ELECTRIFIED BODIES.

76. Conductors and Non-conductors.-- It was known more than two thousand years ago that when a piece of amber is rubbed with silk, it attracts light bodies; and Dr. Gilbert, about three hundred years ago, showed that many other things, such as sulphur, sealing-wax and glass, have the same property as amber.

Here you see the faint and small beginning of our knowledge of electricity, a knowledge which has of late years grown so wonderfully as to enable us to send messages between Europe and America in less than one second of time.

*EXPERIMENT 53.-Let us take a metal rod, having a glass stem, and rub the glass with a piece of silk, both silk and glass being well heated and quite dry. The glass will now have the power of attracting little bits of paper or elder pith, but only at that place where it has been rubbed. The glass has, in fact, by rubbing, acquired a new property, but this property cannot spread itself over its surface. So much for glass. Suppose now that we take the metal rod and touch with it the prime conductor of an electric machine in action, we shall find that the metal rod has acquired the same proper ties as the glass; that is to say, it will attract light bodies like paper or elder pith, but all parts of the rod of metal will have the same property, and not merely that part which touched the electric machine. In fact, the electric influence can spread itself over a surface of metal, though it cannot over one of glass. Glass, therefore, is said to be a non-conductor of

electricity, while metal is called a conductor. In fact, neither heat nor electricity can easily spread itself over glass, but both can easily spread themselves over metal; charcoal, acids, soluble salts, water, and the bodies of animals are likewise good conductors of electricity, although not so good as the metals; while, on the other hand, india-rubber, dry air, silk, glass, wax, sulphur, amber, shellac, are all very bad conductors.

If we wish to succeed in experiments with electricity, it is absolutely necessary to keep the electricity once we have got it; we must, in fact, surround it on all sides by non-conducting bodies. It is, therefore, of great importance to make our experiments in dry air, and to make the body which has the electricity stand upon a glass support.

77. Two kinds of Electricity. EXPERIMENT 54.—I have now to convince you that there are two opposite kinds of electricity. To prove this let us make use of the apparatus you see in fig. 39, consisting of a small pith ball suspended by means of a silk thread to a glass support. First of all let us rub a glass rod with silk, and with the rod so rubbed touch the pith ball. The glass end will compith ball, and it will not

municate electricity to the be able to get away, because the silk thread, the glass support, and the air (if dry) around the pith ball are all non-conductors. Now, if you notice, you will see that after the glass rod has been made to touch the pith ball, this ball will no longer be attracted to the glass rod, but will, on the other hand, be repelled by it. Let us next rub a stick of sealing-wax with a piece of warm, dry flannel, and bring the stick so rubbed

near to the pith ball. It will now be found that the pith ball, which was repelled by the excited glass, will be attracted to the excited sealing-wax.

It thus appears that a pith ball first touched with excited glass will be afterwards repelled by excited glass, but will be attracted by excited sealing-wax.

Now if we had reversed our plan of operations, and had first of all touched the pith ball with excited seal

ing-wax, instead of excited glass, it would then have been repelled by excited sealing-wax, but attracted by excited glass.

We learn from this that there are two kinds of electricity; namely, that which we get from excited glass, and that which we get from excited sealing-wax.

Now when we touched the pith ball with excited glass, we communicated to it part of the electricity of the glass; and as it was afterwards repelled by excited

glass, we conclude that bodies charged with the same kind of electricity repel one another. On the other hand, the pith ball, if charged with excited glass, will be attracted to excited sealing-wax ; or if charged with excited sealing-wax, it will be attracted to excited glass, and hence we conclude that bodies charged with opposite kinds of electricity attract one another.

78. They exist combined in unexcited bodies. We may suppose that every substance has in it a quantity of these two kinds of electricity mixed together, and that what we do in rubbing is merely to separate the two electricities from one another. Accordingly, when we rub a piece of sealing-wax with a piece of flannel, all that we do is to separate the two kinds of electricity-the one kind keeping to the sealing-wax, while the other remains behind upon the flannel. In like manner all that we do when we excite glass with silk is to separate the two electricities, one remaining on the glass while the other adheres to the silk. The same thing holds in all cases where electricity is developed by friction, and it is impossible to produce the one electricity without, at the same time, producing just as much of the other also. In fine, we do not create electricity; but, according to this view of it, we merely separate the two opposite kinds from one another.

The electricity which appears in a stick of glass when it has been rubbed with silk is called positive, and that which appears in a stick of sealing-wax, when it has been rubbed with flannel, is called negative. These are merely terms used in order to distinguish between the two kinds of electricity.

79. Action of excited on unexcited bodies. -We have seen that electricities of the same kind repel, while electricities of opposite kinds attract each other, but we have still to learn what will happen in the following case. Let A (fig. 40), be a large ball of hollow brass, and let the tube to the left hand of it be also of brass; also let these stand upon a glass support, so that any electricity which A has may not be able to get away.

Now let B and c be two vessels having their upper parts made of brass, only capable of being separated from one another at the middle part, where

you see the line in the figure; and let both B and c stand upon glass supports, so that any electricity which either of them has, may not be able to get away.

Let us begin by supposing that A has received a charge of positive electricity, and that in the meantime B and C are unelectrified. Now push в and c up towards A. Since в and c are not electrified, their two

B