140 OUTLINES OF FORESTRY. 



XIV. PURIFICATION OF THE ATMOS- 

 PHERE. 



THE atmosphere covers the earth's surface as a 

 vast ocean of air that extends upwards for a dis- 

 tance of several hundred miles. 



It is composed mainly of a mixture of two gas- 

 eous substances, namely, of nearly seventy-seven 

 per cent, by weight of nitrogen and about twenty- 

 three per cent, of oxygen. Besides these there is 

 a nearly constant quantity of carbonic acid gas, 

 and a variable quantity of the vapor of water. 



The carbonic acid is nearly in the proportion of 

 four parts to ten thousand of air ; or very nearly 

 one cubic inch of carbonic acid gas to each cubic 

 foot of ordinary air. 



The different ingredients of the atmosphere 

 serve various purposes in the economy of the 

 earth. 



The oxygen is necessary for the existence of 

 animal life. 



The carbonic acid is necessary for the existence 

 of plant life. 



