Section Ii.—Constitution of the Organic Part of Plants and Animals.

The organic part of plants, when in a perfectly dry state, constitutes therefore from 85 to 99 per cent. of their whole weight. Of those parts of plants which are cultivated for food, it is only hay and straw, and a very few others, that contain as much as 10 per cent. of inorganic matter.

This organic part consists of four substances, known to chemists by the names of carbon, hydrogen, oxygen, and nitrogen. The first of these, carbon, is a solid substance, the other three are gases or peculiar kinds of air.

1. CARBON. When wood is burned in a covered heap, as is done by the charcoal burners, or is distilled in iron retorts, as in making wood-vinegar, it is charred and converted into common wood charcoal. This charcoal is the most usual and best known variety of carbon. It is black, soils the fingers, and is more or less porous according to the kind of wood from which it has been formed. Coke obtained by charring or distilling coal is another variety. It is generally denser or heavier than the former, though less pure. Black lead is a third variety, still heavier and more impure. The diamond is the only form in which carbon occurs in nature in a state of perfect purity.

This latter fact, that the diamond is pure carbon—that it is essentially the same substance with the finest and purest lamp-black—is very remarkable; but it is only one of many striking circumstances that every now and then present themselves before the inquiring chemist.

Charcoal, the diamond, lamp-black, and all the other forms of carbon, burn away more or less slowly when heated in the air, and are converted into a kind of gas known by the name of carbonic acid. The impure varieties leave behind them a greater or less proportion of ash.

2. HYDROGEN.—If oil of vitriol (sulphuric acid) be mixed with twice its bulk of water, and then poured upon iron filings, the mixture will speedily begin to boil up, and bubbles of gas will rise to the surface of the liquid in great abundance. These are bubbles of hydrogen gas.

If the experiment be performed in a bottle, the hydrogen which is produced will gradually drive out the atmospheric air it contained, and will itself take its place. If a bit of wax taper be tied to the end of a wire, and when lighted be introduced into the bottle, it will be instantly extinguished; while the hydrogen will take fire, and burn at the mouth of the bottle with a pale yellow flame. If the taper be inserted before the common air is all expelled, the mixture of hydrogen and common air will burn with an explosion more or less violent, and may even shatter the bottle and produce serious accidents. This experiment, therefore, ought to be made with care. It may be safely made in an open tumbler, covered by a plate or a piece of paper, till a sufficient quantity of hydrogen is collected, when, on the introduction of the taper, the light will be extinguished, and the hydrogen will burn with a less violent explosion.

This gas is also an exceedingly light substance, rising through common air as wood does through water. Hence, when confined in a bag made of silk, or other light tissue, it is capable of sustaining heavy substances in the air, and even of transporting them to great heights. For this reason it is employed for filling and elevating balloons.

Hydrogen gas is not known to occur anywhere in nature in any sensible quantity. It is very abundant, as we shall hereafter see, in what by chemists is called a state of combination.

3. OXYGEN.—When strong oil of vitriol is poured upon black oxide of manganese, and heated in a glass retort: or when red oxide of mercury, or chlorate of potash, is so heated alone; or when saltpetre, or the same oxide of manganese, is heated alone in an iron bottle;—in all these cases a kind of air is given off, which, when collected and examined by plunging a taper into it, is found to be neither common air nor hydrogen gas. The taper, when introduced, burns with great rapidity, and with exceeding brilliancy, and continues to burn till either the whole of the gas disappears, or the taper is entirely consumed. If a living animal is introduced, its circulation and its breathing become quicker—it is speedily thrown into a fever—it lives as fast as the taper burned—and, after a few hours, dies from excitement and exhaustion. This gas is not light like hydrogen, but is about one-ninth part heavier than common air.

In the atmosphere, oxygen exists in the state of gas. It forms about one-fifth of the bulk of the air we breathe, and is the substance which, in the air, supports all animal life and the combustion of all burning bodies. Were it by any cause suddenly removed from the atmosphere of our globe, every living thing would perish, and all combustion would become impossible.

4. NITROGEN.—If a saucer be half filled with milk of lime, formed by mixing slaked quicklime with water, a very small tea-cup containing a little burning sulphur then placed in the middle, and a common large tumbler inverted over the whole, the sulphur will burn for a while, and will then gradually die out. On allowing the whole to remain for some time, the fumes of the sulphur will be absorbed by the milk of lime, which will rise a certain way into the tumbler. When the absorption has ceased, a quantity of air will remain in the upper part of the tumbler. This air is nitrogen gas.

If the whole be now introduced into a large basin of water, the tumbler being held in the left hand, the cup and saucer may be removed from beneath. The saucer may then be inverted and introduced with its under side into the mouth of the tumbler, which may thus be lifted out of the water and restored to its upright position, the saucer serving the purpose of a cover. By carefully removing this cover with the one hand, a lighted taper may be introduced by the other. It will then be seen that the taper is extinguished by this air, and that no other effect follows. Or if a living animal be introduced into it, breathing will instantly cease, and it will drop without signs of life.

This gas possesses no other remarkable property. It is a very little lighter than common air, and is known to exist in large quantity in the atmosphere only. Of the air we breathe it forms nearly four-fifths of the entire bulk.

These three gases are incapable of being distinguished from common air, or from each other, by the ordinary senses; but by the aid of the taper they are readily recognised. Hydrogen extinguishes the taper, but itself takes fire; nitrogen simply extinguishes it; while in oxygen the taper burns with extraordinary brilliancy and rapidity.

Of this one solid substance, carbon, and these three gases, hydrogen, oxygen, and nitrogen, all the organic part of vegetable and animal substances is made up.

Into these substances, however, they enter in very different proportions. Nearly one-half the weight of all vegetable productions which are gathered as food for man or beast—in their dry state—consists of carbon; the oxygen amounts to rather more than one-third, the hydrogen to little more than five per cent., while the nitrogen rarely exceeds two and a half or three per cent. of their weight.

This will appear from the following table, which exhibits the actual constitution by analysis of some varieties of the more common crops when perfectly dry.

Carbon. Hydrogen. Oxygen. Nitrogen. Ash. Hay, 458 50 387 15 90 Potatoes, 441 58 439 12 50 Wheat Straw, 485 52 389½ 3½ 70 Oats, 507 64 367 22 40

These numbers represent the weights of each element in pounds, contained in 1000 lbs. of the dry hay, potatoes, &c.; but in drying by a gentle heat, 1000 lbs. of hay from the stack, lost 158 lbs. of water, of potatoes wiped dry externally 722 lbs., wheat straw 260 lbs., and oats 151 lbs.

Both potatoes and turnips contain about four-fifths of their weight of water, or five tons of either of these roots contain nearly four tons of water.