112 EXPERIMENTAL PHYSIOLOGY 



four tubes. In which is the milk still fresh? What is 

 the effect of heat on bacteria? of cold? 



Remove the stopper from No. 2 and the ice from No. 1, 

 and place with No. 3 for several days longer. After that 

 time examine the tubes. In which is the milk still fresh? 

 From these experiments, what are your conclusions as to 

 the source of bacteria? Is air that is freed from bacteria 

 by being strained through cotton, a cause of decay? 



C. Prepare two tubes as follows: Boil some milk and fill 

 two tubes that have been previously boiled in water. Stop- 

 per with sterilized cotton and allow the tubes to cool to 

 room temperature. Now introduce hi to one tube some scum 

 from the hay infusion and res topper quickly. Set it in a 

 warm place. Prepare a two per cent solution of corrosive 

 sublimate. Mix a teaspoonful of this with some scum from 

 the hay infusion and let it stand for ten minutes. Then 

 introduce this into the second tube and stopper again quickly ' 

 with the cotton. Place it with the other tube. Examine 

 after a day. In which tube has the milk decomposed? What 

 must the corrosive sublimate have done to the bacteria? 

 What substance may be used to kill bacteria in place of 

 heat? Killing bacteria by heat is called sterilizing; chemical 

 substances that perform the same action are called antisep- 

 tics. What are some of the common antiseptics? 



Summarize your results under the following heads : (1) con- 

 ditions favorable to bacteria growth; (2) conditions unfavor- 

 able to bacteria growth; (3) methods of killing bacteria. 



