SUMMARY 133 



necessary for animal life; nitrogen, which dilutes the oxy- 

 gen; and carbon dioxide, which is indispensable to plant 

 life. 



Water exposed to air evaporates. Through this process,, 

 the atmosphere always contains moisture. Warm air has 

 a greater capacity for moisture than cold air. The property 

 that air has of taking up a large amount of moisture when 

 heated and of depositing it when cooled is the cause of dew, 

 fog, clouds, rain, frost, snow, and sleet. When a liquid 

 evaporates it takes up heat from its surroundings. This 

 principle is employed by man in ice and cold storage plants 

 and by nature in evaporation of moisture from the surfaces 

 of animals and plants. Care should be taken in winter to 

 keep the air in houses supplied with sufficient moisture. 



Air, like every other substance, has weight. Air expands 

 as it is heated, and so warm air is lighter than cold air. Since 

 the particles of air or any other gas move freely over one 

 another, cold air will sink and force up warmer air that sur- 

 rounds it. Hot air furnaces, circulation in a refrigerator, 

 and ventilation of houses depend on this principle. 



Since anything that has weight exerts pressure on the 

 surface on which it rests, air exerts pressure at the surface 

 of the earth, which amounts to about 15 pounds to the 

 square inch. Lift pumps, siphons, and vacuum cleaners 

 are among the mechanical devices that make use of air 

 pressure. 



The volume of air decreases directly as the pressure in- 

 creases. When a given volume of air is compressed, it 

 exerts corresponding outward pressure. This principle is 

 applied in operating brakes, steam whistles, ventilating 

 systems, heavy hammers, and force pumps. 



When air or any other gas is compressed it gives out heat 



