CHAPTER XXVI 

 BACTERIA OF THE AIR 



Introduction. Air practically always contains bacteria. 

 In crowded cities, dusty streets, and elsewhere, the numbers 

 may be very large. On high, snow-capped mountains and 

 over the sea at some distance from the shore the bacteria are 

 very few. The bacteria are not constant in their presence in 

 the air, but vary at different times of the year and under 

 different conditions. It has been supposed in the past that 

 the bacteria of the air frequently include those of great disease- 

 producing power, and it has been supposed that certain dis- 

 eases could be carried for long distances through the air. 

 Modern views, however, seem to indicate that the bacteria 

 of the air are far less dangerous than was formerly thought. 



Condition in Air. The bacteria of the air are not in a 

 state of active growth, but are in a dormant condition while 

 in the air. A moment's reflection will show that this is of 

 necessity so. Bacteria would be quite unable to get their 

 food in a dried condition. Furthermore, the bacteria are usu- 

 ally attached while in the air to some dust particle, and very 

 frequently they ride on these dust particles in small masses 

 and not as individuals. 



Origin of Bacteria. The air bacteria are derived very 

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