ck yellow
vapour?

MRS. B.

It is the acid, which being abandoned by the greatest part of its
oxygen, is converted into a weaker acid, which escapes in the form of
gas.

CAROLINE.

And whence proceeds this heat?

MRS. B.

Indeed, Caroline, I think you might now be able to answer that question
yourself.

CAROLINE.

Perhaps it is that the oxygen enters into the metal in a more solid
state than it existed in the acid, in consequence of which caloric is
disengaged.

MRS. B.

If the combination of the oxygen and the metal results from the union of
their opposite electricities, of course caloric must be given out.

EMILY.

The effervescence is over; therefore I suppose that the metal is now
oxydated.

MRS. B.

Yes. But there is another important connection between metals and acids,
with which I must now make you acquainted. Metals, when in the state of
oxyds, are capable of being dissolved by acids. In this operation they
enter into a chemical combination with the acid, and form an entirely
new compound.

CAROLINE.

But what difference is there between the _oxydation_ and the
_dissolution_ of the metal by an acid?

MRS. B.

In the first case, the metal merely combines with a portion of oxygen
taken from the acid, which is thus partly deoxygenated, as in the
instance you have just seen; in the second case, the metal, after being
previously oxydated, is actually dissolved in the acid, and enters into
a chemical combination with it, without producing any further
decomposition or effervescence. --This complete combination of an oxyd
and an acid forms a peculiar and important class of compound salts.

EMILY.

The difference between an oxyd and a compound salt, therefore, is very
obvious: the one consists of a metal and oxygen; the other of an oxyd
and an acid.

MRS. B.

Very well: and you will be careful to remember that the metals are
incapable of entering into this combination with acids, unless they are
previously oxydated; therefore, whenever you bring a metal in contact
with an acid, it will be first oxydated and afterwards dissolved,
provided that there be a sufficient quantity of acid for both
operations.

There are some metals, however, whose solution is more easily
accomplished, by diluting the acid in water; and the metal will, in this
case, be oxydated, not by the acid, but by the water, which it will
decompose. But in proportion as the oxygen of the water oxydates the
surface of t