It is true, that it is not by the solar rays that subterraneous heat is produced; but still, from this instance, we see, that there is no incongruity in supposing the production of heat to be independent of combustible bodies, and of vital air. We are indeed, in all cases, strangers to the origin of heat; philosophers dispute, at this moment, concerning the source of that which is produced by burning; and much more are they at a loss to determine, what upholds the light and heat of the great luminary, which animates all nature by its influence. If we would form any opinion on this object, we shall do well to attend to the suggestions of that great philosopher, who was hardly less from others by his doubts and conjectures, than by his most rigorous and profound investigations. "May not great, dense, and fixed bodies, when heated beyond a certain degree, emit light so copiously, as, by the emission and reaction of its light, and the reflections and refractions of its rays within its pores, to grow still hotter, till it comes to a certain period of heat, such as is that of the sun? And, are not the sun and fixed stars great earths, vehemently hot, whose heat is conserved by the greatness of the bodies, and the mutual action and reaction between them and the light which they emit?"[76]
[76] Newton's Optics, ubi supra.
168. Some recent experiments, seem to make the suggestions in this query applicable to an opaque body like the earth, as well as to luminous bodies, such as the sun and fixed stars. The radiation of heat, where there is no light, was first rendered probable by the experiments of M. Pictet of Geneva;[77] and the only objections to which the conclusions from those experiments seemed liable, are removed by the late very important discoveries of Dr Herschel.[78] From these it appears, that heat is capable of refraction and reflection, as well as light, so that it is not absurd to suppose, that the heat of great, dense, and faced bodies, may be conserved by the greatness of the bodies, and the mutual action and reaction between them and the heat which they emit.
[77] Essai sur le Feu.
[78] Phil. Trans. 1800, p. 84.
The existence of subterraneous heat is still further rendered probable from the researches of Mairan, which tend to show, that there is another source of terrestrial heat besides the influence of the solar rays.[79]
[79] Mém. de l'Acad. des Sciences, 1765, p. 143.
Whatever be the truth with regard to these conjectures, it is certain, that the first and original source of heat is independent of burning. Burning is an effect of the concentration of heat; and though, by a certain reaction, it has the power of continuing and augmenting that heat, it never can be regarded as its primary and material cause. When, therefore, we suppose a source of heat, independent of fire and of burning, we suppose what certainly exists in nature, though we are not informed of the manner of its existence, nor of its place, otherwise than from considering the phenomena of the mineral kingdom.
169. Lastly, we are not entitled, according to any rules of philosophical investigation, to reject a principle, to which we are fairly led by an induction from facts, merely because we cannot give a satisfactory explanation of it. It would be a very unsound view of physical science, which would induce one to deny the principle of gravitation, though he cannot explain it, or even though the admission of it reduces him to great metaphysical difficulties. If indeed a downright absurdity, or inconsistency with known and established facts, be involved in any principle, it ought not to be admitted, however it may seem calculated to explain other appearances. If, for instance, Dr Hutton held, that combustion was carried on in a region where there was no vital air, we should have said, that he admitted an absurdity, and that a theory founded on such postulata was worse than chimerical. But, if the only thing imputable to him is, that, being led by induction to admit the fusion of mineral substances in the bowels of the earth, he has assumed the existence of such heat as was sufficient for this fusion, though he is unable to assign the cause of it, I believe it will be found, that his system only shares in an imperfection, which is common to all physical theories, and which the utmost improvement of science will never completely remove.
170. Thus, then, we are led, it must be allowed, into the region of hypothesis and conjecture, but by no means into that of chimeras. Indeed, the reproach of flying into the latter region, may be said to come but ill from one, who has trode so often the crude consistence of the chaos, and who delights to dwell beyond the boundaries of nature. By sojourning there long, it is not impossible that the eye may become so accustomed to fantastic forms, that the figures and proportions of nature shall appear to it deformed and monstrous.