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duced by the electric fluid, are accomplished on the same principle, and it appears that, in general, combustible substances go to the negative wire, while oxygen is evolved at the positive. 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. Becquerel has succeeded in forming crystals of a great proportion of the mineral substances precisely similar to those produced by 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 crystallization. Crystals which form in different liquids


are generally more abundant on the side of the jar exposed to the light; and it is a well-known fact 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. Becquerel found that even years of constant voltaic action were necessary for the crystallization of some of the hard substances. If this law be general, how many ages may be required for the formation of a diamond!

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 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.


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, and 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 north. Lines drawn on the globe, through all the places where the needle points due north and south, are called lines of no variation, and they are extremely complicated. The direction of the needle is not even constant in the same place, but changes in a few years according to a law not yet determined. In 1657, the line of no variation passed through London; from that time it has moved slowly, but irregularly, westward, and is now in North America. In the year 1819, Sir Edward Parry, in his voyage to discover the northwest passage round America, sailed near the magnetic pole; and in 1824, Captain Lyon, on an expedition for the same purpose, found that the magnetic pole was then situate in 63° 26' 51" north latitude, and in 80° 51' 25" west longitude. It appears, from later researches, that the law of terrestrial magnetism is of considerable complexity and the existence of more than one magnetic pole in either hemisphere has been rendered highly probable; that there is one in Siberia seems to be decided by the recent observations of M. Hansteen,-it is in longitude 102° east of Greenwich, and a little to the north of the 60th degree of latitude so that, by these data, the two magnetic poles in the northern hemisphere are about 180° distant from each other; but Captain Ross, who

is just returned from a voyage in the polar seas, has ascertained that the American magnetic pole is in 70° 14' north latitude, and 96° 40' west longitude. The magnetic equator does not exactly coincide with the terrestrial equator; it appears to be an irregular curve inclined to the earth's equator at an angle of about 12°, and crossing it in at least three points in longitude 113° 14' west, and 66° 46′ east of the meridian of Greenwich, and again somewhere between 156° 30' of west longitude, and 116° east.

The needle is also subject to diurnal variations ; in our latitudes it moves slowly eastward during the forenoon, and returns to its mean position about ten in the evening; it then deviates to the westward, and again returns to its mean position about ten in the morning. M. Kupffer, of Casan, ascertained, in the year 1831, that there is a nightly, as well as a diurnal variation, depending, in his opinion, upon a variation in the magnetic equator.

A magnetic needle, suspended so as to be moveable only in the vertical plane, dips, or becomes more and more inclined to the horizon the nearer it is brought to the magnetic pole, and there becomes vertical. At the magnetic equator it is horizontal, and between these two positions it assumes every degree of inclination. Captain

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