The tendency of an increased pressure on the bodies to which heat is applied, is to restrain the volatility of those parts which otherwise would make their escape, and to force them to endure a more intense action of heat. At a certain depth under the surface of the sea, the power even of a very intense heat might therefore be unable to drive off the oily or bituminous parts from the inflammable matter there deposited, so that, when the heat was withdrawn, these principles might be found still united to the earthy and carbonic parts, forming a substance very unlike the residuum obtained after combustion under a pressure no greater than the weight of the atmosphere. It is in like manner reasonable to believe, that, on the application of heat to calcareous bodies under great compression, the carbonic gas would be forced to remain; the generation of quicklime would be prevented, and the whole might be softened, or even completely melted; which last effect, though not directly deducible from any experiment yet made, is rendered very probable, from the analogy of certain chemical phenomena. 16. An analogy of this kind, derived from a property of the barytic earth, was suggested by that excellent chemist and philosopher, the late Dr BLACK. The barytic earth, as is well known, has a stronger attraction for fixed air than common calcareous earth has, so that the carbonate of barytes is able to endure a great degree of heat before its fixed air is expelled. Accordingly, when exposed to an increasing heat, at a certain temperature, it is brought into fusion, the fixed air still remaining united to it: if the heat be further increased, the air is driven off, the earth loses its fluidity, and appears in a caustic state. Here, it is plain, that the barytic earth, which is infusible, or very refractory, _per se_, as well as the calcareous, owes its fusibility to the presence of the fixed air; and it is therefore probable, that the same thing would happen to the calcareous earth, if by any means the fixed air were prevented from escaping when great heat is applied to it. This escape of the fixed air is exactly what the compression in the subterraneous regions is calculated to prevent, and therefore we are not to wonder if, among the calcareous strata, we find marks of actual fusion having taken place.[9] [Footnote 9: NOTE VII.]