26 LABORATORY GUIDE IN BACTERIOLOGY 



6. Adjust the reaction in the same manner as above. 



7. Fill into an Erlenmeyer flask and autoclav, for 

 10 minutes at 120° 



8. Keep the Erlenmeyer flask and contents for 24 

 hours in a cool place, filter, and then distribute into 



Fig. 16 



Potato Cylinder 



showing diagonal for cutting 



twenty-five tubes, and autoclav these at 120° for 5 min- 

 utes. This is "peptone-broth." 



Note. — The reason for exposing broth to a heat of 120° 

 twice is this: The solution contains a considerable quantity of 

 substances, which are precipitated by heat and appear as a sedi- 

 ment after cooling. As it is important to have a perfectly clear 

 broth in tubes, these substances are precipitated by the first 

 heating, and, if tubed later, the second steriUzation will not 

 affect the appearance of the medium. 



Broth was originally prepared from chopped beef, 

 and for many purposes this is preferable. The method 

 of preparation is as follows: 



