The Atmosphere. 



25 



factor in the development of the present extensive Weather 

 Bureau service of the United States government. The records 

 of this Bureau are of great service not only in predicting storms 

 and frosts, but in mapping restricted areas, such as the sugar beet 

 belt, which will be favorable for certain crops dependent upon 

 uniform temperature and proper amounts of sunshine and rain- 

 fall. 



Air is a mixture. A chemical compound is characterized by 

 uniform composition. That is, the constituents of a single com- 

 pound occur in the same proportions throughout its mass. This 

 is not true for air, as the following table shows : 



Percentage Composition of the Atmosphere at Different Levels. 



The air is a mixture of water vapor, gases, and solids in which 

 the gases form far the greatest part. Since it is a mixture, the 

 constituents are free to separate and, as the above table shows, the 

 heavier constituents are absent in the higher layers. 



Composition of air. The average composition of dry air is as 

 shown in the table on page 26. 



Water of the atmosphere. The water used by plants is taken 

 up from the soil by way of the roots. Its passage through the 

 plant and the escape of excess of water are regulated by the 

 process of transpiration or evaporation from the surface of the 

 leaves into the air. From the current of water thus maintained 

 from the soil to the plant, growing crops assimilate all of their 

 food except carbon dioxide. When the air is dry it absorbs water 

 readily and promotes transpiration. Moist air, on the contrary, 



