the other two; and he has named his apparatus the multiplying condenser (Fig. 51.). The lower plate, as before, is screwed

Fig. 51.

upon the electroscope; the second is furnished with an insulating handle, and is varnished on its two faces; the third, which is varnished only upon its lower surface, is pierced at its centre with a hole, within which is sealed a glass tube that serves as an insulating handle, and at the same time affords a passage to a glass rod, forming the insulating handle of the second. We can thus easily superpose the three plates, and successively raise the third and the second. In operating, we put the source in communication into the third, and touch the second with the finger by means of a small metal appendage that forms part of its border. Then, this being charged, we raise the third and touch the first with the finger: the electricity of the second is thus found almost entirely disguised; it is charged again by replacing the third, which is still in communication with the source; the third is then to be removed, and the first to be touched, and so on, until the electricity of the second can no longer be disguised by the first. We then remove first the third, and then the second, and the first is found to be charged with a considerable amount

of free electricity, and of the same nature in this case as that of the source; for it is of the contrary nature to the electricity of the second plate, which is itself different from that of the third, which receives its charge directly from the source. It is easy to comprehend that this apparatus condenses a second time the product of a former condensation; and thus very feeble sources of electricity may be rendered sensible. But, like all the instruments, it is too delicate and rather dangerous in its use, and is moreover very tedious and inconvenient in manipulation. It must therefore be reserved for exceptional cases, and be only used with the greatest precaution. We should add, that the plates employed by M. Peclet for the construction of his condenser are made of roughened glass, ground with great care, and covered with gold-leaf, which is applied without cement, it being sufficient to moisten the surface of the glass slightly with the breath. It is not necessary to remark that the same apparatus that serves as a multiplying condenser may be employed as a simple condenser. For this purpose, we have merely to remove the third plate, and the second, being furnished with its insulating handle, is put in direct communication with the source by means of the appendix affixed to its edge.

Magic Picture and Leyden Jar.

The magic picture, called also the sparkling pane, consists of a pane of glass, the two faces of which are coated with a

sheet of tin-foil (Fig. 52.), care being

taken to leave bare upon each of the faces of the glass pane an edge of 2 or 3 inches in width, so that the contrary electricities accumulated on the sheets of tin-foil cannot unite immediately by the edges of the sheets. One of the sheets of tin-foil is put into communication with the source of electricity, and the other with the ground. Then, when they are charged, the two plates may be connected by means of a dis

Fig. 52.

charging rod, and a very vivid spark is obtained. This apparatus is termed the sparkling pane, because metal filings, secured to the glass by gum, are sometimes substituted for one of the metal sheets: now, when the charge is made in the dark, there arises on the face so coated a brilliant light, produced from the sparks which escape between all the grains of metal, at the moment of the neutralisation of the two electricities.

The Leyden jar is nothing more than a magic picture made into a cylinder; the insulating stratum is also of glass; but, instead of being a plane surface,

Fig. 53.

it has the form of a cylinder or bottle (Fig. 53.). One of the sheets of tin-foil is on the outside of the bottle, and is termed the outer coating of the Leyden jar; the other is withinside, and is termed the inner coating. This latter receives electricity by means of a metal rod, terminated on the exterior by a knob, and in the interior by a cluster of wire, which, diverging by virtue of their elasticity, come thus into contact with the metal coating. The rod is fixed by means of a plug of wood or cork, through which it passes tightly, and which is itself inserted firmly into the neck of the jar. A thick coating of wax covers the cork and the exterior part of the glass, surrounding it so as altogether to prevent the possibility of a reunion between the electricity of the exterior coating and that of the inner coating and of the rod that is in connection with it. In order to charge the jar, it is held in the hand by the outer coating, and the knob is presented to the conductor of the electrical machine. The positive electricity of the machine penetrates to the inner coating, decomposes through the glass the natural electricity of the outer coating, the positive of which goes into the ground through the hand and the body of the operator, and the negative of which is disguised. At the end of the operation there remains a small quantity of free electricity upon

the inner coating, as an electroscope indicates. If, after

having carefully placed the jar upon an insulating support, we draw this away with the fingers, it is manifested under the form of a small spark; and, as soon as it is gone, the balls of the electroscope fall down, while the electroscope, placed in communication with the outer coating, indicates the presence of free electricity upon this coating (Fig. 54.).

Fig. 54.

The free electricities of contrary natures, possessed by the two coatings, may be made manifest by placing between the two knobs, with which they are each in communication, the pith ball of an electric pendulum; being attracted by the

Fig. 55.

free electricity of one of the knobs, this ball comes into contact with it, and is electrised by contact; being immediately repelled, it goes to the other, charged with a free electricity of a contrary nature; then it loses its own electricity, and is charged with that of the second knob, by which, being repelled, it returns to the former, and so on (Fig. 55.) Thus the ball executes between the two knobs a series of oscillations, which may be prolonged for several hours, at the end of which, the two coatings having lost their electricity by this succession of small discharges, the phenomenon ceases. The pith ball is sometimes constructed in the form of a spider, of which it is itself the body, and small very fine wire are the limbs; on which account this apparatus is called the electric spider. If, instead of employing the pith-ball, the two knobs of the jar are touched successively with the finger, the discharge is brought about by a series of small sparks arising from the portion of electricity alternately made free on each of the

coatings; but it is only after having drawn a very considerable number of sparks that we succeed in completely discharging the jar. It is important in this experiment to take care never to touch the two knobs of the jar at the same time, which might easily happen if both hands were employed; we should, under such circumstances, ourself serve as the course for the reunion of the two electricities that are accumulated upon the two coatings, and we should experience a violent, and often a dangerous, shock. It is also with a view of avoiding this inconvenience that it is essential to place the Leyden jar, when it is charged, upon an insulating stool; for, if we merely place it on an ordinary table, we are in the route that the two electricities of the two coatings would pursue at the moment we touch the knob; as they are now separated by conducting bodies alone.

To show the different electricity with which the two coatings of a Leyden jar are charged, Faraday employed two concentric cylinders made of metal gauze, separated by a stratum of air about in. thick, and resting on a base of resin. The exterior is put in communication with the ground, while the interior is charged with electricity. They are touched successively with the proof-plane, and are found charged each with a different electricity. Care must be taken not to touch the exterior cylinder on its inner surface, nor the interior on its outer.

The inner coating of a Leyden jar is easily charged with negative electricity. For this purpose we have merely to hold it by the knob, and let the electricity of the machine arrive on its outer coating; but in this case we must take care, after the jar is charged, to place it upon an insulating support; for if, while holding it by the knob, we were to place it upon a table, we should receive the whole of the discharge.

There is a very elegant experiment, for which we are indebted to Lichtenberg, and which is called Lichtenberg's figures, that makes manifest without an electroscope, and in a directly visible form, the nature of the electricity with which the inner coating of a jar is charged. This experiment