the electric atmosphere to extend to any sensible distance from those surfaces of the bodies which touch one another, so that the electric fluid which before surrounded those surfaces would be forced round to the opposite sides, which would thereby become overcharged with electricity, and consequently appear electrical, which is contrary to experience.

198] Many Electricians seem to have thought that electrified bodies were surrounded with atmospheres of electric matter extending to great distances from them. The reasons which may have induced them to think so may be first, that an electrified body affects other bodies at a considerable distance. But this may, with much more probability, be supposed owing to the attraction and repulsion of the electric matter within the body or close to its surface. And, secondly, because a body placed near a positively electrified body receives electricity itself, whence it is supposed to receive that electricity from the electrified body itself, and therefore to be within its atmosphere. But, in all probability, the body in this case receives its electricity from the contiguous air, and not immediately from the electrified body, as will be further explained in its place.

199] Let any number of bodies which conduct electricity with perfect freedom be connected together by substances which also conduct electricity. It is plain that the electric fluid must be equally compressed* in all these bodies, for if it was not, the electric fluid would move from those bodies in which it was more compressed to those in which it was less compressed till the compression became equal in all. But yet it is possible that some of these bodies may be made to contain more than their natural quantity of electricity, and others less. For instance, let some power be applied to some of these bodies which shall cause the electric fluid within their pores to expand and grow rarert, those bodies will thereby be made to contain less electric matter than they would otherwise do, but yet the electric matter within them

*Note by Editor. [That is, must sustain an equal pressure. In modern scientific language the words compression, extension, distortion, are used to express strain, or change of form, while pressure, tension, torsion, are reserved to indicate the stress or internal force which accompanies this change of form. Cavendish uses the word compression to indicate stress. The idea is precisely that of potential.]

+ [No such power has been discovered. There is nothing among electrical phenomena analogous to the expansion of air by heat.-ED.]

will be just as much compressed as it would be if this power were not applied.

On the other hand, if some power were applied which shall diminish the elasticity of the electric fluid within them and thereby make it grow more dense, those bodies will be made to contain more electricity, but yet the compression will remain still the

same.

200] To make what is here said more intelligible, let us suppose a long tube to be filled with air, and let part of this tube, and consequently the air within, be heated, the air will thereby expand, and consequently that part of the tube will contain less air than it did before, but yet the air in that part will be just as much compressed as in the rest of the tube.

In like manner, if you suppose the electric fluid to be not only confined within the pores of bodies, but also to surround them in the form of an atmosphere, let some power be applied to some of those bodies which shall prevent this atmosphere from extending to so great a distance from them, those bodies will thereby be made to contain less electricity than they would otherwise do, but yet the electric fluid that surrounds them will be just as much compressed as it would [be] if that power was not applied.

It will surely be needless to warn the reader here not to confound compression and condensation.

201] I now proceed to my hypothesis.

DEF. 1. When the electric fluid within any body is more compressed than in its natural state, I call that body positively electrified when it is less compressed, I call the body negatively electrified.

It is plain from what has been here said that if any number of conducting bodies be joined by conductors, and one of the bodies be positively electrified, that all the others must be so too.

DEF. 2. When any body contains more of the electric fluid than it does in its natural state, I call it overcharged. When it contains less, I call it undercharged.

202] HYP. 1st. Every body overcharged with electricity repels an overcharged body, and attracts an undercharged one.

HYP. 2nd. Every undercharged body attracts an overcharged body, and repels an undercharged one.

HYP. 3rd. Whenever any body overcharged with electricity is brought near any other body, it makes it less able to contain electricity than before.

HYP. 4th. Whenever an undercharged body is brought near another it makes it more able to contain electricity.

203] COR. I. Whenever any body at a distance from any other electrified body is positively electrified it will be overcharged, and if negatively electrified it will be undercharged.

COR. II. If two bodies, both perfectly insulated, so that no electricity can escape from them, be positively electrified and then brought near to each other, as they are both overcharged they will each, by the action of the other upon it, be rendered less capable of containing electricity, therefore, as no electricity can escape from them, the fluid within them will be rendered more compressed, just as air included within a bottle will become more compressed either by heating the air or by squeezing the bottle into less compass; but it is evident that the bodies will remain just as much overcharged as before.

