14 



A MANUAL OF BACTERIOLOGY 



usually supplied also with a safety valve. The following table 

 gives the temperatures corresponding to the pressure shown 

 on the gauge. 



TEMPERATURE CORRESPONDING TO STEAM PRESSURES 



An exposure of twenty minutes with a temperature of 120°'C. 

 is sufficient to kill all germ life. After cutting off the heat the 

 stopcock is opened and the steam allowed to escape slowly, else 

 the culture media may boil up and force the plugs out. 



Exercise 13. Results of Fractional Sterilization 



Select eight tubes of sterile nutrient agar and give successive 

 numbers. Melt the agar and cool until the tubes can be held m 

 the palm of the hand. Inoculate all the tubes from a hay infu- 

 sion. Pour tubes 1 and 2 immediately into sterile Petri dishes. 

 Place the remainder in the Arnold sterilizer and steam them 

 twenty-five minutes at 100° C. ; remove and immediately pour 

 3 and 4 into sterile Petri dishes, setting away the remainder. The 

 following day heat the remaining tubes as before and pour 5 and 6. 

 On the following day repeat with 7 and 8. Count the colonies on 

 each plate the third day after pouring, and average the duplicates. 

 Why were not all germs killed after the first heating ? What 

 explanation does this experiment give for the old theory of 

 spontaneous generation ? 



Colonies deTeloping in 

 forty-eight hours 



Agar not 

 heated 



Agar once 

 heated 



Agar twice 

 heated 



Agar three 

 times heated 



