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 the common and somewhat loose manner in which the matter had hitherto been treated, it was not certain that the same would hold when the conclusions were accurately and mathematically deduced. To apply this test was what Æpinus undertook, and what he has executed in a manner very satisfactory and complete. He has treated very fully, and perhaps has nearly exhausted what may be called the statics of electricity and magnetism, or the equilibrium of their forces. A great field yet remains, where the motion of the electric fluid is to be considered, and its distribution over the surfaces of bodies of a given figure; where greater difficulties are to be encountered, and where the latest improvements of the integral calculus in the hands of La Place and Poisson have begun to be applied. The investigations of Æpinus in their own department led to very satisfactory results, and the exact agreement between them and the phenomena actually exhibited was extensively observed. Notwithstanding this agreement, we cannot consider the theory of positive and negative electricity as being yet sufficiently established. Though the assumption on which it is founded appear very simple at first, it is found more complex on a nearer inspection. The assumption is, that a fluid resides in the surfaces of all the bodies termed electrics, which is highly elastic, and strongly attracted, at the same time, by the particles of the body, and that while this fluid remains equally diffused over the surface of the body, no phenomenon whatever gives any information of its existence. By certain mechanical operations, however, the equilibrium of this fluid may be destroyed; the fluid may be accumulated at one end, or on one side of a body, and entirely withdrawn from the opposite. It is when an electric is brought into this state that it exhibits the phenomena of electricity, between which, and the calculus instituted on the suppositions just laid down, Æpinus has everywhere remarked the most exact agreement. One great difficulty, however, still remains. The negative ends of two electrified bodies repel one another, just as much as the ends which are reckoned positive. But such an effect cannot result from the mere absence of a substance; when the electric fluid is withdrawn, if repulsion still continue, it must arise from the mutual action of the particles of the body itself. Thus it would appear, that, in the absence of the electric fluid, the tendency of the particles of matter is to repel one another. This is an essential part of the theory; and it is not accurate to say, that the doctrine of Franklin or Æpinus supposes no more than the existence of an elastic fluid diffused over the surfaces, and strongly attracted by the particles of bodies. It supposes, besides, that these particles, in the absence of this fluid, mutually repel one another. This not only takes away from the simplicity of the hypothesis, but it is obviously a very unnatural, not to say a contradictory supposition; because, when the electric matter is removed, how comes it to pass that the particles of the body, notwithstanding their mutual repulsion, still cohere together as firmly as before? This difficulty is acknowledged by Æpinus himself; but it would seem that the theory had taken a strong hold of his mind before he was aware of this consequence from it, so that he became by degrees reconciled to a supposition which appeared to him aat [sic] first not a little incongruous. This must not surprise us: It is not always that, even among philosophers, we meet with the candour, or perhaps we should say the courage, with which Newton suspended his belief in his own great discovery, the principle of universal gravity, as long as the erroneous opinion then existing, about the magnitude of the earth, made the moon's motion in her orbit appear inconsistent with the descent of falling bodies.

Another remark, made by Æpinus himself, involves in it a difficulty which should have induced him to view his theory with considerable diffidence. Though he considers the difference of the two electricities to be the same as between excess and defect, or to consist in this, that the fluid which is deficient in the one part is in excess in the other, he admits that no phenomenon points out on which side the excess, or on which the defect lies. This is a strong indication that the difference is not of the kind supposed. We are not left at a loss to tell whether cold is the absence of a substance which we call heat, or heat the absence of a substance which we call cold. If there were just as much reason for asserting the one of these propositions as the other, one would certainly be inclined to reject both. The same should be done with respect to electricity and magnetism.

The investigations of Æpinus, however, are by no means rendered useless, even if the theory of positive and negative electricity, or of positive and negative magnetism be exchanged for that of two elastic fluids, each attracting the other, and both attracted by the particles of bodies. Most of his investigations may be easily accommodated to this supposition, and, therefore, they are, fortunately for themselves and for their author, of a more permanent nature than the principles from which they were deduced.

It is to be added to this, that Æpinus was the first who saw the affinity between electricity and magnetism, in its full extent, and perceived the light that these two mutually cast on one another. He instituted a regular series of experiments on the nature of the Tourmaline, on which he wrote a small treatise, published in 1762. He is to be regarded also as the inventor of the Condenser of Electricity, and of the Electrophorus, of which he gave the complete theory.

A very excellent view of the theory of Æpinus was published at Paris by M. Haüy, in 1787, in 8vo. The same author has, however, adopted the theory of the two fluids in his own treatise, Leçons de Physique. There is a remarkable coincidence between Æpinus’s work on electricity and magnetism, and that of Mr Cavendish, given in the Philosophical Transactions for 1771, p. 584. The principles from which they set out, and the conclusions at which they arrive, are in a great measure the same. It appears, however, quite certain, that Mr Cavendish knew nothing of the work of the Russian philosopher till his own was completed. His mode of proceeding is more geometrical, and in some parts he has gone farther.

The researches of Æpinus were not confined to the subjects now mentioned, but extended to most of the branches of natural philosophy. Beside the treatise Rh