Elements of Agricultural Chemistry and Geology

CHAPTER X.. The Products of Vegetation—Importance of _chemical_

The Products of Vegetation—Importance of Chemical quality as well as quantity of Produce—Influence of different Manures on the quantity and quality of the Crop—Influence of the Time of Cutting— Absolute quantity of Food yielded by different Crops —Principles on which the Feeding of Animals depends —Theoretical and Experimental Value of different kinds of Food for Feeding Stock—Concluding Observations. 216

ELEMENTS OF AGRICULTURAL CHEMISTRY, &c.

CHAPTER I.. Distinction Between Organic and Inorganic

Distinction between Organic and Inorganic Substances.—The Ash of Plants.—Constitution of the Organic Parts of Plants.—Preparation and Properties of Carbon, Hydrogen, and Nitrogen.—Meaning of Chemical Combination.

The object of the practical farmer is to raise from a given extent of land the largest quantity of the most valuable produce at the least cost, and with the least permanent injury to the soil. The sciences either of chemistry or geology throw light on every step he takes or ought to take, in order to effect this main object.

Section I.—of the Vegetable and Earthy or the Organic and Inorganic

PARTS OF PLANTS.

In the prosecution of his art, two distinct classes of substances engage his attention—the living crops he raises, and the dead earth from which they are gathered. If he examine any fragment of an animal or vegetable, either living or dead, he will observe that it exhibits pores of various kinds arranged in a certain order—that it has a species of internal structure—that it has various parts or organs—in short, that it is what physiologists term organized. If he examine, in like manner, a lump of earth or rock, he will perceive no such structure. To mark this distinction, the parts of animals and vegetables, either living or dead—whether entire or in a state of decay, are called organic bodies, while earthy and stony substances are called inorganic bodies.

Organic substances are also more or less readily burned and dissipated by heat in the open air; inorganic substances are generally fixed and permanent in the fire.

But the crops which grow upon it, and the soil in which they are rooted, contain a portion of both of these classes of substances. In all fertile soils, there exists from 3 to 10 per cent. of vegetable or other matter of organic origin; while, on the other hand, all vegetables, as they are collected for food, leave, when burned, from one-half to twenty per cent. of inorganic ash.

If we heat a portion of soil to redness in the open air, the organic matter will burn away, and, in general, the soil, if previously dry, will not be materially diminished in bulk. But if a handful of wheat, or of wheat straw, or of hay, be burned in the same manner, the proportion that disappears is so great, that in most cases a comparatively minute quantity only remains behind. Every one is familiar with this fact who has seen the small bulk of ash that is left when weeds, or thorns, or trees, are burned in the field, or when a hay or corn-stack is accidentally consumed. Yet the ash thus left is a very appreciable quantity, and the study of its true nature throws much light, as we shall hereafter see, on the practical management of the land on which any given crop is to be made to grow.

Thus the quantity of ash left by a ton of wheat straw is sometimes as much as 360 lbs.; by a ton of oat straw as much as 200 lbs.; while a ton of the grain of wheat leaves only about 40 lbs.; of the grain of oats about 90 lbs.; and of oak wood only 4 or 5 lbs. The quantities of inorganic matter, therefore, though comparatively small, yet, in some cases, amount to a considerable weight in an entire crop. The nature, source and uses of this earthy matter will be explained in a subsequent chapter.

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.

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