tions come to an end; and we shall find their number to be the same as before. The cause of this is also obvious from the theory. We may suppose A to be negative, and C positive. One of them will attract the ball into contact, and will repel it, having put it into an electric state opposite to that of the other conductor. It now becomes a carrier of fluid from the positive to the negative conductor, till it nearly restore both to their primitive state of neutrality.

79. There is frequently a seeming capriciousness in those attractions and repulsions. A pith-ball, or a down feather, hung by silk, will cling to the conductor, or otherwise electrified body, and will not fly off again, at least for a long while. This only happens when those bodies are so dry as to be almost non-conductors. They acquire a positive and negative pole, like an iron nail adhering to a magnet, and are not repelled till they become almost wholly positive or negative. It never happens with conducting light bodies.

80.-Fifth, It should follow from the theory, that the electric attractions and repulsions will not be prevented by the intervention of non-conducting substances in their neutral state. Accordingly, it is a fact, that the interposition of a thin pane of glass, let it be ever so extensive, does not hinder the electrometer from being affected. Also, if an insulated electric be covered with a glass bell, an electrometer on the outside will be affected. Nay, a metal ball, covered to any thickness with sealing wax, when electrified, will affect an electrometer in the same way as when naked. We cannot see how these facts can be explained by the action of electric atmotspheres. It is indeed said, that the atmo sphere on one side of the glass produces an atmosphere on the other; but we have no explanation of this production. If the interposed plate be a non-conductor, how does the one atmosphere produce the other? It must produce this effect by acting at a distance on the particles which are to form this atmosphere. Of what use, then, is the atmosphere, even if those atmospheres could effect the observed

motions of the electrometer in consistency with the laws of mechanics! The atmospheres only substitute millions of attractions or repulsions in place of one. We must observe, however, that the motions of the electrometer are modified, and sometimes greatly changed, by the interposed non-conducting plate; but this is owing to the electricity induced on the plate. If the electric is positive, the adjacent surface of the plate becomes faintly negative, and the side next the electrometer slightly positive. This affects the electrometer even more than the more remote electric does. That this is the cause of the difference between the state of the electrometer when the plate is there and when it is removed, will appear plainly by breathing gently on the glass plate to damp it, and give it a small conducting power. This will make some change in the position of the electrometer. Continue this more and more, till the plate will no longer insulate. The changes produced on the electrometer's position will form a regular series, till it is seen to assume the very position which it would have taken had the plate been brass. Then, considering those changes in a contrary order, and supposing the series continued a little farther, we shall always find that it leads to the position which it would have taken when no plate whatever is interposed. We consider this as an important fact, shewing that the electric action is similar to gravitation, and that there is no more occasion for the intervention of an atmosphere for explaining the phenomena of electricity than for explaining those of gravitation.

81.-Sixth, Since non-electrics are conductors, and since electrics may be excited by friction with a non-electric, it follows, that if this non-electric be insulated, and separated from the electric, it will exhibit signs of electricity; but when they are together, there must not appear any marks of it, however strong the excitation may be. We do not pretend to comphrehend distinctly the manner in which friction, or the other modes of excitation, operate in changing the con

nection between the particles of the fluid and those of the tangible matter; nor is this explained in any electric theory that we know: but if we are satisfied with the evidences which we have for the existence of a substance, whose presence or absence is the cause of the electric phenomena, we must grant that its usual connection with the tangible matter of bodies is changed in the act of excitation, by friction, or by any other means. In the case of friction producing positive electricity on the surface of the electric, we must suppose that the act of friction causes one body to emit or absorb the fluid more copiously than the other, or perhaps the one to emit, and the other to absorb. Which ever is the case, the adjoining surfaces must be in opposite states, and the one must be as much overcharged as the other is undercharged. When the bodies (which we may suppose to have the form of plates) are joined, and the one exactly covers the other, the assemblage must be inactive; for a particle of moveable fluid, situated any where on the side of the overcharged plate, will be as much attracted by the undercharged surface of the remote plate as it is repelled by the overcharged surface of the near plate. The surfaces are equal, and equally electric, and act on either side with equal intensity; and they are coincident. Therefore their actions balance. The action is expressed by the formula of § 43; namely, F' m' x z-z'; and z-z' is 0, by reason of the equal distances of these surfaces from the particle of exterior fluid.

