half a crown-piece be placed upon the tongue, and one edge of a plate of
zinc about the same size beneath the tongue, and if their opposite edges
are then brought into contact before the point of the tongue, a taste is
perceived at the moment of their coming into contact; secondly, if one of
the above plates be put between the upper lip and the gum of the
fore-teeth, and the other be placed under the tongue, and their exterior
edges be then brought into contact in a darkish room, a flash of light is
perceived in the eyes.

These effects I imagine only shew the sensibility of our nerves of sense to
very small quantities of the electric fluid, as it passes through them; for
I suppose these sensations are occasioned by slight electric shocks
produced in the following manner. By the experiments published by Mr.
Bennet, with his ingenious doubler of electricity, which is the greatest
discovery made in that science since the coated jar, and the eduction of
lightning from the skies, it appears that zinc was always found minus, and
silver 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