cupied the attention of philosophers ever since Dr. Franklin demon. strated that thunder is occasioned by the agency of electricity. 1. The most complete set of observations on the electricity of the atmosphere were made by Professor Beccaria of Turin. He found the air almost always positively electrical, especially in the day time and in dry weather. When dark or wet weather clears up, the electricity is always negative. Low thick fogs rising into dry air carry up a great deal of electric matter. 2. In the morning, when the hygrometer indicates dryness equal to that of the preceding day, positive electricity obtains even before. sunrise. As the sun gets up, this electricity increases more re. markably if the dryness increases. It diminishes in the evening. 3. The mid-day electricity of days equally dry is proportional to the heat. 4. Winds always lessen the electricity of a clear day, especially if damp. 5. For the most part, when there is a clear sky and little wind, a considerable electricity arises after sunset at dew falling. 6. Considerable light has been thrown upon the sources of atmo spherical electricity by the experiments of Saussure and other phi. losophers. Air is not only electrified by friction like other electric bodies, but the state of its electricity is changed by various chemical operations which often go on in the atmosphere. Evaporation seems in all cases to convey electric matter into the atmosphere; and Saussure has ascertained, that the quantity of electricity is as much increased when water is decomposed, as when water is dropt on a red hot iron. On the other hand, when steam is condensed into vesicular vapour, or into water, the air becomes negatively electric. Hence it would seem that electricity enters as a component part into water; that it separates when water is decomposed or expanded into steam, and is reunited when the steam is condensed again into water. Farther, Mr. Canton has ascertained that dry air, when heated, becomes negatively electric, and positive when cooled, even when it is not permitted to expand or contract: and the expansion and contraction of air also occasion changes in its electric state. Thus there are four sources of atmospheric electricity known: 1. Friction; 2. Evaporation; 3. Heat and cold; 4. Expansion and contraction: not to mention the electricity evolved by the melting, freezing, solution, &c. of various bodies in contact with air. 7. As air is an electric, the matter of electricity, when accumulated in any particular strata, will not immediately make its way to the neighbouring strata, but will induce in them changes similar to what is induced upon plates of glass or similar bodies piled upon each other. Therefore if a stratum of air be electrified positively, the stratum immediately above it will be negative, the stratum above that positive, and so on. Suppose now that an imperfect conductor were to come into contact with each of these strata, we know, from the principles of electricity, that the equilibrium would be restored, and that this would be attended with a loud noise, and with a flash of light. Clouds which consist of vesicular vapours mixed with particles of air are imperfect conductors; if a cloud therefore come into contact with two such strata, a thunder-clap would follow. If a positive stratum be situated near the earth, the intervention of a cloud will, by serving as a stepping-stone, bring the stratum within the striking distance, and a thunder-clap will be heard while the electrical fluid is discharging itself into the earth. If the stratum be negative, the contrary effects will take place. It does not appear, that thunder is often occasioned by a discharge of electric matter from the earth into the atmosphere. The accidents, most of them at least, which were formerly ascribed to this cause, are now much more satisfactorily accounted for by Lord Stanhope's Theory of the Returning Stroke. Neither does it appear that electricity is often discharged into the earth, as the effects of few thunder-storms are visible upon the earth; that it is so sometimes, however, is certain. In examining and detailing this curious phænomena, there is one remark entitled to particular attention: it is this, that, during every discharge of electricity, whether natural or arti. ficial, through air, some change similar to combustion undoubt. edly takes place. The light and the peculiar smell with which all electrical discharges are accompanied demonstrate this; for no light is perceptible when electricity is discharged in a va cuum. What the change is which electricity produces in air, or how it produces it, are questions which, in the present state of our knowledge, are altogether insoluble. But the very extraordi nary galvanic phænomena which at present occupy the attention of philosophers promise not only to throw light upon this important subject, but to demonstrate a much closer connection between chemistry and electricity than has hitherto been suspected. [Thomson. CHAP. XLIII. ELECTRICITY OF THUNDER AND LIGHTNING, SECTION 1. General history of this curious and interesting discovery. THERE is no subject in natural history that has more attracted or more deserved to attract the attention of philosophers, than the cause of Thunder and Lightning. The magnificence, power and splendour of these combined meteors-the tremendous sound-the brilliant and rapid corruscation, and the awful effects produced without any visible instrumentality, have all concurred in fixing the mind in all ages upon these stupendous phænomena. Among the