Again, it is found that the respective quantities requisite to produce a discharge between two balls, vary directly as the distances; so that, while the distances of discharge increase in the simple ratio of the quantity, the attractive forces increase as its square.

§ 371. Mr. Harris also found, that the attractive force is not influenced by the form of the unopposed portions of the acting conductors, and that it was the same, whether simple circular planes were opposed to one another, or cones by their similar plane bases. Two hemispheres also attracted one another, with the same force as spheres of the same diameter.

The attractive force between two unequal circular areas is no greater than that between two similar areas, each equal to the lesser ; and the attractive force of a mere ring and a circular area, is no greater than that between two rings of the same diameter.

$372. The general result may thus be stated:-the force of attraction is as the number of points in immediate opposition directly, and as the square of their respective distances inversely; hence the attractive force between two parallel plane circles being found, the force between any other two similar planes will be given.

The respective quantities requisite to a discharge through a given interval of air, vary in the simple ratio of the density of the air: when the density is one-half, the discharge occurs with one-half the quantity accumulated, or, which is the same thing, with one-fourth the intensity. Again, when the intensity is constant, the discharge occurs in air of one-half the density at double the distance.

The influence of heat is not in any way opposed to the restraining power of air, provided its density is not allowed to vary; but rarefaction by heat produces exactly the same effect as mechanical rarefaction.

§ 373. These results are in perfect accordance with the molecular theory of induction: for, in the case of an atmosphere rarefied to one-half, only one-half of the dielectric particles remain, and these are brought up to the discharging intensity with one-half the quantity of electricity. And with regard to distance from an inductive centre, the number of particles

which become polarized within any concentric sphere, or any section of such sphere, must increase as the square of the diameter, and the force will be lowered in the same degree.

§ 374. We have hitherto directed our attention solely 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 inductric; 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 disruption 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.

§ 375. 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 plate 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.

§ 376. Crystals of tourmaline, again, whose optical properties have been described (§ 247), 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 ninesided 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 temperature 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 (§ 124), and those crystals are alone electrical, the opposite extremities of whose axes differ with regard to the number, disposition, and figure of their facets.

§ 377. 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.

§ 378. 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. From some late experiments M. Pouillet considers 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, if confirmed, would not detract from the probability of evaporation being the principal source of atmospherical electricity; for all the water upon the face of the globe is impregnated more or less with different salts. The

vitreous electricity which 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.

Some very interesting facts have lately been added to our stock of knowledge upon these points, which originated in accidental observation, but have led to active experiment now in progress, which promises greatly to elucidate this obscure part of our subject. In a colliery near Newcastle, it happened that the boiler of a steam-engine was out of order, and steam of about 35 lb. pressure was escaping from a leak. The engine-man standing in the steam, and endeavouring to repair it, happened to touch the boiler, and drew from it a spark of about half an inch, or one inch, in length. He was greatly alarmed, and reported to the engineer that the boiler was in danger of exploding, as from there being fire on the outside he could not tell what might not be going on in the inside. The phenomenon was investigated; jars were charged, and sparks were drawn from persons placed upon insulating stools, &c. The boilers of locomotive engines have been insulated upon barrels of rosin, and have been found to be in the negative state while the steam was positive. But in this respect changes and alternations have been observed which have not yet been explained, and it is at present doubtful whether the disturbance of electric equilibrium results from the change of physical state which the water undergoes or from the friction of the effluent steam.

§379. At a very early period of electrical science the identity of the electric spark and lightning was suspected; for Dr. Wall, in 1640, in contemplating the light and crackling noise produced by the friction of a large piece of amber, was led to remark that this crackling and light may be supposed in some degree to represent thunder and lightning. But it was Franklin who, in 1747, established the close analogy of the two agents, by strict inductive reasoning, and experimentally verified his conclusions by imitating, on a small scale, the awful phenomena of nature. He had also devised the crucial experiment, of drawing electricity from the clouds, by means of an insulated pointed rod of metal, for the erection of which he only waited the completion of a church steeple in Philadelphia. But M. Dalibard, in France, preceded him by about a month in obtain

ing the actual result, by repeatedly charging a Leyden jar by means of a similar apparatus during the passage of a thunder cloud. Without any knowledge of this experiment it occurred to Franklin, whilst occupied with the subject, that by means of a common school-boy's kite he might obtain ready access to the region of the clouds. He watched the opportunity of an approaching storm and went into the fields, and, with the assistance of his son, raised the kite into the air. He confined it by two or three feet of silken ribbon, but a considerable time elapsed without any result. One very promising cloud passed over his head without producing any effect upon his simple apparatus, when, being about to give up the attempt in despair, he observed, after a sharp shower, some loose threads of the string to repel one another; on this he fastened a key to the string, and was gratified by drawing an electric spark from it. He afterwards raised an insulated metallic rod from the end of his house, so arranged as to communicate with two bells by means of a pendulum which, striking against them as they were alternately attracted and repelled, warned him of the passage of an electric cloud (93).

The experiment with the kite has often since been repeated, with the addition of a thin copper wire twisted with the string; but it is not unaccompanied with danger, for experimenters have frequently received violent shocks, and the electricity has been known to discharge itself to the ground in sparks ten feet long, and two or three inches in diameter. A fatal catastrophe from incautious experiments upon atmospheric electricity occurred to Professor Richman, of St. Petersburg, in 1753. He had erected an apparatus in the air, and was examining it with a friend, when a flash of lightning passed from the insulating rod through his body, and instant death was the consequence.

(93) The chime of bells, here represented, is suspended to the electrified body by the metallic hook, a. The two exterior bells, bb, are in metallic communication with it, and the centre one is insulated by a silk thread, but is in metallic communication with the ground, by the chain, c. The metallic clappers, c c, are suspended by silk threads, and when the exterior bells are charged they are alternately attracted and repelled, and discharge the electricity by convection to the centre conducting-bell.