the distance inversely. It is indeed ascertained by careful experiment, that when dry air and other gases are gradually compressed, their densities vary as the compressing force; and from this it has been inferred, that the repulsive power of the atoms of gases varies inversely in the simple ratio of the distance of the atoms. This inference rests on the supposition, that all the repelling atoms continue in the compressed gas, and without this it cannot be true.

Now during the compression, much caloric, which is universally allowed to be of a repulsive nature, escapes from the compressed gas; and from this well known fact, it clearly follows, that the repulsive force of the atoms of gas increases inversely as some higher power than that of the simple distance, probably as that of the square of the distance inversely; that is, according to the same law which regulates the force of attraction. For a more particular view of the effects of the compression of gases, see ph. 17, sect. iv. We have no instance of the repulsion of matter at sensible distances, although attraction is known to act at least to the utmost boundaries of the solar system, and probably indefinitely beyond those boundaries. It might indeed be said, are Inot electrical and magnetical repulsions instances of the operation of this force at considerable distances? I answer,-By no means; these apparently distant repulsions are more naturally and

clearly explained by the operation of repulsion at insensible distances, (see Electrical Phenomena, sect. vii.); and so is also the apparent repulsion between two balls on the surface of a liquid, (see ph. 24, sect. v.) Electrical and magnetical attractions and repulsions are considered as being of a peculiar and specific nature.

In order to explain electrical phenomena, many hypotheses have been advanced. The two principal of these have been modified, and greatly improved, and philosophers are yet divided in their opinions as to the merits of these electrical theories: many of the phenomena are solved by both, and nearly with an equal degree of facility and elegance. The one is Dr. Franklin's Theory reduced to precision, and much improved, by Epinus and also by the Hon. Mr. Cavendish. See his Paper in vol. Ixi, of Philosophical Transactions. This Theory, in its improved state, admits of one distinct electric fluid, such that, 1. Its particles mutually repel one another, even at sensible distances. 2. Its particles attract those of all other bodies, and are mutually attracted by them at sensible distances. 3. The attraction of one of two electrical bodies, and the fluid of the other, when both are in their natural state, is equal to the emutual repulsion of the fluids; that is, of the fluid of one body upon that of the other. 4. That the same attraction of the first body on the fluid of the second is equal to the attraction of the second

body on the fluid of the first. 5. That the particles of the two bodies, imbued with electricity, repel each other, so that, when in their natural state, the sum of the attractions of each body on the fluid of the other is equal to the sum of the repulsions of the two portions of fluid, and of the particles of the two bodies.

The other Theory, is that of Du Fay, improved by M. Coulomb; it is embraced by many modern philosophers of the first rank, and supposes the existence of two peculiar electric fluids called vitreous and resinous, such that, 1. The particles of the same fluid mutually repel one another at sensible distances. 2. That the two different fluids attract each other at a considerable distance. 3. The particles of each fluid are attracted by those of all bodies. 4. That if two bodies be in their natural state, the vitreous fluid of the one attracts the resinous of the other, as much as the resinous fluid of the first repels it, and again, in like manner, the resinous fluid of the first attracts the vitreous fluid of the second, as much as the vitreous fluid of the first repels the vitreous of the second.

It is not my province to decide on the merits of these hypotheses; there is certainly a great want of simplicity in the supposition of two fluids, especially when we are obliged to give such similar, and at the same time opposite, qualities to them The first theory also labours under a great

objection by requiring, that the particles of distant bodies should act on each other by repulsion. Also there is a great want of simplicity in both, by requiring repulsion, and perceptible attraction, at sensible distances.

Both these theories, in the present Work, are totally discarded, as altogether unnecessary, and also as being quite inadequate to explain many electrical phenomena..

There is doubtless an electric fluid, or ethereal matter, concerned in all the appearances exhibited in electricity; but there is no need to attach to it any peculiarity, or to suppose that it differs from other matter, except as other kinds of matter differ from each other, viz. in the force of their atoms, and the magnitude of the sphere of repulsion.

In the Explanations given in the several parts of this Work, no repulsion of any kind is allowed at sensible distances, except that which is apparent only, and produced by the medium of adjacent matter; nor is there admitted any perceptible attraction of single atoms, particles, or small masses, on each other at sensible distances; for although it is stated that every atom of matter attracts every other, even at immense distances; yet, the attractions of the few atoms which are contained in the bodies, which we employ in our experiments, are so small, that, even unitedly, the effects of their actions at sensible distances are not per

b

ceptible. It will be found, that in this performance, no peculiar electrical theory is substituted, for those which are thus freely dismissed as inefficient, and it is confidently expected, that the Reader will find the electrical phenomena clearly explained, including many, which, it is allowed, that neither the theory of one fluid, nor that of two, as mentioned above, can solve. Similar observations might be made respecting Galvanism, Magnetism, and Electro-magnetism, which have still more puzzled the enquirers into nature; no new theory is here advanced, in order to unfold the mysteries of these Sciences; and yet, perhaps, it will be found that their several phenomena are as clearly accounted for, as by the most refined speculations, which have been directed to this subject. Nay, had not the Author thought the explications better, and more general, without additional principles, and separate theories, than with them, his present labours would have been concealed from. the eye of the Public. How far he is right in his sentiments does not, however, now belong to him to judge.

The Explanations throughout the Work, are to be considered as resting on the general theory of physics laid down at the commencement ; no principles are required but those which are presented to the Reader, and such as are fairly deduced from them; these of course the Reader is expected to admit as the ground-work. And