THE BACTERIA. 141 



III. LIGHT. 



Materials. 10 c.c. of nutrient gelatin. 

 Apparatus. A sterilized Petri dish. Black paper. 



Pour the liquid nutrient gelatin into the dish and expose 

 it two or three minutes to the air. Why? Cover and 

 paste a piece of black paper over half of the dish and 

 expose it to strong sunlight in an ordinary room. Watch 

 it from day to day and notice the result. Explain. 



a. Is light or darkness more favorable to the growth of 

 bacteria ? 



b. Which are more healthful, sunny or dark rooms? 

 Why? 



c. What three conditions have you found essential to 

 the growth of bacteria? 



E. Milk. 



Since milk is a good food and furnishes proper nutri- 

 ment for the growth of bacteria, it becomes a serious matter 

 to keep milk free from bacteria. 



I. THE CONTAMINATION OF MILK. 



a. Sources of contamination. 



b. Prevention of contamination. 



c. Laws in regard to handling milk by milk-men. 



d. Adulteration of milk. 



1. Why is milk adulterated? 



2. What chemicals are most often used? 



II. THE PRESERVATION OF MILK. 



a. Preservatives. Sometimes various chemicals are added 

 to prevent souring. What is used for this purpose ? 



