ATMOSPHERE 29 



Though the amount in the air is very small, it is of 

 some importance to plants. 



30. Moisture. — Water is found in the air as a gas 

 known as w v 1 1 i; VAPOR. The amount of water vapor 

 in the air is very variable, but warm air contains more 

 than cold air. In «•«»!<! air the water vapor condenses to 

 form clouds or mist, in which form it is no longer a gas, 

 but a liquid. Notice how drop* of water collect on the 

 outside of a :J;i>> of Ice-cold water; the warm air which 

 contain! wat. r vapor comes in contact with the cold sur- 

 face of the glass, is cooled down, and the water vapor 

 condenses to form drops of water. This is a good ex- 

 ample of how dew is formed. At night when no heat 

 comes from the sun the air becomes quite cool — cooler 

 than the earth, which is warmed by the heat it has 

 ! np <hiring the day. Now, when the air becomes 

 cool enough, the water vapor it contains condenses to 

 form drops of water, which are gently deposited on the 

 surface of the earth and form dew. Dew is rain, formed 

 the earth's surface. When the air becomes sufti- 

 ( i« ntlv cold, the dew freezes and forms frost. 



Moisture in the air e^rves a useful purpose in helping 

 to keep the earth warm. The heat waves which come 

 from the 6un move with wonderful rapidity, about 

 186,000 miles in a second of time, and pass through the 

 atmosphere as easily as water through a sieve. Striking 

 the earth's surface, these best waves warm it up; in 

 other won -tart up other heal among the 



surfn. of the earth. The he* 1 waves thus 



started in the earth's surface move mucTi more slowly 

 than the heat waves from the sun, and they in time 



