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 quant.i.ties. 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 chemistry and physiology of vegetation, subjects on which we have yet but very imperfect notions. Still I hope that I shall be able, in some measure, to satisfy your curiosity. But, in order to render the subject more intelligible, I must first make you acquainted with the various changes which vegetables undergo, when the vital power no longer enables them to resist the common laws of chemical attraction.

The composition of vegetables being more complicated than that of minerals, the former more readily undergo chemical changes than the latter: for the greater the variety of attractions, the more easily is the equilibrium destroyed, and a new order of combinations introduced.

EMILY.

I am surprised that vegetables should be so easily susceptible of decomposition; for the preservation of the vegetable kingdom is certainly far more important than that of minerals.

MRS. B.

You must consider, on the other hand, how much more easily the former is renewed than the latter. The decomposition of the vegetable takes place only after the death of the plant, which, in the common course of nature, happens when it has yielded fruit and seeds to propagate its species. If, instead of thus finishing its career, each plant was to retain its form and vegetable state, it would become an useless burden to the earth and its inhabitants. When vegetables, therefore, cease to be productive, they cease to live, and nature then begins her process of decomposition, in order to resolve them into their chemical const.i.tuents, hydrogen, carbon, and oxygen; those simple and primitive ingredients, which she keeps in store for all her combinations.

EMILY.

But since no system of combination can be destroyed, except by the establishment of another order of attractions, how can the decomposition of vegetables reduce them to their simple elements?

MRS. B.

It is a very long process, during which a variety of new combinations are successively established and successively destroyed: but, in each of these changes, the ingredients of vegetable matter tend to unite in a more simple order of compounds, till they are at length brought to their elementary state, or, at least, to their most simple order of combinations. Thus you will find that vegetables are in the end almost entirely reduced to water and carbonic acid; the hydrogen and carbon dividing the oxygen between them, so as to form with it these two substances. But the variety of intermediate combinations that take place during the several stages of the decomposition of vegetables, present us with a new set of compounds, well worthy of our examination.

CAROLINE.

How is it possible that vegetables, while putrefying, should produce any thing worthy of observation?

MRS. B.

They are susceptible of undergoing certain changes before they arrive at the state of putrefaction, which is the final term of decomposition; and of these changes we avail ourselves for particular and important purposes. But, in order to make you understand this subject, which is of considerable importance, I must explain it more in detail.

The decomposition of vegetables is always attended by a violent internal motion, produced by the disunion of one order of particles, and the combination of another. This is called FERMENTATION. There are several periods at which this process stops, so that a state of rest appears to be restored, and the new order of compounds fairly established. But, unless means be used to secure these new combinations in their actual state, their duration will be but transient, and a new fermentation will take place, by which the compound last formed will be destroyed; and another, and less complex order, will succeed.

EMILY.

The fermentations, then, appear to be only the successive steps by which a vegetable descends to its final dissolution.

MRS. B.

Precisely so. Your definition is perfectly correct.

CAROLINE.

And how many fermentations, or new arrangements, does a vegetable undergo before it is reduced to its simple ingredients?

MRS. B.

Chemists do not exactly agree in this point; but there are, I think, four distinct fermentations, or periods, at which the decomposition of vegetable matter stops and changes its course. But every kind of vegetable matter is not equally susceptible of undergoing all these fermentations.