he metal, the acid combines with it, washes it off, and
leaves a fresh surface for the oxygen to act upon: then other coats of
oxyd are successively formed, and rapidly dissolved by the acid, which
continues combining with the new-formed surfaces of oxyd till the whole
of the metal is dissolved. During this process the hydrogen gas of the
water is disengaged, and flies off with effervescence.

EMILY.

Was not this the manner in which the sulphuric acid assisted the iron
filings in decomposing water?

MRS. B.

Exactly; and it is thus that several metals, which are incapable alone
of decomposing water, are enabled to do it by the assistance of an acid,
which, by continually washing off the covering of oxyd, as it is formed,
prepares a fresh surface of metal to act upon the water.

CAROLINE.

The acid here seems to act a part not very different from that of a
scrubbing-brush. --But pray would not this be a good method of cleaning
metallic utensils?

MRS. B.

Yes; on some occasions a weak acid, as vinegar, is used for cleaning
copper. Iron plates, too, are freed from the rust on their surface by
diluted muriatic acid, previous to their being covered with tin. You
must remember, however, that in this mode of cleaning metals the acid
should be quickly afterwards wiped off, otherwise it would produce fresh
oxyd.

CAROLINE.

Let us watch the dissolution of the copper in the nitric acid; for I am
very impatient to see the salt that is to result from it. The mixture is
now of a beautiful blue colour; but there is no appearance of the
formation of a salt; it seems to be a tedious operation.

MRS. B.

The crystallisation of the salt requires some length of time to be
completed; if, however, you are so impatient, I can easily show you a
metallic salt already formed.

CAROLINE.

But that would not satisfy my curiosity half so well as one of our own
manufacturing.

MRS. B.

It is one of our own preparing that I mean to show you. When we
decomposed water a few days since, by the oxydation of iron filings
through the assistance of sulphuric acid, in what did the process
consist?

CAROLINE.

In proportion as the water yielded its oxygen to the iron, the acid
combined with the new-formed oxyd, and the hydrogen escaped alone.

MRS. B.

Very well; the result, therefore, was a compound salt, formed by the
combination of sulphuric acid with oxyd of iron. It still remains in the
vessel in which the experiment was