88 LABORATORY GUIDE IN BACTERIOLOGY 



through the broth in flask a. The flask is then dis- 

 connected, and I c.c. is plated in agar and i c.c. in 

 gelatin. The former is incubated at 37° C, and the 

 latter kept at room temperature. After 48 hours the 

 colonies are counted, and the result is multipUed by 

 50. This then represents the amount of bacteria in 

 5 liters of air. 



» t -jL 



Fig. 20 

 Apparatus for Determining tlie Number of Bacteria in a Definite Volume of Air 



a. Erienmeyer flasli /. Five-liter flask 



b. Rubber stopper g, h. Glass tubes 



c. d. Glass tubes i. Finchcock 



EXERCISE 3. BACTERIOLOGICAL STUDY OF MILK 



The method for determining the number of bacteria 

 in milk is fundamentally the same as for water, except 

 that dilutions must be carried higher, as milk generally 

 contains larger numbers of bacteria. 



Sterilization and pasteurization of milk. — Some of 

 the germs in milk are saprophytes (which under favor- 



