Page:Handbook of Meteorology.djvu/16

4 lower shell of air. Below this plane practically all the local movements, especially the upward and downward, or convectional movements of the air occur. The lower or convectional shell is the troposphere; the upper shell is the stratosphere.

Constituents of the Air.—Nitrogen, the constituent of greatest volume at the surface of the rock sphere, is very inert. It does not combine with the oxygen of the air, except in very minute quantities when influenced by lightning discharges. The nitrogen of the air is now used in the manufacture of ammonium nitrate, the basis of certain explosives. Nitrogen is the chemical base of nitric acid, HNO3, and of several other oxygen compounds. It is a constituent of ammonia gas, NH3, and of cyanogen, CN, all of which enter into the structure of many thousand other compounds. Many of these compounds are very unstable; hence the rapid decomposition of animal and vegetable compounds, commonly known as putrefactive decay. The instantaneous dissociation of the nitrogen constituents of such compounds as nitroglycerine and tri-nitrotoluol, or TNT, give to such compounds their value as explosives.

Atomic weight 13·93; sp. gr. .971; temperature of liquefaction &#8722;231° F (&#8722;146° C) at 35 atmospheres pressure.

Oxygen is the active chemical element of the air. It unites readily with pretty nearly every other chemical element. Its union with carbon is the ordinary process of combustion. Iron wire in free oxygen burns about as freely as a match in the open air. The oxygen of respiration oxidizes the impurities of the blood.

The percentage of oxygen is slightly greater in the air of northerly winds of the north temperate zone than in southerly winds. It is slightly below normal over cities, as compared with open spaces. In crowded auditoriums the proportion of oxygen sometimes falls to 20 per cent; in mine tunnels it is sometimes as low as 18 per cent. Candles burn with difficulty with the oxygen content at 18 per cent; and human life cannot long exist with the proportion of oxygen as low as 17 per cent.

Atomic weight 15·88; sp. gr. 1.106; temperature of liquefaction — 182° F (119° C) at 51 atmospheres.

Carbon dioxide (carbonic acid gas), CO2, is the heaviest