CONTAGIOUS DISEASES 367 



at this rate indefinitely because the limit of their multiplica- 

 tion is soon reached through lack of food and through cer- 

 tain conditions unfavorable to growth. But it is this tre- 

 mendous power of multiplication that makes them such a 

 factor in man's life. 



Occurrence and food. Bacteria are found in a great variety 

 of places : in the soil, in the air, in water, in food, and in the 

 human body. Some kinds are injurious to man, but the 

 great majority are beneficial or harmless. Fifteen hundred 

 kinds of bacteria are known to science but only about fifty 

 to seventy-five produce disease. Although these little 

 organisms are plants, they contain no chlorophyll and so 

 cannot make their own food. Some bacteria feed upon 

 the living bodies of plants and animals. These are called 

 parasites. Others live on the dead bodies of plants and 

 animals. They are called saprophytes. 



DEMONSTRATION 29 



Purpose. To see under what conditions air contains the 

 fewest bacteria. 



Materials. See demonstration 27 and read I under Directions. 



Directions, i. Melt as many tubes of the medium as there 

 are samples of air to be tested. Pour each into a sterilized petri 

 dish and cover at once. Allow to stand till the gelatin hardens. 



2. Following are suggested some of the various localities 

 and times for testing the air : in the schoolroom before school, 

 and after school ; in the hall while classes are passing, and just 

 after sweeping ; outdoors on a windy day, and on a still day ; on 

 a street that is much traveled, on one that is little traveled. 



3. In each of the cases to be tried remove the cover from 

 the dish, keeping it off three minutes, then replace. 



4. Allow the dishes to stand for several days, note the 

 appearance of colonies. If possible, count them. If too numer- 

 ous for this, determine the relative number of bacteria found 

 in the various localities. Or the experiment may be tried again 

 with a shorter exposure so as to reduce the number of colonies. 



