excited by friction, mutually attract each other; but that a vitreous body repels a vitreous body, and a resinous a resinous.

The preceding experiments would seem to prove that there are two kinds of electricity-vitreous or positive, and resinous or negative electricity: the one developed by rubbing vitreous substances with silk, and the other by rubbing resinous substances with flannel. And they also show that positive and negative attract each other; while positive repels positive, and negative repels negative.

It is right, however, to mention that the existence of two kinds of electricity is not universally admitted. Some philosophers hold that there are not two distinct kinds, but only two different states of electricity. Hence they suppose that when a substance exhibits what is called positive electricity, it possesses more of the fluid than its ordinary proportion, and that when it exhibits negative electricity, it possesses less than its ordinary proportion. Thus, when glass is rubbed with a piece of silk, they suppose that some of the electricity contained in the silk is transferred to the glass, which then has more than its ordinary share; whilst the silk, having been deprived of part of its electricity, has less than its usual quantity. And in all cases where electricity passes suddenly from one body to another-as, for instance, from cloud to cloud in a thunder-storm-they explain the phenomena by saying that the one body is overcharged and the other undercharged, and that the temporary disturbance takes place in order to restore the electrical equilibrium.

But in whatever way the transference of the electric fluid from one body to another may be explained, there can be no dispute as to the fact of such transference. All the experiments which have just been performed, show that electricity passes from one substance to another. But it deserves notice that all bodies do not afford equal facilities for its passage. Some bodies permit it to pass along them or through them with the utmost ease, and some offer great resistance. The former are called Conductors, and the latter, Non-conductors or Insulators. The metals are good

conductors; while the vitreous and resinous bodies are nonconductors. Silver is understood to be the best conductor, and shellac the best insulator. But no body is a perfect conductor or a perfect insulator. Under certain conditions, the very metals become, to some extent, non-conductors and insulators; and even air and the gases, which rank high among insulators, give way before an electrical current of great intensity, though the strong resistance to be overcome is sufficiently evinced by the disruptive discharge which always occurs, as in the electric spark and in a flash of lightning.

The conductibility of this force through some bodies and not through others admits of being easily illustrated. Here, on this stand, is a glass rod with a pole of wood above it, and a paper tassel at the top of the pole. To show that the wood is a conductor, and the glass a non-conductor or insulator, we have only, by means of a wire, to connect the foot of the wooden pole with an electrical machine—that is, a machine for developing and transferring electricity. For as soon as the handle of the machine is turned, the electricity which is thereby evolved travels along the wire till it reaches the point where the glass and wood meet, and then, instead of passing down the glass, goes up the wood to the tassel, which it endows with such a power of repulsion that the strips of paper spread outwards in all directions.

Again, to show that the human body is a conductor, you have only to stand upon an insulator-for example, a stool with glass legs-in order to prevent the electricity from going away to the earth. Place yourself on such a stool, and receive the electric current from the machine; you forthwith feel that you are electrified, you feel your hair rising up as the paper tassel formerly did; and if there is near you a jet from which gas is issuing, you will probably be able to light the gas by merely touching the jet with your finger. It is thus demonstrable that this power of electricity can be transferred from the matter in which it is generated, and

conducted along wire and other bodies, and thereby made to serve purposes unattainable by any of the forces previously described.

Of the vast power which this force can exert when in intense action, it must suffice for the present to say, that it is able to rend the oak of a hundred winters, to strike men and animals with instant death, to decompose the most intractable compounds, and in a very powerful manner to excite heat, light, and magnetism.

Magnetism, like Electricity, is a dual force, possessing the power both of attraction and of repulsion; but, unlike Electricity, it is not to be found in all bodies, but only in a few, and those chiefly iron and steel. Its manifestation was first observed in Loadstone—an ore of iron, which abounds in Sweden; and for centuries the remarkable property of attracting iron was supposed to be peculiar to that mineral. But it is now well ascertained that any piece of iron or steel has only to be rubbed with loadstone in order to acquire and exhibit the same powers as that natural magnet.

