PART II. 



PHYSIOLOGY. 



CHAPTER VI. 

 GENERAL CONDITIONS FOR GROWTH. 



OCCURRENCE. 



Bacteria are probably the most widely distributed of 

 living organisms. They are found practically everywhere 

 on the surface of the earth. Likewise in all surface waters, 

 in streams, lakes and the sea. They occur in the air imme- 

 diately above the surface, since they are carried up mechan- 

 ically by air currents. They cannot fly of themselves. 

 There is no reason to believe that any increase in numbers 

 occurs to an appreciable extent in the air. The upper air, 

 for example, on high mountains, is nearly free from them. 

 So also is the air over mid-ocean and in high latitudes. As 

 a rule, the greater the amount of dust in the air the more 

 numerous are the bacteria. Hence they are found more 

 abundantly in the air in cities and towns than in the open 

 country. The soil is especially rich in numbers in the upper 

 few feet, but they diminish rapidly below and almost disap- 

 pear at depths of about six feet unless the soil is very porous 

 and open, when they may be carried farther down. Hence 

 the waters from deep wells and springs are usually devoid 

 of these organisms. In the sea they occur at all levels and 

 have been found in bottom ooze dredged from depths of 

 several miles. It is perhaps needless to add that they are 

 found on the bodies and in the alimentary tract of human 

 beings and animals; on clothing, utensils; in dwellings, 

 stables, outhouses, etc. From one-fourth to one-half of the 

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