very clearly explained, though not difficult to be either given or comprehended.

By far the greatest part of the light we possess is derived from the sun: but there are also numerous kinds of bodies that possess it in a latent or dormant state, and readily give it forth upon being properly excited, or under a proper elevation of temperature; and others that give it forth spontaneously, in the common temperament of the atmosphere. Of the former division are oils, bitumens, vinous spirits, carbon, phosphorus, and hydrogen gas. Of the latter various vegetable and animal substances, on the commencement of putrefaction, which consists in a slow decomposition not wholly unlike combustion; touch.wood, which is wood in a peculiar state of decay; various species of boletus or agaric, and especially that which is denominated spunk; and a multiplicity of worms and insects in a state of perfect health, as pyrosmas, phloades, fire-balls, glow-worms, and elaters*.

of

It was at one period, as we shall find in the ensuing section, a very general opinion among the learned, that the luminous appear. ances we refer to, were in every instance produced by phosphorescent insects and worms; but the situations and periods of the year in which they are perhaps most frequently met with, together with a variety of other circumstances, strongly militate against many such an idea; and sufficiently prove, that the source of these meteors is to be sought for in the light exhaled by the decomposition of animal or vegetable materials, magnified, and deepened in hue by the haze or vapoury atmosphere of the moist and swampy low lands in which they are chiefly beheld; and which, in consequence of their moisture and swampiness, are particularly favourable to the process of decomposition.

We may thus account for many of them, and particularly for those that evince no sensible heat during their illumination; for the light exhaled or thrown off from the substances we now allude to, possesses no sensible heat whatever.

It not unfrequently happens, however, that a greater or less degree of heat, a proof of actual though slow combustion, has been evinced during the existence of these phænomena; as also that they have extended more widely than any local decomposition

* See for a fuller account of these curious facts, Books II. and III. of the present work.

would induce us to expect, and have even appeared to change their situation, and to dance about from place to place.

To explain meteors of this kind it is only necessary to observe, that the earth is perpetually exhaling a variety of inflammable gases and other materials, as hydrogen gas or inflammable air, phosphorus, carbonic acid gas, and occasionally sulphurous va. pour, sometimes separately, and sometimes in a state of union: and that the most active of these, are particularly evaporating in the low stagnant marsh grounds where these phænomena chiefly make their appearance; and may at any time be collected with the greatest ease, by placing over the surface of the soil an inverted wine-glass or tumbler. Now although these gases will not inflame spontaneously, in the ordinary temperature of the atmosphere, they readily inflame from a great variety of natural causes to which they are perpetually exposed; and hence, in effect, those nume. rous fire-damps, in coal-mines, and other caverns of which we have given a few tremendous examples in a former part of this work. Electricity may be a common cause of such inflammation; the heat generated during the decomposition of the animal or vege. table materials that may be locally decomposing, may be far more than sufficient for this purpose; for we know it to be sufficient to set hay-stacks on fire, when the grass has been put together in a state sufficiently damp to favour such decomposition. And it is not improbable, that some of these materials may catch the illumi. nation, as from a candle, from a body in the immediate vicinity that is in the act of spontaneous illumination.

Now the ball or general mass of inflammable vapour being once lighted or inflamed, from whatever cause, will continue to burn as long as its inflammable principle continues without being destroyed; and its combustible power may be more or less in proportion to its purity or state of concentration; whence in some instances it may pour forth light with little or no sensible heat; in others, the heat combined with it may be sufficient to produce slow combustion like that of a dung-hill; and in others palpable and rapid flame. From the levity of the illumined or burning vapour, moreover, it must necessarily change its place in various instances according to the current of air which it either finds, or by burning makes for itself: and hence it must appear to move in various directions, upwards and downwards, to the right and to the left, from object to object,

in a constant dance before the spectator, according to the motion, because that operates upon it: while its colours and dimensions must vary according to the varying density of the fog, or haze, through which, in different places or situations, it is seen, or according to its actually increasing or diminishing and decaying bulk hence, in our opinion, the usual origin of this class of singular meteors.

We hazard the opinion, however, with much diffidence, and shall leave it to our readers to adopt, or reject, as its merit may respectively strike them: and shall only further observe, before we proceed to offer a few examples of them, that while in all countries they are for the most part to be met in wet swampy lowlands, and stagnant marshes or morasses; they more usually occur, and with greater lustre, in hot climates, upon the approach of winter, when the sky, after wearing a fiery brightness, begins to be overcast, and the whole horizon to be wrapped as in a muddy cloud. Mists and vapours, says an intelligent writer, at this time continue to rise with peculiar density throughout all the regions under the line: the air which so lately before was clear and elastic, now becomes humid, obscure, and stifling: the fogs become so thick, that the light of the sun seems in a manner excluded; nor would its presence be known, but for the intense and suffocating heat of its beams, which dart through the gloom, and, instead of dissipating, only serve to increase the mist. After this preparation, there follows an almost continual succession of thun der, rain, and tempests. During this dreadful season, the streets of cities flow like rivers, and the whole country wears the appearance of an ocean. The inhabitants often make use of this oppor tunity to lay in a stock of fresh water, for the rest of the year; as the same cause which pours down the deluge at one season, denies the kindly shower at another. The thunder which attends the fall of these rains, is much more terrible than that we are generally acquainted with. Among ourselves, the flash is seen at some distance, and the noise shortly after ensues; our thunder generally rolls on one quarter of the sky, and one stroke pursues another. But here the whole sky appears abruptly illuminated with unremitted flashes of lightning; every part of the air seems productive of its own thunders; and every cloud produces its own shock. The strokes come so thick, that the inhabitants can scarce