THE BACTERIA. 135 



should be neutralized again and boiled for fifteen minutes. 

 If thoroughly sterile, this nutrient gelatin should keep a 

 long time. 



I. BACTERIA IN WATER AND ICE. 



Materials. 3 c.c. of water from the faucet. 20 c.c. of nutrient 

 gelatin. A small piece of ice. 



Apparatus. Two sterilized Petri dishes. (Caution. Keep the 

 dishes covered, and do not remove the cover till your materials are 

 ready to use.) 



a. Pour the water into one of the Petri dishes, and then 

 add 10 c.c. of the nutrient gelatin which has been warmed 

 and so become liquid. Cover as soon as possible, to pre- 

 vent the bacteria in the air from falling into the gelatin. 

 Mix the gelatin and the water by gently rotating the dish. 

 Label the dish carefully, giving the material examined, 

 the date, and your name on a piece of paper placed under 

 the dish. Keep in a darkened place at about the tempera- 

 ture of 24 C. Watch from day to day, and as the colonies 

 of bacteria appear, study them. In performing the experi- 

 ments alone, several check experiments should be made, 

 but the results of the other members of the class will 

 serve as check experiments. 



Notice : 



1. The number of colonies. Each colony started 



from a single bacterium in the water. How 

 many bacteria were there in the water? 



2. The shape of the colonies as seen from the sur- 



face and from the side. What effect do they 

 have on the gelatin ? 