If an artificial magnet—that is, a bar or needle of steel which has acquired the magnetic property by being rubbed with loadstone-be suspended by the centre, so that it may move freely in all directions, one of its ends will be found to point towards the north, and the other towards the south. When left to itself, it invariably takes this position; and if you forcibly turn its north end southward or its south end northward, it will resume its original position whenever the constraining force is removed. This singular property is termed polarity; and the ends of the magnet are called, respectively, its north and its south pole. The direction which it takes is not, indeed, always exactly north and south, for there is what is called the variation of the needle-that is, a deviation, within certain limits, and according to a fixed law, to the west and east. In this country at present, the needle points considerably to the west of due north; in 1657, it pointed due north; previous to that period its variation had been on the east side of north; it then took a westerly direction; in 1814, it attained its greatest westward varia

tion; and from that time till the present it has been slowly returning. Philosophers predict that in little more than a century hence it will again point due north.

In pointing north and south, a magnetized bar or needle does not maintain a horizontal position. The end which has a northerly direction always inclines downward, as if it were heavier than the opposite end, though there is, in reality, no difference in weight between the two. This descent of the north pole of the needle is owing to the greater nearness of that end of the needle to the north pole of the earth, and is termed the dip.

If two magnetized bars are freely suspended by their centres, and brought near each other, the north end of the one and the south end of the other mutually attract each other; while, on the other hand, the north pole of the one repels the north pole of the other, and the south pole of the one repels the south pole of the other. In this respect there is a striking analogy between the magnetic and the electric force; for in both cases the parts of a body which are in a similar state of excitement repel, and those which are in a dissimilar state attract. But are magnetic power and polarity confined to the two ends or extremities of a magnetized bar? At first sight this might seem to be the case, for, in point of fact, the middle of the bar does neither attract nor repel the magnet which you bring near it: only towards its extremities does the bar manifest any force. But in reality the magnetic force is not thus limited. It resides in every part and throughout the whole mass, though exhibited only at or near the extremities. And all we have to do to prove this is to break the bar in two. The moment this is done, it will be found that the two new ends formed by the fracture display magnetic power and polarity as really as the original ends of the bar N -the one showing the

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n

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properties of a south pole and the other the properties of a north pole.

It has been already said that any piece of iron or steel may be magnetized by merely rubbing it with loadstone. But this is not the only method of magnetization. An

artificial magnet has, equally with the loadstone, the power of communicating magnetism, and that by contact alone, without the process of rubbing. If a small bar of steel remains a short time in contact with any artificial magnet, and is then removed, it will be found to have acquired permanent magnetism. If a small bar of iron is similarly treated, a like result will ensue, but with this difference, that, on being removed, it parts with its temporary magnetism and returns to its unmagnetic condition. The iron readily receives the magnetic energy, and as readily parts with it; but the steel is slow to receive, and tenacious in retaining it. Once duly magnetized, bars of steel will act as magnets for many years, and, singular to say, will have their energy rather increased than diminished by their being used in communicating it to others. Indeed, a magnet, whether natural or artificial, never loses by imparting magnetism: it rather gains,—differing in this respect from electrization, which never takes place without causing one body to lose what another body gains.

The earth itself is a magnetizer. If an iron poker is held in the position of the dip, and struck sharply, its lower end will show signs of north polarity, and its upper end of south; and, so long as it is kept in this position, it will retain its magnetic energy. A steel poker, owing to its greater resistance to the reception of magnetism, is not polarized with the same facility as an iron one. Yet even steel bars have been magnetized by the magnetic influence of the earth.

Of all magnetizers, however, the most powerful is electricity. And, what is remarkable, this force not only communicates magnetic properties to iron and steel, but imparts new properties to these bodies after they are magnetized. For example: If a magnetic needle is brought near a wire which is traversed by a current of voltaic electricity, the effect of the electric current upon the needle is to turn it out of its ordinary line of magnetic direction, and cause it to arrange itself across the wire. Before the current reaches it, the needle points from north to south; but the moment it is brought within the influence of the current, it points