SECTION VI 



BACTERIA IN AIR, SOIL, WATER, AND MILK 

 CHAPTER LI 



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; 

 humidity 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, demonstrated 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 bacteriological work and in sur- 

 gery, 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 condi- 



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