The specific gravity of the gases in the following table was determined experimentally by De la Roche and Berad, who took air as the standard for gases.

Manner of finding the weight of a gas compared with that of air is illustrated by the following problem. Example. If 1,000 cubic feet of air weigh 80.728 pounds when the temperature is 32° and the barometer 30", what will 1,000 cubic feet of nitrogen weigh under the same conditions?

Solution. From the table we find the density of nitrogen to be 0.9713 when air is one: hence we have the proportion 1: 0.9713 80.728: Ans., or 78.415+

pounds.

2. The miner has to deal with several gases, espe cially the coal-miner. It is therefore imperative that he should know the composition of these gases, so as to be able to distinguish them. To this end, therefore, a brief synopsis of some of the gases so often met with in mines, together with their properties, is here inserted. Nitrogen.

Symbol, N. Equivalent, 14. Specific gravity, 0.9713.

The name signifies nitre-maker. It constitutes about four-fifths of the atmosphere, and enters into a great variety of combinations. Nitrogen is somewhat lighter than air; a cubic foot of the gas weighing 0.0784167 pounds, while a cubic foot of air weighs 0.080728 pounds. Nitrogen may be obtained by burning the oxygen from a confined portion of air. It is incapable of sustaining combustion or animal life: not that it has positive poisonous properties; but flame is extinguished, and animals smother, for want of oxygen. It is best characterized by its passiveness; as it has very little affinity or attraction for other elements, and upon the slightest provocation will free itself if possible. For instance, it may be induced to combine with iodine, and form "nitric-iodide," a black, insoluble powder, which will explode if moved, jarred, or even touched with a feather. It enters into the composition of gun

powder, nitro-glycerine, and dynamite. With oxygen it forms five distinct compounds:

a, Nitrous oxide, N2O.
b, Nitric oxide, NO.

c, Nitrous anhydride, N2O3.
d, Nitrogen peroxide, NO2.
e, Nitric anhydride, N2O5.

a, Nitrous oxide, or nitrogen sub-oxide, when pure, may be respired for a few minutes with impunity. When inhaled in large quantities, it produces a lively intoxication, accompanied with violent laughter: whence it derives the name of "laughing-gas."

b, Nitric oxide, or nitrogen protoxide, may be prepared by treating copper filings, or turnings, with nitric acid. The gas obtained in this manner is colorless and transparent: in contact with air or oxygen, it produces deep-red fumes.

c, Nitrous anhydride is an obscure body, and forms, with the elements of water, an acid known as "nitrous acid."

d, Nitrogen peroxide is the chief constituent of the deep-red fumes noticed when nitrogen protoxide is brought in contact with air.

e. Nitric anhydride is a very unstable, white, solid compound, decomposing spontaneously into nitrogen peroxide and oxygen. When treated with water, it

forms nitric acid.

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.

Pounds.

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