e extricate both the free and
combined caloric, yet we extricate them only to a certain degree, which
is the freezing point; and we know not how much they contain of either
below that point.

EMILY.

According to the theory of latent heat, it appears to me that the
weather should be warm when it freezes, and cold in a thaw: for latent
heat is liberated from every substance that it freezes, and such a large
supply of heat must warm the atmosphere; whilst, during a thaw, that
very quantity of free heat must be taken from the atmosphere, and return
to a latent state in the bodies which it thaws.

MRS. B.

Your observation is very natural; but consider that in a frost the
atmosphere is so much colder than the earth, that all the caloric which
it takes from the freezing bodies is insufficient to raise its
temperature above the freezing point; otherwise the frost must cease.
But if the quantity of latent heat extricated does not destroy the
frost, it serves to moderate the suddenness of the change of temperature
of the atmosphere, at the commencement both of frost, and of a thaw. In
the first instance, its extrication diminishes the severity of the cold;
and, in the latter, its absorption moderates the warmth occasioned by a
thaw: it even sometimes produces a discernible chill, at the breaking up
of a frost.

CAROLINE.

But what are the general causes that produce those sudden changes in the
weather, especially from hot to cold, which we often experience?

MRS. B.

This question would lead us into meteorological discussions, to which I
am by no means competent. One circumstance, however, we can easily
understand. When the air has passed over cold countries, it will
probably arrive here at a temperature much below our own, and then it
must absorb heat from every object it meets with, which will produce a
general fall of temperature.

CAROLINE.

But pray, now that we know so much of the effects of heat, will you
inform us whether it is really a distinct body, or, as I have heard,
a peculiar kind of motion produced in bodies?

MRS. B.

As I before told you, there is yet much uncertainty as to the nature of
these subtle agents. But I am inclined to consider heat not as mere
motion, but as a separate substance. Late experiments too appear to make
it a compound body, consisting of the two electricities, and in our next
conversation I shall inform you of the principal facts on which that
opinion is founded.