DISTRIBUTION OF BACTERIA 1 35 



arise from clothes and skin. In the air of schoolrooms 

 or audience rooms the number of bacteria is large, and 

 there are more at the close of a school session than 

 at the beginning. There are more bacteria in the air 

 of a poorly ventilated schoolroom than in the air of a 

 sewer. The presence of animals as well as of men 

 always increases the number of bacteria in the air. 

 Wherever we find dust, there we find bacteria. By this 

 it is not meant that dust is composed wholly of bacteria, 

 for many other things go to 

 constitute what we know as 

 dust; but among the dust 

 particles we may be sure to 

 find bacteria in great num- 

 bers. In short, all air in the 



vicinity of habitation contains 



. . r u- u Fig. 55. A group of bacteria 



bacteria. The air of high from water. 



mountains far from the habi- „ .„ 



tation of animals is found frodighms; c, Pseudommas jamhi- 



to be moderately free from nus ; d, Pnteus vulgaris ,- j, Mkro- 



. , . . . coccus aquatiUs; /, Bacillus coll. 



these ubiquitous organisms. 



Elsewhere they are present in abundance. Since this is 

 the case it is quite impossible for any material exposed to 

 the air for even a short time to escape a rapid contami- 

 nation with microorganisms. 



Water. Practically all bodies of water on the surface 

 of the earth are filled with bacteria (Fig. 55). The 

 number found in water, however, is widely variable. In 

 spring water which comes fresh from the ground the 

 number is small, and in some cases they may be wholly 

 absent. The same thing is true of the water of artesian 