Greek philosophers the sources of thunder and lightning were separated from each other. The former was ascribed to a variety of causes, of which Lucretius enumerates not less than ten, the principal of them being the shock of clouds against clouds, and of winds against winds, meeting together from adverse points. The latter was attributed to an accumulation of pure etherial particles of elementary fire, of exquisite minuteness, concentrated in the cloud or clouds, whence the thunder-storm issued, and there creating a gass or vapour of a peculiar and indi. vidual quality*, an idea strongly congruous with the discoveries of modern times. It was long afterwards supposed, that both phænomena had one common origin, and proceeded from sulphureous, nitrous, bituminous or other inflammable vapours ascending from the bowels of * See Good's Lucretius and Notes, book vi. v. 98, and following. the earth, and fermenting together in the atmosphere. Thomson, that he might be sure of being right, enlists the whole of these into service; first describing the ascent of inflammable substances into the atmosphere; and then telling us that they there gradually Ferment; till by the touch etherial rous'd, SUMMER, 1103. Within the last half century, however, philosophers have confined themselves to the etherial touch alone; to the accumulation of particles of elementary fire of a peculiar quality, to which alone the school of Epicurus ascribed the origin of lightning, and they have abundantly succeeded in proving, that this peculiar quality is that of the electric fluid; or, in other words, that thunder and lightning are altogether electrical phænomena. The philosophers of the middle of the last century had not proceeded far in their experiments and enquiries on this subject, before they perceived the obvious analogy between lightning and electricity, and they produced many arguments to evince their identity. But the method of proving this hypothesis beyond a doubt, was first proposed by Dr. Franklin, who, about the close - of the year 1749, conceived the practicability of drawing lightning down from the clouds. Various circumstances of resemblance between lightning and electricity were remarked by this ingenious philosopher, and have been abundantly confirmed by later discoveries, such as the following: flashes of lightning are usually seen crooked and waving in the air; so the electric spark drawn from an irregular body at some distance, and when it is drawn by an irregular body, or through a space in which the best conductors are disposed in an irregular manner, always exhibits the same appearance; lightning strikes the highest and most pointed objects in its course, in preference to others, as hills, trees, spires, masts of ships, &c.; so all pointed conductors receive and throw off the electric fluid more readily than those that are terminated by flat surfaces: lightning is observed to take and follow the readiest and best conductor; and the same is the case with electricity in the discharge of the Leyden phial; whence the doctor infers, that in a thunder-storm, it would be safer to have one's clothes wet than dry: lightning burns, dissolves metals, rends some bodies, some. times strikes persons blind, destroys animal life, deprives magnets of their virtue, or reverses their poles; and all these are well. known properties of electricity. But lightning also gives polarity to the magnetic needle, as well as to all bodies that have any thing of iron in them, as bricks, &c. and by observing afterwards which way the magnetic poles of these bodies lie, it may thence be known in what direction the stroke passed. Persons are sometimes killed by lightning without exhibiting any visible marks o injury; and in this case Signior Beccaria supposed that the lightning does not really touch them, but only produces a sudden vacuum near them, and the air rushing violently out of their lungs to supply it, they cannot recover their breath again and in proof of this opinion he alleges, that the lungs of such persons are found flaccid; whereas these are found inflated when, the persons are really killed by the electric shock; his hypothesis, however, was controverted by Dr. Priestley and a better theory, that of the returning stroke, has since been introduced by Lord Stanhope to explain the phænomenon. To demonstrate, by actual experiment, the identity of the elec tric fluid with the matter of lightning, Dr. Franklin contrived to bring lightning from the heavens, by means of a paper kite, properly fitted up for the purpose, with a long fine wire string, hence called an electrical kite, which he raised when a thunder-storm was perceived to be coming on: and with the electricity thus obtained, he charged phials, kindled spirits, and performed all other such electrical experiments as are usually exhibited by an excited glass globe or cylinder. This happened in June 1752, a month after the electricians in France, in pursuance of the method which he had before proposed, had verified the same theory, but without any knowledge of what they had done. The most active of these were Messrs. Dalibard and Delor, followed by M. Mazeas and M. Monnier. In April and June 1753, Dr. Franklin discovered that the air is sometimes electrified negatively, as well as sometimes positively : and he even found that the clouds would change from positive to negative electricity several times in the course of one thunder-gust. This curious and important discovery he soon perceived was capa ble of being applied to practical use in life, and in consequence he |