140 PHYSIOLOGY FOR THE LABORATORY, 



d. Compare the three from day to day. Souring of 

 milk is due to the presence of certain bacteria. They 

 convert the milk sugar into lactic acid which gives the 

 sour taste and curdles the milk. 



1. In which of the three bottles is the best growth 



of bacteria? 



2. What is the effect of extreme heat and cold 



upon bacteria? 



3. Why does food keep in refrigerators? 



4. Does frozen beef decay? Why not? 



II. MOISTURE. 



Material. 10 c.c. of nutrient gelatin. 

 Apparatus. A Petri dish. 



Pour the warm nutrient gelatin into a Petri dish and 

 allow it to stand uncovered for several days. 



a. Do the colonies of bacteria appear ? 



b. How long do they grow ? 



c. What happens to them? 



d. What happens to the bacteria if the disk is placed in 

 a windy place ? 



e. Are the bacteria blown about when moist or dry? 

 This is a common source of infection in contagious 

 diseases. 



/. Should infected matter from the sick room be allowed 

 to dry? 



g. What should be done with it ? Is moisture necessary 

 to the growth of bacteria ? 



'h. Why does ham keep? 



