phere is forty-five miles thick about the earth: which, however, is merely supposition, as the height has not as yet been computed with accuracy, although it has been proven that Mariotte's law is conformed to by the gases which constitute the air; their density varying according to the pressure.

That the atmosphere varies in pressure was recognized at an early period: even the Florentine pumpmakers were acquainted with the fact that water could not be raised by suction from a depth of more than thirty to thirty-three feet.

Galileo explained this phenomenon, and clearly demonstrated that the pressure of the atmosphere was equal to the weight of thirty-three feet of water.

Torricelli argued from this, that, if the atmosphere would support thirty-three feet of water, it would not support more than thirty inches of mercury; as mercury is about fourteen times heavier than water. The result of Torricelli's investigations and experiments gave us the instrument known as the barometer, by means of which we can measure the density of the atmosphere, which is on an average equal to the weight of a column of mercury thirty inches in height at sea-level. The temperature of the atmosphere is not the same throughout: it becomes colder as we ascend; hence on the top of high mountains we find snow the year round.

That air has weight may be shown by the following experiment. Take a vessel whose capacity, say, is 100 cubic inches, exhaust it of air, and then weigh it. Let it now be filled with dry air at the ordinary temperature and pressure, then weighed again. Upon second weighing it is found to be 31,074 grains heavier than at the first. As the weight of the atmosphere will sustain a column of mercury whose base is one inch. square, and whose height is thirty inches, it must press down with a weight equal to the weight of the mercury of the above dimensions to balance it. The weight of this mercury is 14.7225 pounds, and hence the atmosphere has a weight or pressure equal to 14.7 on each square inch of surface exposed to it.

Air is taken as the standard of comparison for all gases and vapors. The chemical composition of air in its natural state is given by Dr. Frankland as follows:

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, especially 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.