40 METHODS OF STUDYING MICROORGANISMS 



at body temperature (98 F. or 37.5 C.) in the incu- 

 bator. An incubator is a doubly insulated metal box, 

 heated by gas or electricity and controlled by an 

 automatic device by which the temperature is kept 

 constantly where desired. Practically all pathogenic 



FIG. 12 



Method of transferring cultures from one tube to another. (Hiss and Zinsser.) 



bacteria develop best at this temperature. The 

 bacteria of the soil and water probably grow best at 

 about 70 F. or 20 C. After these tubes and plates 

 have been "incubated" for twenty-four or forty-eight 

 hours the bacteriologist observes them and takes note 

 of the evidences of growth. He will make stained 



