[Illustration: Fig. 234.—Medico-galvanic Battery.] [Illustration: Fig. 235.—Battery in case.] ELECTROTYPING may be briefly explained as follows:—Take two vessels, A and B, and in one, A, put some dilute sulphuric acid and two plates, one of zinc, Z, the other of copper, D, but be sure they are not touching each other; each of these plates must have a piece of wire fastened, by soldering to their upper parts. In the vessel, B, put some solution of sulphate of copper and a small quantity of dilute sulphuric acid, and attach another copper plate to the wire which comes from the copper plate in the acid; this second copper plate is to be immersed in the solution of sulphate of copper, and to the wire from the zinc plate is to be fixed the object to be coated. If a medallion or other object in plaster, it should be soaked in very hot wax and then brushed over with blacklead until the surface is perfectly blackened and bright; the wire should be bound all round the margin and soldered (as it were) with melted wax to the medallion, taking care that this wax also is well coated with blacklead. If the object be now immersed in the sulphate of copper solution and kept at a short distance from the plate (it must not touch it), a coating of copper will soon cover the surface and form a perfect cast, which, when of sufficient thickness, may be removed by filing the edge all round. If instead of the plaster cast a copper coin or other copper object be used, the blackleading is not required, but the surface must be first made clean and bright. Many uses are made of the galvanic current by medical men. If the circuit of the pile is closed and we take a wire in each hand and break contact, a concussion will be felt in the joints of the arm and fingers, and a certain contraction of the muscles. The currents of electricity cause the shocks, and by a peculiar arrangement by which the circuit can be closed or broken at pleasure, a series of shocks can be sent through the body when it forms the connection between the poles of the battery. We give illustrations of a medico-galvanic machine. In fig. 235 there are two batteries, A and B, with cells, C D. Each battery consists of a central plate of platinized silver separated from the zinc plates by a piece of wood, E and F; the binding-screws are fastened to the silver plates, and G H retain the zinc plates; I is a copper band connecting the zinc plate of one battery with the silver plate of the other. At Z and opposite are wires leading to the coil machine. The quantity and intensity of the current are regulated respectively by the indicator, O, and the wires, Q. There is a point, R S, for the breaking of the contact; P N are screws retaining the wires which lead to the handles, U V, grasped by the patients. [Illustration: Fig. 236. Fig. 237. Fig. 238. Horse-shoe magnets.]