the subject seems to have died, for nothing more appears on the page of history, till Theophrastus, who notices the attractive power of the excited Tourmalin; and from this period till the beginning of the eighteenth century, a space of 1900 years, there is almost a complete chasm in the history of electricity. In the year 1700, Dr. Gilbert, who is esteemed the father of the English electricians, wrote a work entitled "De Magnete," which contains a variety of electrical experiments, and by which it appears he took much pains to discover substances that possessed the power of electric excitation. The great Mr. Boyle, in the year 1670, had applied himself to this subject, and enlarged the catalogue of these substances. Sulphur globes were now made use of to obtain greater powers of electricity than had ever before been had. Otto Guericke, the contemporary of Boyle, discovered electric repulsion: he not only saw the electric light, that had been observed by Boyle, but heard the hissing sound which was emitted. Sir Isaac Newton observed that the electric attraction and repulsion penetrated through glass. Mr. Hawksbee was the first person who applied a glass-globe to the machine, and he made many very important discoveries. In 1729, Mr. Gray found out the difference between electrics and conductors, and soon after this was discovered the difference between positive and negative electricity. Muschenbroek, in the year 1746, accidentally discovered the accumulation of the electric power in the Leyden phial. From this time electricity became the general subject of conversation, and, as a science, it advanced with great rapidity under the auspices of Franklin, Priestley, and others. Of electricity, as well as of optics, the last named philosopher is known as the interesting and elaborate historian.

General principles. The earth, and all bodies with which we are acquainted, are supposed to contain a certain quantity of an exceedingly elastic invisible fluid, called the electric fluid. This certain quantity belonging to all bodies, may be called their natural share; and which of itself, in its dormant

state, produces no sensible effect; but as soon as the equilibrium is disturbed, and any body, or surface of a body becomes possessed of more or less than its natural share, it exhibits the phenomena of attraction and repulsion, and other remarkable effects are produced according to the quantity of excitation.

The equilibrium could never be disturbed, or if it were, it would be immediately restored, if the bodies in, or on which it was disturbed, admitted to it a free passage, either along their surfaces, or through their pores. As this is not the case with some bodies, whenever any of these has acquired an additional quantity of electric matter, and is at the same time every where surrounded with bodies through which it cannot pass, it must remain overcharged: or if it has lost a part of its natural quantity, it must in like circumstances remain exhausted.

All substances are divided by electricians into two classes called electrics and non-electrics. Electrics do not suffer the electric matter to pass readily over their surfaces, and hence they obtain the name of non-conductors. Non-electrics do permit the electric matter to pass readily over their surfaces, and on that account they have been denominated conductors.

Glass, resin, sealing-wax, sulphur, bees-wax, and baked wood, are the most perfect non-conductors among solids, and oils, and dry air among fluids. But heat and moisture render all substances conductors.

According to the theory of Franklin, which seems best adapted to the explanation of facts, when a body has more than its natural quantity of this fluid, it is said to be positively electrified, and the electricity which it contains is called positive electricity; and when it has less than its natural share, it is said to be negatively electrified. When a conductor is so surrounded with non-conductors, that the electric fluid cannot pass from it into the earth, it is said to be insulated, such is the case of a piece of metal supported upon a glass pillar.

The electric fluid is generally brought into action by friction, though there are other means of disturbing the equilibrium. The chief electric appearances are the attraction and

repulsion of light bodies; fiery sparks attended with a snapping noise; pencils of light proceeding from, or entering into an electrified body upon the approach of other substances.

If a glass tube two or three feet in length, and an inch or more in diameter, be briskly drawn through the hand, or rubbed with a piece of dry and warm silk, the effect of that friction is, that the electric matter leaves the hand, and passes upon the glass, where it will remain; for as the glass, and the dry air which surrounds it, are non-conductors of electricity, this redundancy of the electric matter cannot flow away; but if any conducting substances, as the finger, or a piece of metal, be presented to the glass, the electric fluid will pass from that part into them, attended with a crackling noise. The tube thus excited will attract any light substances, as morsels of paper, &c.; which will, when brought near, spring to it; and after a few seconds, when they seem to have obtained a certain portion of the electricity of the glass, they will be repelled from it. This, and all electrical experiments are best seen in a darkened room.

