To account for these phenomena, two theories have been dvanced, one by Dr. Franklin, who supposes there is only one electrical fluid, and the other by Du Fay, who supposes there are two distinct fluids.

Dr. Franklin supposed that all terrestrial substances were pervaded with the electrical fluid, and that by exciting an electric, the equilibrium of this fluid was destroyed, so that one part of the excited body, contained more than its natural quantity of electricity, and the other part less. If in this state a

conductor of electricity, as a piece of metal, be brought near the excited part, the accumulated electricity would be imparted to it, and then this conductor would receive more than its natural quantity of the electric fluid. This he called positive electricity. But if a conductor be connected with that part which has less than its ordinary share of the fluid, then the conductor parts with a share of its own, and therefore will then contain less than its natural quantity. This he called negative electricity. When one body positively, and another negatively electrified, are connected by a conducting substance, the fluid rushes from the positive to the negative body, and the equilibrium is restored. Thus bodies which are said to be positively electrified contain more than their natural quantity of electricity, while those which are negatively electrified contain less than their natural quantity.

The other theory is explained thus. When a piece of glass is excited and made to touch a pith ball, as above stated, then that ball will attract another ball, after which they will mutually repel each other, and the same will happen if a piece of sealing wax be used instead of the glass. But if a piece of excited glass, and another of wax, be made to touch two separate balls, they will attract each other; that is, the ball which received its electricity from the wax will attract that which received its electricity from the glass, and will be attracted by it. Hence Du Fay concludes that electricity consists of two distinct fluids, which exist together in all bodies-that they have a mutual attraction for each other-that they are separated by

How will two bodies act, one having more, and the other less than the natural quantity of electricity, when brought near each other? How will they act when both have more or less than their natural quantity? Explain Dr. Franklin's theory of electricity. What is meant by positive, and what by negative electricity? What is the consequence, when a positive and a negative body are connected by a conductor? Explain Du Fay's theory. When two balls are electrified, one with glass, and the other with wax, will they attract or repel each other?

the excitation of electrics, and that when thus separated, and transferred to non-electrics, as to the pith balls, their mutual attraction causes the balls to rush towards each other. These two principles he called vitreous and resinous electricity. The vitreous being obtained from glass, and the resinous from wax, and other resinous substances.

Dr. Franklin's theory is by far the most simple, and will account for most of the electrical phenomena equally well with that of Du Fay, and therefore has been adopted by the most able and recent electricians.

It is found that some substances conduct the electric fluid from a positive to a negative surface with great facility, while others conduct it with difficulty, and others not at all. Substances of the first kind are called conductors, and those of the last, non-conductors. The electrics, or such substances as, being excited, communicate electricity, are all non-conduct ors, while the non-electrics, or such substances as do not communicate electricity on being merely excited, are conductors. The conductors are the metals, charcoal, water, and other fluids, except the oils; also, smoke, steam, ice, and snow. The best conductors are gold, silver, platina, brass, and iron.

The electrics, or non-conductors, are glass, amber, sulphur, resin, wax, silk, most hard stones, and the furs of some animals.

A body is said to be insulated, when it is supported, or surrounded by an electric. Thus, a stool, standing on glass legs, is insulated, and a plate of metal laid on a plate of glass, is insulated.

When large quantities of the electric fluid are wanted for experiment, or for other purposes, it is procured by an electrical machine. These machines are of various forms, but all consist of an electric substance, of considerable dimensions; the rubber by which this is excited, the prime conductor, on which the electric matter is accumulated, the insulator, which prevents the fluid from escaping, and machinery by which the electric is set in motion.

What are the two electricities called? From what substances are the two electricities obtained? What are conductors? What are non-conductors? What substances are conductors? What substances are the best conductors? What substances are electrics, or non-conductors? When is a body said to be insulated? What are the several parts of an electrical machine?

