viduals who have been fortunate enough to lay open to the world a new law of nature.

It has been doubted by many persons, whether the voltaic and electrical energy were the same: but thousands of experiments might be offered to prove them to be such. M. de Luc's very simple aerial electroscope, or electrical column, as he calls it, may be adverted to, as sufficient of itself to establish this fact. This column consists of zinc-plates and Dutch gilt paper, in regular suc. cession, like the metallic plates of the voltaic pile, the groups being from one thousand to ten thousand. When two of these columns are placed horizontally, the one insulated, and the other communicating with the ground, each being terminated with a small bell, and a small brass ball is suspended between the two bells by a silken thread, the ball, by the mere influence of the electricity contained in the atmosphere, will chime, by striking alternately from column to column, and consequently from bell to bell, sometimes more or less rapidly, and sometimes more or less loudly, and sometimes scarcely at all, according to the state and proportion of the electric aura; and the instrument, which is a genuine voltaic pile, not only proves the identity of the electric and voltaic power, but may be conveniently employed as a measurer of the electricity which the at. mosphere contains. It should he observed, however, that as there are no fluids known, except such as contain water, that are capable of being made the medium of connexion between the metals, or metal of the voltaic apparatus, the effect in this, and in all similar instances, is resolved by Sir Humphry Davy into some small quan. tity of moisture, or water still existing in the substances employed, which he asserts will not act if each of the substances be made perfectly dry.

The first distinct experiment upon the igniting powers of large voltaic plates was performed by MM. Fourcroy, Vauquelin, and Thenard; but a much grander combination for exhibiting the effects of extensive surface was constructed by Mr. Children, and consists of a battery of twenty double plates four feet by two; of which the whole surfaces are exposed, in a wooden trough, in cells covered with cement, to the action of diluted acids.

The most powerful combination, however, that exists, in which numbers of alternations is combined with the extent of surface, is that constructed by subscriptions of a few zealous cultivators and patrons of science, in the laboratory of the Royal Institution. It consists of two hundred instruments, connected together in regular order, each composed of ten double plates, arranged in ce lls of por celain, and containing in each plate thirty-two square inches; so that the whole number of double plates is 2000, and the whole surface 128,000 square inches. This battery when the cells were filled with sixty parts of water mixed with one part of nitric acid, and one part of the sulphuric acid, afforded a series of brilliant and impressive effects. When pieces of charcoal about an inch long and one-sixth of an inch in diameter were brought near each other (within the thirtieth part or fortieth part of an inch), a bright spark was produced, and more than half the volume of the char. coal became ignited to whiteness; and by withdrawing the points from each other, a constant discharge took place through the heated air, in a space equal at least to four inches, producing a most brilliant ascending arch of light, broad, and conical in form in the middle. When any substance was introduced into this arch it in. stantly became ignited; platina melted as readily in it as wax in the flame of a common candle: quartz, the sapphire, magnesia, lime, all entered into fusion; fragments of diamond, and points of char. coal and plumbago, rapidly disappeared, and seemed to evaporate in it, even when the connexion was made in a receiver exhausted by the air-pump; but there was no evidence of their having previously undergone fusion.

When the communication between the points positively and negatively electrified was made in air, rarefied in the receiver of the air-pump, the distance at which the discharge took place increased as the exhaustion was made; and when the atmosphere in the vessel supported only one-fourth of an inch of mercury in the barometrical gage, the sparks passed through a space of nearly half an inch; and by withdrawing the points from each other, the discharge was made through six or seven inches, producing a most beautiful coruscation of purple light, the charcoal became intensely ignited, and some platina wire attached to it fused with brilliant scintillations, and fell in large globules upon the plate of the pump. All the phænomena of chemical changes were produced with intense rapi. dity by this combination. When the points of charcoal were brought near each other in nonconducting fluids, such as oils, ether, and oxymuriate compounds, brilliant sparks occurred, and elastic matter was rapidly generated: and such was the intensity of the electricity, that sparks were produced, even in good imperfect conductors, such as the nitric and sulphuric acids.

[Editor. Pantologia.

T

[53]

CHAP. III.

MAGNETISM.

HE theory of magetism bears a very strong resemblance to that of electricity, and it must therefore be placed near it in a system of natural philosophy. We have seen the electric fluid not only exerting attractions and repulsions, and causing a peculiar distribution of neighbouring portions of a fluid similar to itself, but also excited in one body, and transferred to another, in such a manner as to be perceptible to the senses, or at least to cause sensible effects, in its passage. The attraction and repulsion, and the peculiar distribution of the neighbouring fluid, are found in the phænomena of mag. netism; but we do not perceive that there is any actual excitation, or any perceptible transfer of the magnetic fluid from one body to another distinct body; and it has also this striking peculiarity, that, metallic iron is very nearly, if not absolutely, the only substance capable of exhibiting any indicatious of its presence or activity.

