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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

from A, or will appear to be repelled by it, just as a body heavier than water tends to descend in it, and, on the contrary, if B is made to contain less than its natural quantity of the fluid, it will have a tendency towards A, or will appear to be attracted by it.

215] Let now the space A be made to contain less than its natural quantity of the fluid (as the fluid in B is now repelled from A with less force than it is in any other direction, id est, apparently attracted towards it), if B also contain less than its natural quantity of the fluid it will tend to recede from A, id est, appear to be repelled by it; but if B contain more than its natural quantity, it will then tend to approach towards A, id est, appear to be attracted by it.

216] If the electric fluid is diffused uniformly through all bodies not appearing electrical and the repulsion of its particles extends to considerable distances, it is plain that the consequences are such as are here described; but how far that supposition will agree with experiment I am in doubt *.

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EXPERIMENTS ON ELECTRICITY.

EXPERIMENTAL DETERMINATION OF THE LAW OF ELECTRIC FORCE.

217] I now proceed to give an account of the experiments, in all of which I shall suppose, according to the received opinion, that the electricity of glass is positive, but it is not at all material to the purpose of this paper whether it is so or not, for if it was negative, all the experiments would agree equally well with the theory.

218] EXPERIMENT I. The intention of the following experiment was to find out whether, when a hollow globe is electrified,

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a smaller globe inclosed within it and communicating with the outer one by some conducting substance is rendered at all over or undercharged; and thereby to discover the law of the electric attraction and repulsion.

219] I took a globe 121 inches in diameter, and suspended it by a solid stick of glass run through the middle of it as an axis, and covered with sealing-wax to make it a more perfect nonconductor of electricity. I then inclosed this globe between two hollow pasteboard hemispheres, 13.3 inches in diameter, and about

of an inch thick, in such manner that there could hardly be less than of an inch distance between the globe and the inner surface of the hemispheres in any part, the two hemispheres being applied to each other so as to form a complete sphere, and the edges made to fit as close as possible, notches being cut in each of them so as to form holes for the stick of glass to pass through.

By this means I had an inner globe included within an hollow globe in such manner that there was no communication by which the electricity could pass from one to the other.

I then made a communication between them by a piece of wire run through one of the hemispheres and touching the inner globe, a piece of silk string being fastened to the end of the wire, by which I could draw it out at pleasure.

220] Having done this I electrified the hemispheres by means of a wire communicating with the positive side of a Leyden vial, and then, having withdrawn this wire, immediately drew out the wire which made a communication between the inner globe and the outer one, which, as it was drawn away by a silk string, could not discharge the electricity either of the globe or hemispheres. I then instantly separated the two hemispheres, taking care in doing it that they should not touch the inner globe, and applied a pair of small pith balls, suspended by fine linen threads, to the inner globe, to see whether it was at all over or undercharged.

221] For the more convenient performing this operation, I made use of the following apparatus. It is more complicated, indeed, than was necessary, but as the experiment was of great importance to my purpose, I was willing to try it in the most

accurate manner.

h

b

Fig. 12.

M

H

B

N

ABCDEF and Abc Def (Fig. 12) are two frames of wood of the same size and shape, supported by hinges at A and D in such manner that each frame is moveable on the horizontal line AD as an axis. H is one of the hemispheres, fastened to the frame ABCD by the four sticks of glass Mm, Nn, Pp, and Rr, covered with sealing-wax. h is the other hemisphere fastened in the same manner to the frame AbcD. G is the inner globe, suspended by the horizontal stick of glass Ss, the frame of wood by which Ss and the hinges at A and D are supported being not represented in the figure to avoid confusion.

Tt is a stick of glass with a slip of tinfoil bound round it at x, the place where it is intended to touch the globe, and the pith balls are suspended from the tinfoil.

The hemispheres were fixed within their frames in such manner that when the frames were brought near together the edges of the hemispheres touched each other all round as near as might be, so as to form a complete sphere, and so that the inner globe was inclosed within them without anywhere touching them, but on the contrary being at nearly the same distance from them in all parts.

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