roasting_
and _smelting_ of the lead; but, I confess, that I do not understand in
what these operations consist.

MRS. B.

Roasting is the process by which the volatile parts of the ore are
evaporated; smelting, that by which the pure metal is afterwards
separated from the earthy remains of the ore. This is done by throwing
the whole into a furnace, and mixing with it certain substances that
will combine with the earthy parts and other foreign ingredients of the
ore; the metal being the heaviest, falls to the bottom, and runs out by
proper openings in its pure metallic state.

EMILY.

You told us in a preceding lesson that metals had a great affinity for
oxygen. Do they not, therefore, combine with oxygen, when strongly
heated in the furnace, and run out in the state of oxyds?

MRS. B.

No; for the scoriae, or oxyd, which soon forms on the surface of the
fused metal, when it is oxydable, prevents the air from having any
further influence on the mass; so that neither combustion nor
oxygenation can take place.

CAROLINE.

Are all the metals equally combustible?

MRS. B.

No; their attraction for oxygen varies extremely. There are some that
will combine with it only at a very high temperature, or by the
assistance of acids; whilst there are others that oxydate spontaneously
and with great rapidity, even at the lowest temperature; such is in
particular manganese, which scarcely ever exists in the metallic state,
as it immediately absorbs oxygen on being exposed to the air, and
crumbles to an oxyd in the course of a few hours.

EMILY.

Is not that the oxyd from which you extracted the oxygen gas?

MRS. B.

It is: so that, you see, this metal attracts oxygen at a low
temperature, and parts with it when strongly heated.

EMILY.

Is there any other metal that oxydates at the temperature of the
atmosphere?

MRS. B.

They all do, more or less, excepting gold, silver, and platina.

Copper, lead, and iron, oxydate slowly in the air, and cover themselves
with a sort of rust, a process which depends on the gradual conversion
of the surface into an oxyd. This rusty surface preserves the interior
metal from oxydation, as it prevents the air from coming in contact with
it. Strictly speaking, however, the word rust applies only to the oxyd,
which forms on the surface of iron, when exposed to air and moisture,
which oxyd appears to be united with a small portion of carbonic acid.

EMILY.

When metals oxydat