rtunate circumstance that air should be a bad conductor,
as it tends to preserve the heat of the body when exposed to cold
weather.

MRS. B.

It is one of the many benevolent dispensations of Providence, in order
to soften the inclemency of the seasons, and to render almost all
climates habitable to man.

In fluids of different densities, the power of conducting heat varies no
less remarkably; if you dip your hand into this vessel full of mercury,
you will scarcely conceive that its temperature is not lower than that
of the atmosphere.

CAROLINE.

Indeed I know not how to believe it, it feels so extremely cold. --But
we may easily ascertain its true temperature by the thermometer. --It is
really not colder than the air;--the apparent difference then is
produced merely by the difference of the conducting power in mercury and
in air.

MRS. B.

Yes; hence you may judge how little the sense of feeling is to be relied
on as a test of the temperature of bodies, and how necessary a
thermometer is for that purpose.

It has indeed been doubted whether fluids have the power of conducting
caloric in the same manner as solid bodies. Count Rumford, a very few
years since, attempted to prove, by a variety of experiments, that
fluids, when at rest, were not at all endowed with this property.

CAROLINE.

How is that possible, since they are capable of imparting cold or heat
to us; for if they did not conduct heat, they would neither take it
from, nor give it to us?

MRS. B.

Count Rumford did not mean to say that fluids would not communicate
their heat to solid bodies; but only that heat does not pervade fluids,
that is to say, is not transmitted from one particle of a fluid to
another, in the same manner as in solid bodies.

EMILY.

But when you heat a vessel of water over the fire, if the particles of
water do not communicate heat to each other, how does the water become
hot throughout?

MRS. B.

By constant agitation. Water, as you have seen, expands by heat in the
same manner as solid bodies; the heated particles of water, therefore,
at the bottom of the vessel, become specifically lighter than the rest
of the liquid, and consequently ascend to the surface, where, parting
with some of their heat to the colder atmosphere, they are condensed,
and give way to a fresh succession of heated particles ascending from
the bottom, which having thrown off their heat at the surface, are in
their turn displaced. Thus ev