100 THE ATMOSPHERE AND ITS SERVICE TO MAN 



are low and sheltered from the wind, the heavy gas accumu- 

 lates in sufficient quantities to be fatal to animals that at- 

 tempt to pass through them. 



Moisture in the Air : Evaporation. The atmosphere 

 at all times and under all conditions contains some mois- 

 ture. In the air of even the driest desert there is some 

 water vapor. Plants and animals both need it. Were 

 it not for the moisture in the air there would be no rain ; 

 and without rain no land life could exist. Thus the air, 

 which contains oxygen and water vapor for both plants 

 and animals, carbon dioxide for plants, and nitrogen to 

 dilute the oxygen, is one of the most important life factors 

 of the earth. 



Experiment 36. Carefully weigh a dish of water and put it in 

 a convenient place where there is free access of air. After some 

 hours weigh it again. What causes the change of weight ? Try 

 this experiment with a test tube, a watch crystal, and a wide- 

 mouthed beaker, under various conditions and in various places. 



When water is exposed to the air, it gradually disappears 

 into the surrounding atmosphere. This process is called 

 evaporation. Evaporation takes place only from the sur- 

 face of a body of water. It may occur at any temperature ; 

 but since heat is absorbed in the process of evaporation 

 (page 85), the more heat there is available, the more 

 rapid will be the evaporation. 



Evaporation must not be confused with boiling. Heat 

 is absorbed in both processes; but boiling takes place only 

 at a definite temperature and goes on inside the liquid. 



If the water surface is large and the temperature high, 

 there is a large amount of evaporation and the water rapidly 

 rises into the air. In the tropics the evaporation from the 



