being merely a mode of expressing the absence of the former quality. But it is otherwise with regard both to electricity and to magnetism; for all the phenomena included under these heads result, in either case, from the predominance of one or other of two contrary kinds of influence, which act only when separate-but which neutralise each other, or become inoperative, when combined, Electricity, for instance, is either of a vitreous or resinous quality; or, to use a different form of expression, is either of a positive or negative species the state of neutrality, or point of zero, being a determinate point, intermediate between these opposite conditions.

Magnetic agencies are in like manner referrible to one of two contrary polarities, the northern and the southern, or, as they are sometimes termed, the austral and the boreal. These contrary electric or magnetic influences have, in either case, the very same relations with regard to each other: whether separate, or in combination, they must always co-exist. Vitreous electricity can never be produced without the accompanying production of an equal degree of resinous electricity; nor do we ever find a northern polarity in a magnetic body, but in association with a southern polarity in some other part of that body. In both cases the two associated agents are mutually attractive; while each is repulsive of the agent of its own kind. Vitreous and resinous electricities, for example, attract each other; but the vitreous repels the vitreous, and the resinous repels the resinous. In like manner, the north and south poles of magnets attract each other; but repulsion takes place between poles of the same kind.

It is a well-known fact, that electrified bodies tend to render all the bodies in their vicinity also electrified; and the very same kind of influence is likewise exerted by magnets on all ferrugineous matter situated within a certain distance. This class of phenomena is termed induction. An exact correspondence is observed to exist between the laws of electric and of magnetic induction. Each kind of electricity, as well as each magnetic polarity, tends to induce in bodies on which it exerts its influence, an electricity, or a polarity of the opposite kind.

Although the phenomena of electricity are more easily and naturally explained on the hypothesis of there being two fluids of opposite natures, there can be no doubt that they are also explicable on the theory of Franklin, which supposes the agency of but one electric fluid. But this latter hypothesis, although apparently the more simple and satisfactory, is found, on the whole, when applied to the explanation of the phenomena, to be encumbered with more serious difficulties, and to involve more arbitrary assumptions, than the admission of two kinds of electricity. Even here, however, the analogy with magnetism is still preserved. Epinus, who has

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so ably traced the consequences of the Franklinian theory of elec→ tricity, has also shown that a hypothesis precisely similar with regard to a single magnetic fluid might be framed, with certain modifications, so as to account for all the phenomena of magnetism. More recent investigations, especially those of Coulomb, have also determined, that the law by which electric and magnetic actions vary in relation to distance, is in both cases the same-namely, that the intensity of force is always in the inverse ratio of the square of the distance. In consequence of this perfect similarity between the laws, as well as modes of action, of magnetic poles, and of bodies charged with electricity of either kind, appearances similar to those resulting from the mutual action of magnets may be obtained by means of electricity alone. Thus if two bodies, composed of a slowlyconducting substance, having the form of magnetic needles or bars, and suspended on their centres, be charged at both ends with electricities of opposite kinds, they will influence each other precisely in the same way as magnetic needles would do, when placed in similar circumstances. The extremities so electrified will act like magnetic poles on the extremities of the other needle, and will assume exactly the same relative positions with regard to each other, as if they had been actuated by magnetic instead of electric polarities.

But the example just given, while it illustrates the striking simi→ larity in the laws and modes of action of the two principles, marks still more pointedly the distinction which exists between them, and furnishes a powerful argument in favour of their being separate and independent powers. However readily one magnetic bar may affect another magnetic bar, and however great may be the mutual action between the two electrised bars, yet no corresponding influence is perceptible between the magnetic and the electric bodies. A body charged with electricity, no matter of what species, will, it is true, exert some action on the magnetic needle, because its influence extends generally to all bodies in its vicinity; but its action is merely electric, and is neither increased nor diminished in consequence of the magnetic properties of the needle: for the same needle, when deprived of all magnetism, will still be subject to the same degree of electric action as before.

A bar of steel, already magnetised, may receive in addition a charge of electricity: it will then act on other bodies in the double capacity of a magnet and of an electrified body; but both agencies will still remain perfectly distinct, and will appear to have no influence on each other. From all the direct experiments that were made prior to the discovery of Ersted, no other conclusion could be deduced, than that electricity and magnetism could in no way be considered as modifications of a common principle, but

that

that they were powers totally independent of one another, although marked by characters of similarity in their modes of action. On prosecuting the inquiry, we find that even the resemblance we have already traced exists only to a very limited extent. Magnetism appears to reside exclusively in iron, and in bodies which contain it in a metallic state; but electricity affects all bodies whatsoever. Both kinds of electricity are conducted with great facility by various bodies, and pass from the one to the other, not only when they are in contact, but even when a certain distance intervenes. Thus bodies, when insulated, may become charged with either the vitreous or the resinous electricity in a separate state; and the properties of each may thus be exhibited without the interference of the other. If, by the influence of an electrified body in the vicinity, a series of connected conductors have different states of electricity induced upon its different parts, this difference will appear on separating them, and removing them from the influence of the neighbouring bodies, by which that electricity was induced. When the conductor is divided in the middle, and the parts removed from one another, the one half will be found to be positively electrified, and the other negatively.

