MINE VENTILATION.

CHAPTER I.

1. THE atmosphere is composed of nitrogen and oxygen, with a trace of carbonic-acid gas.

These three gases, essential to the existence of all animal and vegetable life, when taken separately will not support life. A mechanical mixture of these gases, in the proportion of four parts of nitrogen to one part of oxygen, is the air we breathe; which, if mixed with deleterious gases (or, as we say, impure), will cause serious physical disorders, and not unfrequently premature death. Carbonic-acid gas rarely exceeds one part in sixteen hundred of pure air; being present in the atmosphere, so say our best chemists, in the ratio of four parts in ten thousand. The term "atmosphere" designates that immense expanse or, ocean of gaseous matter which envelops or surrounds our earth, commonly called "air." It is supposed that this atmos

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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: