The object to be plated is connected with the negative electrode of a battery or a magneto-electric machine and suspended in a hot solution of the cyanide, and a plate of silver connected with the positive electrode is suspended near it, as represented in fig. 30. The object to be plated, now forming the negative electrode of the electrolyte cell, attracts the metallic silver of the cyanide; the potassium, the most electropositive of the three bodies, remaining combined with the cyanogen, the two forming the electro-negative constituent of the compound AgKCy2. The reactions which take place may be represented in the following diagram:

Fig. 25.

Fig. 26.

Fig. 27.

Fig. 28.

Fig. 29.-Gas Battery.

Fig. 30.-Simple Bath.

The body KCy2, which is liberated upon the surface of the silver plate, immediately combines with that metal, and thus the solution is continually replenished with the double cyanide. For gilding, a solution of auro-cyanide of gold, AgKCy2, corresponding to the silver salt, may be formed in a similar manner. A plate of gold forms the positive, and the object which is to be coated thie negative electrode.

A number of objects may be suspended upon one rod, the positive electrode being enlarged so as to offer a sufficient surface for the action of the cyanide of potassium; or the compound cell system may be adopted, as represented in fig. 31, where the bath is divided into separate cells, like those of a trough battery, the negative plate in one cell being connected with the positive plate in the next. This arrangement requires the addition of electromotive force to the battery, and is moreover found not to be so manageable or economical as the simple cell system. Electroplating in nickel has recently been introduced by Dr. Isaac Adams of Boston. The following bath has been found to work well: Add one part of a solution of neutral tartrate of ammonia to 20 parts of a solution of the double sulphate of nickel and ammonia, both solutions being in water and standing at 70° Baume. After mixing and standing a few hours, the bath is ready for use. A plate of nickel forms the positive electrode. Several applications of nickel-plating have been patented; as to gas burners to protect the tips from oxidation; to culinary utensils for the same purpose; to facing printing type, to harden the surface and prevent the action of colored inks; and for covering portions of firearms.

It is also used in plating surgical instruments. Electroplating is not confined to the deposition of one metal upon another, but alloys may be coated with one metal, or a single metal may be coated with an alloy. Brass and bronze, with careful management, and by the use of positive electrodes of the alloy, may be successfully deposited upon copper plates.-Electrotyping, which consists in making a cast of a metal upon a mould by galvanic action, is performed as follows: The "form" containing the type, woodcut, or other engraving which is to be electrotyped, is cleaned and slightly dusted with finely powdered black lead. It is then laid face upward on a bed plate of a hydraulic or toggle-joint press of great power. A brass case in the form of a shallow pan, and rather larger than the form, has turned into it about a half inch thickness of melted beeswax, which after cooling is placed over the form, and the two are powerfully pressed together. This forms a wax mould, which after being separated from the form is built up, in places which are to be blanks, by running on more wax with a building iron; an elevation upon the mould, of course, forming a depression in the electrotype plate.

The surface of the mould is now coated over with fine black lead powder, to give it a conducting surface for the galvanic current, which operation is performed with a kind of stippling brush of badger's hair, moved by machinery in a box containing black lead dust. When a good surface has been formed the loose particles are blown off, and it is washed with a weak solution of sulphate of copper, after which it is dusted with fine iron filings, by which means a film of metallic copper is deposited over the black lead surface, which increases its compactness and conducting power. It is then suspended in a bath composed of sulphate of copper and sulphuric acid, in the proportion of two of the salt to one of the acid, in enough water to stand at about 14° Baume, and is connected with the negative electrode of a battery or of a magneto-electric machine. Opposite the face of the mould there is suspended a sheet of copper, connected with the positive electrode. From its greater constancy Daniell's battery is usually preferred, but others may be used, and where great rapidity is required powerful magneto-electric machines like that of Wilde (see Magneto-Electeicity) are employed. A series of moulds may be suspended back to back, the copper sheet being placed between opposite pairs.

When the circuit is closed the sulphate of copper is decomposed, the metallic copper, the positive constituent, going to the negative plate, which is the plumbago surface of the mould, while the body SO4 unites with the copper plate, forming sulphate of copper. The arrangement of bath and battery is similar to that shown in fig. 30. The moulds may be made of gutta percha instead of wax. The time required to form the electrotype plate depends upon the strength of* the solution and the electromotive force of the battery or magneto-electric machine. With a saturated solution of sulphate of copper, and a machine requiring a six-horse power to drive it, several large plates may be made in less than one hour. After the shell, as the deposit is called, is taken off the mould, it is placed in water to protect it from oxidation until the workmen are ready to perform the operation of backing. A shell is laid upon its face in a shallow vessel having a plane bottom, and its back is washed with a solution of chloride of tin, called soldering fluid. A sheet of tin foil is then laid over it, and heat applied to the bottom of the pan until the foil melts and spreads over the surface.

A fusible alloy of tin and zinc or type metal is then melted and turned upon the back to the thickness of about an eighth of an inch. After cooling the plate is taken out, cleaned, and polished with marble dust and benzine, or with clay and soft soap; and it may be faced with nickel if desired.

Fig. 31.-Compound Bath.