b. A porous diaphragm, containing the cylinder or plate of zinc (c), and filled with dilute sulphuric acid.
d. A small bar of brass or copper fastened to the vessel by the binding screws (e, e), and supporting the plate of zinc (c), by the hook of copper wire (f), and the mould (g), by the hook (h).
i. A small shelf or partition to support crystals of sulphate of copper, to keep up the strength of the solution.
a. A constant battery cell.
b. Decomposition cell, a cubical vessel made of wood or earthenware, and filled with a mixture of 1 part of dilute sulphuric acid (1 acid + 9 water), and 2 parts of saturated solution of sulphate of copper by measure.
c, c, c. Moulds suspended to the brass rod (f), and connected with the zinc or positive element of the battery (a), by means of the screw (g.)
d, d. Pieces of sheet copper suspended on the brass rod (h), and connected with the zinc end of the battery, by means of the screw (i), employed to keep up the strength of the cupreous solution in the decomposition cell.
To connect the moulds with the zinc or positive element, stout copper wires or strips of thin sheet copper are employed. In the case of a non-metallic mould, the wire must lead directly to the plumbagoed surface, or, what amounts to the same thing, the plumbago must be extended to the point of attachment. The connecting wires, and the backs and edges of metallic moulds, must be covered with sealing-wax varnish, or other non-conducting substance, to prevent them receiving the deposit. Before a mould is placed in the copper solution it is advisable that everything should be arranged, so that the immersion may occasion immediate voltaic action. If the connection between the zinc and the mould is not effected until after the immersion, the solution may act chemically on the surface of the mould, and cause the deposit to appear dark and dirty. When a mould has remained in the solution long enough to receive a complete coating of copper, it may be lifted out with impunity for examination. If everything is going on well, the deposited metal will present a brilliant, light, copper-coloured surface. When sufficiently thick, the deposit is removed with care, washed and placed to dry. Electrotype medals may be polished with wash-leather and the plate brush, or bronzed. Various natural objects such as insects, fruits, &c.; small works of art, such as busts and statuettes; chemical vessels, particularly glass flasks and retorts; and numerous classes of articles, may be rendered less fragile by coating them with copper by the electrotype process.
II. Deposition of the precious metals—