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The oxides are also decomposed: the oxygen appears at the positive pole, and the metal at the negative. The decomposition of the alkalies and earths by Sir Humphry Davy formed a remarkable era in the history of science. Soda, potass, lime, magnesia, and other substances heretofore considered to be simple bodies incapable of decomposition, were resolved by electric agency into their constituent parts, and proved to be metallic oxides, by that illustrious philosopher. All chemical changes produced by the electric fluid are accomplished on the same principle, and it appears that, in general, combustible substances, metals, and alkalies go to the negative wire, while acids and oxygen are evolved at the positive. The transfer of these substances to the poles is not the least wonderful effect of the voltaic battery. Though the poles be at a considerable distance from one another, nay, even in separate vessels, if a communication be only established by a quantity of wet thread, as the decomposition proceeds the component parts pass through the thread in an invisible state, and arrange themselves at their respective poles. The powerful efficacy of voltaic electricity in chemical decomposition arises from the continuance of its action, and its agency appears to be most exerted on fluids and substances which, by conveying the electricity partially and imperfectly, impede its progress. But it is now proved to be as efficacious in the composition as in the decomposition or analysis of bodies.

It had been observed that, when metallic solutions are subjected to galvanic action, a deposition of metal, generally in the form of minute crystals, takes place on the negative wire. By extending this principle, and employing a very feeble voltaic action, M. Becquerrel has succeeded in forming crystals of a great proportion

of the mineral substances, precisely similar to those produced by nature. The electric state of metallic veins makes it possible that many natural crystals may have taken their form from the action of electricity bringing their ultimate particles, when in solution, within the narrow sphere of molecular attraction already mentioned as the great agent in the formation of solids. Both light and motion favour crystallisation. Crystals which form in different liquids are generally more abundant on the side of the jar exposed to the light; and it is well known that still water, cooled below 32°, starts into crystals of ice the instant it is agitated. Light and motion are intimately connected with electricity, which may therefore have some influence on the laws of aggregation; this is the more likely, as a feeble action is alone necessary, provided it be continued for a sufficient time. Crystals formed rapidly are generally imperfect and soft, and M. Becquerrel found that even years of constant voltaic action were necessary for the crystallisation of some of the hard substances. If this law be general, how many ages may be required for the formation of a diamond!

Common electricity, on account of its high tension, passes through water and other liquids, as soon as it is formed, whatever the length of its course may be. Voltaic electricity, on the contrary, is weakened by the distance it has to traverse. Pure water is a bad conductor, but ice absolutely stops a current of voltaic electricity altogether, whatever be the power of the battery, although common electricity has sufficient tension to overcome its resistance Dr. Faraday has discovered, that this property is not peculiar to water, that, with a few exceptions, bodies which do not conduct electricity when solid, acquire that property and are

immediately decomposed when they become fluid, and, in general, that decomposition takes place as soon as the solution acquires the capacity of conduction, which has led him to suspect that the power of conduction may be only a consequence of decomposition.

Heat increases the conducting power of some substances for voltaic electricity, and of the gases for both kinds. Dr. Faraday has given a new proof of the connection between heat and electricity, by showing that, in general, when a solid which is not a metal becomes fluid, it almost entirely loses its power of conducting heat, while it acquires a capacity for conducting electricity in a high degree.

The galvanic fluid affects all the senses. Nothing can be more disagreeable than the shock, which may even be fatal if the battery be very powerful. A bright flash of light is perceived with the eyes shut, when one of the wires touches the face and the other the hand. By touching the ear with one wire and holding the other, strange noises are heard, and an acid taste is perceived when the positive wire is applied to the tip of the tongue and the negative wire touches some other part of it. By reversing the poles the taste becomes alkaline. It renders the pale light of the glowworm more intense. Dead animals are roused by it, as if they started again into life, and it may ultimately prove to be the cause of muscular action in the living.

Several fish possess the faculty of producing electrical effects. The most remarkable are the gymnotus electricus, found in South America, and the torpedo, a species of ray, frequent in the Mediterranean. The electrical action of the torpedo depends upon an apparatus perfectly analogous to the voltaic pile, which the animal has the power of charging at will, consisting

of membranous columns filled throughout with laminæ, separated from one another by a fluid. The absolute quantity of electricity brought into circulation by the torpedo is so great, that it effects the decomposition of water, has power sufficient to make magnets, and gives very severe shocks. It is identical in kind with that of the galvanic battery, the electricity of the under surface of the fish being the same with the negative pole, and that in the upper surface the same with the positive pole. Its manner of action is, however, somewhat different, for, although the evolution of the electricity is continued for a sensible time, it is interrupted, being communicated by a succession of discharges.

SECTION XXIX.

TERRESTRIAL MAGNETISM.-MAGNETIC MERIDIANS.

VARIATION

OF THE COMPASS. LINES OF NO VARIATION. MAGNETIC POLES. THEIR NUMBER AND POSITION. DIURNAL AND NOCTURNAL VARIATIONS. — THE DIP. — THE MAGNETIC EQUATOR. ITS POSITION. — VARIATION IN THE DIP. CAUSE OF MAGNETIC CHANGES UNKNOWN.-ORIGIN OF THE MARINER'S COMPASS.- NATURAL MAGNETS. ARTIFICIAL MAGNETS. - POLARITY. INDUCTION. INTENSITY. -HYPOTHESIS OF TWO MAGNETIC FLUIDS. DISTRIBUTION OF THE MAGNETIC FLUID. ANALOGY BETWEEN MAGNETISM AND ELECTRICITY.

In order to explain the other methods of exciting electricity, and the recent discoveries that have been made in that science, it is necessary to be acquainted with the general theory of magnetism, and also with the magnetism of the earth, the director of the mariner's compass, his guide through the ocean. Its influence extends over every part of the earth's surface, but its action on the magnetic needle determines the poles of this great magnet, which by no means coincide with the poles of the earth's rotation. In consequence of their attraction and repulsion, a needle freely suspended, whether it be magnetic or not, only remains in equilibrio when in the magnetic meridian, that is, in the plane which passes through the north and south magnetic poles. There are places where the magnetic meridian coincides with the terrestrial meridian. In these a magnetic needle freely suspended points to the true north; but, if it be carried successively to different places on the earth's surface, its direction will deviate sometimes to the east and sometimes to the west of

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