be oxydated, in order to form the salt;
this, therefore, is much the most easy way of procuring it on a large
scale.

EMILY.

I am surprised to find that both acids and compound salts are generally
obtained from their various combinations, rather than from the immediate
union of their ingredients.

MRS. B.

Were the simple bodies always at hand, their combinations would
naturally be the most convenient method of forming compounds; but you
must consider that, in most instances, there is great difficulty and
expense in obtaining the simple ingredients from their combinations; it
is, therefore, often more expedient to procure compounds from the
decomposition of other compounds. But, to return to the sulphat of iron.
--There is a certain vegetable acid called _Gallic acid_, which has the
remarkable property of precipitating this salt black--I shall pour a few
drops of the gallic acid into this solution of sulphat of iron--

CAROLINE.

It is become as black as ink!

MRS. B.

And it is ink in reality. Common writing ink is a precipitate of sulphat
of iron by gallic acid; the black colour is owing to the formation of
gallat of iron, which being insoluble, remains suspended in the fluid.

This acid has also the property of altering the colour of iron in its
metallic state. You may frequently see its effect on the blade of a
knife, that has been used to cut certain kinds of fruits.

CAROLINE.

True; and that is, perhaps, the reason that a silver knife is preferred
to cut fruits; the gallic acid, I suppose, does not act upon silver.
--Is this acid found in all fruits?

MRS. B.

It is contained, more or less, in the rind of most fruits and roots,
especially the radish, which, if scraped with a steel or iron knife, has
its bright red colour changed to a deep purple, the knife being at the
same time blackened. But the vegetable substance in which the gallic
acid most abounds is _nutgall_, a kind of excrescence that grows on
oaks, and from which the acid is commonly obtained for its various
purposes.

MRS. B.

We now come to the PHOSPHORIC and PHOSPHOROUS ACIDS. In treating of
phosphorus, you have seen how these acids may be obtained from it by
combustion?

EMILY.

Yes; but I should be much surprised if it was the usual method of
obtaining them, since it is so very difficult to procure phosphorus in
its pure state.

MRS. B.

You are right, my dear; the phosphoric acid, for general purposes, is
extract