126 THE INVISIBLE WORLD 



But what have microbes to do with the oxygen 

 cycle? As much as, more, if possible, than they 

 have to do with the other cycles. True, oxygen in 

 the air inhaled into the breathing organs of every 

 living animal on the earth, there unites with car- 

 bon and is exhaled back into the air as carbon 

 dioxide. And the change in the organs is strictly 

 chemical. It is two atoms of oxygen uniting with 

 one atom of carbon, forming the molecule of the 

 carbon dioxide. Microbes have nothing to do with 

 it. 



True, in every fire that burns the same change 

 takes place. It is chemical. Microbes do not 

 touch it. Oxygen from the air unites with the car- 

 bon as the fuel burns, and comes back into the 

 air as carbon dioxide. 



True, too, in the decaying of every vegetable or 

 animal substance oxygen from the air unites with 

 the carbon of the burning fuel, and returns to the 

 air as carbon dioxide. The change here, too, is 

 chemical, exactly like the changes in the two 

 previous cases. Two atoms of oxygen lock arms 

 with one atom of carbon, thus making up the 

 molecule of carbon dioxide. 



True, it may be, that, in supporting these three 

 natural fires, 7,000,000 tons of oxygen are taken 

 from the atmosphere every day. If so, true it also 

 is that 14,200,000 tons of carbon dioxide are poured 

 from these fires back into the atmosphere every 

 day. True it is that all this sea of gas, spreading 

 itself in the air over the land, is used up every 



