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negative, as employed in electricity, and particularly in this Treatise, it may be proper to give an example by way of illustration. Thus, if smooth glass be rubbed with silk, both become electrical, and will attract and repel bits of straw, feathers, and other light bodies. An insulated light body which has been first attracted, and then repelled by the one, say the glass, will continue to be repelled by it, but will be attracted by the other. The glass in this case is said to be positive, and the silk negative: also any body is said to be positive, when it agrees with the glass in repelling the body, which has been repelled by the glass, and it is said to be negative, when it agrees with the silk by attracting the body which has been repelled by the glass. There are good reasons for concluding that, during the process of friction, the glass has received a portion of electric matter, from the silk, on which account the above distinction is made, and it is introduced here for the sake of giving a distinct idea of the terms.

How it may be known when a body is electrified positively or negatively, will appear from the phenomena relating to electrical attraction and repulsion to be explained in this section.

Diffusion of the Fluid.

PH.1. The principle, whatever it may be, which produces electrical phenomena, is universally diffused throughout the material world, having been noticed in all kinds and forms of bodies, and clearly evinced by an innumerable multitude of experiments and observations.

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Exp. Ethereal matter of all kinds, constituted accord

ing to the first postulate, and designated in def. 32, if existing in sufficient quantity, will tend to universal diffusion, while the centers of its atoms are in each other's spherules, and its constituent forces may be so small, and the extent of its spherules such, that it shall answer to the description of light, caloric, and the electric fluid, according to the differences in its spherules and constituent forces, it will adhere to the atoms of bodies, and may be transmitted from one body to another with greater or less facility according to circumstances, hence one species of it may be capable of producing the electrical phenomena, as a little reflection will shew, and as will appear from the following explanations. It is probable that the spherules of its atoms are less, and its constituent forces greater than those of light and caloric. We denominate this species of ethereal matter, the electric fluid.

PH. 2. When two bodies of different kinds are rubbed together, under proper circumstances, both become electrical; but always the one is positive, and the other negative.

Exp. Both the bodies are at first imbued with the ethereal matter, which we call the electric fluid, so as to be in a state of equilibrium with each other, and with the surrounding bodies, also this fluid, which may be said to be natural to them, will be most diffused over the surfaces of the bodies (prop. 17, and its cors. sect. ii.), hence when they are rubbed together, partly from the difference in the surfaces, and partly from that of the sort of bodies, the connection of their parts, their degrees of hardness, &c. it is to be expected, a priori, as a natural consequence, that a portion of the fluid of the one will pass to that of the other, and hence, when separated, the one will be disposed to part with some of its fluid to the surrounding bodies, and the other to receive a portion. Hence both

are excited, there is a tendency, in both cases, of the electric fluid to pass between either of them, and such bodies as are in their natural state, and so far similar effects will occur: but a marked difference will arise from this circumstance, that the fluid tends to leave the one body, but to enter the other. When they are put nearly in contact they destroy each other's effects, because now the fluid tends to pass from the one to the other, rather than to the adjacent bodies.

PH. 3. If a dry smooth glass tube be rubbed with a piece of dry woollen cloth, the tube is rendered positive, and the cloth negative.

Exp. The electric fluid strongly adheres to glass, as will be seen in some of the following phenomena, hence it will receive a portion of the fluid from the more yielding and fibrous cloth, and thus will become positive, while it leaves the cloth in a negative state.

PH. 4. If a dry glass tube, whose surface has been rendered rough by grinding with emery, sand, &c. be rubbed with a dry woollen cloth, the tube will, in this case, become negative, and the cloth positive.

Exp. The electric fluid adheres strongly to the glass, and hence it will contain much of that fluid, as its natural quantity, and since the surface presents an innumerable multitude of points, these points will be fraught with the electric fluid, (prop. 17, and its cors.) it follows that the cloth will easily now take, or, if we may so speak, wipe off much of that fluid from the projecting points, and hence the glass is rendered negative, and the cloth positive.

Obs. This and the two preceding phenomena are introduced in this place, for the sake of giving a more easy explanation of the following ones; the subject of excitation will be more fully developed in due order. The elec

tricity induced on a smooth glass tube when rubbed with flannel or silk is called positive, because many phenomena shew that in this case it receives an increase to its natural quantity of electric fluid.

PH. 5. If the electric virtue be excited in one body to a certain degree, and another body be brought into contact with it, the body presented will receive more or less of the electrical influence.

Exp. Let A and B, fig. 50, be two bodies, on which, as well as on all the surrounding ones, such a proportion of electric fluid is diffused, that they are in a state of equilibrium amongst themselves, and, in this case, it is evident there can be no disposition or tendency of the ethereal matter to pass from the one to the other. Now let a sufficient quantity of the fluid be communicated, by some means, to A, and diffused over its surface, the atmospherules of its particles will be extended, and will press outward (prop. 27, sect. ii.) and will therefore tend to pass off to any body, suppose B, connected with the earth and placed near it, and much more when in contact. On the contrary let a sufficient quantity of fluid by some means be abstracted from A, the atmospherules of its particles will be diminished, and those of the surrounding bodies will tend towards it, (prop. 28, sect. ii.) and hence it will be in a state ready to receive the electric fluid, from any body, as B, in its natural state, and to which it is contiguous. Hence in the first case an adjacent body in contact will become positive, in the second case, negative.

Conductors and Non-conductors.

PH. 6. Some bodies will receive and retain the electric virtue at and near the part to which it is communicated, and these are called non-conductors; such are resins, sulphur, glass, silk, wool, dry air, and other gases, dry paper, baked wood, oils, dry metallic oxides, &c.; again other bodies suffer the electric fluid to be diffused with more or less facility and rapidity over their several parts, such are called conductors, among which are the metals, charcoal, acids, water, and moist bodies, &c.

Exp. As it was shewn, that, in cases of cohesion, and of chemical affinity, some atoms or particles adhere or combine with more freedom and firmness than others, so, for the same reason, the electric fluid, or ethereal matter in general, will more readily adhere to, or unite with, the atoms or particles of one body, than with those of another, in consequence of differences in the atmospherules retained by the particles or atoms of the body: also from the connection, closeness, and relative position, of the tenacious atoms, which compose different bodies, as well as from their different forces, and extent of their spherules, the electric fluid will more completely diffuse itself around the particles of the one, than about those of the other, and will more firmly adhere to them. Now let A and B, fig. 66, be two bodies, the quantum of their fluid being such that they are in equilibrium with each other, and with the surrounding bodies; and let the electric fluid be confined to A, till it has a given tension, or tendency to escape, (which may take place by prop. 27, sect. ii.), so that when B is brought into contact it will receive a portion of the fluid at the touching point: now if the

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