DISTRIBUTION OF BACTERIA 115 



arise from clothes and skin. In the air of schoolrooms 

 or audience rooms the number of bacteria is very great, 

 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 OQ o *? 

 dust; but among the dust abed 



particles we may be sure to _ 



x? = a <? ff *=-, x //- 

 find bacteria in great num- Jj|/ ^jJ ~"5^ 



bers. In short, all air in the e ^ ' 



vicinity of habitation contains 



* FIG. 55. A group of bacteria in 



bacteria. The air of high water. 



mountains far from the habi- 



tat ion of animals is found lans '> c > bacillus **t*rficiaiis ; <t, ba- 



cillus rubescens ; e, bacillus hyalinus ; 



to be moderately free from /, badiius deiicatuius , g, badiiusjan- 

 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 



