halations from the earth.

MRS. B.

Such substances may be considered rather as heterogeneous and
accidental, than as forming any of its component parts; and the
proportion they bear to the whole mass is quite inconsiderable.

ATMOSPHERICAL AIR is composed of two gasses, known by the names of
OXYGEN GAS and NITROGEN or AZOTIC GAS.

EMILY.

Pray what is a gas?

MRS. B.

The name of gas is given to any fluid capable of existing constantly in
an aeriform state, under the pressure and at the temperature of the
atmosphere.

CAROLINE.

Is not water, or any other substance, when evaporated by heat, called
gas?

MRS. B.

No, my dear; vapour is, indeed, an elastic fluid, and bears a strong
resemblance to a gas; there are, however, several points in which they
essentially differ, and by which you may always distinguish them. Steam,
or vapour, owes its elasticity merely to a high temperature, which is
equal to that of boiling water. And it differs from boiling water only
by being united with more caloric, which, as we before explained, is in
a latent state. When steam is cooled, it instantly returns to the form
of water; but air, or gas, has never yet been rendered liquid or solid
by any degree of cold.

EMILY.

But does not gas, as well as vapour, owe its elasticity to caloric?

MRS. B.

It was the prevailing opinion; and the difference of gas or vapour was
thought to depend on the different manner in which caloric was united
with the basis of these two kinds of elastic fluids. In vapour, it was
considered as in a latent state; in gas, it was said to be chemically
combined. But the late researches of Sir H. Davy have given rise to a
new theory respecting gasses; and there is now reason to believe that
these bodies owe their permanently elastic state, not solely to caloric,
but likewise to the prevalence of either the one or the other of the two
electricities.

EMILY.

When you speak, then, of the simple bodies oxygen and nitrogen, you mean
to express those substances which are the basis of the two gasses?

MRS. B.

Yes, in strict propriety, for they can properly be called gasses only
when brought to an aeriform state.

CAROLINE.

In what proportions are they combined in the atmosphere?

MRS. B.

The oxygen gas constitutes a little more than one-fifth, and the
nitrogen gas a little less than four-fifths. When separated, they are
found to possess qualities totally different from each other. For ox