132. The circumstance, however, which gives to this theory its peculiar character, and exalts it infinitely above all others, is the introduction of the principle of pressure, to modify the effects of heat when applied at the bottom of the sea. This is in fact the key to the grand enigma of the mineral kingdom, where, while one set of phenomena indicates the action of fire, another set, equally remarkable, seems to exclude the possibility of that action, by presenting us with mineral substances, in such a state as they could never have been brought into by the operation of the fires we see at the surface of the earth. These two classes of phenomena are reconciled together, by admitting the power of compression to confine the volatile parts of bodies when heat is applied to them, and to force them, in many instances, to undergo fusion, instead of being calcined or dissipated by burning or inflammation. In this hypothesis, which some affect to consider as a principle gratuitously assumed, there appears to me nothing but a very fair and legitimate generalization of the properties of heat. Combustion and inflammation are chemical processes, to which other conditions are required, besides the presence of a high temperature. The state of the mineral regions makes it reasonable to presume, that these conditions are wanting in the bowels of the earth, where, of consequence, we have a right to look for nothing but expansion and fusion, the only operations which seem essential to heat, and inseparable from the application of it, in certain degrees, to certain substances. Though this principle, therefore, had no countenance from analogy, the admirable simplicity, and the unity, which it introduces into the phenomena of geology, would sufficiently justify the application of it to the theory of the earth. As another excellence of this theory, I may, perhaps, be allowed to remark, that it extends its consequences beyond those to which the author of it has himself adverted, and that it affords, which no geological theory has yet done, a satisfactory explanation of the spheroidal figure of the earth.[40] [Footnote 40: NOTE XXV.]