ke the volatile
product?

MRS. B.

Yet when the fire burns best, and the quantity of volatile products
should be the greatest, there is no smoke; how can you account for that?

EMILY.

Indeed I cannot; therefore I suppose that I was not right in my
conjecture.

MRS. B.

Not quite: ashes, as you supposed, are a fixed product of combustion;
but smoke, properly speaking, is not one of the volatile products, as it
consists of some minute undecomposed particles of the coals that are
carried off by the heated air without being burnt, and are either
deposited in the form of soot, or dispersed by the wind. Smoke,
therefore, ultimately, becomes one of the _fixed_ products of
combustion. And you may easily conceive that the stronger the fire is,
the less smoke is produced, because the fewer particles escape
combustion. On this principle depends the invention of Argand's Patent
Lamps; a current of air is made to pass through the cylindrical wick of
the lamp, by which means it is so plentifully supplied with oxygen, that
scarcely a particle of oil escapes combustion, nor is there any smoke
produced.

EMILY.

But what then are the volatile products of combustion?

MRS. B.

Various new compounds, with which you are not yet acquainted, and which
being converted by caloric either into vapour or gas, are invisible; but
they can be collected, and we shall examine them at some future period.

CAROLINE.

There are then other gases, besides the oxygen and nitrogen gases.

MRS. B.

Yes, several: any substance that can assume and maintain the form of an
elastic fluid at the temperature of the atmosphere, is called a gas. We
shall examine the several gases in their respective places; but we must
now confine our attention to those that compose the atmosphere.

I shall show you another method of decomposing the atmosphere, which is
very simple. In breathing, we retain a portion of the oxygen, and expire
the nitrogen gas; so that if we breathe in a closed vessel, for a
certain length of time, the air within it will be deprived of its oxygen
gas. Which of you will make the experiment?

CAROLINE.

I should be very glad to try it.

MRS. B.

Very well; breathe several times through this glass tube into the
receiver with which it is connected, until you feel that your breath is
exhausted.

CAROLINE.

I am quite out of breath already!

MRS. B.

Now let us try the gas with a lighted taper.

EMILY.

It is very pure