Here is a comparison formed of two mineral substances, to which it is of the highest importance to attend.  The solidity and present state of the one of these is commonly thought to be the operation of fire; of the other, again, it is thought to be that of water.  This, however, is not the case.  The immediate state and condition of both these bodies is now to be considered as equally the effect of fire or heat.  The reason of our forming such a different judgment with regard to these two subjects is this; we see, in the one case, the more immediate connection of the cause and the effect, while, in the other, we have only the effects from whence we are in science to investigate the cause.

But, if it were necessary always to see this immediate connection, in order to acknowledge the operation of a power which, at present, is extinguished in the effect, we should lose the benefit of science, or general principles, from whence particulars may be deduced, and we should be able to reason no better than the brute.  Man is made for science; he reasons from effects to causes, and from causes to effects; but he does not always reason without error.  In reasoning, therefore, from appearances which are particular, care must be taken how we generalise; we should be cautious not to attribute to nature, laws which may perhaps be only of our own invention.

The immediate question now before us is not, If the subterraneous fire, or elevating power, which we perceive sometimes as operating with such energy, be the consolidating cause of strata formed at the bottom of the sea; nor, if that power be the means of making land appear above the general surface of the water? for, though this be the end we want to arrive at ultimately, the question at present in agitation respects the laws of nature, or the generality of particular appearances.

Has the globe within it such an active power as fits it for the renovation of that part of its constitution which may be subject to decay?  Are those powerful operations of fire, or subterraneous heat, which so often have filled us with terror and astonishment, to be considered as having always been?  Are they to be concluded as proper to every part upon the globe, and as continual in the system of this earth?  If these points in question shall be decided in the affirmative, we can be at no loss in ascertaining the power which has consolidated strata, nor in explaining the present situation of those bodies, which had their origin at the bottom of the sea.  This, therefore, should be the object of our pursuit; and in order to have demonstration in a case of physical inquiry, we must again have recourse to the book of nature.

The general tendency of heat is to produce fluidity and softness; as that of cold is, on the contrary, to harden soft and fluid bodies.  But this softening power of heat is not uniform in its nature; it is made to act with very different effect, according to the nature of the substance to which it is applied.  We are but limited in the art of increasing the heat or the cold of bodies; we find, however, extreme difference in their substances with respect to fusibility.