CHAPTER XXXII 



FOOD CYCLES AND MICROBES 



As represented in Fig. 68, the atmosphere, as a 

 whole, surrounds the earth as an envelope 45 miles, 

 237,600 feet, high. It is composed chiefly of 

 four parts,- water, carbon dioxide, oxygen, and 

 nitrogen. These parts are not chemically com- 

 bined, but only mixed together mechanically. 



The air is densest at the sea level. From this 

 point upward, it becomes rarer and rarer, until, at 

 the top, it is exceedingly rare. 



Now it is estimated that, if compressed from the 

 top downward until the density should be the same 

 at every point, the atmosphere would be but five 

 miles high. It is further estimated that, if its 

 different parts should then be separated, one from 

 another, and arranged in layers from the bottom 

 according to their specific gravities, the layer of 

 water would come first, and cover the earth five 

 inches deep. Next would come the layer of carbon 

 dioxide, thirteen feet deep. Next the oxygen layer, 

 5,280 feet deep. Finally would come the nitrogen 

 layer, 21,120 feet deep. Fig. 69 represents simply 

 the order of the layers, not their relative thickness. 

 But each layer further represents a distinct cycle 



of food. i 



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