In all these particulars, the parallel which we had drawn between magnetism and electricity entirely fails us; and instead of analogy, we meet with decided contrast. The magnetic fluid, or whatever else be the principle of magnetic action, is incapable of transference from one body to another. Its action on bodies in contact, or in the vicinity, is limited to induction only; hence the two polarities can never be separately obtained.

If a magnetic bar, of which the poles are situated at the two extremities, be broken across in the middle, instead of obtaining each pole separately, as happened with regard to the two electricities in the experiment just referred to, we find that each fragment has become a complete magnet with two poles, like the original one; and all that we get by successive fractures is the multiplication of the number of magnets.

The question respecting the identity or the independence of electricity and magnetism has attracted much attention among philosophers, from a very early period in the history of these comparatively modern sciences. It was proposed as the subject of a prize dissertation, by the Academy of Bavaria, in 1774; on which occasion several elaborate memoirs were drawn up by Van Swinden, Steiglehner, and Hubner. A collection of these memoirs, in three volumes, was published, ten years afterwards, by Van Swinden, containing a large mass of materials, from which some valuable facts may be gleaned.

In following the laborious investigations which were undertaken

to

to resolve the proposed problem, it is often amusing to perceive how near an approach was sometimes made to the discovery of the truth, which was lurking, as it were, in ambush, close to the very path of the inquirer. An unexpected result would sometimes startle the philosopher in the midst of his experiments, and afford him a momentary glimpse of some new region in the science, into which he had not power to penetrate, and which, after fruitless efforts, he ended in persuading himself was a mere illusion.

That some connexion actually exists between electricity and magnetism was proved by a great variety of circumstantial, at least, if not of direct, evidence. It was observed that lightning, on some occasions, rendered the iron which it met with in its passage magnetic. This effect had sometimes also resulted in experiments in which steel bars had been made the channel for the discharge of artificial electricity, especially when powerful batteries were employed. But the success of the experiments made with this view was by no means constant. They often failed of producing any magnetism; and still less could any certain conclusion be established, with respect to the particular polarity, which each species of electricity tended to produce. When magnetic compass-needles were subject to the electric discharge, their power was often weakened, and sometimes destroyed, and the poles of the needle were occasionally found to be reversed by the operation; so that if the needle were suspended on its centre, that end which before turned towards the north now pointed towards the south, and vice versâ. A similar effect has been known to result from lightning, when it passed near to a ship's compass; and many stories are related of disastrous accidents arising from this cause.

So numerous were the facts that proved the tendency of electricity to produce magnetism, that they could scarcely have failed to excite more accurate scrutiny, and to lead to the discovery of the real nature of their connexion, had it not been for the exclusive attention that was given to certain phenomena, which appeared to afford a plausible solution of the difficulty. It was well known that the globe of the earth exerts on all ferruginous bodies an influence, similar to what would arise from a powerful magnet situated near its axis. Iron has, in consequence of this influence, a tendency to acquire spontaneous polarity; the direction of the poles of iron so affected being the same with that assumed by the dipping-needle: that is, by a magnetic bar suspended accurately on its centre of gravity, and allowed perfect freedom of motion, not merely in a horizontal plane, (as is the case with the compassneedle,) but also in a vertical plane. The direction it will then assume, in London, is a line inclined 70° to the horizon; and the lower end deviating 24° to the west of the meridian.

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If a bar of steel be held in this position, and struck repeatedly with a hammer, or if it be heated red hot, and then suddenly cooled by immersion in water while in this same position, it will be found to have acquired permanent magnetic properties. That end, which was nearest to the north magnetic pole of the earth, will have acquired the property of directing itself to the north, when suspended so as to allow of its moving freely in a horizontal plane while the other end will have acquired the opposite polarity. It was, therefore, natural to conclude that it was merely by its mechanical action that the electric discharge induced magnetism, in those instances where it was found to have this effect. The electricity was supposed to operate, by giving to the particles of the solid body the same kind of vibratory motion which is produced by any other mode of concussion, whereby the inductive influence of the earth's magnetism was promoted. Such was the notion universally entertained on the subject, not only in the time of Van Swinden, but down to the period of Ersted's discovery. Those who undertook experiments on this subject were too much prepossessed with this idea of the mechanical operation of the passage of electricity, to observe, with sufficient impartiality, the real nature of the effects which they witnessed, and which, from a natural propensity to attach superior importance to all the instances that favoured their previous views, and to disregard whatever opposed them, were received as confirmations of their truth.

Some facts, indeed, there were to which this explanation could not apply; such as those of the diurnal variation of the compass, apparently connected with the periodic changes of temperature in different portions of the earth, and oscillations observed in the needle during thunder storms, and during the prevalence of the aurora borealis. No clue could be found to a rational explanation of these anomalies, which continued to baffle the ingenuity of philosophers for many centuries. D'Alibard imagined he had established, by his experiments, that when an electric battery is discharged through a needle, the end at which the positive electricity enters becomes a north pole, whatever be the situation of the needle. Wilke allowed a large share of influence to terrestrial magnetism; but still imagined he had traced a connexion between northern polarity and negative electricity, which is contrary to the conclusion of D'Alibard. Van Swinden undertook a series of experiments in order to reconcile these discrepancies, but met with anomalies which were even greater and more perplexing than the former. Beccaria also occupied himself with experiments to solve this point, about the year 1777, but with no better success. There occurred an observation in the course of his attempts, which might have enlightened him as to the true

cause.