obscurity, this is not the case with regard to the more common effects
of heat; and, tho' the actual existence of subterraneous fire, as the
cause of light and heat, might be a thing altogether problematical in
our opinion; yet, as to other effects, there are some of these from
which the action of that liquefying power may be certainly concluded as
having taken place within the mineral region, although the cause should
be in every other respect a thing to us unknown. In that case, where the
operation or effect is evident, and cannot be disputed, to refuse to
admit the power in question, merely because we had not seen it act, or
because we know not every rule which it may observe in acting, would
be only to found an argument upon our ignorance; it would be to
misunderstand the nature of investigating physical truths, which must
proceed by reasoning from effect to cause.

Our knowledge is extremely limited with regard to the effects of heat in
bodies, while acting under different conditions, and in various degrees.
But though our knowledge in these respects is limited, our judgment with
regard to the efficacy of this power of heat is in its nature positive,
and contains not any thing that is doubtful or uncertain. All mankind,
who have the opportunity, know that the hard substance of ice is by heat
converted into water, wherein no hardness remains; and the profound
philosophy of Dr Black, in relation to the subject of _latent heat_, as
that of Sir Isaac Newton, in relation to the weight of bodies, is not
necessary to convince the world that in the one case ice will melt, and
in the other, that heavy bodies will move when unsupported.

But though, in the abstract doctrine of _latent heat_, the ingenuity
of man has discovered a certain measure for the quantity of those
commutable effects which are perceived; and though this be a progress of
science far above the apprehension of the vulgar, yet still, that solid