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

that the fluid within it be as much compressed as it was before by virtue of the insulation. Therefore if the body A be now not insulated, but positively electrified, and an overcharged body be brought to such a distance from it that the electric fluid in the adjacent air be equally compressed with that in A, such a quantity of electricity will thereby be driven out of A that it will retain only its natural quantity. So that A will be neither overcharged nor undercharged, nor will the electricity have any disposition to run either in or out of it.

209] If the overcharged body be now brought nearer, A will become undercharged, and the electricity will run into it from the surrounding air. If the overcharged body be not brought so near A will be overcharged, and the electricity will run out of it. If an undercharged body be brought near A it will become more overcharged than before, and the electricity will run out stronger than before.

COR. III. If the body A be negatively electrified, and an undercharged body be brought near it till the electric fluid in the adjoining air is as much compressed as that in the body A, the electricity will have no disposition to run either in or out of A, nor will it be either overcharged or undercharged, as will appear from the same way of reasoning as was used with regard to the 2nd Corollary.

If the undercharged body be now brought nearer, A will become overcharged, and the electricity will also run out of it. If the undercharged body be removed farther off, A will become undercharged, and the electricity will also run into it. If an overcharged body be brought near to A, it will become more undercharged than before, and the electricity will also run in faster than before.

On the whole, therefore, it appears that whenever a body is undercharged the electricity will run into it, and whenever it is overcharged it will run out.

210] It has usually been supposed that two bodies, whenever the electricity either runs into or out of both of them, repel each other; but that when it runs into one and out of the other, they attract. In the beginning of this paper I laid down a different rule for the electric attraction and repulsion, namely, that when

the two bodies are both overcharged or both undercharged they repel, but attract when one is overcharged and the other undercharged.

But by what has been just said it appears that these two rules agree together, or at least if they do differ, they differ so little that there is no reason to think my rule will agree less with experiment than the other.

The reasoning here used would have been more satisfactory if the bodies were capable of containing electricity only on one side, namely, on that which is turned towards the other body. But I do not imagine, however, that this will make much difference in the effect.

211] What has been here said holds good only in cases where the size of the body A is small in respect of the distance of the electrified body from it, so that the influence of the electrified body may be nearly the same on all parts of the body A as is the case in bits of cork held near an excited tube; but when the size of the body A is such that the influence of the electrified body may be much greater on that part of A which is directly under it than on that which is farther removed from it, as is the case in electrifying a prime conductor by an excited tube, then the case is very different, for then on approaching the electrified tube, part of the electric fluid will be driven away from that part of the prime conductor which is nearest the excited tube to the remoter parts where its influence is weaker, whereby that part of the conductor nearest the tube will be undercharged, and consequently the compression of the electric fluid in that part will be less than in the contiguous air, consequently some electric matter will flow into it from the adjoining air, whereby the conductor will be overcharged, and therefore on taking away the tube will be positively electrified.

Thus if the excited tube or other electrified body is not brought within a certain distance, the conductor receives its electricity only from the contiguous air, as was before said, and not immediately from the electrified body; but if the body be brought near enough, the electric matter jumps from the electrified body to the conductor in form of a spark.

212] The means by which this is brought about seems thusWhen the part of the conductor nearest the excited tube has

received any electricity from the contiguous air, that air will be undercharged, and will receive electricity from the adjacent air between it and the tube, by which means the electric matter will flow in gentle current between the particles of air from the excited tube to the conductor. It seems now as if the particles of air were by this means made to repel each other with more force, and thereby to become rarer, this will suffer the electric fluid to flow in a swifter current, which again will increase the repulsion of the particles of air, till at last a vacuum is made, upon which the electric fluid jumps in a continued body to the conductor.

213] That a vacuum is formed by the electric fluid when it passes in the form of a spark through air or water appears, I think, from the violent rising of the water in Mr Kinnersley's electrical air-thermometer (Priestley, p. 216), and still more strongly from the bursting the vial of water, in Mr Lane's experiment, by making the electrical fluid pass through the water in the form of a spark.

If I am not much mistaken I have frequently observed, in discharging a Leyden vial, that if the two knobs are approached together very gently, a hissing noise may be perceived before the spark, which shews that the electricity does begin to flow from one knob to the other before it moves in the form of the spark, and may therefore induce one to think that the spark is brought about in the gradual manner here described.

214] The attraction and repulsion of electrified bodies, according to the law I have laid down, may perhaps be accounted for in the following manner. Let a fluid consisting of particles mutually repelling each other, and whose repulsion extends to considerable distances, be spread uniformly all over the globe, except in the space A, which we will suppose to contain more than its proper quantity of the fluid. The fluid placed in any space B within reach of the repulsion of A will be repelled from A with more force than it will [be] in any other direction. But as it cannot recede from A without an equal quantity of the fluid coming into its room which will be equally repelled from A, it is plain that it will have no tendency to recede from A, any more than a body of the same specific gravity as water has any tendency to sink in water. Let now the space B be made to contain more than its natural quantity of this fluid, it will then really have a tendency to recede