BACTERIA 253 



will harden in a thin layer. The part of the bottle 

 which touches the cotton must be clean and free from 

 gelatin. 



After placing the cotton in the bottles we put them 

 back into the pail and sterilize for half an hour more. 

 Let them stand for two or three days. If any spots 

 appear on the gelatin or potato, we must sterilize 

 again. Each spot contains thousands and thousands of 

 bacteria. So long as spots appear, we must repeat 

 the sterilizing. If after standing for a few days, no 

 more spots appear, we know that the bacteria are all 

 dead. If the bottles or tumblers are well sterilized 

 before the gelatin is put into them, it is seldom neces- 

 sary to sterilize more than once after adding the 

 gelatin. 



One of our bottles or tumblers we will leave just as 

 it is. We shall not expect to see any decay in this, 

 because we suppose that all the bacteria are killed. 

 The other bottles we will open so as to put bacteria 

 into them. A good way is to dip a long needle (fastened 

 in a soft wooden handle) or a hat pin into a little water 

 which contains decaying plants, and then stab it right 

 down to the bottom of the jelly, or make a cross with 

 the needle on the surface of a sterilized slice of potato. 



How do light and warmth affect bacteria? Put 

 bacteria into four of the bottles of gelatin in the way 

 just described. Keep one of these in the light. Light 

 kills many bacteria, and that is one reason why sun- 



