2

Now if the apparatus in this state were exposed in a perfectly non-conducting medium, it is evident that this state of equilibrium would continue without change; but if it were surrounded by an absorbing medium, as the air, it would gradually lose its electricity. To understand how this would take place, we must remember that in the same state of the air, and for a surface of the same form, the waste is proportional to the whole quantity of free electricity which resides upon it. Thus, in the first instants, the loss will be greater for the first face A, than for the second A,, because the latter has less free electricity; so also it will be greater for A, than for A,, and so on to the last face B, where it will be nothing, because upon this face there is no free electricity. But by this series of unequal losses, free electricity will be developed. For the equilibrium before established did not exist between the portions of free electricity of the different faces, but between their absolute charges; and since the first charge A, is weakened, it can no longer neutralize upon B, all which it neutralized before; it is the same with respect to the action of A, upon B,, and so on to the face B. The electricity of this face being no longer completely neutralized, a portion becomes free, and this portion, at first very small, gradually augments. For although, from the instant that it first appears, it is continually exposed to the absorbing action of the air, yet from its weakness, it loses at first less than the free portions of the other faces; hence the change of equilibrium goes on gradually in the same way, the loss of free electricity diminishing more and more upon the first face and increasing upon the last, and upon the intermediate faces, varying between these two extremes. No limit can be assigned, therefore, to these variations, except it be the equality of the quantities of free electricity residing upon the two extreme faces of the apparatus, which will also reduce their charges to an equality. Then the disposition of the electricity will generally be symmetrical, as we proceed from these two faces toward the centre of the pile; the quantities of free electricity will be of a contrary nature on each side of this centre, gradually decreasing as we approach it; and at the centre they will be nothing, and we may touch the plate which is placed there without experiencing any shock. But if we break the pile at this place, or at any other, and insulate

the parts, there will gradually be developed at the broken extremity, a certain quantity of free electricity, which will be of á contrary nature to that of the other extremity which was left untouched.

This result is agreeable to theory, and, as I have satisfied myself, is perfectly confirmed by actual experiment.

The phenomena which are presented by minerals capable of being electrified by heat, are analogous to those we have described; and we can scarcely doubt that nature has provided them with a similar apparatus, that is, with an electric pile composed of an infinite number of parallel plates. The mere detail of the facts will be sufficient to establish this truth.

I shall take as an example the variety of the tourmaline denominated by M. Haüy isogone; it has the form of a prism with nine faces, terminated at one end by a summit of three faces, and at the other by a summit of six faces. When this stone is exposed to a temperature less than 98° of Fahrenheit, it offers no signs of electricity; but if we immerse it for some minutes in boiling water, and then, holding it with a pair of small pincers applied to the middle of the prism, we present it to the disc of an electroscope or to the small pendulum, already charged with a known electricity, we shall see that it is attracted by one end and repelled by the other. The summit with three faces possesses the resinous electricity, and the summit with six faces the vitreous. By making the electroscope very sensible, we find that each kind of electricity goes on decreasing rapidly from the summit where it resides; that it becomes very feeble at a small distance from each extremity of the prism; and that from this point to the centre, the mineral appears to be in its natural state; in a word, the effects are absolutely the same as in the insulated electric pile described above.

Many other crystals have since been found to exhibit similar phenomena. Several are more sensible in this way than the tourmaline, a small increase of heat being sufficient to electrify them. M. Haüy, who has made many curious researches on this subject, has remarked that the property in question belongs only to crystals whose forms are not symmetrical, and that the parts where the opposite electric poles reside, vary always from symmetry, as the two extremities of the prism of the

tourmaline.

It is possible that a very great depression of temperature in the case of the tourmaline might destroy its electrical equilibrium, as an elevation of temperature is known to do, or that it might be destroyed by a less degree of heat, if the stone were previously exposed to extreme cold. These particulars, which might serve to clear up the mystery of the electrification of this mineral, deserve to be examined.

When melted sulphur is poured into an iron basin, and suffered to cool in this basin while insulated, we find that it acquires the resinous electricity, and the iron the vitreous. This fact seems to indicate what takes place in each element of the tourmaline and of the other crystals which are electrified by heat. A series of such elements, being placed in contact with each other, would probably form a true electric pile, in which the insulation and separation of the plates would be effected by the nonconductibility of the substance of the crystal.

