to thunder-storms, upon the mastheads of vessels, the points of soldiers' spears, and even upon the outstretched fingers of the hand. Upon the high table-land of Mexico, where the atmosphere at times is extremely dry, the horses' manes become luminous and emit a crackling noise. These lights used formerly to be regarded with feelings of dread and superstition, and went in Italy by the name of the Fire of St. Elmo.

$245. The atmosphere, though not usually charged to the excess which is necessary to produce the phenomena to which we have just adverted, will commonly afford indications of electrical excitement. In calm, dry weather, when no clouds are visible, the gold leaves of an electrometer, armed with a pointed rod or flame, will always indicate vitreous electricity; and it has been ascertained that its intensity is subject to regular variations, reaching a maxinium about seven or eight o'clock in the morning, and falling to a minimum between one and two. In high winds and damp weather, without rain, electrical indications can rarely be obtained; and in cloudy weather, and in times of showers of hail, rain, or snow, they vary very much both as to kind and intensity.

246. We have hitherto directed our attention principally to one method of exciting the electrical forces, namely, the friction of dissimilar substances, by which their particles are rapidly brought into close contact and as rapidly separated. By this operation we have found particularly that the rubbed surface of a dielectric becomes inductive; its charge being sustained both through its own substance and through the air to surrounding conductors. But there are many other methods by which this extraordinary agent may be developed; and, indeed, the forcible disturbance of the established equilibrium of the particles of bodies in any way seems sufficient to call it forth in various degrees. Thus the forcible dis

ruption of cohesion; mere pressure upon certain crystallized substances; the heating of others; changes of physical state; crystallization and evaporation, are all capable of producing electrical excitement.

247. If we break a roll of sulphur, we shall find a charge of electricity upon its two fresh surfaces; and if we pound it in a dry mortar, and pour the fragments upon the glass of an electrometer, the leaves will diverge very forcibly; and if we renew the contact with fresh surfaces upon a fresh plate, we shall find that it is not easy to deprive it of the whole quantity which it has thus acquired.

If we take a rhombohedron of Iceland spar, and, holding it by two opposite edges, press upon two of its opposite faces, it will manifest a decided power of attraction upon light substances.

248. Crystals of tourmaline, again, whose optical properties will be described (§ 392), exhibit a very remarkable state of electrical excitement when gently heated. The tourmaline is a hard, crystallized mineral, which occurs in granite and other primitive rocks, in the form of three, six, or nine sided prisms, terminated by three or six sided pyramids. It was discovered by the Dutch in Ceylon, who called it Aschen trikker, from its property of attracting ashes when thrown into the fire. It appears, however, to have been known to Theophrastus. When the stone is of considerable size and warmed, flashes of light may be seen to dart across its surface when laid upon a hot iron. If a crystal of this mineral be mounted upon a pivot, or otherwise suspended with freedom of motion, its excitement will be found to be polar, and one end will be attracted by excited glass and the other repelled. The polar arrangement of its particles exists throughout its substance; for, when broken in two, each half will prove to be likewise polar. It is during the rise of its tempera ture that these phenomena take place: during the

process of cooling they also occur, but with the opposite direction of the forces.

The poles of the mineral have reference to the axis of symmetry, and those crystals are alone electrical, the opposite extremity of whose axes differ with regard to the number, disposition, and figure of their facets.

§ 349. Boracite is another mineral which possesses the property of becoming electric by heat in a high degree. It crystallizes in the form of a cube, but the edges and angles are generally replaced by secondary planes, and four of the angles are always observed to present a greater number of facets than the other four: the most complex angles are rendered vitreous, and the simplest resinous, by heat, and these are always found at the opposite extremities of the axis of symmetry.

250. Electricity may also be developed by the near contact of heterogeneous substances. Thus, if a circular disk of platinum be fitted with an insulating handle, and pressed against a similar disk of zinc, both being held by the insulating rods, both will be electrified, as may be rendered manifest by the condenser and gold-leaf electrometer. The zinc will have vitreous, and the platinum resinous electricity. Silver or copper may be used instead of platinum, and like effects, although in a less degree, will be apparent.

§ 251. If melted sulphur be poured into a glass vessel, it becomes electrical in the process of crystallization; and if it be removed from the glass and examined after solidification, it will be found in the resinous state, and the glass in the corresponding vitreous state. Water, also, in the act of freezing, becomes electrical.

The evaporation of water likewise excites electricity; and if a heated platinum vessel be placed upon the cap of a gold-leaf electrometer, and water dropped into it as the steam flies off, the leaves will

expand with resinous electricity. The effect is rendered very decided with the assistance of the condenser. The late experiments of M. Pouillet have rendered it probable that the evaporation of perfectly pure water is not accompanied by any development of the electrical forces, but that a very minute portion of saline matter in solution is sufficient to determine the effect. This does not detract from the probability of evaporation, being the principal source of atmospheric electricity; for all the water upon the face of the globe is impregnated more or less with different salts. The vitreous electricity that corresponds to the resinous charge, which, in the experiment, is left upon the electrometer, is carried into the air, and probably communicates a charge to the minute drops into which it is again condensed, and which float in the atmosphere.

GALVANIC ELECTRICITY.

252. The phenomenon we have just cited is made the basis of a method of developing a modification of electricity, far more important in its action and in its connexion with other branches of physical science than that which is excited by friction.

The history of the origin of this most important branch of science must, almost necessarily, be included in their description. In the year 1790, Professor Galvani, of Bologna, was accidentally occupied with the dissection of a frog, at the time when some other person was experimenting with an electrical machine in its neighbourhood, and observed that, whenever the point of his scalpel was in contact with one of the crural nerves, and a spark was drawn from the machine, violent convulsions were occasioned in the limbs.

We now know that, under these circumstances, the frog formed part of a system of bodies under induction, by the polarization of the particles of which, the charge was sustained upon the prime

conductor; when the latter was discharged, their state of tension was relieved, and, in returning to their former state, commotion was produced similar to that which takes place with more violence when an animal is placed in the course of the discharge of a Leyden jar. In seeking to vary the circumstances of the experiment, the explanation of which was unknown to him, Galvani armed the muscles and the nerves with different metals, and found that, whenever a metallic communication was made between the two, similar convulsive movements were produced without the co-operation of the electrical machine.(53) The new branch of science which sprang from this capital observation has been called GALVANISM, in honour of the mind which discerned the importance of phenomena of such apparently trifling import.

253. There are many ways now known of varying the experiment. By placing a live flounder upon

a slip of zinc, with a shilling upon b its back, whenever a metallic

communication is made between a the two metals by a metallic wire, strong muscular contractions are produced in the fish. If a piece of silver be placed upon a person's tongue, and a piece of zinc

(53) This figure represents the legs of a frog prepared for galvanic experiments. The skin is removed, and the crural nerves, a a, are easily found by gently separating the muscles on the back of the thighs: b is a silver wire passed under both the nerves, by which metallic contact is ensured. The legs may be laid upon a plate of zinc; and, when the circuit is completed by a metallic communication between the two metals, violent convulsions are produced.