Page:Handbook of Meteorology.djvu/18

6 table, p. 280, shows the maximum weight of water vapor at different temperatures.

Although the proportion of water vapor mingled with the air differs from time to time, the per cent of total volume decreases from the equator toward the poles. The average annual per cent at the equator is 2.63; in latitude 70° it is only 0.22. The proportional water vapor content of the air is commonly expressed as “per cent of humidity.” Thus, with half the maximum proportion of vapor, the humidity is 50 per cent.

Sp. gr. 0.62; “boils” with vapor tension equal to that of the air at sea level, at 212° F (100° C); solidifies or “freezes” at 32° F (0° C).

Argon and the related group of elements, neon, krypton and xenon, constitute practically 8 parts per thousand of air. The gases of the argon group are chemically inert; no compounds with other elements are known to exist. This is true also of the other elements of the group. If they have any specific influence not possessed by nitrogen, the influence is not known.

Atomic weight of argon 39.88; sp. gr. 1.21; liquefies at &#8722;184° F (&#8722;120° C) under pressure of 40 atmospheres. Hydrogen is the lightest of the chemical elements, and the weight of its atom is the unit of atomic weights. Ignited with oxygen it forms water:

2H+O = H2O

Hydrogen is a constituent of all chemical compounds containing water, and of the various hydrides and hydrates. It occurs in the lower air in variable but very minute proportions which may be due to the chemical dissociation of organic matter. It is a constituent of natural gases, and of certain volcanic