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_. [Footnote 77: Essai sur le Feu.] [Footnote 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] [Footnote 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.