Fig. 215,

B

A

Fig. 215 represents such a machine, of which A is the electric, being a cylinder of glass; B the prime conductor, R the rubber or cushion, and C a chain connecting the rubber with the ground. The prime conductor is supported by a standard of glass. Sometimes, also, the pillars which support the axis of the cylinder, and that to which the cushion is attached, are made of the same material. The prime conductor has several wires inserted into its side, or end, which are pointed, and stand with the points near the cylinder. They receive the electric fluid from the glass and convey it to the conductor. The conductor is commonly made of sheet brass, there being no advantage in having it solid, as the electric fluid is always confined entirely to the surface. Even paper, covered with gold leaf, is as effective in this respect, as though the whole was of solid gold. The cushion is attached to a standard, which is furnished with a thumb screw, so that its pressure on the cylinder can be increased or diminished. The cushion is made of leather, stuffed, and at its upper edge there is attached a flap of silk, F, by which a greater surface of the glass is covered, and the electric fluid

What is the use of the pointed wires in the prime conductor? How is it accounted for, that a mere surface of metal will contain as much electric uid, as though it were sond? When are of glass, or sealing wax, is excited, by rubbing it with the hand, or a piece of silk, whence comes the electricity?

thus prevented, in some degree, from escaping. The efficacy of the rubber in producing the electric excitation is much in creased by spreading on it a small quantity of an amalgam of tin and mercury, mixed with a little lard, or other unctuous substance.

The manner in which this machine acts, may be inferred from what has already been said, for when a stick of sealing wax, or a glass tube, is rubbed with the hand, or a piece of silk, the electric fluid is accumulated on the excited substance, and therefore must be transferred from the hand, or silk, to the electric. In the same manner, when the cylinder is made to revolve, the electric matter, in consequence of the friction, leaves the cushion, and is accumulated on the glass cylinder, that is, the cushion becomes negatively, and the glass positively electrified. The fluid, being thus excited, is prevented from escaping by the silk flap, until it comes to the vicinity of the metallic points, by which it is conveyed to the prime conductor. But if the cushion is insulated, the quantity of electricity obtained, will soon have reached its limit, for when its natural quantity has been transferred to the glass, no more can be obtained. It is then necessary to make the cushion communicate with the ground, which is done by laying the chain on the floor, or table, when more of the fluid will be accumulated, by further excitation, the ground being the inex haustible source of the electric fluid.

If a person who is insulated, takes the chain in his hand, the electric fluid will be drawn from him, along the chain to the cushion, and from the cushion will be transferred to the prime conductor, and thus the person will become negatively electrified. If then, another person, standing on the floor, hold his knuckle near him who is insulated, a spark of electric fire will pass between them, with a crackling noise, and the equilibrium will be restored; that is, the electric fluid will pass from him who stands on the floor, to him who stands on the stool. But if the insulated person takes hold of a chain, connected with the prime conductor, (he may be considered

When the cushion is insulated, why is there a limited quantity of electric matter to be obtained from it? What is then necessary, that more electric matter may be obtained from the cushion? If an insulated person takes the chain, connected with the cushion, in his hand, what change will be produced in his natural quantity of electricity? If the insulated person takes hold of the chain connected with the prime conductor, and the machine be worked, what then will be the change produced in his electrical state?

as forming a part of the conductor, and therefore the electric fluid will be accumulated all over his surface, and he will be positively electrified, or will obtain more than his natural quantity of electricity. If now, a person standing on the floor touch this person, he will receive a spark of electrical fire from him, and the equilibrium will again be restored.

If two persons stand on two insulated stools, or if they both stand on a plate of glass, or a cake of wax, the one person being connected by the chain with the prime conductor, and the other with the cushion, then, after working the machine, if they touch each other, a much stronger shock will be felt, than in either of the other cases, because the difference between their electrical states will be greater, the one having more and the other less than his natural quantity of electricity. But if the two insulated persons both take hold of the chain connected with the prime conductor, or with that connected with the cushion,(no spark will pass between them, on touching each other, because they will then both be in the same electrical state.)

We have seen, fig. 213, that the pith ball is first attracted and then repelled, by the excited electric, and that the ball so repelled will attract, or be attracted, by other substances in its vicinity, in consequence of having received from the excited body more than its ordinary quantity of electricity.

Fig. 216.

These alternate movements are amusingly exhibited, by placing some small light bodies, such as the figures of men and women, made of pith, or paper, between two metallic plates, the one placed over the other, as in fig. 216, the upper plate communicating with the prime conductor, and the other with the ground. When the electricity is communicated to the upper plate, the little figures, being attracted by the electricity, will jump up, and strike their heads against it, and having received a portion of the fluid, are instantly repelled, and again attracted by the lower plate, to which they impart their elec

if two insulated persons take hold of the two chains, one connected with the prime conductor, and the other with the cushion, what changes will be produced?