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 employed? 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?

Action of an Electrical Current upon a Magnet.

427.

ERSTED discovered the fact that an electrical current has a directive power over the magnetic needle, tending always to direct it at right angles to its own direction.

This action may be shown by the apparatus represented in Fig. 293. If a wire be placed parallel to and pretty near a magnetic needle, and then a current of electricity be passed through it, the needle will turn around, and after a

few oscillations will come to rest in a position sensibly at right angles to the current. That it does not take a position absolutely perpendicular to that of the current, is because of the directive force of the earth, which partially counteracts that of the current.

The direction towards which the austral pole, that is, the north end of the needle, will turn, depends upon the direction of the cur

(427) What discovery was made by ERSTED? Explain the action of the electrical current on the needle. Which way does the north end turn?

rent. If that flows from south to north, and above the needle, the needle deviates towards the west; if it flows towards the south, and above the needle, the latter deviates towards the east. When the current flows below the needle, the phenomena are reversed.

Ampere's Law.

428. AMPERE, to whom the discovery of the greater portion of electro-magnetic phenomena is due, gave a simple expression to the law which governs the action of a current upon a magnet. He supposes an observer lying down upon the wire along which the current flows, the current entering at the head and going out at the feet. Then, if he turn his face towards the needle, the austral pole will in all cases be deviated towards his right hand.

Action of Magnets upon Currents, and of Currents upon Currents.

429. AMPERE established the following principles:

1. Magnets exercise a directive force upon currents.

To illustrate this, we bend a copper wire into a circular form, and then dip its extremities, which should be pointed with steel, into cups of mercury, one above the other, as shown in Fig. 294. These cups communicate with the two poles of a battery, by means of which a current may be generated, flowing as indicated by the arrows. Now if a bar magnet be brought near this current, the axis being in the plane of the current, we shall see the hoop turn about the steel points in the cups, and come to rest, with its plane perpendicular to the axis of the magnet. This experiment, which is due to AMPERE, is the reverse of that made by ERSTED.

2. The earth, which acts like a huge magnet upon a magnetic needle, acts in the same manner upon movable cur

(428.) Explain AMPERE'S law. (429.) What is AMPERE's first principle? How illustrated? His second principle?