260 CRYPTOGAMS 



Set apart one dish for a control and label it A. Then 

 expose a second dish to the air for ten minutes. Close it 

 and label it B. Set it aside with A, in a warm place where 

 the thermometer reads 36 C. 



Leave the preparations and examine them from day to 

 day until spots appear on the surface of the culture 

 medium. 



Describe the color and any differences in appearance of 

 these spots. 



Look for spots on the control dish. What does this prove 

 regarding the source of the spots ? 



Conclusion. How can you explain the presence of the 

 spots in the dish which was exposed if no spots appear in 

 the control dish? 



222. BACTERIA 



Object. To demonstrate the fact that the air in some 

 localities is richer than others in bacteria. 



Apparatus. Same as in the preceding Experiment. Several 

 Petri dishes will be required. 



Method. Prepare dishes as before, setting one away as 

 a control. Then expose each of the remaining dishes for 

 ten minutes in different places, such as the cellar, pantry, 

 parlor, attic, and the street. Dishes may be exposed in 

 different houses and in different parts of the city, as streets, 

 parks, or public buildings. 



Label each dish and bring all together with the control 

 in a place where the temperature is 36 C. and again make 

 observations from day to day. 



Conclusion. State what localities give most and what 

 ones the fewest colonies of bacteria. 



