bodies are changed into fluids, by the power of heat, is the same
unalterable judgment, which the savage forms as well as the philosopher.
Here, therefore, are evident effects, which mankind in general attribute
to the power of heat; and it is from those known effects that we are to
investigate subterraneous fire, or to generalise the power of heat, as
acting in the interior parts, as well as on the surface of this earth.

If, indeed, there were any other cause for fluidity besides the
operation of fire or the power of heat, in that case the most evident
proof, with regard to the flowing, or former fluidity, of mineral
bodies, would draw to no conclusion in proving the existence of mineral
fire; but when we have not the smallest reason for conjecturing any
other cause, or the least doubt with regard to that which, in the
doctrine of latent heat, has been properly investigated, the proofs
which we shall bring, of fusion in all the minerals of this earth,
must be held as proofs of mineral fire, in like manner as the proof of
subterraneous fire would necessarily imply mineral fusion as its natural
effect.

Thus we have, in our physical investigation, several points in view.
First, from the present state of things, to infer a former state of
fusion among mineral bodies. Secondly, from that former fusion, to infer
the actual existence of mineral fire in the system of the earth. And,
lastly, from the acknowledged fact of subterraneous fire as a cause, to
reason with regard to the effects of that power in mineral bodies.

But besides the power or effect of subterraneous heat in bodies which
are unorganised, and without system, in the construction of their
different parts, we have to investigate the proper purpose of this great
agent in the system of this world, which may be considered as a species