CONTAGIOUS DISEASES 381 



DEMONSTRATION 30 



Purpose. To see if ice contains bacteria. 



Materials. See demonstration 27 and read i under Direc- 

 tions. 



Directions. Melt a piece of ice in a sterilized beaker. Take 

 one cubic centimeter of the water and add it to a test tube con- 

 taining melted culture medium. Shake and pour into a steril- 

 ized petri dish. Cover. Allow to stand a few days and see 

 if any colonies appear. 



Disposal of sewage. Besides the question of the puri- 

 fication of drinking water, the matter of the disposal of 

 sewage must be considered in this connection. The details 

 of this matter in cities must be left for experts to solve. 

 There are, however, methods by which each city may dis- 

 pose of its sewage without emptying it into streams. They 

 may filter it by means of sand beds or burn it in cremating 

 furnaces; the present method of contaminating our beauti- 

 ful streams and rivers is unnecessary. 



Pasteurization of milk. Milk may be a means of carry- 

 ing the germs of typhoid fever, tuberculosis, diphtheria, and 

 scarlet fever. In large cities it is usually impossible to know 

 exactly the place from which the milk comes, and when it 

 is fed to children it is safer to treat it in the home so as to 

 destroy the bacteria. This may be done in two ways, by 

 boiling or by pasteurization. The objection to boiling is 

 that it so changes the character of the milk as to render it 

 more indigestible. 



In the process of pasteurizing, the milk is not heated to 

 so high a temperature as the boiling point, only to about 

 1 60 degrees. This temperature is sufficient to kill the 

 bacteria and does not affect the digestibility of the milk, 

 and is, therefore, to be preferred to boiling. At this tem- 

 perature the bacteria which cause milk to sour are destroyed 

 also, and pasteurized milk will keep sweet longer. Pasteur- 



