METHODS OF STUDY 25 



in the hot-air sterilizer at 150 C. (302 F.) for sixty 

 minutes, when the cotton plugs will be charred to a 

 light brown color and will retain their shape. 



Culture Media. Certain food substances are nec- 

 essary for the cultivation of bacteria, which are 

 known as culture media, and by the behavior of dif- 

 ferent species upon the various media it is possible for 

 the observer to recognize their kind. 



The most common media are bouillon, gelatin, 

 agar, potato, and blood-serum, besides a multitude 

 of special media for more advanced work. 



Formula for making culture media are a part of 

 the equipment of every laboratory, and must be so 

 closely followed that no attempt will be made to 

 embody them here. 



The making of culture media is excellent practice, 

 as the most accurate measurements and management 

 are required ; otherewise failure is a sure result. 



Staining. As before mentioned, the staining of 

 bacteria is an important method of distinguishing a 

 species, as various kinds stain in a fixed and char- 

 acteristic manner. The aniline dyes used most 

 commonly are gentian-violet, methylene-blue, and 

 fuchsin (reddish pink). 



A sterilized slide has placed upon it a drop of 

 water to which is added a small amount of bacterial 



