56 LOUIS PASTEUR 



these are present even in ordinary air in astonish- 

 ingly large numbers. 



It is no wonder then, even if only an occasional 

 dust particle contains a bacterium, that it is so 

 difficult to keep out all of these intruders. The 

 bacteriologist who wishes to have air admitted to 

 his materials employs plugs of cotton wool which 

 allow air to enter, but filter out all solid particles. 

 Bacteria may be grown in many kinds of media, 

 some species in one medium and others in other 

 media; a few forms cannot be cultured artificially 

 at all. A very convenient and much employed 

 method of culture is partly to fill a test-tube with 

 the culture medium, plug it with cotton wool, 

 sterilize it by heating, and then to introduce from 

 the point of a needle a minute amount of material 

 containing bacteria. The growth of the colony 

 may often be followed by observing the clouded 

 area in the culture medium. Colonies may be 

 transplanted from one tube to another and kept 

 going for an indefinite time. 



The varied applications and refinements of the 

 technique of bacteriology would require a volume 

 to describe. Perfection of technique has been 

 brought about as bacteriology has progressed and 

 become applied to different fields. Undoubtedly 



