ilver was always found plus, when both of them were in their separate
state. Hence, when they are placed in the manner above described, as soon
as their exterior edges come nearly into contact, so near as to have an
extremely thin plate of air between them, that plate of air becomes charged
in the same manner as a plate of coated glass; and is at the same instant
discharged through the nerves of taste or of sight, and gives the
sensations, as above described, of light or of saporocity; and only shews
the great sensibility of these organs of sense to the stimulus of the
electric fluid in suddenly passing through them.

VI. _Of the Sense of Heat._

There are many experiments in chemical writers, that evince the existence
of heat as a fluid element, which covers and pervades all bodies, and is
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 h