CHAPTER III. 



HOW THE LIVING ELEMENTS OF DUST ARE 

 STUDIED. 



HOW do we find out how many living- 

 germs there are and of what kinds in 

 a given volume of air ? It will suffice for our 

 purposes here to say that the bacteria are so 

 extremely small that the search for them as 

 they occur in nature is ordinarily of little avail 

 by the simple use of the microscope. 



We have recourse in such studies to what is 

 called the " culture method." ' By this method, 

 instead of bringing a portion of fluid or of the 

 air in which we wish to seek for bacteria di- 

 rectly under the microscope, we mix a small 

 portion of the fluid or air with some material 

 which serves as food for the germs, and on or 

 in which they will readily grow. 



1 See " The Story of the Bacteria." 



ii 



