We have already observed, that if the plate of metal be covered, even with varnish, the particles will not descend or form upon it; nevertheless, if some slight substance be not interposed, the depositing particles adhere so firmly to it as to be inseparable, and it is upon this property that one of the processes--that of engraving in relief on a plate of copper--entirely depends for success. When a cast of an engraved plate is required, the plate must be coated with bee’s-wax, mixed with a little spirits of turpentine. It is laid on the plate in a lump and melted, and when just cooling is wiped off, when, although apparently clean, enough remains to interpose between the new and original plates, and prevent a too strong cohesion. It is not necessary that the engraved plate should be copper: it may be for instance lead or type metal, in which case it need not be waxed, as the application of heat, expanding the metals unequally, causes them at once to start asunder. A piece of wire having been soldered to the back of the plate, its back and edges should be covered with a double coat of thick varnish, or it may be embedded in a box with plaster-of-Paris or Roman cement. This precaution is necessary, to prevent the plate from being inclosed, and to limit the deposition to a proper extent. It may now be suspended in the apparatus, and the wires being placed in contact, the operation begins. Particle by particle the new metal is formed, until the plate is of sufficient thickness, when it is withdrawn, and heat being applied, the two plates are separated, one being the exact counterpart, in relief, of the other. Care must be taken in all cases to change the solution of copper frequently, for by merely _adding_, the separated particles of the sulphuric acid would accumulate to such extent as to mar or injure the operation. From the plate thus formed in relief, as many casts as may be required can be obtained, by making it the mould. To copy or multiply medals and coins the operation is very simple, for a mould can be easily obtained by compressing the medal or coin between two plates of milled sheet lead, and by varnishing the lead round the impression, the deposit will be formed in the hollow only; and for this purpose a very simple apparatus will suffice, and one that may be very easily made. For the outer vessel an ordinary glass tumbler or finger-bowl will answer; and for the inner, a cylindrical gas-glass, having a bottom made of plaster-of-Paris. The solution of copper being in the tumbler, and the acid with the zinc in the gas-glass, the mould should be suspended by its conducting wire between the bottoms, the wire of the zinc connected with it, and the operation will proceed. In all cases it must be observed that the edge of the mould should be up, as, if it be placed horizontally, extraneous substances, sinking by their own weight, may be deposited upon it.