We can form an opinion, therefore, only by the result of these operations. The sap is evidently composed of water, absorbed by the roots, and holding in solution the various principles which it derives from the soil. From the roots the sap ascends through the tubes of the alburnum into the stem, and thence branches out to every extremity of the plant. Together with the sap circulates a certain quant.i.ty of carbonic acid, which is gradually disengaged from the former by the internal heat of the plant.

CAROLINE.

What! have vegetables a peculiar heat, a.n.a.logous to animal heat?

MRS. B.

It is a circ.u.mstance that has long been suspected; but late experiments have decided beyond a doubt that vegetable heat is considerably above that of unorganised matter in winter, and below it in summer. The wood of a tree is about sixty degrees, when the thermometer is seventy or eighty degrees. And the bark, though so much exposed, is seldom below forty in winter.

It is from the sap, after it has been elaborated by the leaves, that vegetables derive their nourishment; in its progress through the plant from the leaves to the roots, it deposits in the several sets of vessels with which it communicates, the materials on which the growth and nourishment of each plant depends. It is thus that the various peculiar juices, saccharine, oily, mucous, acid, and colouring, are formed; as also the more solid parts, fecula, woody fibre, tannin, resins, concrete salts; in a word, all the immediate materials of vegetables, as well as the organised parts of plants, which latter, besides the power of secreting these from the sap for the general purpose of the plant, have also that of applying them to their own particular nourishment.

EMILY.

But why should the process of vegetation take place only at one season of the year, whilst a total inaction prevails during the other?

MRS. B.

Heat is such an important chemical agent, that its effect, as such, might perhaps alone account for the impulse which the spring gives to vegetation. But, in order to explain the mechanism of that operation, it has been supposed that the warmth of the spring dilates the vessels of plants, and produces a kind of vacuum, into which the sap (which had remained in a state of inaction in the trunk during the winter) rises: this is followed by the ascent of the sap contained in the roots, and room is thus made for fresh sap, which the roots, in their turn, pump up from the soil. This process goes on till the plant blossoms and bears fruit, which terminates its summer career: but when the cold weather sets in, the fibres and vessels contract, the leaves wither, and are no longer able to perform their office of transpiration; and, as this secretion stops, the roots cease to absorb sap from the soil. If the plant be an annual, its life then terminates; if not, it remains in a state of torpid inaction during the winter; or the only internal motion that takes place is that of a small quant.i.ty of resinous juice, which slowly rises from the stem into the branches, and enlarges their buds during the winter.

CAROLINE.

Yet, in evergreens, vegetation must continue throughout the year.

MRS. B.

Yes; but in winter it goes on in a very imperfect manner, compared to the vegetation of spring and summer.

We have dwelt much longer on the history of vegetable chemistry than I had intended; but we have at length, I think, brought the subject to a conclusion.

CAROLINE.

I rather wonder that you did not reserve the account of the fermentations for the conclusion; for the decomposition of vegetables naturally follows their death, and can hardly, it seems, be introduced with so much propriety at any other period.

MRS. B.

It is difficult to determine at what point precisely it may be most eligible to enter on the history of vegetation; every part of the subject is so closely connected, and forms such an uninterrupted chain, that it is by no means easy to divide it. Had I begun with the germination of the seed, which, at first view, seems to be the most proper arrangement, I could not have explained the nature and fermentation of the seed, or have described the changes which manure must undergo, in order to yield the vegetable elements. To understand the nature of germination, it is necessary, I think, previously to decompose the parent plant, in order to become acquainted with the materials required for that purpose. I hope, therefore, that, upon second consideration, you will find that the order which I have adopted, though apparently less correct, is in fact the best calculated for the elucidation of the subject.

CONVERSATION XXIII.

ON THE COMPOSITION OF ANIMALS.

MRS. B.

We are now come to the last branch of chemistry, which comprehends the most complicated order of compound beings. This is the animal creation, the history of which cannot but excite the highest degree of curiosity and interest, though we often fail in attempting to explain the laws by which it is governed.

