upon the caloric contained in it. Sometimes the watery vapour diffused
in the atmosphere is but imperfectly dissolved, as is the case in the
formation of clouds and fogs; but if it gets into a region sufficiently
warm, it becomes perfectly invisible.

EMILY.

Can any water dissolve in the atmosphere without its being previously
converted into vapour by boiling?

MRS. B.

Unquestionably; and this constitutes the difference between
_vaporization_ and _evaporation_. Water, when heated to the boiling
point, can no longer exist in the form of water, and must necessarily be
converted into vapour or steam, whatever may be the state and
temperature of the surrounding medium; this is called vaporization. But
the atmosphere, by means of the caloric it contains, can take up a
certain portion of water at any temperature, and hold it in a state of
solution. This is simply evaporation. Thus the atmosphere is continually
carrying off moisture from the surface of the earth, until it is
saturated with it.

CAROLINE.

That is the case, no doubt, when we feel the atmosphere damp.

MRS. B.

On the contrary, when the moisture is well dissolved it occasions no
humidity: it is only when in a state of imperfect solution and floating
in the atmosphere, in the form of watery vapour, that it produces
dampness. This happens more frequently in winter than in summer; for the
lower the temperature of the atmosphere, the less water it can dissolve;
and in reality it never contains so much moisture as in a dry hot
summer's day.

CAROLINE.

You astonish me! But why, then, is the air so dry in frosty weather,
when its temperature is at the lowest?

EMILY.

This, I conjecture, proceeds not so much from the moisture being
dissolved, as from its being frozen; is not that the case?

MRS. B.

It is; and the freezing of the watery vapour which the atmospheric heat
could not dissolve, produces what is called a hoar frost; for the
particles descend in freezing, and attach themselves to whatever they
meet with on the surface of the earth.

The tendency of free caloric to an equilibrium, together with its
solvent power, are likewise connected with the phenomena of rain, of
dew, &c. When moist air of a certain temperature happens to pass through
a colder region of the atmosphere, it parts with a portion of its heat
to the surrounding air; the quantity of caloric, therefore, which served
to keep the water in a state of vapour, being diminishe