into the fine metal wire, and into the ball that is suspended to it, and not into the silk thread, and consequently not into the ball which terminates it; we may convince ourselves of this, by seeing that the former of the two balls attracts light bodies and the latter does not. We can operate, in the same manner, upon all substances by drawing them into rods of the same diameter and length; each rod must carry a ball at one of its extremities, whilst the other extremity is fixed to the insulated metallic support that receives the electricity.

The following is an approximative table of the conducting and insulating faculty of different bodies. This table is composed of two columns; the first contains the conducting bodies, placed in the order of their degree of conductibility, beginning by the best conductors; and the second contains the insulating bodies, placed in the order of their insulating faculty, beginning by the worst insulators. It hence follows that the second column may be regarded as a continuation of the first.

Bodies, therefore, are all capable of becoming electrical by friction; but they differ among themselves with regard to the faculty they possess of transmitting electricity; some transmit it promptly and freely; others more slowly and with difficulty; others seem to be almost incapable of transmitting it. However, they are all susceptible of taking electricity from an electrised body with which they are touched; only if the touched body is an insulator, it takes electricity on that part of its surface alone which has been touched; whilst, if it is a conductor, it acquires it throughout its whole extent, although it has received it in only one point. This is a means of electrising, which is termed to electrise by communication.

We may further remark that, if the electrised body is an insulator, it gives the electricity it possesses to the conducting body that touches it only at the points in which the contact takes place; but, if it is a conductor, there occurs a division of its electricity between itself and the touched body, a di

* Gutta Percha appears to be of known substances one of the best insulators; its place, however, cannot exactly be assigned in this table.

vision which is subject to a very simple law, viz. that each of the two bodies, which we necessarily suppose insulated, takes a part of the total electricity proportional to its own surface. This law, which we shall demonstrate hereafter*, explains why an insulated and electrised conducting body, when put into communication with the ground, loses all its electricity; the electricity that it possessed is really divided between itself and the earth, proportional to their respective surfaces; but its surface being infinitely small in comparison with that of the earth, there must, therefore, remain to it, after contact, infinitely less electricity, or none at all.

It often happens that when we bring an electrised body near to a body that is not so, the electricity of the former passes into the latter before there is any contact between them, under the form of a spark that traverses the space of air by which they are separated. This circumstance in no degree modifies the final result, which is the same as if the communication had been made by contact.

*This law, as we shall see further on, is not mathematically rigorous; but it so nearly approaches to accuracy, as to be able to be admitted, without too great an error, into the cases that are more frequently presented.

CHAP. II.

NATURE AND FORM OF ELECTRICITY.

Distinction between the two Electricities.

WE have hitherto, in a general manner, called all bodies electrised, whatever they may be, which, after having been rubbed or put into communication with an electrised body, exercise an attraction at a distance upon light bodies. We are now about to study this kind of action more closely; and we shall see that this study will lead us to recognise that the electricity, manifested by different substances, is not always identical.

If to a film of raw silk, which is itself fixed to a glass support that insulation may be more perfect, we suspend a conducting ball of elder pith, about a quarter of an inch or a little more in diameter, we shall have an apparatus which we will call a simple electroscope or electric pendulum. If we bring an electrised glass rod near to this ball, the ball will be attracted, will touch the glass, and then, after having touched it, will be immediately repelled (Fig. 3.). The ball will have acquired electricity by its contact Fig. 3. with the glass; it will preserve this electricity, because it is insulated; but, if it is touched with the hand, it will immediately lose it, and be restored to the natural condition. If we have a second ball similar to the first, suspended at the same height, and electrised in the same manner, and then bring the two balls thus electrised carefully together, giving them as little movement as possible, when arrived at a certain distance from each other, they

*Balls of elder pith, covered with leaf gold, or very light hollow metal balls may also be employed.

repel each other, and do so the more strongly as we endeavour to bring them more closely together. If we make the same experiment by employing a stick of rubbed wax instead of a glass rod to electrise the two balls, we shall have exactly the same result. But, if we electrise one of the balls with the

Fig. 4.

glass rod, and the other with the stick of wax, and then bring them together carefully, when arrived at a certain distance we shall see them attract and then fly to each other, and, after having touched, recover the vertical position, and give no further signs of electricity (Fig. 4.).*

If one of the balls is electrised either with glass, or wax, and the other is not, they attract each other; and, as soon as they have been in contact, they repel one another; the first having communicated a portion of its own electricity to the second.

We may conclude from the preceding experiments :—

1. That there is attraction between an electrised body and a body that is not so.

2. That there is repulsion between two bodies electrised by the same source of electricity.

3. That there is attraction between two bodies electrised, the one by glass, the other by wax.

There is not, therefore, an identity between the electricity that glass acquires by friction, and that which wax acquires, since, on a body electrised in the same manner, the effect of the one is repulsive, and that of the other attractive; hence, to distinguish them, the former has been named vitreous electricity, and the latter resinous electricity.

Experiment has taught us that all bodies in nature, on being rubbed, acquire one or other of these electricities. Glass, and vitreous bodies in general, acquire the former;

* When the balls are of pith, it sometimes happens that the contact lasts some moments, before the neutralisation is complete. This is because the elder pith, when it is dry, is not a perfect conductor; and the electricity consequently does not circulate in it freely and promptly. With metal balls or pith balls, covered with metal leaf, the same inconvenience is not presented.