For explaining the phænomena of magnetism, we suppose the par. ticles of a peculiar fluid to repel each other, and to attract the par. ticles of metallic iron with equal forces, diminishing as the square of the distance increases; and the particles of such iron must also be imagined to repel each other, in a similar manner. Iron and steel, when soft, are conductors of the magnetic fluid, and become less and less pervious to it as their hardness increases. The ground work of this theory is due to Mr. Aepinus, but the forces have been more particularly investigated by Coulomb, and others. There are the same objections to these hypotheses as to those which constitute the theory of electricity, if considered as original and fundamental properties of matter: and it is additionally difficult to imagine, why iron, and iron only, whether apparently magnetic or not, should. repel similar particles of iron with a peculiar force, which happens to be precisely a balance to the attraction of the magnetic fluid for iron. This is obviously improbable; but the hypotheses are still of great utility in assisting us to generalise, and to retain in memory a number of particular facts which would otherwise be insulated. The doctrine of the circulation of streams of the magnetic fluid has been justly and universally abandoned; and some other theories, much more ingenious, and more probable, for instance that of Mr.

Prévost, appear to be too complicated, and too little supported by facts, to require much of our attention.

The distinction between conductors and nonconductors is, with respect to the electric fluid, irregular and intricate; but in magnetism the softness or hardness of the iron or steel constitutes the only difference. Heat, as softening iron, must consequently render it a conductor; even the heat of boiling water affects it, in a certain degree, although it can scarcely be supposed to alter its temper; but the effect of a moderate heat is not so considerable in magnetism as in electricity. A strong degree of heat appears, from the experiments of Gilbert, and of Mr. Cavallo, to destroy completely all magnetic action.

It is perfectly certain that magnetic effects are produced by quantities of iron incapable of being detected either by their weight or by any chemical tests. Mr. Cavallo found that a few particles of steel, adhering to a hone, on which the point of a needle was slightly rubbed, imparted to it magnetic properties; and Mr. Coulomb has observed, that there are scarcely any bodies in nature which do not exhibit some marks of being subjected to the influence of magnetism, although its force is always proportional to the quantity of iron which they contain, as far as that quantity can be ascertained; a single grain being sufficient to make 20 pounds of another metal sensibly magnetic. A combination, with a large proportion of oxygen, deprives iron of the whole or the greater part of its magnetic proper. ties; finery cinder is still considerably magnetic, but the more per. fect oxids and the salts of iron only in a slight degree; it is also said that antimony renders iron incapable of being attracted by the magnet. Nickel, when freed from arsenic and from cobalt, is decidedly magnetic, and the more so as it contains less iron. Some of the older chemists supposed nickel to be a compound metal containing iron; and we may still venture to assume this opinion as a magnetical hypothesis. There is indeed no way of demonstrating that it is impossible for two substances to be so united as to be incapable of separation by the art of the chemist; had nickel been as dense as platina, or as light as cork, we could not have supposed that it contained any considerable quantity of iron, but in fact the specific gravity of these metals is very nearly the same, and nickel is never found in nature but in the neighbourhood of iron; we may therefore suspect, with some reason, that the hypothesis of the existence of iron in nickel may be even chemically true. The

aurora borealis is certainly in some measure a magnetical phenomenon, and if iron were the only substance capable of exhibiting magnetical effects, it would follow that some ferruginous particles must exist in the upper regions of the atmosphere. The light usually attending this magnetical meteor may possibly be derived from electricity, which may be the immediate cause of a change of the distribution of the magnetic fluid, contained in the ferruginous vapours, that are imagined to float in the air.

We are still less capable of distinguishing with certainty in magnetism, than in electricity, a positive from a negative state, or a real redundancy of the fluid from a deficiency. The north pole of a magnet may be considered as the part in which the mag. netic fluid is either redundant or deficient, provided that the south pole be understood in a contrary sense: thus, if the north pole of a magnet be supposed to be positively charged, the south pole must be imagined to be negative; and in hard iron or steel these poles may be considered as unchangeable.

A north pole, therefore, always repels a north pole, and attracts a south pole. And in a neutral piece of soft iron, near to the north pole of a magnet, the fluid becomes so distributed, by induction, as to form a temporary south pole next to the inagnet, and the whole piece is of course attracted, from the great proximity of the attracting pole. If the bar is sufficiently soft, and not too long, the remoter end becomes a north pole, and the whole bar a perfect temporary magnet. But when the bar is of hard steel, the state of induction is imperfect, from the resistance opposed to the motion of the fluid; hence the attraction is less powerful, and an opposite pole is formed, at a certain distance, within the bar; and beyond this another pole, similar to the first; the alternation being sometimes repeated more than once. The distribution of the fluid within the magnet is also affected by the neighbourhood of a piece of soft iron, the north pole becoming more powerful by the vicinity of the new south pole, and the south pole being consequently strengthened in a certain degree; so that the attractive power of the whole magnet is increased by the proximity of the iron. A weak magnet is capable of receiving a temporary induction of a contrary magnetism, from the action of a more powerful one, its north pole becoming a south pole on the approach of a stronger north pole; but the original south pole still retains its situation at the opposite end, and restores