N placed at a small distance from the former, there are three cases to be considered.

1. The Moveable Body is in a Natural State and a Conductor.In this case the body м acting by influence on the neutral fluid of the body N, attracts the negative fluid and repels the positive, so that the maximum of the tension of the two fluids takes place at the points a and b respectively. Now electric attraction and repulsion being inversely as the square of the distance, the attraction between the points a and c is greater than the repulsion between b and c, and the moveable body approaches the fixed body with a resultant force equal to the excess of the attractive over the repulsive force.

2. The Moveable Body is Electrised and a Conductor.-If the moveable body is charged with contrary electricity to that of the body m, there is always attraction; if it is charged with the same electricity, there is repulsion for a certain distance, but at a shorter distance there may be attraction without contact. To explain this anomaly, it is sufficient to observe that, becides the free fluid which the moveable body already contains, it also contains natural fluid, which being decomposed by the positive influence of the body M, the hemisphere b receives a fresh quantity of positive fluid, while the hemisphere a is charged with negative fluid; consequently there is, as in the preceding case, attraction and repulsion. The latter overpowers the former at first, because the quantity of positive fluid in the body N is greater than that of negative; but as the interval ac is diminished, the attractive force increases quicker than the repulsive, and may at last exceed it.

3. The Moveable Body is a Bad Conductor.-If the moveable body is a bad conductor and electrised, it is repelled or attracted according as it is charged or not with the same electricity as the fixed body. If it is in the natural state, as a powerful source of electricity whose action is prolonged can always more or less decompose the natural fluid even in bad conductors, this fluid is decomposed under the influence of the body M, if M is sufficiently electrised, and then there is attraction.

The Gold Leaf Electroscope.-Electroscopes or electrometers are apparatus for ascertaining whether a body is electrised and with what kind of electricity. The electric pendulum already described is an electroscope. There are various apparatus employed for this pupose, but we shall confine ourselves to the gold leaf electroscope at present. It is also called Bennet's electrometer, and consists of a glass jar n, fig. 385, resting upon a plate of copper, and having its neck stopped by a cork covered with isolating varnish, which also coats all the upper part of the jar. Through the cork passes a copper wire having a ball c of the same metal at one end, and two pieces of gold leaf beaten very thin at the other, inside the jar. When a body charged with any electricity whatever-e. g. negative as indicated in the figure this electricity acting by influence upon the neutral fluid of the ball and the wire, the positive fluid in the ball is attracted and the negative repelled towards the pieces of gold leaf, which, being charged with the same kind of electricity, repel each other, and thus we see that the body a is electrised. It is easy to ascertain the kind of electricity with which the

Fig. 386.

contrary to that of the body. The pieces of gold leaf fall together, but on withdrawing the finger and then the body A, they diverge again. Then to ascertain the kind of electricity in the apparatus, bring a glass stick rubbed with wool slowly near the ball c. If the divergence of the pieces of gold leaf increases, it shows that the electricity of the electroscope is repelled to the lower part, whence we conclude that it is of the same kind as that of glass, that is to say, positive. But if the divergence diminishes, the electricity of the apparatus is attracted by that of the glass; it is therefore contrary to it, that is, negative.

ELECTRICAL MACHINES.

Electrophorus.-Electrical machines are apparatus for obtaining a more or less abundant development of statical electricity. The most simple of electrical machines is the electrophorus. This apparatus, invented by Volta, is composed of a cake of resin B, fig. 387, and a wooden disc A, covered with a tin-foil, and furnished with a glass isolating handle. To obtain electricity by means of this apparatus, first dry the cake of resin and the wooden disc by warming them slightly; then beat the resin hard with cat's skin, which will electrise it negatively. Then placing the disc covered with tin-foil on the resin, fig. 386, the latter, being a very bad conductor, retains its negative electricity, and by its influence on the disc, it attracts the positive fluid to the surface which is in contact with it, while it repels the negative fluid to the other. Then touching the tin-foil with the finger, you withdraw the negative fluid, and the wooden disc remains electrised positively. In fact, if you Fig. 387.

take it away with one hand by the glass handle, and bring the other hand near it fig. 387, a bright spark is produced, owing to the recomposition of the positive fluid of the disc with the negative fluid of the hand.

In dry air the cake of resin, having been once electrised, may retain its electricity for months, and you may obtain all this time as many sparks as you like, without beating the resin again with cat's skin, provided you take care each time first to touch the disc covered with tin-foil while it is in contact with the resin, and again a second time when you hold it by the glass handle. The electrophore is employed in chemistry to make gaseous compounds explode in the eudiometer by means of the electric spark.

Ramsden's Electrical Machine.-The first electrical machine was made by Otto de Guericke, who also invented the airpump. It consisted of a sulphur sphere fixed to an axis turned with one hand, while the other rested on the sphere and served to rub it. The ball of sulphur was soon exchanged for a glass cylinder, the hand being still employed to rub it. Towards 1740, Winkler, a German philosopher, first made use of a horse-hair cushion covered with silk for rubbing. At the same period, Boze, professor in the Duchy of Wurtemberg, collected the electricity disengaged by friction by means of a tin-plate isolated tube. At last, in 1766, Ramsden, in London, substituted for the glass cylinder a circular glass plate rubbed by cushions. The electrical machine then took the form which it still generally retains.

Between two wooden supports, fig. 388, is a circular glass

tively. These latter communicating with the earth by the wooden supports, immediately lose their electricity as fast as it is produced. The positive electricity of the plate acts by influence upon the conductors and attracts the negative fluid, which, being set free at the spikes, combines with the positive electricity of the glass and neutralises it. The conductors, which thus lose their negative electricity, therefore remain positively electrised. Consequently, in the electrical machine the plate communicates nothing to the conductors, it is only necessary to withdraw from them the negative fluid arising from the decomposition of the natural fluid.

The machine being once charged, on placing the hand near it a strong spark is elicited, which is repeated as long as the plate continues to be turned, for the spark being the result of the combination of the negative fluid of the hand with the positive fluid of the machine, the latter tends at each spark to return to the neutral state, but the influence of the plate electrises it afresh. To give an electrical machine all the activity of which it is susceptible, it is of consequence to dry the supports, the plate and the cushions carefully, by gently warming them, or wiping them with a warm cloth. cushions require particular attention both as to their arrangement and keeping them in good condition. Those most in use are made of thin leather stuffed with horse-hair and covered with mosaic gold, a substance which greatly increases the development of electricity, probably by a chemical reaction, if we may judge from the sulphurous odour given out by the cushions on being rubbed.

The

plate P, fixed at its centre to an axis which is turned by means of a winch. This plate is pressed by four leather or silk cushions or rubbers F, at the extremities of its vertical diameter. In a line with its horizontal diameter are two brass tubes bent round as seen in the figure, and called combs, because they are furnished with a number of spikes on the sides facing the plate. These combs are attached to two larger tubes c, called conductors, which stand on four glass legs, and are connected together by one of a smaller diameter. As the glass plate moves round it is electrised positively and the cushions nega

Within the last few years, the old rubbers have been brought into use, which were introduced by Van Marum in 1788, and which give electrical machines a much greater degree of tension than the horse-hair cushions. These cushions, which are represented in the accompanying figure, 388, consist of a flat piece of wood well planed, and pressed against the plate by a double spring or by two screws regulated at pleasure, which is preferable. This flat piece of wood is covered with thick flexible leather, upon which a layer of tin-foil is put, and lastly, upon this a piece of taffetas a, which extends a little

establish a communication with the triangle, while the central bell hangs by a silk thread which isolates it from the machine, but it communicates with the earth by means of a metallic chain. Lastly, between the middle bell and the two others, are two little copper balls hanging by silk threads. When the machine is charged, the bells A and B are electrised positively, attract the copper balls, and repel them as soon as contact has taken place. Now these balls being then electrised positively, approach the bell c, which, although in communication with the earth, is charged with negative electricity through the influence of the others. Directly after contact the balls are therefore repelled towards the bells A and B, and thus keep moving backwards and forwards rapidly, and striking the bells all the time the machine is charged.

