of carbon. This is made either of the graphitoidal carbon deposited in gas retorts, or by calcining in an iron mould an intimate mixture of coke and bituminous coal, finely powdered and strongly

compressed. Both these modifications of carbon are good conductors. Each element consists of the following parts: 1. a vessel, F (fig. 358), either of stoneware or of glass, containing, as in Daniell's, dilute sulphuric acid; 2. a hollow cylinder, Z, of amalga

mated zinc; 3. a porous vessel, V, in which is ordinary nitric acid; 4. a cylinder of carbon, C, prepared in the above manner. In the

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vessel, F, the zinc is first placed, and in it the carbon as seen in P. To the carbon is fixed a binding screw, m, (fig. 359), to which a copper wire is attached; forming the positive pole. The zinc is provided with a similar binding screw, n, and wire, which is thus the negative pole.

In Bunsen's battery the hydrogen resulting from the action is liberated on the surface of the carbon. This being surrounded by nitric acid, the hydrogen decomposes this acid, forming water and hyponitrous acid, which dissolves, or is subsequently disengaged as nitrous fumes. And, though the hydrogen is most completely got rid of by the decomposition of the nitric acid, the production of these nitrous fumes is very noxious.

The elements are arranged to form a battery (fig. 359) by connecting each carbon to the zinc of the following one by means of the clamps, mn, and a strip of copper, c, represented in the top of the figure. The copper is pressed at one end between the carbon and the clamp, and at the other it is soldered to the clamp, n, which is fitted on the zinc of the following element, and so forth. The clamp of the first carbon and that of the last zinc are alone provided with binding screws, to which are attached the wires.

CHAPTER VIII.

EFFECTS OF THE BATTERY.

448. Physiological effects. The remarkable phenomena of the voltaic battery may be classed under the heads physiological, chemical, mechanical, and physical effects; and these latter may be again subdivided into the thermal, luminous, and magnetic effects. All are due to the recombination of the opposite electricities like those of the electrical machine; but they are far more remarkable and more energetic, owing to the continuity of their action. To produce them the body experimented upon must be connected on the one side with the positive and on the other with the negative pole.

The physiological effects consist of shocks and violent contractions which the current produces in the muscles, not only of living, but of dead animals, as has been seen in Galvani's experiment with the frog.

H H

When the electrodes of a strong battery are held in the two hands a violent shock is felt, resembling that of a Leyden jar, especially if the hands are moistened with acidulated or saline water, which increases the conductivity. The shock is more violent in proportion to the number of elements used; with a Bunsen's battery of 50 to 60 couples the shock is very strong, with 150 or 200 couples it is unbearable, and even dangerous when continued. It is less perceptible in the fore part of the arms than the shock of the Leyden jar, and, when transmitted through a chain of several persons, it is generally only felt by those nearest the poles.

The shock, as in the case of the Leyden jar, is due to the recomposition of the two electricities; with this difference, that with the Leyden jar the discharge being instantaneous, the resultant shock is so also; while in the latter case, as the battery is immediately recharged after each discharge, the shocks succeed each other with rapidity.

John Aldini, a nephew of Galvani, was the first to study the

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action of the battery on dead animals. He came to Paris at the beginning of the present century, and repeated on a large scale several of his experiments at the veterinary school of Alfort near Paris.

449. Thermal effects.-When a voltaic current is passed through a metallic wire the same effects are produced as by the discharge of an electric battery; the wire becomes heated and even incandescent if it is very short and thin. With a powerful battery all metals are melted, even iridium and platinum, the least fusible of metals. Carbon is the only body which hitherto has not been fused by it. M. Despretz, however, with a battery composed of 600 Bunsen's elements joined in six series, has raised rods of very pure carbon to such a temperature that they were softened and could be welded together, indicating an incipient fusion.

A battery of thirty to forty Bunsen's elements is sufficient to melt and volatilise fine wires of lead, tin, zinc, copper, gold, silver, iron, and even platinum, with differently coloured sparks. Iron and platinum burn with a brilliant white light; lead with a purple light; the light of tin and of gold is bluish white; the light of zinc is a mixture of white and gold; finally, copper and silver give a green light. The thermal effects of the voltaic current are used in firing mines for military purposes and for blasting operations.

450. Luminous effects.—In closing a voltaic battery a spark is obtained at the point of contact, which is frequently of great brilliance. A similar spark is also perceived on breaking contact. These luminous effects are obtained when the battery is sufficiently powerful, by bringing the two electrodes very nearly in contact; a succession of bright sparks springs sometimes across the interval, which follow each other with such rapidity as to produce a continuous light. With eight or ten of Grove's elements brilliant luminous sparks are obtained by connecting one terminal of the battery with a file, and moving its point along the teeth of another file connected with the other terminal.

The most beautiful effect of the electric light is obtained when two pencils of charcoal are connected with the terminals of a powerful battery in the manner represented in fig. 361. The two charcoals being placed in contact the current passes, and their ends soon become incandescent. If they are then removed to a distance of about the tenth of an inch, according to the intensity of the current, a luminous arc extends between the two points, which has an exceedingly brilliant lustre, and is called the voltaic arc.

The length of this arc varies with the force of the current. In air it may exceed two inches with a battery of 600 elements. If the charcoal attached to the positive pole be examined, it will be found to have become hollowed, and worn away, while the negative charcoal has increased. It thus seems that the carbon is transported from the positive to the negative pole, and that this is the

manner in which the transmission of the electricity between the two poles is effected.

The intensity of the electric light is very great. Bunsen, in experimenting with forty-eight couples, and removing the charcoals