Page:The fairy tales of science.djvu/58

36 This beautiful experiment proves air to be a mixture of oxygen and a certain gas in which no ordinary combustible will burn. This gas has been named azote or nitrogen.

Oxygen forms about one-fifth of the atmosphere, and nitrogen nearly the remaining four-fifths; to these components must be added about one two-thousandth part of a gas called carbonic acid, and traces of another body called ammonia. Though these two last-named constituents bear such a small proportion to the others, we shall presently see that they have important duties to perform in the economy of nature.

The composition of the atmosphere is everywhere uniform; we may bring down air from the summit of the highest mountain and collect it in the deepest valley, but we shall not be able to detect the slightest variation in its composition.

The same uniformity is apparent whether we examine the air of the polar regions or that of the tropics; whether we collect it in the densely populated city or in the untrodden forest. This fact seems all the more wonderful when we consider the contaminating influence of the countless exhalations that are continually rising into the atmosphere. The clouds of smoke poured forth by our chimneys, the expired breath of animals, and the gases that proceed from decaying matters, do not perceptibly disturb the equilibrium of the constituents of the atmospheric ocean.