To explain how hailstones attain to a considerable weight before falling, Volta invented an apparatus, founded, like the preceding, upon electric attraction and repulsion. It consists of a glass bell placed upon a copper plate, with a number of little pith balls upon it, fig. 393. Through the neck of the

It is a sheet iron boiler, with a fire-place inside, isolated by four glass legs. Its length is about five feet, and its diameter about a foot and a half. A glass tube placed vertically on the right of the boiler, and communicating with it at both ends, indicates the level of the water inside. A small gauge, which is not represented in the figure, denotes the degree of pressure. On the boiler is a plug c, which is opened when the steam has acquired sufficient tension. Above this plug is a reservoir B, in which the tubes through which the steam escapes circulate. These tubes have other tubes, called adjutages, of a peculiar form at the end, one of which is represented on a large scale by the cut м on the left of our illustration. The interior of these adjutages is made of hard wood, curved as the arrow shows, which increases the friction. Lastly, the box B is filled with water to cool the escape tubes. The steam before reaching the adjutages is thus partially condensed, and comes forth mingled with small bubbles of water, which is a necessary condition, because the experiments of Mr. Faraday show that electricity is not obtained from the passage of dry vapour.

Electric Chime; Hail Apparatus.-The electric chime is a small apparatus composed of three bells attached to a horizontal tringle, in communication with an electrical machine, fig. The bells A and в hang by metallic chains which

392.

tus consisting of five or six metallic rays bent at the end, all in the same direction, each terminating in a point, and fastened to a cap which is moveable on a pivot, fig. 394. This apparatus being placed on an electrical machine, the rays revolve rapidly in the direction opposite to that of the points as soon as the machine is charged. This motion is not the effect of reaction, like that of the hydraulic tourniquet, as some philosophers have maintained, but of repulsion between the electricity of the points and that which they communicate to the air. The electric fluid collecting towards the points, escapes in the air, and as the air is then charged with the same electricity as the points, it repels them and is repelled by them.

Fig. 390.

the contrary fluid which is separated from the point by the influence of the machine.

Volta's Pistol. This is a small apparatus which serves to show that the electric spark has the property of determining chemical combinations. It consists of a tin-plate vase, in which is placed an explosive combination of two volumes of hydrogen and one of oxygen. It is then hermetically sealed. On one side is a tubular opening, through which passes a metallic rod terminated by two small balls A and B, as represented in the cut on the left of the accompanying illustration, fig. 397, and enclosed in a glass tube which isolates it from the rest of the instrument. Holding the instrument in the hand, Fig. 396.

It is found that the tourniquet does not move in vacuo, and if you place the hand near it while it turns in the air, a slight current of air is felt, owing to the displacement caused by the electrised air.

When the electricity thus escapes at a point, the electrised air is repelled strongly enough to give rise to a current which is not only perceptible by the hand, but capable of blowing out a candle or wax taper with a powerful machine. Fig. 395 shows how the experiment is performed. The same effect may be obtained by placing the candle or taper on one of the conductors, and bringing a metallic point that is held in the hand near to it, fig. 396. The current in this case arises from

as shown in the figure, bring it near an electrical machine. The knob a being then electrised negatively by influence, and the knob в positively, the spark comes out between the knob A and the machine, and at the same moment a second spark appears between the knob в and the side of the vase, which communicates with the earth through the hand. It is this latter spark which determines the combination of the two gases. This combination being accompanied by a lively disengagement of heat, the steam which is produced acquires such an expansive force, that the cork is projected with great force and a loud report like that of a pistol.

LESSONS IN MORAL SCIENCE.-No. VI. THE KIND OF INDIFFERENCE WHICH HAS BEEN CONSIDERED ESSENTIAL TO FREE AGENCY.

IN every act of choice or will, it is implied that the person willing might, if he pleased, act in a different way from what he does, for otherwise he would be under a necessity of acting in one way only, and there could be no freedom in such an action. There is no freedom in the pulsations of the heart, for they are not voluntary, but go on whether we will it or not. In all actions where the will is exercised there must be at least two things which may be done. This liberty was by the ancients distinguished into two kinds, the liberty of contradiction, and the liberty of contrariety. In the first we have the choice

of doing or not doing some proposed act. In the second, we have the liberty to do one thing or another, or one thing or several others. In regard to such objects of choice, there was said to be indifference, by which it was not meant that the mind was indifferent at the moment of choice. This would be a contradiction, because indifference towards an object, and the choice of an object, are opposite and irreconcilable states of mind. But the meaning was, that, abstractedly from the feelings of the agent, the contrary or different actions were indifferent. It was in the power of the agent, if he were disposed, to do or not do, to do this or that; but it was never understood to imply, that with the inclination in one direction a choice might be made in the opposite direction. A man may do what he pleases, but it is absurd to suppose that he can will to do what it does not please him to do.

The doctrine of a power of contrary choice, as the thing has