Conversations on Chemistry Part 95

MRS. B.

Precisely. It is equally impossible for either animal or vegetable substances to be converted into stone. They may be reduced, as we find they are, by decomposition, to their const.i.tuent elements, but cannot be changed to elements, which do not enter into their composition.

There are, however, circ.u.mstances which frequently prevent the regular and final decomposition of vegetables; as, for instance, when they are buried either in the sea, or in the earth, where they cannot undergo the putrid fermentation for want of air. In these cases they are subject to a peculiar change, by which they are converted into a new cla.s.s of compounds, called _bitumens_.

CAROLINE.

These are substances I never heard of before.

MRS. B.

You will find, however, that some of them are very familiar to you.

Bitumens are vegetables so far decomposed as to retain no organic appearance; but their origin is easily detected by their oily nature, their combustibility, the products of their a.n.a.lysis, and the impressions of the forms of leaves, grains, fibres of wood, and even of animals, which they frequently bear.

They are sometimes of an oily liquid consistence, as the substance called _naptha_, in which we preserved pota.s.sium; it is a fine transparent colourless fluid, that issues out of clays in some parts of Persia. But more frequently bitumens are solid, as _asphaltum_, a smooth, hard, brittle substance, which easily melts, and forms, in its liquid state, a beautiful dark brown colour for oil painting. _Jet_, which is of a still harder texture, is a peculiar bitumen, susceptible of so fine a polish, that it is used for many ornamental purposes.

_Coal_ is also a bituminous substance, to the composition of which both the mineral and animal kingdoms seem to concur. This most useful mineral appears to consist chiefly of vegetable matter, mixed with the remains of marine animals and marine salts, and occasionally containing a quant.i.ty of sulphuret of iron, commonly called pyrites.

EMILY.

It is, I suppose, the earthly, the metallic, and the saline parts of coals, that compose the cinders or fixed products of their combustion; whilst the hydrogen and carbon, which they derive from vegetables, const.i.tute their volatile products.

CAROLINE.

Pray is not _c.o.ke_, (which I have heard is much used in some manufactures,) also a bituminous substance?

MRS. B.

No; it is a kind of fuel artificially prepared from coals. It consists of coals reduced to a substance a.n.a.logous to charcoal, by the evaporation of their bituminous parts. c.o.ke, therefore, is composed of carbon, with some earthy and saline ingredients.

_Succin_, or _yellow amber_, is a bitumen which the ancients called _electrum_, from whence the word electricity is derived, as that substance is peculiarly, and was once supposed to be exclusively, electric. It is found either deeply buried in the bowels of the earth, or floating on the sea, and is supposed to be a resinous body which has been acted on by sulphuric acid, as its a.n.a.lysis shows it to consist of ah oil and an acid. The oil is called _oil of amber_, the acid the _succinic_.

EMILY.

That oil I have sometimes used in painting, as it is reckoned to change less than the other kinds of oils.

MRS. B.

The last cla.s.s of vegetable substances that have changed their nature are _fossil-wood_, _peat_, and _turf_. These are composed of wood and roots of shrubs, that are partly decomposed by being exposed to moisture under ground, and yet, in some measure, preserve their form and organic appearance. The peat, or black earth of the moors, retains but few vestiges of the roots to which it owes its richness and combustibility, these substances being in the course of time reduced to the state of vegetable earth. But in turf the roots of plants are still discernible, and it equally answers the purpose of fuel. It is the combustible used by the poor in heathy countries, which supply it abundantly.

It is too late this morning to enter upon the history of vegetation. We shall reserve this subject, therefore, for our next interview, when I expect that it will furnish us with ample matter for another conversation.

CONVERSATION XXII.

HISTORY OF VEGETATION.

MRS. B.

The VEGETABLE KINGDOM may be considered as the link which unites the mineral and animal creation into one common chain of beings; for it is through the means of vegetation alone that mineral substances are introduced into the animal system, since, generally speaking, it is from vegetables that all animals ultimately derive their sustenance.

CAROLINE.

I do not understand that; the human species subsists as much on animal as on vegetable food, and there are some carnivorous animals that will eat only animal food.

MRS. B.

That is true; but you do not consider that those that live on animal food, derive their sustenance equally, though not so immediately, from vegetables. The meat that we eat is formed from the herbs of the field, and the prey of carnivorous animals proceeds, either directly or indirectly, from the same source. It is, therefore, through this channel that the simple elements become a part of the animal frame. We should in vain attempt to derive nourishment from carbon, hydrogen, and oxygen, either in their separate state, or combined in the mineral kingdom; for it is only by being united in the form of vegetable combination, that they become capable of conveying nourishment.

EMILY.

Vegetation, then, seems to be the method which Nature employs to prepare the food of animals?

MRS. B.

That is certainly its princ.i.p.al object. The vegetable creation does not exhibit more wisdom in that admirable system of organisation, by which it is enabled to answer its own immediate ends of preservation, nutrition, and propagation, than in its grand and ultimate object of forming those arrangements and combinations of principles, which are so well adapted for the nourishment of animals.

EMILY.

But I am very curious to know whence vegetables obtain those principles which form their immediate materials?

MRS. B.

This is a point on which we are yet so much in the dark, that I cannot hope fully to satisfy your curiosity; but what little I know on this subject, I will endeavour to explain to you.

The soil, which, at first view, appears to be the aliment of vegetables, is found, on a closer investigation, to be little more than the channel through which they receive their nourishment; so that it is very possible to rear plants without any earth or soil.

CAROLINE.

Of that we have an instance in the hyacinth and other bulbous roots, which will grow and blossom beautifully in gla.s.ses of water. But I confess I should think it would be difficult to rear trees in a similar manner.

MRS. B.

No doubt it would, as it is the burying of the roots in the earth that supports the stem of the tree. But this office, besides that of affording a vehicle for food, is far the most important part which the earthy portion of the soil performs in the process of vegetation; for we can discover, by a.n.a.lysis, but an extremely small proportion of earth in vegetable compounds.

CAROLINE.

But if earths do not afford nourishment, why is it necessary to be so attentive to the preparation of the soil?

MRS. B.

In order to impart to it those qualities which render it a proper vehicle for the food of the plant. Water is the chief nourishment of vegetables; if, therefore, the soil be too sandy, it will not retain a quant.i.ty of water sufficient to supply the roots of the plants. If, on the contrary, it abound too much with clay, the water will lodge in such quant.i.ties as to threaten a decomposition of the roots. Calcareous soils are, upon the whole, the most favourable to the growth of plants: soils are, therefore, usually improved by chalk, which, you may recollect, is a carbonat of lime. Different vegetables, however, require different kinds of soils. Thus rice demands a moist retentive soil; potatoes a soft sandy soil; wheat a firm and rich soil. Forest trees grow better in fine sand than in a stiff clay; and a light ferruginous soil is best suited to fruit-trees.

CAROLINE.