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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.
ELECTRICITY OF THUNDER AND LIGHTNING.
General history of this curious and interesting discovery. Tuere 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 phænoniena.
Among the Greek philosophers the sources of thunder and lightning were separated from each other. The former was as. cribed 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, bitu. minous 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,
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 pro. ceeded 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 disco. veries, 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 thas 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 exhi. biting any visible marks of 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 electric 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
proposed a method, which he soon accomplished, of securing build. ings from being damaged by lightning, by means of conductors.
Nor had the English philosophers been inattentive to this sub. ject : but, for want of proper opportunities of trying the necessary experiments, and from some other unfavorable circumstances, they had hitherto failed of success. Mr. Canton, however, succeeded in July 1752; and in the following month, Dr. Bavis and Mr. Wil.
son observed nearly the same appearances as Mr. Canton had done . before. By a number of experiments Mr. Canton also soon after
observed, that some clouds were in a positive, while some were in a negative state of electricity; and that the electricity of his conductor would sometimes change, from one state to the other, five or six times in less than half an hour.
But Sig. Beccaria discovered this variable state of thunder. clouds, before he knew that it had been observed by Dr. Franklin or any other person; and he has given a very exact and particular account of the external appearances of these clouds. From the observations of his apparatus within doors, and of the lightning abroad, he inferred that the quantity of electric matter in a com. mon thunder-storm, is inconceivably great, considering how many pointed bodies, as spires, trees, &c. are continually drawing it off, and what a prodigious quantity is repeatedly discharged to or from the earth. This matter is in such abundance, that he thinks it impossible for any cloud or number of clouds to contain it all, so as either to receive or discharge it. He observes also, that during the progress and increase of the storm, though the lightning fre. quently struck to the earth, the same clouds were the next moment ready to make a still greater discharge, and his apparatus conti. nued to be as much affected as ever; so that the clouds must have received at one part, in the same moment when a discharge was made from them in another. And from the whole he concludes, that the clouds serve as conductors to convey the electric fluid from those parts of the earth that are overloaded with it, to those that are exhausted of it. The same cause by which a cloud is first raised, from vapours dispersed in the atmosphere, draws to it those that are already formed, and still continues to form new ones, till the whole collected mass extends so far as to reach a part of the earth where there is a deficiency of the electric fluid, and where the electric matter will discharge itself on the earth. A
channel of communication being thus produced, a fresh supply of electric matter is raised from the overloaded part, which continues to be conveyed by the medium of the clouds, till the equilibrium of the fluid is restored between the two places of the earth. Beccaria observes further, that a wind always blows from the place from which the thunder-cloud proceeds; and that the sudden accumulation of such a prodigious quantity of vapours must displace the air, and repel it on all sides. Indeed, many observations of the descent of lightning, confirm his theory of the manner of its ascent; for it often throws before it the parts of conducting bodies, and distributes them along the resisting medium, through which it must force its passage ; and upon this principle, the longest flashes of lightning seem to be made, by forcing into its way part of the vapours in the air. One of the chief reasons why these Aashes make so long a rumbling is, that their production by the vast length of a vacuum made by the passage of the electric matter : for although the air collapses the moment after it has passed, and the vibration, on which the sound depends, com. mences at the same moment; yet when the flash is directed toward the person who hears the report, the vibrations excited at the nearer end of the track, will reach his ear much sooner than those from the more remote end; and the sound will, without any echo er repercussion, continue till all the vibrations have successively reached him.
How it happens that particular parts of the earth, or the clouds, come into the opposite states of positive and negative electricity, is a question not absolutely determined : though it is easy to conceive that when particular clouds, or different parts of the earth, possess opposite electricities, a discharge will take place within a certain distance ; or the one will strike into the other, and that in the dis. charge a flash of lightning will be seen. Mr. Canton queries whether the clouds do not become possessed of electricity by the gradual heating and cooling of the air ; and whether air suddenly rarefied may not give electric fire to clouds and vapours passing through it, and air suddenly condensed receive electric fire from them.-Mr. Wilcke supposes, that the air contracts its electricity in the same manner that sulphur and other substances do, when they are heated and cooled in contact with various bodies. Thus, the air being heated or cooled near the earth, gives electricity to the earth, or