crescence, which affords a defensive
covering for these eggs. The insect, when come to life, first feeds on
this excrescence, and some time afterward eats its way out, as it
appears from a hole which is formed in all gall-nuts that no longer
contain an insect. It is in hot climates only that strongly astringent
gall-nuts are found; those which are used for the purpose of making ink
are brought from Aleppo.

EMILY.

But are not the oak-apples, which grow on the leaves of the oak in this
country, of a similar nature?

MRS. B.

Yes; only they are an inferior species of galls, containing less of the
astringent principle, and therefore less applicable to useful purposes.

CAROLINE.

Are the vegetable acids never found but in their pure uncombined state?

MRS. B.

By no means; on the contrary, they are frequently met with in the state
of compound salts; these, however, are in general not fully saturated
with the salifiable bases, so that the acid predominates; and, in this
state, they are called _acidulous_ salts. Of this kind is the salt
called cream of tartar.

CAROLINE.

Is not the salt of lemon, commonly used to take out ink-spots and
stains, of this nature?

MRS. B.

No; that salt consists of the oxalic acid, combined with a little
potash. It is found in that state in sorrel.

CAROLINE.

And pray how does it take out ink-spots?

MRS. B.

By uniting with the iron, and rendering it soluble in water.

Besides the vegetable materials which we have enumerated, a variety of
other substances, common to the three kingdoms, are found in vegetables,
such as potash, which was formerly supposed to belong exclusively to
plants, and was, in consequence, called the vegetable alkali.

Sulphur, phosphorus, earths, and a variety of metallic oxyds, are also
found in vegetables, but only in small quantities. And we meet sometimes
with neutral salts, formed by the combination of these ingredients.

CONVERSATION XXI.

ON THE DECOMPOSITION OF VEGETABLES.

CAROLINE.

The account which you have given us, Mrs. B., of the materials of
vegetables, is, doubtless, very instructive; but it does not completely
satisfy my curiosity. I wish to know how plants obtain the principles
from which their various materials are formed; by what means these are
converted into vegetable matter, and how they are connected with the
life of the plant?

MRS. B.

This implies nothing less than a complete history of the chemist