132 THE AIR 



the air on each acre is in round numbers 45,000 tons. The 

 pressure or weight of the air, however, is never constant; it 

 varies from day to day, from season to season, and from 

 latitude to latitude. It is lower, for example, at the poles 

 and at the equator than it is between these two latitudes. 



110. Properties of the Air. — The air is usually a trans- 

 parent, colorless, odorless, mechanical mixture of gases, 

 vapor, and solids, the latter existing in exceedingly fine 

 particles. 1000 cc. of air weigh 1.293 grams. By cooling 

 and pressure it can be condensed to a bluish, mobile liquid, 

 whose boiling point is about — 195° C. Its specific gravity is 

 0.9. There exist in it particles of ice from frozen water, and 

 solid carbon dioxide. These can be removed bv filtration. 



111. Water Vapor. — Ordinary air contains varying 

 amounts of water vapor, on the average about 1.3 per cent, 

 by volume or 0.84 per cent, by weight. There is a limit to 

 the amount of water vapor that the air will retain. When 

 that limit is reached water is condensed to drops and we 

 have rain, or snow if it is cold enough to freeze the drops. 

 The higher the temperature the more water vapor can be 

 held by the air. For instance, at 0° C, 1 cubic meter of 

 air will hold 4.8 grams, whereas at 20° C, " ordinary room 

 temperature," 1 cubic meter will hold 17.1 grams. When the 

 air is saturated at any given temperature, a lowering in the 

 temperature will result in precipitation. A glass of ice water 

 " sweats," that is, moisture is condensed from the surrounding 

 air by a lowering of the temperature below which the moisture 

 can be retained. That temperature at which air begins to 

 deposit water is called the dew point and, of course, will 

 vary with the amount of water vapor present in the air. 

 Dew is deposited at night when objects are cooled off by 

 radiation to such an extent that their temperature is below 

 the dew point of the surrounding air. 



112. Temperature of the Air. — The presence of water 

 vapor in the air modifies the temperature to a very great 

 extent. Perfectly dry air absorbs practically no heat from 

 the sun's rays. They pass through and warm up the earth. 

 And in the same way at night, heat radiates from the earth, 

 passing through dry air with but little absorption. In the 



