276. The disruptive discharge of the voltaic battery through air is dependant upon the same principles as that of the Leyden battery; but the phenomena are modified by the lower intensity, greater quantity, and perpetual renewal of the force. When passing between two charcoal points, its duration renders it the most splendid source of light which is under the command of art. Rarefaction of the medium through which it passes, either by heat or by mechanical means, produces the same effects as upon the common electric spark from friction. When the poles of a powerful battery are gradually separated after contact, the discharge takes place through an interval which increases with the heating of the air by the ignited charcoal. With the original battery of the Royal Institution of 2000 plates, the discharge passed through four inches of air; and with the constant battery of 70 cells, the flame is much more voluminous, and extends to the distance of one inch.

It would, however, appear, that the air is not the only form of matter which is concerned in the phe

Figure 3 exhibits two glasses, connected together by a siphon of large bore, filled with an electrolyte, and two tubes inverted in the glasses and filled with the same liquid; into the upper ends of which platinum wires, terminating in long slips of the

same metal, are inserted. The opposite products of the decomposition may thus be separately obtained; the convective cur rent passing through the interposed liquid in the siphon,

coal, the latter became studded with globules of fused platinum; and when this arrangement was reversed, a considerable protuberance of carbon was formed upon the platinum platinode.

278. The colour of the light varies with the substances between which the discharge passes. The beautiful effects produced by the deflagration of thin metallic leaves interposed in the course of the discharge are not owing to the combustion of the metals, though in some cases increased by this cause, but arise from a dispersion of their particles analogous to that of the more momentary explosion of the Leyden battery. They equally, in both cases, take place in vacuo.

Gold leaf emits a white light tinged with blue; silver, a beautiful emerald green light; copper, a bluish white light with red sparks; lead, a purple; and zinc, a brilliant white light fringed with red.

The disruptive discharge of the voltaic battery will take place with great brilliancy under the surface of distilled water; some electrolytic effect will at the same time occur, but the greater part of the charge will pass in a brilliant stream of light.

THERMO-ELECTRICITY.

279. We have seen that when the electric current meets with obstruction to its course, the equilibrium of heat is disturbed even in good conductors; it might, therefore, have been a priori expected that a disturbance in the equal flow of heat would produce an electric current, in such forms of matter as are capable of transmitting it.

We are indebted to Professor Seebeck, of Berlin, for the experimental confirmation of this conclusion in 1832, and the discovery of the phenomena of

THERMO-ELECTRICITY.

§ 280. If a platinum wire be carefully soldered to the two extremities of a delicate galvanometer, and it be heated at any point remote from the junctions,

no disturbance of the electric equilibrium will be produced; from the homogeneous structure of the wire, the heat will flow equally to the right and left of the heated point. But it will be very different if a knot or spiral turn be made in the wire without breaking it for if the focus of heat be applied to the right of such obstruction, an electrical current will be established from right to left, and will be indicated by the needle. This must arise from the unequal rate at which the heat will obviously be propagated on two sides of the obstructing mass. Wires of copper and silver will act in the same way, only in a very inferior degree. The same effect will be produced if the wire, instead of being continuous, be divided, and each end being twisted into a spiral to increase the surfaces, one be heated red in a spirit lamp, and brought into contact with the other. The deviation of the magnetic needle will indicate, as before, that a current is passing from the hotter to the colder point. That these effects do not depend upon any chymical action of the air, is proved by performing the experiment under the surface of well-purified oil, under which circumstances the same results will be obtained.

Those metals which possess a decidedly crystalline texture, present even more marked electrical phenomena from the unequal propagation of heat in their masses. If a ring or rectangle be cast of antimony or bismuth, of the diameter of three or four inches, and of the substance of about the third of an inch, and one half of its surface be kept cool by ice and the other heated, a current of electricity will be immediately established of sufficient power to affect the magnetic needle without the assistance of a coil.

281. These thermo-electric effects, again, are very much increased by combinations of two metals. If a bar of antimony have a copper wire soldered to it, or merely twisted round one of its ends,

and attached to the other in the form of a loop, when heated at the contact of the metals at one extremity, it will strongly deflect a magnetic needle placed above or below it. (65)

It is found that similar circuits may be formed of combinations of other metals, and that they may be ranged in the following order according to their thermo-electric efficiency; the most powerful combination being formed by those metals which are the most distant from each other in the series: bismuth, platinum, lead, tin, copper, or silver, zinc, iron, antimony. When heated together, the current is found to proceed from those which stand first to the last.

§ 282. There can be little doubt that the specific heat and conducting power of the metals must be concerned in these effects; but, in comparing the tables of each, the connexion does not immediately appear. Structure or crystalline arrangement has also much influence upon them. With some combinations, as, for example, zinc and silver, the current will go on increasing with the temperature to a certain point, 2480; will then become null, and, by increasing the heat, will be re-established in the contrary direction. This singular phenomenon is most probably referrible to a change in the arrangement of the particles of the zinc.

283. Similar circuits may also be formed, according to the experiments of M. Nobili, with substances whose conducting power is lower than that of the metals. He made small cylinders of porcelain clay, of the length of three or four inches, and three or four lines in diameter, and wrapped round

(65) a represents a bar of antimony with a piece of copper wire twisted round one end of it, b, and looped over the other end, c. When heated by the flame of a spirit-lamp at the contact of the metCals, b, a magnetic needle placed at d will be deflected.