which we shall suppose to be of copper, a DANIEL's battery of two or three couples is usually employed.

A couple of DANIEL's battery differs from one of BUNSEN's in the following particulars. The carbon cylinder is replaced by one of zinc, denoted by Z, in Fig. 291, and the zinc cylinder is replaced by one of copper, denoted by C, in the same figure. The outer vessel is of glass, and is filled with a solution of sulphate of copper (blue vitriol), which is kept saturated by some crystals of the sulphate

placed at the bottom of the vessel. The porous vessel is filled with dilute sulphuric acid. When this battery is in action, water is decomposed; the oxygen goes to the zinc, forming oxyde of zinc, which is dissolved by the sulphuric acid, giving sulphate of zinc. The hydrogen of the water goes to the sulphate of copper in P, and decomposes it. The result of these decompositions and recompositions is to keep up a current of electricity, as shown by the arrows, which will continue as long as the vessel, P, is kept full of the saturated solution of sulphate of zinc.

Fig. 291 shows the method of depositing the metal upon

the mould. M is a vessel filled with a solution of sulphate of copper; A and B are metallic rods communicating with the two poles of the battery; the mould is suspended from. the rod, B, and facing it is a plate of pure copper suspended from the rod, A; these constitute the electrodes, the mould being the negative one.

The current which is set up through the solution of copper between the electrodes, decomposes the sulphate into sulphuric acid, oxygen, and pure copper. The sulphuric acid and oxygen go to the copper plate, and uniting with it, produce sulphate of copper; the pure copper goes to the negative electrode, that is, to the mould, and is there depos ited. After about two days the coating of copper becomes thick enough to be removed from the mould, and it then presents a fac-simile of the object to be copied. In copying medals, each face is copied separately, and the two are united by means of some fusible metal placed between them

Electro-gilding and Electro-plating.

425. The process of covering bodies with thin coatings. of gold or silver is analogous to that of electrotyping. The perfection of the process consists in making the coating of gold or silver, not only of uniform thickness, but also closely adherent.

The object to be gilded or silvered is first heated upon a charcoal fire to remove all fatty matter; it is next plunged into dilute sulphuric acid, and then rubbed with a hard brush to remove any oxyde that may exist upon the surface; it is next plunged into common nitric acid, and then into nitric acid into which a small quantity of salt and soot has been thrown; it is then washed in pure water and carefully dried in sawdust, and is ready for use.

Explain the chemical changes which take place. (425.) What is the process of electro-silvering and electro-gilding? How is the object cleaned?

The method of silvering, or electro-plating, is shown in Fig. 292. The object to be silvered is suspended in a bath of a silver solution by a metallic rod which connects with the negative pole of a BUNSEN's battery. Immediately below it is a plate of pure silver, which is connected with the positive. pole of the battery. The object to be silvered and the silver plate, a, constitute the electrodes, a being the positive one. The explanation of the process is analogous to that in the preceding article.

The salt of silver generally employed is a double cyanuret of silver and potassium. The thickness of the coating deposited will depend upon the power of the battery and upon the time of immersion.

The process of gilding is the same as that of silvering, except that we use a cyanuret of gold and potassium, and a plate of gold at a, instead of a silver one.

Explain the process of silvering as shown in Fig. 292. What salt of silver is em ployed? What is the process of gilding? What salt of gold is used?

The history of electro-plating and electro-gilding is briefly as follows: In 1803, BRUGNATELLI first gilded a silver medal by suspending it in a solution of gold from the negative pole of a battery, but proceeded no further. In 1840, DE LA RIVE, of Geneva, discovered a process of gilding metals with a battery, but by his process much gold was wasted, and the work was unsatisfactory. In the same year, ELKINGTON, an Englishman, discovered the process of gilding by means of the cyanuret of gold and potassium. A few months later, RUOLZ succeeded in silvering and platinizing metals by the methods now in general use. The arts of electro-plating, electrogilding, and electrotyping are now of general application, and afford occupation to thousands of artisans.

Give an outline of the history of electro-plating and electro-gilding.

Relation between Magnetism and Electricity.

426. Ir was observed at an early period that the magnetic and electrical fluids had many analogous properties. In each case fluids of the same name repel, whilst those of an opposite name attract. It was also observed that a stroke of lightning often reversed the poles of a magnetic needle, and sometimes completely destroyed its magnetism. The two have also points of dissimilarity. Magnetic fluids are not transmitted like electrical fluids through conductors. A magnet does not, like an electrified body, return to a neutral state when brought into communication with the earth. Magnetism can only be developed in a few, whereas electricity may be developed in all bodies.

Between these analogies and dissimilarities nothing positive could be affirmed with respect to the identity of magnetism and electricity, until, in 1819, ERSTED made a discovery which showed that these physical agents are most intimately allied, if not identical.

(426.) What early observations were made on the relation of the phenomena of electricity and magnetism? What dissimilarities were noticed?