124 A YEAR IN SCIENCE 



Green plants. use the carbon dioxide for food, and in so 

 doing they remove it from the air. 



The little solid particles in the air we call dust. The 

 amount of dust in the atmosphere is always great. It 

 is constantly settling everywhere, indoors and out, 

 Avhenever the air is dry. Much of the dust about us 

 is harmless, but some of it is very dangerous because 

 it consists of minute organisms which produce diseases. 



Heat is received by the air from several sources, but 

 the heat from the sun is much greater than that from 

 all other sources. That the atmosphere depends chiefly 

 upon the sun for its heat is shown by the variations in 

 temperature from day to night, from cloudy days to 

 sunny ones, and from season to season. 



Moisture in atmosphere. Water vapor is supplied by 

 evaporation. It is constantly entering the atmosphere 

 from all damp surfaces and from all bodies of water. Its 

 presence in the air may be proved in various ways. If 

 a pitcher of ice water stands in a warm room, drops of 

 water appear on the outside of it. Window panes in the 

 winter are covered with moisture. 



The amount of water vapor in the atmosphere is* 

 extremely variable. It varies from place to place, and 

 from time to time in the same place. Some water vapor, 

 however, is always present, even in the desert where the 

 air seems driest. Since we can not see nor smell this 

 vapor we are usually not conscious of its presence. 



The term humidity is used in referring to the amount 

 of moisture in the air. If there is much moisture, we say 



