228 CIVIC BIOLOGY 



iii the same room just after the class has left. (3) Expose a 

 Petri dish of gelatin in a room for five minutes immediately 

 after wiping up the dust with a dry cloth or after using a 

 feather duster. Compare this plate with one that was exposed 

 for the same length of time in a room immediately after it 

 had been dusted with a damp cloth. (4) Expose a plate in 

 a living room for five minutes and compare with the air in 

 the yard. (5) Compare the number of bacteria in the air 

 upon the ground with that of the first and fourth stories of 

 the same building. Is it true that a child breathes less pure 

 air than a man ? Is it more desirable to sleep upstairs, as 

 far as air is concerned ? (6) Expose a plate of gelatin in a 

 busy street before and after it has been sprinkled, or before 

 and after a rain. (7) Compare the number of bacteria in a 

 well-cleaned street with the number in one that is not cleaned. 

 What do you think of the system that cities are using for 

 flushing their streets ? (8) Compare as to number of bacte- 

 ria the air before and after a snowstorm or rainstorm. Inocu- 

 late plates with rain or fresh snow. Keep these experiments in 

 a drawer in the laboratory. In a day or so count the colonies 

 of bacteria and record results. (9) Sweeten and cook fruit, 

 such as apples, in a test tube. Plug with cotton. Does 

 canned fruit keep if air is present and bacteria are excluded ? 

 (10) Discuss the desirability of having children's playgrounds 

 upon the roofs in large cities. 



Experiments for bacteria of water. (1) Make a culture of 

 water from a stream (dip your platinum loop three times) 

 and compare with the same amount of water from the reser- 

 voirs and lakes of the locality. (2) Compare the water above 

 and below the point where the sewage is emptied. (3) Com- 

 pare the different drinking waters of the locality. (4) Make 

 cultures of water that is rich in organic debris and compare 

 with the same water that has been boiled for fifteen minutes. 

 (5) Filter some of the water used 'in the above experiment 



