194]

THEORY OF EXPERIMENT V.

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192] Consequently, in trying either the large circle or the two small ones, the trial plate must be opened to very nearly the same surface to contain the same charge as them as it must be if they were placed at an infinite distance from the trial plate, and consequently no sensible alteration can be produced in the phenomena of the experiment by the repulsion and attraction of the circles and trial plate on each other.

193] Thirdly, for the same reason it appears that as the circles and the trial plate are both at much the same distance from the ground and walls of the room, no sensible alteration can be produced in the experiment by the ground near the circles being rendered undercharged and that near the trial plate overcharged.

It must be observed, indeed, that the distance of the circles and trial plate from the ground is much less than their distance from each other, and consequently the alteration of the charge of the two circles and trial plate produced by this cause will not be so nearly alike as that caused by their attraction and repulsion on each other; but as, on the other hand, the whole alteration of their charge produced by this cause is, I imagine, much less than that produced by the other, I imagine that this cause can hardly have a more sensible effect in the experiment than the preceding.

194] Fourthly, we have not as yet taken notice that the canals by which the jars Aa communicate with the ground are but short, and meet the ground at no great distance from the jars.

But it may be shewn by the same kind of reasoning used in Prop. [II. Art. 178], with the help of the second corollary to the preceding proposition, that the quantity of redundant fluid in the circles will bear very nearly the same proportion to that in the positive side of the jar A, whether the canal by which A communicates with the ground is long or short.

Besides that, if it was possible for this circumstance to make much alteration in the proportion which the redundant fluid in the circles bears to that in A, it would in all probability have very nearly the same effect in trying the two small circles as in trying the large one, so that no sensible alteration can be produced in the experiment from this circumstance.

It appears, therefore, that none of the above-mentioned circumstances can cause any sensible alteration in this experiment*.

Therefore take the point a so that the repulsion of a particle at a on that canal shall be a mean between the repulsions of the same particle thereon when placed at B and C, the charge of T will be increased in the same proportion as it would be by the repulsion of a plate containing as much redundant fluid as the two plates together whose centre was a, and the charge of the two circles together will also be increased in the same proportion as that of the circle whose centre is a would be thereby.

[Note 17.]

[195

THOUGHTS CONCERNING ELECTRICITY.

195] Electricity seems to be owing to a certain elastic fluid interspersed between the particles of bodies, and perhaps also surrounding the bodies themselves in the form of an atmosphere.

196] This fluid, if it surrounds bodies in the form of an atmosphere, seems to extend only to an imperceptible distance from them*, but the attractive and repulsive power of this fluid extends to very considerable distances.

197] That the attraction and repulsion of electricity extend to considerable distances is evident, as corks are made to repel by an excited tube held out at a great distance from them. That the electric atmospheres themselves cannot extend to any perceptible distance, I think, appears from hence, that if two electric conductors be placed ever so near together so as not to touch, the electric fluid will not pass rapidly from one to the other except by jumping in the form of sparks, whereas if their electric atmospheres extended to such a distance as to be mixed with one another, it should seem as if the electricity might flow quietly from one to the other in like manner as it does through the pores of any conducting matter.

But the following seems a stronger reason for supposing that these atmospheres cannot extend to any perceptible distance from the body they surround, for if they did it should seem that two flat bodies whenever they were laid upon one another should always become electric thereby, for in that case there is no room for

* There are several circumstances which shew that two bodies, however smooth and strongly pressed together, do not actually touch each other. I imagine that the distance to which the electric atmospheres, if there are any, extend must be less than the smallest distance within which two bodies can be made to approach.

199]

HYPOTHESIS.

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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.

205]

HYPOTHESES.

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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