referred to, they will produce in the system no motion of rotation about its centre of gravity, and all the pressures being decomposed in any direction, will mutually destroy each other on the opposite sides. Now let us suppose that at a certain part of the needle, either the point or any other part, we remove the insulating wrapper, so that the electricity may escape through this aperture; then the pressure at this part being nothing, the opposite pressure will act without a counterpoise, and cause the needle to turn in the direction in which the force is exerted.

This result could scarcely take place in an absolute vacuum, because the electricity of the stratum would be instantly dissipated when the insulating wrapper was perforated; but it may be obtained in the free air; it is only necessary to sharpen the points of the needle to such a degree that the electricity accumulated there, may overcome the atmospheric pressure. In this case, the air serves as a wrapper, and the aperture is made by the electricity itself; whereas, in the other case, we supposed it to be made artificially. The phenomenon would be precisely similar, if the needle, instead of being electrified, were a hollow vessel, filled with water or mercury, and its extremities, being bent and pointed, were two little canals whose orifices had been formed by the pressure of the fluid. The pressure then becoming nothing at these orifices, that which is exerted on the opposite element of the interior surface, would impel the needle, and thus cause it to turn in the opposite direction.

58. In this case, if we take the product of the masses into the velocities of all the liquid particles which escape, the product will be constantly equal to the sum of the products of the masses into the velocities of the other parts of the needle, and of the liquid which turns with it in the opposite direction. The same equality must, therefore, obtain in the motion of the electrified needle; but the mass of the electrical particles is absolutely insensible, since the most highly electrified bodies do not appear to have their weight increased by a quantity capable of being detected by the nicest balances; it follows, then, that the velocity of these particles must be infinitely great; and no example is, perhaps, better fitted to give us a just idea of this velocity.

Before we were made acquainted with the true laws of electrical equilibrium, it was not known by what means the attraction and repulsion, which actually take place between electrical particles, could transmit themselves to the material particles of bodies; and this effect was vaguely designated by the word tension, which represented the electricity as a spring placed between the electrified bodies, and tending to make them approach to, or depart from each other. The details into which we have now gone, serve to explain how this transmission of force takes place, by means of the pressure which the electricity exerts upon the surrounding atmosphere, or generally upon the obstacles which oppose its dispersion.

Of the Construction of Electrical Machines.

59. It has been apparent from our first experiments, that to render electrical phenomena conspicuous, it is necessary to apply the friction to surfaces of some extent. We accordingly make use of a large glass plate or cylinder fitted to turn against one or more rubbers, by means of a winch; and provided with an insulated metallic body placed near it, to receive the electricity, as it is developed, and to transmit it to other conductors, also insulated, as the experiment to be performed may require. But, knowing as we now do, that several bodies, thus electrified, exert always a mutual action upon cach other, we have to inquire what is the best arrangement that can be given to the several parts of the apparatus; of what substance ought the rubber to be; what should be the form of the prime conductor and the other conductors; what the form, substance, and dimensions of the insulating supports, in order that they may res pectively answer their purpose in the best manner. These important questions we shall answer very briefly.

There are three principal things to be considered; namely, the plate, the rubber, and the conductors.

60. Let us first consider the rubber. Whatever may be its substance, it is necessary, in order that it may produce an extensive and continued friction, that it should exactly fit the surface

of the plate or cylinder, and that it should press it in a great number of points. Nothing is better adapted to this purpose than cushions stuffed with hair, and covered with simple leather, which are pressed by a spring against the surface of the glass. The leather alone, thus rubbing upon the glass, excites but little electricity. We obtain it much more abundantly by covering the cushions with a dry amalgam of mercury, zinc, and tin triturated together; so that the amalgam is in fact the rubber, and the glass the body rubbed. If we insulate the cushions during the friction, and examine the electricity acquired by the glass, we shall perceive that it is vitreous; consequently the cushions take the contrary electricity, that is, the resinous, as may be easily shown. But in the ordinary use of the machine, we must be careful not to insulate the cushions; on the contrary, they must be made to communicate with the ground by a metallic conductor; for we thus obtain the electricity much more copiously.

