Oxygen.

Symbol, O. Equivalent, 16, Specific gravity, 1.1056.

3. Oxygen forms one-fifth part of the atmosphere. It is transparent and colorless, not to be distinguished by its aspect or smell from atmospheric air. It is the most widely diffused of all the elements, forming about one-third of the solid crust of the globe. It unites with all the other elements to form compounds, which are sometimes gaseous, sometimes solid, sometimes liquid. The name signifies acid-former; and, with one exception, oxygen enters into the combination of acids. All the ordinary phenomena of fire and light which we daily witness depend upon the union of the body burned. with the oxygen of the air: in fact, the term "oxidation" may, for all ordinary purposes, be regarded as synonymous with "combustion.”

Faraday has roughly estimated that the amount of oxygen required daily to supply the lungs of the human race is at least one thousand millions of pounds; that required for the respiration of the lower animals is at least twice as much as this; while the always active process of decay requires certainly no less than four times as much. Faraday also estimates that one thousand millions of pounds are sufficient to sustain all the artificial fires lighted by man, from the camp-fire of

the savage to the roaring blaze of the blast-furnace, or the raging flames of a grand conflagration.

Amount of Oxygen required Daily.

Whole population

Animals

Combustion and fermentation

Decay and other processes

Total amount of oxygen required daily .

Pounds.

1,000,000,000

2,000,000,000

These figures are inconceivable; and, when we reduce the oxygen consumed to tons, we fail to grasp it, as it is no less than 3,571,428 tons.

Although the consumption of oxygen is so great, yet there is no fear of its being exhausted; as, at the present rate of consumption, there is enough to last nine hundred thousand years. Oxygen is the active principle of the atmosphere. It devours every thing with which it can unite: it corrodes metals, decays fruits, promotes combustion, and is a prime necessity for health.

The body is a stove, in which fuel is burned; the chemical action being the same as in any other stove. We take into our lungs air, and give out a poisonous gas,carbonic-acid, the waste products of the combustion of our bodies. From this we may learn how important a factor oxygen is for health, and how necessary it is that we have plenty of fresh, pure air, if we

wish to be free from disease. One man breathes into his lungs at each inspiration about 230 cubic inches of air, or one gallon. In the delicate cells of the lungs the air gives up its oxygen to the blood, receiving, in turn, carbonic acid and water, foul with waste matter which the blood has picked up in its circulation through the body. Should we rebreathe it into our lungs, the blood will leave the lungs, not bearing invigorating oxygen, but refuse matter to obstruct the whole system. Without oxygen the muscles become inactive, the heart acts slowly, food is undigested, brain is clogged, and at last such fatal results as were manifested in the "Black Hole of Calcutta " implore us not to be stingy or afraid of "God's blessing,"-pure air.

4. Having examined slightly the constituent parts of the atmosphere, let us briefly examine the principal gases met with in coal-mining.

Carbonic-Acid Gas.

Symbol, CO2. Equivalent, 22. Specific gravity, 1.53.

One cubic foot of the gas at 32° F., and barometer at 30", weighs 0.12845 of a pound.

This gas is composed of carbon and oxygen. Miners have given it several names, such as "stythe," "chokedamp," "black-damp," and "after-damp." This gas is

always produced when compounds containing carbon are burnt in air or oxygen. It may be produced by treating limestone or marble with hydrochloric or sulphuric acids. The occluded gases in all coal contain carbonic acid. Carbonic acid is considered poisonous, on account of the many deaths which have resulted from burning charcoal and carbonaceous materials in places where there was a deficiency of ventilation, and by reason of the fatal nature of after-damp of explosions in coal-mines. Its specific gravity is 1.524; so that it is a little more than one and a half times as heavy as air. It lodges near the floor of places in which it is evolved when little more than mutual diffusion is going on. Owing to its great density, it may be poured from one vessel to another. It is the only gas, except nitrogen, which is evolved by most bituminous coals; and, when it is given off in quantity, active ventilation is required to carry it off.

Le Blanc, and many other chemists, affirm that air containing more than five parts in a thousand is injurious to breathe. Mr. J. W. Thomas of England, while not asserting that it is not injurious, says, that "in levels and seams of semi-bituminous and bituminous coals in South Wales, in part or wholly worked to the dip, with scanty ventilation in some particular spots, through the non-completion of air-splits, or conveyances, men often

work in an atmosphere containing from two to five per cent of this gas for hours." Be that as it may, the system, uninspired by the energizing oxygen, is sensitive to cold. The pale cheek, the lustreless eye, the languid step, shortness of breath, speak but too plainly of oxygen starvation. "In such a soil, catarrh, scrofula, miners' asthma, and consumption run riot."

Miners call the carbonic acid produced by the explosion of fire-damp, "after-damp.' They fear it almost as much as fire-damp, as it instantly destroys the lives of all who may have escaped the flames of the explosion. This property of carbonic acid, of choking or smothering, has of late years been made use of for putting out fires in coal-mines. In one case, an English mine which had been burning twenty years was smothered by pouring into it eight billion cubic feet of carbonic acid, and then closing it up for one month. At the end of the month the mine was opened, and found to be ready for the resumption of labor.

When found alone in a mine, carbonic acid is not considered as dangerous as fire-damp, since it will not burn. Carbonic acid, at the ordinary temperature and pressure, is a gas. It solidifies when subjected to great pressure; but, as soon as the pressure is removed, it returns to the gaseous state: therefore the term “ carbonic acid" is applied as well to the gas as to the acid.