THE BACTERIA. 



143 



ered for half an hour. A cloth over the pail will help to 

 retain the heat. Take out the milk bottles, cool as quickly 

 as possible in cold water or on ice, and keep in a cold 



1 



FIG. 19. Apparatus for Pasteurizing Milk. 



place. The cotton plugs should be kept dry and not 

 removed until the milk is to be used. 



a. Taste the milk to determine if its flavor has been 

 changed. 



b. Keep the bottles at the ordinary temperature of the 

 room. How long does the milk keep before souring? 



c. Is the milk germ free ? Prove it. 



F. Disinfection. 



By disinfection we mean killing all the bacteria and 

 their spores. Bacteria may be killed in several ways. 



I. THE ACTION OF CHEMICALS. 



Materials. 2 c.c. of a (1 to 1000) solution of corrosive sublimate. 

 4 c.c. of milk. 20 c.c. of nutrient gelatin. 

 Apparatus. Two sterilized Petri dishes. 



