The values of all except the first three are variable; that of floating dust, hydrogen, and helium is empiric and calculated.
The values determined by Humphreys, are those of air from which the water vapor has been removed—that is, of dry air.
The distribution of the constituents of the air.
The foregoing represent the proportions at the surface of the earth. The proportions change with increasing altitude. The nitrogen disappears at a calculated height of 84 miles; the oxygen, at about 60 miles. Water vapor is calculated to exist at an altitude of 60 miles, but it is not observable above 7 or 8 miles. Carbon dioxide is not observable above an altitude of 2 or 3 miles; theoretically it may extend to a calculated height of more than 15 miles. The proportion of hydrogen and helium,
