THE BACTERIA. 145 



d. 1. Recall experiment E. 



2. Is Pasteurized milk sterile ? 



e. How were the flasks and Petri dishes sterilized? 

 This method is hot-air sterilization. The objects need to be 

 subjected to a temperature of 150 C. for three quarters 

 of an hour in dry air. What kind of objects are best 

 sterilized in this way ? 



G. Purification of Water. 

 Read upon the following subjects : 



1. Purification of water by sand filtration. Describe 



a filtration plant such as is used to filter the 

 drinking water of the city of Lawrence, Mass., 

 or Albany, N. Y. 



2. Domestic filters. Their efficiency. 



3. Sterilization of water by heat. 



4. Purification of water by sedimentation. 



5. Purification of water by agitation with solid par- 



ticles and subsequent subsidence. 



6. Chemical precipitation. 



7. Purification of sewage. 



H. Epidemics. 



Give the history of one epidemic, either of typhoid fever 

 or of diphtheria. 



a. Its cause and extent. 



b. Its means of spreading. 



c. Remedies. 



