length, both the hydrogen and the carbon are burnt; and the log is slowly converted into carbonic acid and water, leaving only a handful of earth as the ashes of this lingering combustion. Even the heat generated in this form of burning is found to be precisely the same as in ordinary combustion; the only difference being that, in the latter case, it is all set free in a few hours, while in the former it is so slowly developed that it escapes our notice.

139. Causes of Decay. -Green wood decays much sooner than dry wood. Indeed, if wood be kept perfectly dry, it will not decay for ages. In the dry climate of Egypt, wooden mummy-cases have been preserved for more than three thousand years.

The decay of the green wood is due to the presence of what are called albuminous substances. The most important of these is vegetable albumen, and it is essentially the same thing as the albumen (or white) of an egg. We have said that most vegetable substances are made up of carbon, hydrogen, and oxygen; but these albuminous compounds, which form only a small part of the bulk of plants, contain an additional element, nitrogen. We have seen that the compounds of nitrogen are generally very unstable, and these albuminous substances are peculiarly so. In the presence of moisture they soon putrefy, or break up into simpler compounds. The oxygen of the air takes no part in this process, but, by contact with the putrefying substance, it is, in some mysterious way, awakened to the state of partial activity mentioned above. It is thus enabled to attack and consume the wood and all the other organic compounds present. The decay, or slow combustion, once begun, is self-sustaining; fresh portions of oxygen being continually roused to activity by the process itself, precisely as in ordinary burning (130).

When the nitrogenized vegetable compounds are burnt, the nitrogen passes off in a free state; when they decay, combined with hydrogen as ammonia.

140. Rusting. - The slow combustion of metals is called rusting, and the oxide formed is called rust. All the familiar metals, except silver, gold, and platinum, are tarnished on exposure to the air; that is, they become covered with a film of rust, or oxide.

That heat is developed by rusting, as by other kinds of slow combustion, is shown by the fact that if a large pile of iron-filings be moistened and exposed to the action of the air so that they rust rapidly, the temperature rises perceptibly.

A remarkable case of heat developed by rusting occurred in England during the manufacture of a submarine electric cable. The copper wire of the cable was covered with gutta-percha, tar, and hemp, and the whole enclosed in a casing of iron wire. The cable, as it was finished, was coiled in tanks filled with water: these tanks leaked, and the water was therefore drawn off, leaving about 163 nautical miles of cable coiled in a mass 30 feet in diameter (with a space in the centre 6 feet in diameter) and 8 feet high. It rusted so rapidly that the temperature in the centre of the coil rose in four days from 66 to 79°, though the temperature of the air did not rise above 66° during the period, and was as low as 59' part of the time. The mass would have become even hotter, had it not been cooled by pouring on water.

141. Spontaneous Combustion.-When charcoal which has been finely pulverized for making gunpowder is exposed in large heaps, the oxygen of the air combines with it slowly at first; but, as the heat developed accumulates, the oxidation becomes more rapid, until in some cases the mass takes fire and burns.

So too, when cotton or tow, which has been used for

wiping machinery, and has become saturated with oil, is laid aside in heaps, it begins to oxidize slowly; but the heat developed makes the combustion more and more rapid, until sometimes the heap bursts into a flame.

This rapid combustion, developed gradually from slow combustion, is called spontaneous combustion.

142. Respiration. - The albuminous or nitrogenized compounds, which form but a small part of the plant, make up almost the entire bulk of the animal, so that animal substances are even more prone to decay than vegetable. And this decay is not confined, as we might suppose, to dead animal matter, but is constantly going on in the living animal. The only difference is, that in the latter case the loss from decay is continually repaired, while in the former case it is not. The materials for the repair of the living body are furnished by the food.

This food is mainly made up of three classes of substances: (1) non-nitrogenized, as starch and sugar; (2) nitrogenized, as lean meat; and (3) fatty substances, as butter. All three kinds are absolutely necessary to the life of man. They are all contained in milk, which may be regarded as "the type of animal food." It contains sugar, which belongs to the first of the above classes; caseine, or curd, which belongs to the second; and butter, which belongs to the third. Bread also contains all three kinds of food; being made up (1) starch, (2) gluten (the most important nitrogenized constituent of wheat and other grain), and (3) a small quantity of oil. A man cannot live for any length of time on any one kind of food, as starch or butter, or on any mixture of kinds of food, which does not contain all three classes of substances.

of

The different classes of food serve different purposes in the body. The nitrogenized and a part of the fatty substances supply the waste which results from the action

of the varied machinery of the body. They renew the muscles, sinews, and nerves, which are constantly wearing out. On the other hand, the non-nitrogenized substances, as starch and sugar, probably take no part in repairing the machinery, but merely furnish fuel to keep up the heat of the body. To produce heat they must be burnt up, and we shall see that the process is another example of slow combustion.

Starch and sugar make up by far the greatest part of this fuel, and in fact of our food generally. They are almost identical in composition, and starch can readily be converted into sugar. When taken into the stomach, starch undergoes this change, and the sugar then readily dissolves in the water present. The solution is absorbed by the veins, and becomes mingled with the blood, which carries it to the heart. By the heart, which is made up of two force-pumps, the blood is forced through the lungs. These are composed of millions of little membranous bags or air-cells, closely packed together, and all connected by means of tubes with the windpipe, and thus with the nose and mouth. The membrane of the cells is very thin, so that they are easily compressed. The whole mass of the lungs is also very elastic, and by the action of muscles they are alternately expanded and contracted as we breathe. When they expand, the air from without rushes in and fills the cells; and when they contract, this air is forced out again.

We have said that the blood charged with sugar is forced through the lungs. The tube, or artery, which conveys it, divides and subdivides, until it is reduced to very small capillary tubes, which form a delicate network on the surfaces of the air-cells. The walls of these capillaries are very thin, and the oxygen of the air readily passes through them, by a process called osmose, and mingles with the blood. At the same time, and in the

same way, carbonic acid is given out by the blood, and mixes with the air. The blood, holding in solution both sugar and oxygen, now goes back to the heart; and by the second force-pump it is sent to all parts of the body. In the mean time, the sugar is burnt up by the oxygen which was absorbed in the lungs.

Sugar, like wood, consists of carbon, hydrogen, and oxygen. The last two are present in the proportions to form water, so that sugar may be said to be composed of charcoal and water. Of these two substances the charcoal only is combustible. This, during the circulation of the blood, is slowly burnt up by the dissolved oxygen, and converted into carbonic acid, which remains in solution until it is discharged, when the blood returns again to the lungs, or else escapes through the skin.” *

Respiration, then, is a kind of combustion, in which "the fuel is sugar, and the smoke carbonic acid and aqueous vapor." The presence of carbonic acid in the air, from the lungs, may be proved by breathing through a glass tube`into lime-water, which soon becomes milky. The presence of the watery vapor may be shown by breathing upon any cold substance.

The weight of carbonic acid breathed out by a fullgrown man, in a day, varies from 1 to 3 pounds, or from 9 to 27 cubic feet; and the weight of carbon burnt is from 5 to 15 ounces. The amount of heat produced is of course the same as would be set free by burning the same weight of charcoal in a stove. The temperature of the body is thus kept above that of the air; the heat

*Cooke's "Religion and Chemistry." The greater part of §§ 135-137 and 140-142 has been condensed from the 3d and 4th Lectures in this admirable series. We cannot too strongly commend the book to teachers. They will be certain to draw from it, for the purposes of oral instruction, far more largely than our limits have permitted here.