204] COR. III. If two bodies be placed near together, and then equally positively electrified, they will each be overcharged, but less so than they would [be] if they had not been placed near together.

It may perhaps be said that this is owing to the electric atmosphere not having so much room to spread itself when the two bodies are brought near together as when they are at a distance; but I think it has already been sufficiently proved that these atmospheres cannot extend to any sensible distance from their respective bodies.

COR. IV. If two bodies are placed near together and then equally negatively electrified, they will each be undercharged, but less so (id est, they will contain more electricity) than if placed at a distance.

This phenomenon cannot be accounted for on the foregoing supposition.

205] COR. V. If a body overcharged with electricity be brought near a body not electrified and not insulated, part of the

electric fluid will be driven out of this body, and it will become undercharged.

But if the body be insulated, as in that case the electric fluid cannot escape from it, it will not become undercharged, but the electric fluid within it will be more compressed than in its natural state, id est, the body will become positively electrified, and will remain so as long as the overcharged body remains near it, but will be restored to its natural state as soon as the overcharged body is taken away, provided no electricity has escaped during the mean time.

This is in effect the same case as that described in the 5th experiment of Mr Canton's paper in the 48th vol. of [the Philosophical] Transactions, p. 353, and is explained by him much in the same manner as is done here.

206] COR. VI. If a body positively electrified in such a manner that if it is by any means made more or less capable of containing electricity, the electric fluid shall run into it from without or shall run out of it, so as to keep it always equally electrified, be brought near another body not electrified and not insulated, the second body will thereby be rendered undercharged, whereby the first body will become more capable of containing electricity, and consequently will become more overcharged than it would otherwise be with the same degree of electrification. This again will make the second body more undercharged, which again will make the first body more overcharged, and so on.

It must be observed here, that if the two bodies are brought so near together that their action on one another shall be considerable, the electricity will jump from one to the other; otherwise if the two bodies were brought so near together that their distance should not be greater than the thickness of the glass in the Leyden bottle, it seems likely that the first body might receive many times as much additional electricity as it would otherwise receive by the same degree of electrification; and that the second body would lose many times as much electricity as it would by the same degree of negative electrification.

If the second body be negatively electrified, the same effect will be produced in a greater degree.

It may also happen that the second body shall be made undercharged though it is positively electrified, provided it be much less

electrified than the first body, and that the two bodies be placed near enough to each other.

207] The shock produced by making a communication between the two surfaces of the Leyden vial seems owing only to the glass prepared in that manner containing vastly more electricity on its positive side than an equal surface of metal equally electrified, and vastly less on its negative side than the same surface of metal negatively electrified to the same degree, so that if two magazines of electricity were prepared, each able to receive as much additional electricity by the same degree of electrification as one of the surfaces of a Leyden vial, and one of the magazines was to be positively electrified and the other negatively, there is no doubt but what as great a shock would be produced by making a communication between the two magazines as between the two surfaces of the Leyden vial.

I think, therefore, that the phenomena of the Leyden vial may very well be accounted for on the principle of the 6th Corollary, for in the Leyden vial the two surfaces of the glass are so near together, that the electric matter on one surface may act with great force on that on the other, and yet the electricity cannot jump from one surface to the other, by which means perhaps the positive side may be made many times more overcharged, and the negative side many times more undercharged, than it would otherwise be.

208] HYP. 5th. It seems reasonable to suppose that when the electric fluid within any body is more compressed than it is in the air surrounding it, it will run out of that body, and when it is less compressed it will run into the body.

COR. I. Let the body A, not electrified, be perfectly insulated, and let an overcharged body be brought near it. The body A will thereby be rendered less capable of containing electricity, and therefore the electric fluid within it, as it cannot escape, will be rendered more compressed. But the electricity in the adjoining air will, for the same reason, be also compressed, and in all probability equally so, therefore the electricity will have no disposition. either to run in or out of the body.

COR. II. It is evidently the same thing whether A be insulated, or whether it be not insulated, but electrified in such manner