120 EXPERIMENTAL GENERAL SCIENCE 



matter, has weight. At sea level, a cubic foot of air weighs 

 about an ounce and a quarter. The total weight of the atmos- 

 phere is some 5,000,000,000,000,000 tons. A column of air an 

 inch square, extending from sea level upward as far as the air 

 goes, weighs nearly 15 pounds; that is, it presses downward 

 or, rather, is pulled downward by gravity, with a force of 

 nearly 15 pounds to the square inch. Since air, like other 

 gases, is easily compressed, the air at sea level is densest, 

 owing to the pressure of other air above it, but this density 

 and pressure grows steadily less as we go upward. The pres- 

 sure of the air at sea level is often taken as a standard for 

 measuring other pressures, and is spoken of as the pressure 

 of one atmosphere. 



105. Function of the Air Constituents. Oxygen is the life- 

 giving principle of the air, for without it neither animals nor 

 plants could exist. Its union with carbon in our bodies keeps 

 us warm, and similar unions elsewhere supply most of the 

 energy used in the world. Though oxygen is of such great 

 importance, it would be harmful if it occurred in greater 

 amounts, since combustion or oxidation would go on altogether 

 too fast. A cook-stove would burn up in pure oxygen. The 

 most important use of nitrogen, so far as man is concerned, is 

 to dilute the oxygen and thus put a damper on its activities. 

 It is the nitrogen which gives the air most of its weight and 

 pressure, and makes it possible for it to turn windmills, move 

 sailboats, and the like. In tornadoes, the air may move at 

 such a high speed as to uproot trees, destroy buildings, and 

 do many curious things, such as driving straws through boards. 

 The pressure of rapidly moving air also makes heavier-than- 

 air flying machines possible. So long as they are passing 

 rapidly through the air, the pressure is sufficient to keep them 

 up. Carbon dioxide, in addition to being the material from 

 which the bulk of the solid parts of plants is made, also absorbs 

 much of the heat radiated from the earth, and together with 



