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Requmur's thermometer, and Mr. de Buffon asserts, that at equal depths the same will always be found in the waters. A late experiment of Mr. de Saufsure in the lake of Geneva contradi&s, however, this assertion; as, at certain depths to be found even in that Jake, 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, says he, “ the air of deep mines is insufferably hot. It arises from their so much greater proximity to the burning centre.” For this, however, more evident and less remote causes inay be alrigued. 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, sufficiently account for it. He pretends that the natural and proper heat of the earth is no less proved by ele&ricity. The heat and fire excited by electric concussion prove not, however, an active heat in the substance electrified, but a dormant fire brought into rapid action by the stroke. Ele&ricity is no doubt more easily excited in a body to which some degree of heat is previously communicated; but it will have its effect on all substances susceptible of it in a cold state. Common fire is also brought forth in the same manner. of steel, both exteriorly and interiorly as cold as ice, struck against a flint in the same state of coldness, draws fire from it as well as . if previously heated. The cold experienced on high mountains is alleged as proof that much the greater portion of heat proceeds
A piece from the central fire. At the height of 1500 toises from the level of the sea, 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 such could be tried, would surely 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 seas, should according to this system, notwithstanding the feebleness of the sun's
rays and its longer absences, possess a much greater interior heat than those which lie under the equator. Neither are volcanic fires or warm springs proofs of constant active heat in the bowels of the earth. These proceed from other causes, and chemical mixtures will produce the like effects where there was no previous heat or diftant communication with it. From all these observations we may, I think, conclude, that the heat of the earth is not occasioned by that of its burning centre, and consequently that Mr. de Buf. fon's idea is an unsubstantiated reverie.
Before we determine the source and causes of heat, fire, and light, it will be proper to scrutinize their natures and qualities, at least as far as their effects may lead us. Hence we may perhaps resolve whether they are only different degrees and appearances of one and the same element, or are various modifications and combinations of that with other elements. From thence we may possibly
be conducted to divine their original source. In this investigation
Fire exists not, says Mr. Wallerius, without a matter proper to receive and maintain it: that matter is the food or aliment of fire which chemists call the inflammable principle, or phlogiston. As fire is always combined with heat, it has been concluded that inflammable matter was scarcely distinct from the matter of heat, calorific matter, or, as Mr. de Luc calls it, the fiery fluid; but they are distinguishable in several circumstances.
ist. The inflammable principle, abounding in certain bodies, as in oils, spirits, &c. may hinder these from freezing, but it cannot liquefy folid bodies. Heat is so fluid and has such force as to liquefy the most solid bodies. The principle of Auidity exists in heat, and without it no fluid could exift.
2dly. The matter of heat is so subtle as to traverse the most folid bodies, leave them and be dispersed without effecting any fenfible change in the bodies heated; but inflammable matter cannot be combined with other bodies without contact and mixture, and
when intimately combined produces a considerable change in those bodies. It requires force to be again separated from them. Calcination and the reduction of metals demonstrate this to chemists. It however of itself abandons liquids in time. The spirit of sulphur loses its phlogiston, and oils, on losing their inflammable principle, become rancid.
3dly. Phlogifton unites itself with difficulty to, and separates slowly from, other bodies; whilft calorific matter, more subtle, easily penetrates the pores of other bodies, and is dissipated with equal facility. Inflamed metals cool promptly, but the inflammable matter remains. These substances are therefore really distinct.
It appears that calorific matter is of a very fluid, subtle, and active nature, and that all natural motion is produced by it; that it is elastic nce it acts expansively, and endued with a certain gravity, though imperceptible, which is proved by the successive diminution of heat; for, if it did not gravitate, it would abandon the atmosphere and rise beyond it.
By fire is understood a matter affording light and warmth, which consumes some substances by destroying the connection of their parts; but it can only attack those bodies which contain inflammable or calorific matter. Fire consists in the motion of inflammable and calorific matter ; it is always accompanied by vapours and
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fmoke. Its three degrees are crystallization, incandescence, and flame. Relatively to these three degrees it must be observed, that the heat and force of fire are not proportional to this motion and to these degrees. A heap of straw inflamed is less hot than a pile of wood which burns, but its flame is stronger. The force of fire decreases in ratio of the tenuity of parts, but light is not proportional to the force of fire. It is well known that flame is nothing, but burning smoke.
Fire exists not without light, but light may exist without fire; witness phosphoric bodies, and the splendour of the sun often. greater in a fine winter's day than in the heat of summer..
The properties of light are ::
Light wants no aliment, no inflammable matter to sustain ito In the concentration of the solar rays, light constantly enjoys the fame force without smoke or vapour.
Light exercises its force not only in the whole space of air but also in vacuo. It extends itself to the bottom of the waters, though our eyes cannot follow it so far. Marine animals would not want the
organs of sight; if light did not exist in the bottom of the waters. Light penetrates glass bodies without expanding them, and without communicating any motion to their parts. The rays