gases. The proportion increases as the height in the air increases. On account of its lightness meteorologists are of the opinion that the rapid movement of the earth in space is constantly throwing it off into space. At all events, there seems to be sufficient evidence that it is the chief if not the sole constituent of the outer part of the atmosphere. It is much used for the inflation of balloons and airships, being about 15 times as buoyant as air.
Atomic weight 1; sp. gr. 069; liquefies at about −375° F (−226° C) under a pressure of 15 atmospheres.
Helium is another inert element. It is a constituent of several minerals, including pitchblende, an oxide of uranium. It occurs by absorption in many deep rocks and also in the gases that escape from deep springs. Cottrell discovered it in the proportion of about 2 per cent in certain Texas gas wells. Because it is non-explosive and non-inflammable, it has been used in the inflation of balloons. Its buoyancy is about 92 per cent of that of hydrogen and it does not readily pass through balloon fabrics. Because of its lightness and also its high molecular speed, it is thought to occur chiefly in the outer shell of the atmosphere—possibly escaping from the earth altogether. If it plays any part in meteorological phenomena, its influence is not known.
Atomic weight 4; sp. gr. approximately .128; liquefies at −452° F (−269° C) at a pressure of about 3 atmospheres.
Nitric acid (HNO3) and ammonia (NH3) are present in the air in very minute proportions. Nitric acid is most readily detected at the time of thunderstorms. From time to time the proportions of these substances vary greatly from the proportions noted in the table. The presence of ammonia is due probably to the decomposition of organic matter.
Ozone (O3) is an allotropic form of oxygen, whose normal molecule is O2. Ozone possesses a pungent odor that frequently is discernible at the time of nearby lightning discharges and the passage of high-potential electric sparks. The normal proportion in the air is exceeded many times over during thunder-storms.
The proportion of ozone varies with environment. It is greater over the sea than over the land—possibly due to the lack of oxidizable matter; and this may explain its greater pro-
