295] This part consists chiefly of experiments made to determine the charges of plates of glass and other electric substances coated in the manner of Leyden vials. The method I used in doing this was nearly of the same nature as that by which I determined the charges of the other sort of bodies in the preceding part, but the apparatus was more compact and portable and is represented in Fig. 20, where Hh is a horizontal board lying on the ground, Ll and Ll are two upright pillars supporting the two horizontal bars Nn and Pp, both at the same height above the ground, and parallel to each other.

To these two bars are fastened four upright sticks of glass covered with sealing wax; they are represented in the figure and shaded black, but are not distinguished by letters to avoid confusion. To these sticks of glass are fastened four horizontal pieces of wire Aa, Bb, Dd, and Ee, and to Bb is fastened another wire mM supported at the further end by a stick of waxed glass.

Rr is a wooden bar reaching from the wire Ee to the pillar Ll, and along the upper edge of this bar runs a wire, one end of which is wound round the wire Ee and the other reaches to the ground and serves to make a communication between Ee and the ground.

98] § 1. It appears from experiment, that some bodies suffer the electric fluid to pass with great readiness between their pores; while others will not suffer it to do so without great difficulty; and some hardly suffer it to do so at all. The first sort of bodies are called conductors, the others non-conductors. What this difference in bodies is owing to I do not pretend to explain.

It is evident that the electric fluid in conductors may be considered as moveable, or answers to the definition given of that term in page 6. As to the fluid contained in non-conducting substances, though it does not absolutely answer to the definition of immoveable, as it is not absolutely confined from moving, but only does so with great difficulty; yet it may in most cases be looked upon as such without sensible error.

99] Air does in some measure permit the electric fluid to pass through it; though, if it is dry, it lets it pass but very slowly, and not without difficulty; it is therefore to be called a non-conductor.

It appears that conductors would readily suffer the fluid to run in and out of them, were it not for the air which surrounds them: for if the end of a conductor is inserted into a vacuum, the fluid runs in and out of it with perfect readiness;

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.

1] Since I first wrote the following paper, I find that this way of accounting for the phænomena of electricity is not new. Æpinus, in his Tentamen Theoriœ Electricitatis et Magnetismi, has made use of the same, or nearly the same hypothesis that I have; and the conclusions he draws from it agree nearly with mine, as far as he goes. However, as I have carried the theory much farther than he has done, and have considered the subject in a different, and, I flatter myself, in a more accurate manner, I hope the Society will not think this paper unworthy of their acceptance.

2] The method I propose to follow is, first, to lay down the hypothesis; next, to examine by strict mathematical reasoning, or at least, as strict reasoning as the nature of the subject will admit of, what consequences will flow from thence; and lastly, to examine how far these consequences agree with such experiments as have yet been made on this subject. In a future paper, I intend to give the result of some experiments I am making, with intent to examine still further the truth of this hypothesis, and to find out the law of the electric attraction and repulsion.

HYPOTHESIS.

3] There is a substance, which I call the electric fluid, the particles of which repel each other and attract the particles of all other matter with a force inversely as some less power of the distance than the cube:

236] The intention of the remaining experiments was to find out the proportion which the quantity of redundant fluid in bodies of several different shapes and sizes, would bear to each other if placed at a considerable distance from each other and connected together by a slender wire, or, which comes to the same thing, to find the proportion which the quantity of redundant fluid in them would bear to each other if they were successively connected by a slender wire to a third body placed at a great distance from them, supposing the quantity of redundant fluid in the third body to be the same each time; and to examine how far that proportion agrees with what it should be by theory if the bodies were connected by canals of incompressible fluid.

237] To avoid circumlocution I shall frequently in the following pages make use of a term the meaning of which is given in the following definition.

Def. When in relating any experiment in which two bodies B and b were successively connected to a third body and overcharged, I say that the charge of B was found to be to that of b as P to 1, I mean that the quantity of redundant fluid in B would have been to that in b in the above proportion,

In this and all the following propositions and lemmata the electric attraction and repulsion is supposed to be inversely as the square of the distance.

140] PROP. XXIX. Let a thin circular plate be connected to a globe [of the same diameter] placed at an infinite distance from it by a straight canal of incompressible fluid such as is described in Pr. xix., perpendicular to the plane of the plate and meeting it in its center, and let them be overcharged.

If we suppose that part of the redundant fluid in the plate is spread uniformly, and that the remainder is disposed in its circumference, and that the part which is spread uniformly is to that which is disposed in the circumference as p to one, the quantity of redundant fluid in the plate will be to that in the globe as p + 1 to 2p + 1.

For by Prop. XXII., Cor. v., the force with which that part of the redundant fluid in the plate which is disposed in the circumference repels the fluid in the canal is the same with which an equal quantity placed in the globe repels it in the contrary direction, and the repulsion of that part which is spread uniformly is the same as that of twice that quantity placed in the globe, and therefore the repulsion of a quantity of fluid equal to p + 1 disposed in the plate as expressed in the proposition is equal to that of the quantity 2p + 1 placed in the globe

395] Although the proofs brought by Mr Walsh, that the phenomena of the torpedo are produced by electricity, are such as leave little room for doubt; yet it must be confessed, that there are some circumstances, which at first sight seem scarcely to be reconciled with this supposition. I propose, therefore, to examine whether these circumstances are really incompatible with such an opinion; and to give an account of some attempts to imitate the effects of this animal by electricity.

396] It appears from Mr Walsh's experiments, that the torpedo is not constantly electrical, but hath a power of throwing at pleasure a great quantity of electric fluid from one surface of those parts which he calls the electrical organs to the other; that is, from the upper surface to the lower, or from the lower to the upper, the experiments do not determine which; by which means a shock is produced in the body of a person who makes any part of the circuit which the fluid takes in its motion to restore the equilibrium.

397] One of the principal difficulties attending the supposition, that these phenomena are produced by electricity, is, that a shock may be perceived when the fish is held under water; and in other circumstances, where the electric fluid hath a much readier passage than through the person's body.

So little is known of the details of the life of Henry Cavendish, and so fully have the few known facts been given in the Life of Cavendish by Dr George Wilson, that it is unnecessary here to repeat them except in so far as they bear on the history of his electrical researches.

He was born at Nice on the 10th October, 1731, he became a Fellow of the Royal Society in 1760, and was an active member of that body during the rest of his life. He died at Clapham on the 24th February, 1810.

His father was Lord Charles Cavendish, third son of William, second Duke of Devonshire, who married Lady Anne Grey, fourth daughter of the Duke of Kent. Henry was their eldest son. He had one brother, Frederick, who died 23rd February, 1812.

Of Lord Charles Cavendish we have the following notice by Dr Franklin. After describing an experiment of his on the passage of electricity through glass when heated to 400° F., he says,

Lord Charles Cavendish has also recorded a very accurate series of observations on the depression of mercury in glass tubes, and these have furnished the basis not only for the correction of the leading of barometers, &c., but for the verification of the theory of capillary action by Young, Laplace, Poisson and Ivory.