Page:Handbook of Meteorology.djvu/17

Rh gaseous constituent of the air. It is derived from carbon in the ordinary process of combustion:

C + 2O = CO2

It is also derived from various hydrocarbons of rotting vegetation by dissociation and combustion, as, for instance, methane (marsh gas):

CH4 +4O = CO2 +2H2O

The normal proportion of carbon dioxide in the air is about 3⋅3 parts in 10,000 of air. In manufacturing districts, where coal is used for power-fuel, the proportion is greater. In the bracing air of a cold wave it is materially less. It is less during winter, when the temperature is below freezing and the ground is snow-covered, than in summer. Over the land the proportion is slightly greater at night than in the day-time, and during foggy weather it is materially greater than in dry weather.

In theaters, churches, schoolrooms, and poorly ventilated rooms the proportion of carbon dioxide may be as high as 12 parts per 10,000 of air; occasionally it is even greater. Breathed air is harmful, not so much on account of its carbon dioxide content as on account of the presence of products of putrefaction. Although carbon dioxide exists in the air at a calculated height of 15 miles, the proportion decreases so rapidly that it may be disregarded as a component of the air above the height of 1 mile.

Sp. gr. 1.53; liquefies and solidifies with moderate pressure at ordinary temperatures.

Water vapor in varying proportions is a constituent of the air. The maximum proportion depends chiefly on temperature. Thus at 30° F there may be nearly 2 grains by weight of water vapor per cubic foot; at 70° F, there may be nearly 8 grains. There may be less in either case, but there cannot be more; any excess will be condensed. When the maximum proportion is present the air is conveniently said to be “saturated.” The