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Properties of Gases

Composition of the Air. - Just as water is the most widely distrib­uted and most important of liquids, so the air is the most important and intimate of gases. It consists for the most part of two elements that are mixed together but not chemically combined. These elements are oxygen and nitrogen. In spite of the fact that there is no chem­ical union between them, the composition of the air is extraordinar­ily constant. Up to a height of 7 miles it always contains about 21 parts of oxygen to 79 parts of nitrogen. Besides oxygen and nitrogen, the air contains small parts of other gases, the most important of which are water vapor and carbon dioxide.

Weight of Air.- To an ordinary observer the air seems to have no weight and to offer little resistance to bodies moving through it. Yet smoke rises through the air and small balloons ascend out of sight. This is because the air is denser than the gas with which the balloon is filled. The heavier air crowds the lighter gas upward as a piece of wood is forced to the surface of the water because it is light­er than water.

If a hollow glass sphere provided with a stopcock is weighed when the stopcock is open and then connected to an air pump by which as much of the air as possible is removed from the sphere, and if now the stopcock is closed and the sphere weighed a second time, it is found that the second weight is less than the first. The difference be­tween these two weights is the weight of the air removed from the sphere. If the volume of the sphere is known and if it is almost com­pletely exhausted, a fair approximation to the density of the air can be obtained by this method. A liter (1,000 cu. cm.) of air at the tem­perature of melting ice and under standard conditions of pressure weighs 1.293 g.

Compressibility of Gases. - If an attempt is made to decrease the volume of a liquid by the application of pressure, it is found that it is necessary to apply an enormous pressure in order to get appreciable changes in volume. The behavior of gases in this respect is quite different. It is easy to compress the body of air so that it occupies only one-third or one-tenth of its original volume. As soon as this pressure is removed, the air or other gas springs back to its original volume. The tires of automobiles are ordinarily filled with air. As more and more air is forced into the tire, the volume of the tire increases very little; but the air taken from the outside is forced to occupy much less volume than it originally occupied. As the air is forced into the tire, the pressure it exerts is more and more increased.

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