SECTION V 



BACTERIA IN AIR, SOIL, WATER, AND MILK 



CHAPTER LII 

 BACTERIA IN THE AIR AND SOIL 



BACTERIA IN THE AIR 



BACTERIA reach the air largely from the earth's surface, borne 

 aloft by currents of air sweeping over dry places. Their presence in 

 air, therefore, is largely dependent upon atmospheric conditions ; humid- 

 ity and a lack of wind decreasing their numbers, dryness and high 

 winds increasing them. Multiplication of bacteria during transit 

 through the air probably does not take place. 



Apart from these considerations the presence of bacteria in air also 

 depends upon purely local conditions prevailing in different places. 

 They are most plentiful in densely populated areas and within buildings, 

 such as theaters, meeting halls, and other places where large numbers of 

 people congregate. On mountain tops, in deserts, over oceans, and in 

 other uninhabited regions, the air is comparatively free from bacteria. 

 A classical illustration of this fact is found in the experiments which 

 Pasteur carried out in his refutation of the doctrine of spontaneous 

 generation. Tyndall also, in working upon the same subject, demon- 

 strated this fact. From the surface of the ground and other places 

 where bacteria have been deposited, they reach the air only after 

 complete drying. It is a fact of much importance, both in bacterio- 

 logical work and in surgery, that bacteria do not rise from a moist 

 surface. From dry surfaces they may rise, but only when the air is 

 agitated either by wind or by air-currents produced in other ways. 

 In closed rooms, therefore, even when bacteria are plentiful and the walls 

 and floors are perfectly dry, there is little danger of the inhalation of 

 bacteria unless the air is agitated in some way. The most favorable 

 conditions for the occurrence of many bacteria in air are the existence 

 of a prolonged drought followed by a dry wind. Under such condi- 



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