Although the electric fluid may be excited with the hand on glass tubes, or on sticks of sealing-wax rubbed briskly with flannel, or on the sleeves of a man's coat; yet the quantities so obtained are very small in comparison of what may be collected by other means, as an electrical machine, and of what may be wanted for experiments on a large scale.

Fig. 1. Plate IV. represents an electrical machine; the construction is evident, almost at first sight. EF is a glass cylinder turned by the handle I, and the whole frame is fastened to a strong table by iron clamps. K is a rubber made of silk, or leather stuffed with horse-hair, and L is a silk flap fastened to it, and covering part of the cylinder, to prevent the dispersion and escape of the electric fluid. The rubber is fastened to a spring, which proceeds from a socket cemented on the top of the glass pillar M. There is another strong glass pillar N, on which is fixed a hollow metallic cylinder O P, called a prime conductor, to one end of which is attached a row of pointed wires

at Q, to collect the electric fluid from the glass cylinder; and at the other end is a knobbed wire, from which electric sparks of great length may be drawn.

When the cylinder is turned briskly, the friction of the glass against the rubber causes the electric fluid, which was in the rubber, to pass to the glass; from whence it is conveyed by means of the points to the prime conductor, which, when other bodies, as the knuckle of a person's hand, are presented to it, will give out visible sparks. To obtain a constant supply of the electrical fluid, a brass chain is hung on the knob K, which communicates with the ground, whence the electric fluid is brought in great quantities.

If instead of a transparent glass cylinder, one of sulphur, of sealing-wax, or of rough glass, were made use of, the effect of the friction would be, that a quantity of electric matter naturally belonging to those substances, passes from them to the hand; and these being surrounded with air, which is a nonconductor, remain exhausted, and ready to take sparks of elec tric fire from any bodies that are presented to them. The sulphur, &c. in one case, though deprived of their share of electricity, are said to be excited, as well as the glass which was overloaded with it; because, though the states they are in be the reverse of one another, the effects produced by them are very similar.

The electricity produced by the excitation of glass is called positive electricity, and by some writers vitreous: that produced by sealing-wax, &c., is called negative or resinous electricity.

To prove that the earth is the grand reservoir of the electric fluid, let the chain be taken from the rubber, and thus the communication with the earth is cut off; then whatever friction is applied, the quantity of electric fluid obtained will be very small. Let the chain now be put on to the conductor R, and made to hang on the ground, or on the table connected with the ground, and the electric fluid will again be abundant, and may be received from the cushion. Now it passes up the chain, along the prime conductor, and so on to the cushion;

whereas, when the cushion is connected with the earth by the chain, and the conductor insulated, it passes from the cylinder through the points to the conductor. Hence an electrical machine may be made to produce negative and positive electricity: the former by connecting the prime conductor with the earth, by a chain, and taking the fluid from the rubber; the latter by making the rubber communicate with the ground, and taking the fluid from the conductor. Electrify two insulated conductors by placing one before the cylinder, and connecting the other with the cushion, and they will exhibit different kinds of electricity; the same substances which the one attracts, will be repelled by the other; and if the two conductors be brought near each other, a strong spark will pass from one to the other, and thus the equilibrium will be restored. But if both the conductors be placed before the cylinder, they will be equally electrified with the same electricity; and what is attracted and repelled by the one, will be attracted and repelled by the other likewise, and no spark will pass from one to the other by their being brought ever so near. In the foregoing experiment, the conductor connected with the cushion is said to be negatively electrified: that placed before the glass cylinder is said to be positively electrified.

Of electrical Attraction and Repulsion. If two bodies be electrified with the same electricity, they repel each other: if one be electrified positively and the other negatively, they attract each other. A body not electrified will be attracted by those that are, whether they be electrified positively or negatively.

Electrify a pair of insulated pith balls, fig. 2, that is, balls suspended on threads of silk, by bringing them near a prime conductor, or any other substance that is in a state of electric excitation, and they will repel each other.

If one pair of insulated pith balls, electrified negatively, and another pair positively, be brought near, they will attract each other, and both pairs will collapse. Light feathers,