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leaves. Among them are some of the most active poisons and some of the most valuable medicines, as morphine, strychnine, and quinine. Tea and coffee owe their peculiar effects to the same alkaloid, theine. Opium is remarkable for the great number of these compounds which it contains. The narcotic power of tobacco is due to a highly poisonous alkaloid called nicotine.

171. Proteine Bodies. - Under the term proteine bodies have been included the most important nitrogenized products of plants, as albumen, caseine, and fibrine. They all have essentially the same composition, containing about 53.6 per cent of carbon, 7.1 of hydrogen, 15.6 of nitrogen, 22.1 of oxygen, and a varying quantity of sulphur not exceeding 1.6 per cent. They are found in animals as well as in plants, and there are several similar compounds found only in animals.

The proteine bodies form the most essential articles of food for animals, since, as we have learned, nitrogenized compounds are indispensable to the repair of the machinery of the body. And for these, as for all other constituents of their food, animals are directly or indirectly dependent on plants.

SUMMARY.

The atmosphere contains oxygen, nitrogen, carbonic acid, and watery vapor, with traces of ammonia and nitric anhydride.

Ordinary combustion, decay, and respiration are chemical processes, differing from one another mainly in the rapidity and the completeness with which they take place.

The division of substances into combustibles and supporters of combustion is convenient, though merely con

ventional; since combustion consists in the chemical union of two or more elements, and, in reality, one of the elements is just as combustible as another.

The present materials of the earth, being mainly compounds of hydrogen and carbon with oxygen, and of the metals with oxygen, chlorine, and sulphur, must be regarded as products of combustion.

The rusting of the metals is a process of slow combustion.

The products of combustion, decay, and respiration are chiefly carbonic acid, water, and ammonia.

The growth of plants is a chemical process, carried on in the leaf by the agency of the sunbeam. In growing, plants remove from the air carbonic acid, water, and ammonia. These compounds are broken

up;
the

oxygen in part given back to the air; and the other elements, with the remaining oxygen, re-arranged so as to form the various vegetable compounds.

By removing the products of combustion, and restoring oxygen, plants purify the air for the respiration of animals.

Animals derive all their food, directly or indirectly, from plants; while plants derive their food, directly or indirectly, from the air.

Most of the natural vegetable products are compounds of carbon, hydrogen, and oxygen ; but some contain nitrogen in addition to these, and are, therefore, called nitrogenized compounds. Of the former class, the most important are starch, sugar, dextrine, the vegetable acids, the fats and oils, wax,

the
gums and re

is, and the

vegetable alkaloids ; of the latter class, the proteine bodies, including albumen, caseine, and fibrine. The latter are the most essential articles of food for animals, since the nitrogenized products make up almost the entire bulk of the animal structure.

DESTRUCTIVE DISTILLATION AND

ITS PRODUCTS.

172. When vegetable substances are heated in closed vessels, so as to exclude the oxygen of the air, they are broken up into a number of compounds, which vary with the temperature to which they have been exposed. When vegetable matter burns or decays, these organic compounds, as we have seen, are broken up by the affinity of oxygen brought to bear upon them from without; while in the other case they are broken up by the internal action of heat. In the first case, new compounds are formed by the addition of new material; in the second case, by subdivision, without any addition of new material. The first process is called combustion; the second, destructive distillation.

173. The Preparation of Charcoal. - We have a very simple case of destructive distillation in the preparation of charcoal. This process may be illustrated by putting pieces of dry wood into a flask, closed with a cork, through which a glass tube passes. This tube passes into a cold glass bottle, and thence to a jar over water. Heat the flask, and the wood turns black. The jar fills with an inflammable gas. Hold a cold glass vessel over the flame of this gas, and moisture collects upon it, showing that water is a product of the burning of this gas. Carbonic acid also is found to be produced. This gas obtained from the wood must,

therefore, be a compound of hydrogen and carbon. A part of the carbon of the wood combines with hydrogen, to form the inflammable gas; while the greater part remains behind as a black solid. This black residue is charcoal, a form of carbon. If the heating is continued, a liquid product is also obtained.

One way of preparing charcoal is to place billets of wood in an iron cylinder, which is closed air-tight and heated to dull redness. The volatile products are driven off and allowed to escape through a flue, and the solid charcoal remains behind.

A ruder method is practised in the country, where wood is plenty. A stake is set in level ground, and brushwood heaped about its base. Wood is then stacked round the stake, so as to form a mound some 20 or 30 feet in diameter. This mound is then covered, first with leaves or turf, and then with earth, leaving only a small opening at the bottom, through which the wood is set on fire. When the fire is well under way, the mound is covered more deeply and allowed to burn slowly out. This requires about a month. The burning of a part of the wood furnishes heat for charring the rest.

174. The Products of the Distillation of Wood. When hard wood, as beech, is subjected to destructive distillation in a retort, and the volatile products are condensed in a suitable vessel, four principal classes of substances are formed: (1) gases; (2) a watery fluid ; (3) a dark resinous fluid; (4) charcoal.

(1) This product is a mixture of inflammable gases, the most important of which are the two hydrocarbons (or compounds of hydrogen and carbon), marsh-gas, H.C, and olefiant gas, H,C.

(2) This product is an acrid liquid, known as pyroligneous acid, or wood vinegar, which is used in the manufacture of acetic acid.

(3) This product is wood tar, a thick liquid, insoluble in water, but soluble in alcohol. Its chief use formerly was for tarring and calking ships, but recently it has become an important source of illuminating and lubricating oils.

These oils will be described hereafter. (4) This product is the charcoal remaining in the retort. It is used chiefly as fuel and in reducing metallic ores.

175. Ingredients of Wood Tar.- When beech-wood tar is distilled, a light oil passes over first, called eupion, or wood naphtha. It is now often sold under the name of benzole, and used as a burning-fluid, for removing oilstains from clothes, and for countless other purposes.

It burns with a brilliant white flame, free from smoke; but its extreme inflammability makes it a dangerous liquid for lamps.

After this light oil has distilled over, a heavy oil follows. It contains various ingredients, the chief of which are creosote and paraffine.

176. Creosote. - This is an oily, colorless liquid, with a peculiar smoky odor. It has remarkable antiseptic (or preservative) properties. A piece of flesh steeped in a very dilute solution of it dries up into a mummy-like substance, which refuses to decay. Meat, as tongues or hams, may be almost instantly cured by dipping it into a solution containing one part of creosote to 100 parts of water or brine. It is this substance which imparts to wood-smoke its property of preserving meat.

It is a compound of carbon, hydrogen, and oxygen.

177. Paraffine. — This is a pearly-white, tasteless, and odorless solid. The most corrosive acids and alkalies have no effect upon it. Hence its name, from parum, little, and affinis, from which affinity is derived.

It burns with a bright white flame, without smoke. It is now much employed as a material for candles, which

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