MICROORGANISMS AND HUMAN WELFARE 15 



of students. Put the closed dishes in an oven and heat to a high 

 temperature (150 C.) for an hour to kill any germs or spores that 

 may be on the dishes. Allow the oven to cool before opening the 

 door ; otherwise the dishes are likely to crack. 



To fill the Petri dishes, melt the agar mixture in a steam sterilizer, 

 then arrange the sterilized Petri dishes along the edge of a horizontal 

 surface. Carefully remove the cotton plug from the flask, lift one 

 edge of the cover of one of the Petri dishes, pour enough of the hot 

 agar mixture into the lower part of the dish to make a layer about an 

 eighth of an inch deep, and quickly replace the cover on the dish. 

 Quickly pour into each of the dishes in turn. After the agar has 

 hardened, the dishes are ready for the experiments. Any agar 

 mixture left in the flasks should be sterilized for thirty minutes on 

 each of three successive days in order to make sure that it will keep 

 for subsequent use. 



Treat several of the Petri dishes of agar as follows : Label 

 the first dish No. 1 and keep it closed throughout the experi- 

 ments. Place a second Petri dish on the desk of a pupil, 

 remove the cover and thus for ten minutes expose the surface 

 of the agar to the air of a classroom or laboratory; label 

 it dish No. 2. In a similar manner expose the surface of 

 dish No. 3 for ten minutes to the air near the floor of a corri- 

 dor through which classes are passing. Put all three dishes 

 aside for a few days in a dark place where the temperature 

 is 80 to 90 (e.g. in a furnace room), and then examine 

 each dish. 



1. State the difference in the treatment of dishes No. 1, No. 2, 



and No. 3. In what respects have all three been 

 treated alike ? 



2. The spots on the surface of the agar are colonies of bacteria, 



each one of which has developed from a single bacterium 

 (see Fig. 11). Which of the three dishes has the largest 

 number of bacteria colonies? 



3. Suggest a reason for the difference in the number of bacteria 



colonies in the three dishes. 



4. What do you infer, therefore, as to the presence of bacteria 



in the air? 



