und by experiment to have _less
capacity for heat_ than arterial blood; whence it follows that the
blood, in gradually passing from the arterial to the venous state,
during the circulation, parts with a portion of caloric, by means of
which heat is diffused through every part of the body.

EMILY.

More and more admirable!

CAROLINE.

The cause of animal heat was always a perfect mystery to me, and I am
delighted with its explanation. --But pray, Mrs. B., can you tell me
what is the reason of the increase of heat that takes place in a fever?

EMILY.

Is it not because we then breathe quicker, and therefore more heat is
disengaged in the system?

MRS. B.

That may be one reason: but I should think that the principal cause of
the heat experienced in fevers, is, that there is no vent for the
caloric which is generated in the body. One of the most considerable
secretions is the insensible perspiration; this is constantly carrying
off caloric in a latent state; but during the hot stage of a fever, the
pores are so contracted, that all perspiration ceases, and the
accumulation of caloric in the body occasions those burning sensations
which are so painful.

EMILY.

This is, no doubt, the reason why the perspiration that often succeeds
the hot stage of a fever affords so much relief. If I had known this
theory of animal heat when I had a fever last summer, I think I should
have found some amusement in watching the chemical processes that were
going on within me.

CAROLINE.

But exercise likewise produces animal heat, and that must be quite in a
different manner.

MRS. B.

Not so much so as you think; for the more exercise you take, the more
the body is stimulated, and requires recruiting. For this purpose the
circulation of the blood is quickened, the breath proportionably
accelerated, and consequently a greater quantity of caloric evolved.

CAROLINE.

True; after running very fast, I gasp for breath, my respiration is
quick and hard, and it is just then that I begin to feel hot.

EMILY.

It would seem, then, that violent exercise should produce fever.

MRS. B.

Not if the person is in a good state of health; for the additional
caloric is then carried off by the perspiration which succeeds.

EMILY.

What admirable resources nature has provided for us! By the production
of animal heat she has enabled us to keep up the temperature of our
bodies above that of inanimate objects; and whenever this sou