw, _d_. The porous jar is then filled to the same height as the copper solution in the jar with diluted sulphuric acid, in the proportions of one of acid to twenty of water. Voltaic action immediately commences, and the copper will continue to be deposited from the solution as long as the supply of fresh crystals of sulphate of copper is continued. In about twenty-four hours the coating of copper will be as thick as a thin card, and it may be then removed. When detached from the medal, it will be found to be an exact counterpart, in the minutest details, of the original; those parts of the medal which are in relief being, of course, the reverse in the mould. [Illustration] The extreme minuteness and delicacy of the electrotype process is strikingly exemplified in its application to the transference of engraved copper-plates. A highly finished engraved copper-plate has a film of metal deposited over its whole surface, which, when detached, exhibits all the lines that are cut into the copper-plate in relief. That electrotype cast then serves as the mould for further depositions, in which every line in the original engraving is so perfectly developed, that it is impossible to detect a difference in the impressions taken from the two plates. By this means any number of casts may be made and worked from, whilst the original is preserved uninjured. The objection to this application is that the metal deposited is not so hard as the hammered plates, and will not, therefore, bear the wear and tear of copper-plate printing so well as the plates made by hand. It was at one time supposed that the depositing of metal on surfaces, by voltaic action, might be applied to the manufacture of numerous kinds of copper articles without manual labour. For this purpose, casts were made of plaster of Paris, which were covered with black lead, to give them the property of conducting electricity, and the metal was then deposited upon them. But, independently of the practical difficulties attending the operation, it was found that the metal was not sufficiently hard, and the cost of the requisite voltaic batteries rendered the economy of the process questionable.