It is important that this mixture of gases that constitutes our air be maintained in the right proportion. Only a slight difference in relative amounts might be disastrous to life. An increase in the oxygen would stimulate mental and physical activities and hold the human faculties at a higher tension. Man would accomplish more in a given time, but his span of life would be shortened; and too great an increase in the proportion of this stimulating element would quickly terminate life. Conversely an increase in the nitrogen would render all life more lethargic and man would be slower to act and to think; and too great an increase would smother every living thing. In addition to the gases named, the air contains small amounts of many other substances,—argon, nitric acid, ammonia, ozone, xenon, krypton, and neon; as well as organic matter, germs, and dust in suspension. Over the land it contains sulphates in minute quantities, and over the sea and near the seashore salt left from the evaporated spray. The proportion of each component of the atmosphere by volume of the total atmosphere is different from its proportion by weight. The percentages for the more abundant gases are as follows: ===============+=============+=========== | BY VOLUME | BY WEIGHT ---------------+-------------+----------- Nitrogen | 78.04 | 75.46 Oxygen | 20.99 | 23.19 Argon | 0.94 | 1.30 Carbon dioxide | 0.03 | 0.05 +-------------+----------- | 100.00 | 100.00 ===============+=============+=========== =Nitrogen.= Its principal functions are to dilute the oxygen and to furnish food to vegetation. It is inert and does not manifest many marked chemical affinities. Its lack of activity is shown by the fact that it will neither support combustion nor burn. =Oxygen.= Oxygen, unlike nitrogen, is an active element that readily enters into chemical combination with many other elements, and it is second in quantity to nitrogen. With hydrogen it constitutes eight ninths, by weight, of water; combined with other elements it constitutes forty to fifty per cent. of the crust of the earth. It burns so readily that were it not greatly diluted by an inert gas like nitrogen it would be difficult if not impossible to stop a conflagration when once started. It is the vitalizing principle in all forms of life. By its chemical union with carbon in the tissues of plants and animals it develops the energy manifested in their movements.