Yes. Though it is usually extracted from galls, on account of its being most abundant in that vegetable substance, it may also be obtained from a great variety of plants. It constitutes what is called the _astringent principle_ of vegetables; it is generally combined with tannin, and you will find that an infusion of tea, coffee, bark, red-wine, or any vegetable substance that contains the astringent principle, will make a black precipitate with a solution of sulphat of iron. CAROLINE. But pray what are galls? MRS. B. They are excrescences which grow on the bark of young oaks, and are occasioned by an insect which wounds the bark of trees, and lays its eggs in the aperture. The lacerated vessels of the tree then discharge their contents, and form an excrescence, 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.