Mechanical and Chemical Effects produced by the Repulsive Force of

accumulated Electricities.

94. We have already remarked more than once, that the electricity spread over the surface of conducting bodies, exerts a contrary pressure upon the atmosphere which retains it at this surface by its weight. We have seen that this reaction, which is always proportional to the square of the thickness of the electric stratum, may become sufficiently powerful to overcome the resistance opposed by the air. Then the electricity escapes through the particles of the air. Hence we infer, that at higher degrees of accumulation, the electricity becomes capable of breaking through substances much more dense than the air, and even of separating their particles. This is confirmed by experiment.

The force of an electric battery, when highly charged, is sufficient to break cylinders of wood through which it is made to pass. It inflames certain combustible bodies, as phosphorus, ether, and other spirits, that is, it causes them to combine with the oxygen of the air, especially if they have been previously

warmed. It destroys life when it is made to pass through the body of an animal, and the flesh soon putrefies like that of ani mals killed by lightning. It passes also through plates of glass lengthwise and breaks them, provided their surfaces are polished; for otherwise the glass would be a conductor and the discharge might pass without breaking it. If transmitted along a fine wire of iron, silver, or copper, it melts it into little globules. With a degree of accumulation still more intense, these wires and even thin leaves of metal are suddenly volatilized.

It is evident that such a force might, by a similar action, produce in liquid or gaseous substances, all the phenomena which result naturally from a strong compression or from a sudden elevation of temperature; and this is in fact observed to take place. Thus the electric discharge, even that of a simple Leyden jar, inflames hydrogen and oxygen when they are mixed together in the proportion of about two parts by bulk of hydrogen to one of oxygen; and the residuum is water, or rather the vapour of water, elevated to a high temperature by the great quantity of caloric which the combination disengages. The most convenient apparatus for this experiment is represented in figure 40. It consists of a large glass globe, kept filled with oxygen gas by making it communicate with receivers having a constant pressure. Into this globe issues a constant current of hydrogen gas through a very fine glass tube. The jet is inflamed by a feeble spark sent through the globe by metallic conductors, and the combustion having once begun, supports itself. This experiment requires much caution to avoid explosions; but when we wish to observe only the fact of the combination of the two gases, we can safely employ the apparatus represented in figure 41. This is a glass tube closed at top with a metal stopper, which is strongly luted and which has a small knob projecting without the tube. A flexible metallic rod rises in the same tube by a spring, and approaches within a small distance of the knob. Then the tube being immersed in a trough of water, is filled with gas like a common receiver; and being drawn partly out and wiped, a spark is given to the metallic cap; it passes through the gaseous mixture, and causes inflammation with a loud noise. The same effect is produced by simple mechanical pressure; and also by an elevation of temperature.

In the same way that we form water by the electric spark, we are able also to decompose it. To this end, recourse was had formerly to violent discharges through the liquid, which produced in it explosions accompanied with sparks. But the able and ingenious Dr Wollaston contrived to produce the same effect in a much more certain, easy, and beautiful manner, by conducting the electric current through the water by means of very fine platina wires, terminating in sharp points, and insulated in glass tubes, or enveloped in resin, that they might not lose their electricity, except at the points themselves. It is evident that a very feeble electricity will, under these circumstances, acquire an extreme intensity, which is confined to the extremity of the point, and acts entirely against the single particle of water with which the point is in contact. Thus the electric current of a feeble machine, being transmitted in this way, is sufficient to disengage a continued stream of little bubbles, which being collected and tried by the electric spark, are found to be the two gases of which water is composed. The effect is rendered more certain and rapid by bringing together at the same time, through two opposite wires, two currents of electricity of different kinds.

If the transmission is made by two very fine points, one of copper, and the other of silver, immersed in a solution of sulphate of copper, the first communicating with the vitreous conductor, the sulphate is decomposed. The copper, being separated from the acid, is deposited in a metallic state upon the silver wire, and the other wire is dissolved. If we invert the communications, so as to cause the silver wire, thus covered, to communicate with the vitreous conductor, the deposit of copper, formed upon its surface, is redissolved, and the precipitation takes place upon the other wire.

These beautiful experiments, and many others of the same kind, due also to Dr Wollaston, prove that the resinous electricity tends to disengage oxygen from the combinations into which it enters, and that the vitreous electricity, on the contrary, favors these combinations. Of the truth of this important result we shall hereafter have abundant proof.