renders the body more subject to the action
of heat afterwards applied, by allowing the excitability to be
accumulated. No person, I believe, ever brought on an inflammation,
or inflammatory complaint, by exposure to cold, however long might
have been that exposure, or however great the cold; but if a person
have been out in the cold air, and afterwards come into a warm room,
an inflammatory complaint will most probably be the consequence.

Indeed coming out of the cold air into a moderately warm room
generally produces a lively and continued warmth in the parts that
have been exposed.

The second general law is, that when the exciting powers have acted
with violence for a considerable time, the excitability becomes
exhausted, or less fit to be acted on; and this we shall be able to
prove by a similar induction.

Let us first examine the effects of light upon the eye: when it has
acted violently for some time on the optic nerve, it diminishes the
excitability of that nerve, and renders it incapable of being
affected by a quantity of light, that would at other times affect it.
When we have been walking out in the snow, if we come into a room, we
shall scarcely be able to see any thing for some minutes.

If you look stedfastly at a candle for a minute or two, you will with
difficulty discern the letters of a book which you were before
reading distinctly. When our eyes have been exposed to the dazzling
blaze of phosphorus in oxygen gas, we can scarcely see any thing for
some time afterwards, and if we look at the sun, the excitability of
the optic nerve is so overpowered by the strong stimulus of his
light, that nothing can be seen distinctly for a considerable time.
If we look at the setting sun, or any other luminous object of a
small size, so as not greatly to fatigue the eye, this part of the
retina becomes less sensible to smaller quantities of light; hence
when the eyes are turned on other less luminous parts of the sky, a
dark spot is seen resembling the shape of the sun, or other luminous
object on which our eyes have been fixed.

On this account it is that we are some time before we can distinguish
objects in an obscure room, after coming from broad daylight, as I
observed before.

We shall next consider the action of heat. Suppose water to be heated
to 90 degrees, if one hand be put into it, it will appear warm; but
if the other hand be immersed in water heated to 120 degrees, and
then put into the water