144 PHYSIOLOGY FOB THE LABORATORY. 



a. Pour 10 c.c. of the warm nutrient gelatin into a 

 sterilized Petri dish. Add the corrosive sublimate and 

 thoroughly mix it with the gelatin by rotating the dish. 

 Add 2 c.c. of the milk and mix it with the gelatin. 



b. Pour 10 c.c. of the warm gelatin into the other Petri 

 dish. Add 2 c.c. of the milk. Place the two dishes 

 under the same conditions of heat and light and watch 

 for results from day to day. Explain. 



1. What other chemicals are used as disinfectants ? 



2. How do they disinfect? 



3. What danger is there in their use ? 



4. When may chemicals be safely used as disin- 



fectants ? 



5. When may they not be used ? 



II. THE EFFECT OF HEAT. 



a. Recall experiment A. I. b. 

 Does freezing kill bacteria? 



b. Recall experiments C. I and D. I. 



Does boiling or exposure for three quarters of an hour 

 at 100 C. by means of steam kill bacteria? 



c. Some foods and liquids would be injured by exposure 

 for three quarters of an hour to so high a temperature as 

 100 C. How was the nutrient gelatin sterilized? This 

 method is called discontinuous sterilization. The gelatin was 

 subjected to a temperature of 56 C. for three or four hours 

 on three successive days. 



1. Were all the bacteria killed on the first day? 



What happened to the spores? 



2. What took place on the second and third days ? 



