BACTERIA 121 



dish, taking care to protect the dish (Why?) by hold- 

 ing the lid directly over it, but far enough above to 

 allow room for the test-tube. Any standard work 

 on bacteriology will show how this is done, or the 

 instructor may demonstrate for the class. After 

 pouring replace the lid, and set the dish outside the 

 window, or in the ice box, to cool. When cool and 

 firm the dish may be infected and put away in a 

 drawer or dark store room for a week. 



2. Infect the agar-agar by removing the lid of the Petri 

 dish and exposing the surface to the air of the labora- 

 tory, hall, library, or other place where people come 

 and go. The exposure should be made for a defi- 

 nite time, uniform for all exposures, preferably three 

 to five minutes. 



3. When the bacteria have grown sufficiently to be 

 visible in colonies (requiring usually a week or ten 

 days, depending upon the temperature), bring out 

 your cultures and examine them. Count the num- 

 ber of colonies visible to the naked eye, or with a 

 hand lens, and, on the basis of one bacterium as 

 the source of each colony, compute the number of 

 bacteria falling per square centimeter per minute 

 at the time of infection. 



4. Observe carefully the differences in the colonies as 

 regards (a) color, (b) elevation, (c) luster, (d) 

 rapidity of growth. 



5. Take two test-tubes of the same liquid culture- 

 medium, preferably bouillon, and infect them in 

 the same way. Boil one over a flame. Label 

 both and set them away where they will not be 

 disturbed. Observe at frequent intervals (e.g., 

 daily), for a week or ten days, noting especially 



