vessel (fig. 328), with metallic caps at each end. The lower cap is provided with a stopcock, so that it can be screwed into an airpump, and also into a heavy metallic foot. The upper metallic rod moves up and down in a leather stuffing box; the lower one is fixed to the cap. An almost complete vacuum having been made, the stopcock is turned, and the vessel screwed into its foot; the upper part is then connected with a powerful electrical machine, and the lower one with the ground. On working the machine, the globe becomes filled with a feeble violet light continuous from one

end to the other, and resulting from the recomposition of the positive luid of the upper cap with the negative of the lower. If the air be gradually allowed to ente: by opening the stopcock, the tension increases with the resistance, and the light which appears white and brilliant is now only seen as an ordinary spark.

409. Magic pane.-The magic pane consists of a glass plate, one side of which is covered with several folds of tinfoil, arranged so as to form a series of metallic bands, arranged parallel and close

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Magic Pane.

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to each other. The pane is supported vertically by two glass supports, and the upper end of the tinfoil is connected with the electrical machine by a conductor, and the lower one with the ground by a chain. In this condition, if the machine be worked, the elec

tricity will pass into the ground by the tinfoil, without any interruption; but if a series of breaks are made in the tinfoil by cutting it away with a penknife, a spark appears at each break; and if these breaks be so arranged as to represent a given object, a flower, or a monument, or words, these objects are reproduced in lines of fire when the electrical machine is set to work. This experiment is really due to the prodigious velocity of electricity, which is not less than about 190,000 miles in a second. Hence, in the above experi

ment, although the sparks are really successive, they follow each other with such rapidity as to seem continuous.

410. Luminous globe and tube.-The luminous globe is a glass globe lined on the inside with a series of small lozenges of tinfoil placed very near each other without actually touching. The

first plate is connected with an electrical machine at work, and the last with the ground, upon which a series of bright sparks appears at each break in the metallic conductor (fig. 330).

If the small metal plates are arranged inside a spiral glass lustre from one end to the other, this arrangement forms a luminous tube.

4II. Volta's cannon.-This is not merely interesting as an experiment, but also as demonstrating an important fact, namely, that

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Volta's Cannon.

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the electrical spark can establish chemical action. Thus, water is formed of two gases, hydrogen and oxygen, in the ratio of one volume of the latter to two volumes of the former. Now, when an electrical spark is passed through a mixture of these two gases, they combine in their proportions, and form water. This combination is, moreover,

attended by a bright flash of light and a loud report, the latter being due to the expansive force of aqueous vapour, due to the high temperature produced by the combination.

On this property which mixtures have of detonating by the electrical spark, Volta's cannon, represented in fig 331, is constructed. It is a small brass cannon resting on an insulating support. In the touchhole is a small glass tube, and in this a brass wire with a small knob at each end; one of which knobs is on the outside, and the other very near the inside of the cannon, but not touching it. Having introduced a mixture of two parts of hydrogen and one of oxygen, the cannon is closed by a cork, and is connected with the ground by a metal chain. If then the charged disc of the electrophorus be approached, a spark passes to the small knob, and at the same time inside the cannon. This latter causes the two gases to combine with a violent explosion, which drives out the cork.

CHAPTER IV.

CONDENSATION OF ELECTRICITY.

412. Electrical condensers.-Condensers are apparatus by which, electricity may be accumulated. Their shape is greatly varied, but they are all composed essentially of two insulated conductors separated by a non-conductor, and their working is an application of the action of induction. Epinus's condenser consists of two metal plates, A and B, insulated by being supported on glass legs (fig. 332); between them is a pane of ordinary glass, of somewhat larger diameter than that of the plates, A and B, which

are about six inches in diameter. The legs can be moved along a support, and fixed in any position.

In explaining the action of the condenser, it will be convenient to call that side of the metal plate nearest the glass the anterior, and the other the posterior, side. And first let A be at such a distance from B as to be out of the sphere of its action. The plate B, which