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new and sublime conclusion, which represents nature as having provided for a constant succession of land on the surface of the earth, according to a plan having no natural termination, but calculated to endure as long as those beneficent purposes, for which the whole is destined, shall continue to exist.

This conclusion, however, was but a suggestion, till the mechanism was inquired into by which this grand renovation may be brought about, or by which loose materials can be converted into stone, and elevated into land. This led to an investigation of the mineralizing principle, or the cause of the consolidation of mineral bodies. And Dr Hutton appears accordingly, with great impartiality, and with no physical hypothesis whatever in his mind, to have begun with inquiring into the nature of the fluidity which so many mineral substances seem to have possessed previous to the acquisition of their present form. After a long and minute examination, he came to the conclusion, that the fluidity of these substances has been what he terms simple, that is to say, not such as is produced by combination with a solvent. The two general facts from which this conclusion follows are, first, that no solvent is capable of holding in solution all mineral substances, nor even all such varieties of them as are often united in the same specimen ; and, secondly, that in the bodies composed of fragments of other bodies, the consolidation is so com

plete that no room is left for a solvent to have ever occupied. The substance, therefore, which was the cause of the fluidity of mineral bodies, and prepared them for consolidation, must have been one that could act on them all, which occupied no space within them, and could find its way through them, whatever was the degree of their compactness and induration. Heat is the only substance which has these properties; and is the only one, therefore, which, without manifest contradiction, can be assigned as the cause of mineral consolidation.

Many difficulties, however, were still to be removed before this hypothesis was rendered completely satisfactory; but in what order Dr Hutton proceeded to remove them, the notes above mentioned do not enable me to state. We may nevertheless conjecture, with considerable probability, what the step was which immediately followed.

It must have occurred to him, as an objection to the consolidation of minerals by subterraneous heat, that many substances are found in the bowels of the earth in a state altogether unlike that into which they are brought by the action of our fires at the surface. Coal, for instance, by exposure to fire, has its parts dissipated; the ashes which remain behind are a substance quite different from the coal itself; and hence it would seem that this fossil can never before have been subjected to the action of fire. But is it certain, (we may suppose Dr Hutton to have said

to himself,) if the heat had been applied to the coal in the interior of the earth, at the bottom of the sea, for example, that the same dissipation of the parts would have taken place? Would not the greater compression that must prevail in that region have prevented the dissipation, at least till a more intense heat was applied? And if the dissipation was prevented, might not the mass, after cooling, be very different from any thing that can be obtained by burning at the surface of the earth? It is plain that there is no reason whatever for answering these questions in the negative. And, on the contrary, if the analogy of nature is consulted, if the fact of water requiring more heat to make it boil when it is more compressed, or the experiments with Papin's digester, be considered, it will appear that the answer must be in the affirmative. Nay, it could not but seem reasonable to proceed a step farther, and, as the mixture of substances is known in so many instances to promote their fusibility, to suppose that, when the volatile parts of bodies were restrained, the whole mass might be reduced into fusion by heat, though, when these same parts were driven off, the residuum might be altogether infusible. Thus coal, when the charcoal and bitumen are forced to remain in union, may very well be a fusible substance, though, when the latter is permitted to escape, the former becomes one of the most refractory of all bodies.

In this way, and probably from this very instance, the effects of compression may have suggested themselves to Dr Hutton. He would soon perceive that the same principle could be very generally applied, and that it afforded the solution of a difficulty concerning limestone similar to that which has been just stated with respect to coal. Limestone is not found in the bowels of the earth having the causticity which it acquires by the action of fire, and hence one might conclude that it had never been exposed to the action of that element. But the experiments of Dr Black, before his friend was engaged in this geological investigation, * had proved that the causticity of lime depends on the expulsion of the aëriform fluid, since distinguished by the name of carbonic gas, which composes no less than two-fifths of the whole. This great discovery, which has extended its influence so widely over the science of chemistry, also led to important consequences in geology; and Dr Hutton inferred from it, that strong compression might prevent the caus

* Dr Black's paper on magnesia, which contained this discovery, was communicated to the Philosophical Society of Edinburgh in June 1755, and was published in the second volume of their Essays, in the year following. Dr Hutton had at this time only begun his geological researches. It was not, I imagine, till after the year 1760 that they came to take the form of a theory.

ticity of lime, by confining the carbonic gas, even when great heat was applied, and that, as has been supposed of coal, the whole may have been melted in the interior of the earth, so as on cooling to acquire that crystallized or sparry structure which the carbonate of lime so frequently possesses. *

* In the view here presented of the principle of compression, as employed in the Huttonian Theory, it is considered as a hypothesis, conformable to analogy, assumed for the purpose of explaining certain phenomena in the natural history of the earth. It rests, therefore, as to its evidence, partly on its conformity to analogy, and partly on the explanation which it affords of the phenomena alluded to. In supposing that it derives probability from the last-mentioned source, we are far from assuming any thing unprecedented in sound philosophy. A principle is often admitted in physics, merely because it explains a great number of appearances; and the theory of Gravitation itself rests on no other foundation.

The degree of this evidence will perhaps be differently appreciated, according to a man's habits of thinking, or the class of studies in which he has been chiefly engaged. To Dr Hutton himself it appeared very strong; for he considered the fact of the liquefaction of mineral substances by heat as so completely established, that it affords a full proof of the fusibility of those substances having been increased by the compression which they endured in the bowels of the earth. In his view of the matter, no other proof seemed necessary, and he did not appear to think that the direct testimony of experiment, could it have been obtained, would have added much to the credibility of this part of his system.

For my part, I will acknowledge that the matter appears

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