attracted by the solutions of some of them, and is detruded from the
combination of others. Thus from the combinations of metals with acids, and
from those combinations of animal fluids, which are termed secretions, this
fluid matter of heat is given out amongst the neighbouring bodies; and in
the solutions of salts in water, or of water in air, it is absorbed from
the bodies, that surround them; whilst in its facility in passing through
metallic bodies, and its difficulty in pervading resins and glass, it
resembles the properties of the electric aura; and is like that excited by
friction, and seems like that to gravitate amongst other bodies in its
uncombined state, and to find its equilibrium.

There is no circumstance of more consequence in the animal economy than a
due proportion of this fluid of heat; for the digestion of our nutriment in
the stomach and bowels, and the proper qualities of all our secreted
fluids, as they are produced or prepared partly by animal and partly by
chemical processes, depend much on the quantity of heat; the excess of
which, or its deficiency, alike gives us pain, and induces us to avoid the
circumstances that occasion them. And in this the perception of heat
essentially differs from the perceptions of the sense of touch, as we
receive pain from too great pressure of solid bodies, but none from the
absence of it. It is hence probable, that nature has provided us with a set
of nerves for the perception of this fluid, which anatomists have not yet
attended to.

There may be some difficulty in the proof of this assertion; if we look at
a hot fire, we experience no pain of the optic nerve, though the heat along
with the light must be concentrated upon it. Nor does warm water or warm
oil poured into the ear give pain to the organ of hearing; and hence as
these organs of sense do not perceive small excesses or deficiences of
heat; and as heat has no greater analogy to the solidity or to the figures