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ON THE

STRUCTURE

OF THIS GLOBE.

LETTER V.

Various Opinions on the Nature of Light, Heat, and Fire.

SIR,

AFTER having in my preceding letter laid before you the celebrated fyftem of Mr. de Buffon, and the several opinions of many other modern philofophers tending with him to prove not only the high antiquity of this globe, but the numberlefs ages required to elaborate and confolidate its component parts, and to fashion its actual ftructure; and having given you with freedom my reafons for diffenting from their pretended irrefragable proofs of this thefis; you will give me leave, previous to any further difcuffion on that subject, to fay, with extreme diffidence, a few words on the nature of Light, Heat, and Fire. On this head natural philosophy, as yet in its infancy, is very undecided, and has produced a num

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ber of difcordant opinions. It is, however, a point which appears to me of great importance, and by no means indifferent for the settling of our ideas, both on the original and actual formation of this earth and on the continued operations of nature. On this difficult fubject, I fhall rather produce the different opinions of others than presume to offer any fixed fentiment of my own. Heat, Fire, and Light, have till very lately been generally looked upon as one and the fame matter, proceeding from one and the fame cause, distinct only in degree-fire generated from heat, and light from fire. All three, however, appear in many inftances perfectly distinct, and discriminated by various and frequently oppofite effects. The fource of all three has been generally placed in the fun, or at leaft that body has been looked upon as their great magazine; but on this too there are different opinions.

It is on the basis of fire first exifting in the fun and purloined from thence by other bodies that Mr. de Buffon has laid his system. Having once kindled and fixed it there as in its feat, he imagines whatever relics remain of it in our planet to be no other than a flackened portion thence originally derived. As we have already feen, he confiders not heat, fire, and light, deemed only different degrees from one and the same cause, as a distinct element from his fole vitreous fubftance. They are, according to him, purely generated from, and the effects of, preffure and friction. By their operation, without the intervention of any other diftin&t matter, the fun

was

was put and remains in fufion, and became a liquid mass of glass producing heat, fire, and light. The poffibility of these emanating from his vitreous matter by mere preffure and friction we have already noticed, and left to his more fagacious difciples to explain. Had he allowed fire to be a diftin&t though yet inactive menftruum, the agency of preffure and friction might have given motion to a more fubtle interspersed element, and have communicated it to a naturally inert and folid mass immersed therein; but without fuch intervention we cannot conceive preffure to have any other effect but that of confolidating. We have feen that this author imagines the earth and other planets to be portions of this liquid fiery mass, which, whilft their parent fun continues in full vigour of fufion, have darkened and cooled, and are ftill cooling by degrees. The centre of our globe is still burning, and he asserts that the greatest part of the heat yet experienced on its furface proceeds from thence, and not, as was generally fuppofed, from the fun. Adopting the calculations of Mr. de Mairan, he determines this heat to be at present 31 times greater than that communicated by the fun; but as the centre cools it will gradually diminish, till at length the whole furface of the earth will become frozen and incapable of preserving animal or vegetable life. Mr. de Mairan's calculations are founded on a scale of degrees between extreme cold and extreme heat, applied to the unvaried temperature of the caves of the obfervatory at Paris, much greater than that of the air in times of frost. The heat of their atmosphere is conftantly at 10 degrees of de

Reaumur's

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Reaumur's thermometer, and Mr. de Buffon afferts, that at equal
depths the fame will always be found in the waters.
A late experi-
ment of Mr. de Sauffure in the lake of Geneva contradicts, how-
ever, this affeṛtion; as, at certain depths to be found even in that
lake, the heat was diminished to four degrees. In Mr. de Buffon's
system, heat must increase as we descend deeper into the bowels of
the earth, and accordingly, fays he, " the air of deep mines is infuf-
ferably hot. It arises from their so much greater proximity to the
burning centre." For this, however, more evident and lefs remote
causes may be affigned. The removing of earth, the breaking
of ftones exciting latent heat and fires where phlogistic matter
abounds, the confined perspiration and breath of the miners, and
the heat of the lights by which they work, fufficiently account for
it. He pretends that the natural and proper heat of the earth is
no less proved by electricity. The heat and fire excited by electric
concuffion prove not, however, an active heat in the substance
electrified, but a dormant fire brought into rapid action by the
ftroke. Electricity is no doubt more eafily excited in a body to
which fome degree of heat is previously communicated; but it will
have its effect on all fubftances fufceptible of it in a cold ftate.
Common fire is also brought forth in the fame manner.
A piece
of fteel, both exteriorly and interiorly as cold as ice, ftruck against
a flint in the fame state of coldnefs, draws fire from it as well as
if previously heated. The cold experienced on high mountains is
much the greater portion of heat proceeds

alleged as proof that

from

from the central fire. At the height of 1500 toifes from the level of the fea, the heat is diminished from 20 to 30 degrees. If this diminution proceeds from that further distance from the centre, the waters at equal depths below that level, if the temperature of any fuch could be tried, would furely be boiling, and those which lay very deep would have at least their proportionable degree of increased heat. The earth and waters under the flattened poles, much nearer to the centre than equatorial lands or feas, should according to this system, notwithstanding the feebleness of the fun's rays and its longer abfences, poffefs a much greater interior heat than those which lie under the equator. Neither are volcanic fires or warm fprings proofs of conftant active heat in the bowels of the earth. These proceed from other caufes, and chemical mixtures will produce the like effects where there was no previous heat or From all these obfervations we diftant communication with it. may, I think, conclude, that the heat of the earth is not occafioned by that of its burning centre, and confequently that Mr. de Buffon's idea is an unfubftantiated reverie.

Before we determine the fource and caufes of heat, fire, and light, it will be proper to scrutinize their natures and qualities, at lead us. Hence we may perhaps leaft as far as their effects may lead us. refolve whether they are only different degrees and appearances of one and the fame element, or are various modifications and combinations of that with other elements. From thence we may poffibly

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