EMILY.

But since all animals ultimately derive their nourishment from vegetables, the chemistry of this order of beings must consist merely in the conversion of vegetable into animal matter.

MRS. B.

Very true; but the manner in which this is effected is, in a great measure, concealed from our observation. This process is called _animalisation_, and is performed by peculiar organs. The difference of the animal and vegetable kingdoms does not however depend merely on a different arrangement of combinations. A new principle abounds in the animal kingdom, which is but rarely and in very small quant.i.ties found in vegetables; this is nitrogen. There is likewise in animal substances a greater and more constant proportion of phosphoric acid, and other saline matters. But these are not essential to the formation of animal matter.

CAROLINE.

Animal compounds contain, then, four fundamental principles; oxygen, hydrogen, carbon, and nitrogen?

MRS. B.

Yes; and these form the immediate materials of animals, which are _gelatine_, _alb.u.men_, and _fibrine_.

EMILY.

Are those all? I am surprised that animals should be composed of fewer kinds of materials than vegetables; for they appear much more complicated in their organisation.

MRS. B.

Their organisation is certainly more perfect and intricate, and the ingredients that occasionally enter into their composition are more numerous. But notwithstanding the wonderful variety observable in the texture of the animal organs, we find that the original compounds, from which all the varieties of animal matter are derived, may be reduced to the three heads just mentioned. Animal substances being the most complicated of all natural compounds, are most easily susceptible of decomposition, as the scale of attractions increases in proportion to the number of const.i.tuent principles. Their a.n.a.lysis is, however, both difficult and imperfect; for as they cannot be examined in their living state, and are liable to alteration immediately after death, it is probable that, when submitted to the investigation of a chemist, they are always more or less altered in their combinations and properties, from what they were, whilst they made part of the living animal.

EMILY.

The mere diminution of temperature, which they experience by the privation of animal heat, must, I should suppose, be sufficient to derange the order of attractions that existed during life.

MRS. B.

That is one of the causes, no doubt: but there are many other circ.u.mstances which prevent us from studying the nature of living animal substances. We must therefore, in a considerable degree, confine our researches to the phenomena of these compounds in their inanimate state.

These three kinds of animal matter, gelatine, alb.u.men, and fibrine, form the basis of all the various parts of the animal system; either solid, as the _skin_, _flesh_, _nerves_, _membranes_, _cartilages_, and _bones_; or fluid, as _blood_, _chyle_, _milk_, _mucus_, the _gastric_ and _pancreatic juices_, _bile_, _perspiration_, _saliva_, _tears_, &c.

CAROLINE.

Is it not surprising that so great a variety of substances, and so different in their nature, should yet all arise from so few materials, and from the same original elements?

MRS. B.

The difference in the nature of various bodies depends, as I have often observed to you, rather on their state of combination, than on the materials of which they are composed. Thus, in considering the chemical nature of the creation in a general point of view, we observe that it is throughout composed of a very small number of elements. But when we divide it into the three kingdoms, we find that, in the mineral, the combinations seem to result from the union of elements casually brought together; whilst in the vegetable and animal kingdoms, the attractions are peculiarly and regularly produced by appropriate organs, whose action depends on the vital principle. And we may further observe, that by means of certain spontaneous changes and decompositions, the elements of one kind of matter become subservient to the reproduction of another; so that the three kingdoms are intimately connected, and constantly contributing to the preservation of each other.

EMILY.

There is, however, one very considerable cla.s.s of elements, which seems to be confined to the mineral kingdom: I mean metals.

MRS. B.

Not entirely; they are found, though in very minute quant.i.ties, both in the vegetable and animal kingdoms. A small portion of earths and sulphur enters also into the composition of organised bodies. Phosphorus, however, is almost entirely confined to the animal kingdom; and nitrogen, but with few exceptions, is extremely scarce in vegetables.