PURIFICATION OF THE ATMOSPHERE. 143 



Jack Sprat and his wife, of nursery lore, who 

 between them kept both plate and platter clean. 

 Each liked and thrived on what the other re- 

 jected. 



If this balance between the plant and animal 

 kingdom is disturbed, the composition of the 

 atmosphere will be altered, and a marked change 

 will be produced in the earth's plant and ani- 

 mal life. Such changes have been observed in 

 the geological past long before the creation of 

 man. 



The earth's atmosphere was originally vaster 

 than at present. The quantity of oxygen and 

 carbonic acid gas was enormously greater. 



A careful estimate places the amount of oxygen 

 that exists, combined with the different substances 

 that form the fifteen or twenty miles of the earth's 

 crust that have been carefully studied, at least at 

 one-half of the total weight. 



At an early age in the earth's life this oxygen 

 existed in a free state in the atmosphere, and be- 

 came fixed by combining with or oxidizing the 

 different materials of the crust. This oxidizing 

 action has now practically ceased, and the quantity 

 of oxygen present in the air is constant. 



Prior to the Carboniferous age carbonic acid 



