PURIFICATION OF THE ATMOSPHERE. 145 



Boil, there will also follow a permanent disturbance 

 from the inability of such section of country to 

 sustain any plant life. 



As to the purification of the atmosphere by 

 plants, Dana, in his " Manual of Geology," * says, 

 on page 353, 



" In the present era, the atmosphere consists essentially of 

 oxygen and nitrogen, in the proportion of twenty-three to 

 seventy-seven parts by volume. Along with these constitu- 

 ents, there are about four parts by volume of carbonic acid in 

 ten thousand parts of air. Much more carbonic acid would 

 be injurious to animal life. To vegetable life, on the contrary, 

 it would be, within certain limits, promotive to growth ; for 

 plants live mainly by means of the carbonic acid they receive 

 through their leaves. The carbon they contain comes princi- 

 pally from the air. 



" This being so, it follows, as has been well argued, that the 

 carbon which is now coal, and was once in plants of different 

 kinds, has come from the atmosphere, and, therefore, that the 

 atmosphere now contains less carbonic acid than it did at the 

 beginning of the Carboniferous period, by the amount stowed 

 away in the coal of the globe. 



******** 



" Such an atmosphere, containing an excess of carbonic acid 

 as well as of moisture, would have had greater density than 



* Reprinted, by permission, from a " Manual of Geology," 

 third edition, by James D. Dana. New York : Ivison, Blake- 

 man, Taylor & Co. London : Trubner & Co. Pp. 911. 

 a k 13 



