Now, in the first place, the high degree of temperature which is here understood, is probably not necessary to the purposes of mineralization, as has just been shown; and, in the second place, it is not FIRE, in the usual sense of the word, but HEAT, which is required for that purpose; and there is nothing chimerical in supposing, that nature has the means of producing heat, even in a very great degree, without the assistance of fuel or of vital air. Friction is a source of heat, unlimited, for what we know, in its extent, and so perhaps are other operations, both chemical and mechanical; nor are either combustible substances, or vital air, concerned in the heat thus produced. So also the heat of the sun's rays in the focus of a burning glass, the most intense that is known, is independent of the substances just mentioned; and, though that heat certainly could not calcine a metal, nor even burn a piece of wood, without oxygenous gas, it would doubtless produce as high a temperature in the absence as in the presence of that gas. 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] [Footnote 76: Newton's Optics, _ubi supra_.]