86. Neither can it be said that the existence of subterraneous heat is a principle assumed without any evidence, but that of the geological facts which it is intended to explain: on the contrary, it is proved by phenomena within the circle of ordinary experience, namely, those of hot springs, volcanoes, and earthquakes. These leave no doubt of the existence of heat, and of a moving and expansive power, in the bowels of the earth; so that the only questions are, at what depth is this power lodged? to what extent, and with what intensity, does it act? That it is lodged at a very considerable depth, is rendered probable by the permanency of some of the preceding phenomena: from the earliest times many fountains have retained their heat to the present day; and volcanoes, though they become extinguished at length, have a very long period allotted for their duration. The cause of earthquakes is certainly a force that resides very deep under the surface, otherwise the extent of the concussion could not be such as has been observed in many instances. 87. The intensity of volcanic fire, is another circumstance that favours the opinion of its being seated deep under the surface. That this intensity is considerable, is certain from the experiments made by Sir James Hall on the fusibility of whinstone and lava; from which it appears, that the lowest temperature in which either of these stones melt, is about 80° of Wedgewood's pyrometer. Some mineralogists have indeed affirmed, that lava is melted, not by the intensity of the heat applied to it, but in consequence of a certain combination formed between it and bituminous substances, in a manner which they do not attempt to explain, and which has indeed no analogy to any thing that is known. That a hypothesis, formed in such direct opposition to the most obvious principles of inductive reasoning, should have been imagined by a philosopher who had examined the phenomena of Etna and Vesuvius with much attention, and described them with great accuracy and truth, is more wonderful than that it should have been adopted by mineralogists, whose views of nature may have been confined within a cabinet or a laboratory. It is, however, a hypothesis, which, having never had any support but from other hypotheses, hardly merited the direct refutation that it has received from the experiments just mentioned.