CHAPTER XXXI 



AIR FOOD AND MICROBES 



THIS kind of food, traced to its original source, 

 is found, in every instance, to be dependent, in one 

 way or another, upon the service of microbes. 



The mixing of the carbon of dead and decaying 

 animal and vegetable bodies with oxygen of the 

 air is a chemical change. But the decaying of 

 such bodies is initiated and carried forward by mi- 

 crobes. Soon after such bodies die, the microbes 

 of putrefaction attack them, tear asunder the 

 molecules of their tissues, and the carbon, thus 

 liberated, unites with oxygen of the air, rises and 

 mingles with the air as carbon dioxide; while the 

 nitrogenous matter, for the most part, returns to 

 the soil. 



The burning of all wood fires, too, on the earth, 

 is the same chemical change carbon of the wood 

 uniting with oxygen of the air, and rising into the 

 air as carbon dioxide. But the growth of the wood 

 itself is made possible by microbes fixing nitrogen 

 in the soil. 



The great quantity of carbon dioxide daily pro- 

 duced by the burning of coal is the final result of 

 the same chemical change; but the coal itself is of 



vegetable origin, and' the growth of the rank vegeta- 



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