STUDY OF BACTERIA. 103 



e. Compare the results obtained in this 

 experiment with those already ob- 

 tained in the experiment with 

 yeast. 

 C. Growth of bacteria on nutrient agar. 



Pour a thin layer of the nutrient agar into 

 several Petri-dishes which have been heated 

 for a half -hour in the steam stenlizer, quickly 

 replacing the covers on the dishes, also the cot- 

 ton plug in the flask. Number the dishes 1,2, 

 etc., and write date of each part of the experi- 

 ment. 



Keep some of the dishes carefully closed 

 throughout the experiments. Label each of 

 these dishes "Not exposed." 



Open several of the other dishes, exposing 

 the gelatin to the air of the room for 10 min- 

 utes. Replace the covers, and label each dish 

 " Exposed to the air 10 minutes." 



Open the other dishes sufficiently to spread 

 on the surface a little of the dust from the 

 floor or from the street. Label each dish " Ex- 

 posed to dirt." 



Open a third set of dishes of the gelatin and 

 pour on the surface a thin layer of the city 

 water obtained from the school faucets. Label 

 each "Exposed to city water." 



Set aside the dishes where the temperature 

 is about 70 F. Examine the dishes at the end 

 of a day or two. 



i. Do you find any differences between the 

 dishes which have been exposed to the 