This is always observed in the development of electricity by the mutual friction of any two bodies. The excess which each of them acquires is always much more sensible when the other communicates with the ground than when they are both insulated. The circumstance is of great importance, because it seems to relate to the manner in which the two electricities are developed by friction. But, for the same reason, it is difficult to be explained, because our theories apply only to electricity already excited, and are as yet but little advanced with respect to electricity in its state of disengagement from bodies. We can there

Mr Singer, a late English electrician, who wrote a complete. treatise on electrical instruments, recommends, as the best amalgam, a compound of two parts, by weight, of tin, four of zinc, and seven of mercury; the mercury to be heated by itself a little above 100° and poured into a wooden box, to which the proper proportions of zinc and tin, in a state of fusion are to be added. The box is then to be closed, and briskly shaken to unite the ingredients as perfectly as possible. When the whole has become cold, it is to be pounded in a mortar and reduced to a fine powder; this powder is then mixed with a portion of hog's lard just sufficient to give it the consistency of paste.

fore only enunciate the fact as it presents itself in the experi ment, and deduce from it the mechanical conditions to which the development of electricity is subject. For this purpose, let us imagine, in the first place, two insulated bodies A and B, which being rubbed, the one against the other, in their natural state, acquire, the one a quantity +e of vitreous electricity, the other a quantity -e of resinous electricity. I give the negative sign to the latter, to indicate that being added to the other, it neutralizes it. It is undoubtedly the nature of the two surfaces, and the power of the friction which determine this proportion between the spaces and the quantities of electricity which attach to each of them; of the nature of the mechanism by which this phenomenon takes place, we are entirely ignorant. But the two electricitiese and e, being once disengaged from their combination, there is no doubt that they preserve their individual properties, so as to exert their own repulsive force, and mutually attract each other. In virtue of their own repulsion, the electricitye, developed upon A, tends to spread itself over B, at the points of contact; and reciprocally, the resinous electricity e, developed upon B, tends to spread itself over A. This double tendency is also favoured by the mutual attraction which+e and e exert for each other, and in virtue of which they endeavour to reunite. Since this diffusion and union do not take place, it follows that the unknown power which disengaged the two electricities+e ande from each other, and separated them, fixing one upon each body, should act also after this separation and with sufficient energy to keep them separate in spite of the two causes which conspire to make them unite. Now it appears that this action of rubbing takes place only at the surface in contact, so that it does not prevent either of the two electricities+e ande from spreading itself over the surface of the body upon which it resides, with the degree of freedom which belongs to the greater or less conducting power of this body. For if B, for example, be a conductor, and it be made to communicate with the ground by different points of the surface in contact, its electricitye will disappear, and B will return to the natural state, without the body A, on that account, losing its excess +e; this is constantly seen when we rub an insulated body A against a body B not insulated. Now it is very evident that in this state

of things, the friction developes and maintains upon A a greater quantity of electricity than it would do if B were insulated. For, in the first case, if A took +e, and B. e, in order to retain +e, it would be necessary to overcome, besides its own repulsive force, its attraction toe; whereas the latter force does not exist whene has passed off into the ground. For a similar reason, if the same body A is successively rubbed against two insulated conducting bodies B and B', both of the same nature, and presenting surfaces of the same kind, but of unequal extent, the larger will give a greater quantity of electricity to A; for the disengaged electricity which may fix itself upon B or B', being spread over the whole surface of these bodies, it will form a thinner stratum, with an equal quantity on the body of the larger bulk, and therefore the proper repulsive force of this electricity, at the surface in contact, will be less on 'this body than on the other; and hence it follows that the electricity can, in this case, be maintained in a greater quantity in a state of separation.

61. Besides these general conditions, the friction of the plate of the electrical machine against the insulated cushions, is attended with a circumstance which renders the effects produced much more feeble than when the cushions communicate with the ground. It consists in this, that the different parts of the plate which present themselves successively in their rotation to the rubber have previously passed before the prime conductor, where the vitreous electricity which they had acquired is entirely or almost entirely neutralized; and thus they are nearly in their natural state when they come again between the cushions. These different parts, therefore, represent so many insu

The way in which this neutralization takes place, is very evident. The parts of the plate which arrive charged with vitreous electricity before the prime conductor, decompose by influence its natural electricities, repel the vitreous and attract the resinous in the points next to the plate. There, this resinous electricity, on account of the form of these points, acquiring a great repulsive force, breaks through the layer of air which separates it from the plate, and goes to neutralize the vitreous electricity adhering to it. The same effect would also take place, although less perfectly, if the extremity of the