×

But let the plates be separated. Part, and probably the greatest part, of the redundant fluid on one of the rubbed surfaces will fly back to the other, being urged both by the attraction of the redundant matter and the repulsion of its own particles. But the electric, being electric because, and only because, it is a non-conductor, must retain some, or will remain deprived of some, in a stratum a little within the surface. The two plates must therefore be left in op

posite states, and the conducting, or non-electric plate, if insulated before separation, must now exhibit electric action.

All this is exactly agreeable to fact. We also know that electrics may be excited by rubbing on each other; and if of equal extent, and equally rubbed, they exhibit no electric powers while joined together; but when parted, they are always in opposite states. The same thing happens when sulphur is melted in a metal dish, or when Newton's metal is melted in a glass dish. While joined, they are most perfectly neutral; but manifest very strong opposite electricities when they are separated. This completely disappears when they are joined again, and re-appears on their separation, even after being kept for months or years in favourable circumstances. We have observed the plates of talc, and other laminated fossils, exhibit very vivid electricity when split

asunder.

82. Attention to these particulars enables us to construct machines for quickly exciting vivid electricity on the surface of bodies, and for afterwards exhibiting it with continued dispatch. The whirling globe, cylinder, or plate, first employed by Mr. Hauksbee, for the solitary purpose of examining the electricity of the globe, was most ingeniously converted by Hausen, a German professor, into a rapid collector and dispenser of electricity to other bodies, by placing an insulated prime conductor close to that part of the surface of the globe which had been excited by friction. Did our limits give us room, we should gladly enlarge on this subject, which is full of most curious particulars, highly meriting the attention of the philosopher. But it might easily occupy a whole volume; and we have still before us the most interesting parts of the mechanical department of electricity, and shall hardly find room for what is essentially requisite for a clear and useful comprehension of it. We must, therefore, request our readers to have recourse to the original authors, who have considered the excitation by friction minutely. And we particularly recommend the

very careful perusal of Beccaria's Dissertations on it, comparing the phenomena, in every step, with this theory of Epinus. Much valuable information is also obtained from Mr. Nicholson's observations. The Epinian theory will be found to connect many things, which, to an ordinary reader, must appear solitary and accidental.

83. Seeing that this very simple hypothesis of Epinus so perfectly coincides in its legitimate consequences with all │ the general phenomena of attraction and repulsion, and not only with those that are simple, but even such as are com pounded of many others-we may listen, without the imputation of levity, to the other evidences which may be offered for the materiality and mobility of the cause of those mechanical phenomena. Such evidences are very numerous, and very persuasive. We have said, that the transference of electricity is desultory, and that the change made in the electric state of the communicating bodies is always considerable. It appears to keep some settled ratio to the whole electric power of the body. When the form of the parts where the communication takes place, and other circumstances, remain the same, the transference increases with the size of the bodies; and all the phenomena are more vivid in proportion. When the conductor is very large, the spark is very bright, and the snap very loud.

1. This snap alone indicates some material agent. It is occasioned by a sonorous undulation of the air, or of some elastic fluid, which suddenly expands, and as suddenly collapses again. But such is the rapidity of the undulation, that when it is made in close vessels it does not exist long enough, in a very expanded state, to affect the column of water, supported in a tube by the elasticity of the air, for the purpose of a delicate thermometer or barometer; just

A full account of Nicholson's Experiments will be found in the Phil. Trans. 1789, p. 265. and in the EDINBURGH ENCYCLOPEDIA, Art. ELECTRICITY, vol. viii. p. 510