PROBLEM 99 



119 



Method. — Expose a number of these Petri dishes containing 

 nutrient for the same length of time in as many of the following 

 conditions, and as many others, as possible : 



(a) to the air of the schoolroom. 



(b) in the halls of the school while pupils are passing. 



(c) in the halls of the school when pupils are not moving. 



(d) at the level of a dirty and much-used city street. 



(e) at the level of a well-swept and little-used city street. 

 (/) in a city park. 



(g) in a factory building. 



(h) to dirt from hands placed in dish. 



(i) to contact with the interior of the mouth. 



(j) to contact with decayed vegetable or meat. 



(k) to contact with 

 dirty coin or bill. 



(I) to contact with 

 two or three hairs 

 from pupil's head. 



Observations. — 

 After three to five 

 days note the condi- 

 tions of the various 

 plate cultures. Each 

 day count the num- 

 ber of spots (colonies) of bacteria and molds growing on the cul- 

 ture medium. Make a table like the above to show your results. 



Conclusion. — 1. Where are bacteria found in greatest num- 

 bers? 



2. What are the factors in your environment by means of which 

 bacteria might get to your body ? 



Problem 99 : To study how rapidly bacteria grow. 



Method. — Imagine that you have inhaled a germ causing cold 

 or consumption (bacillus tuberculosis) while riding on the subway 

 train or street car at 8.30 a.m. You are in such poor physical 

 condition that the bacterium can grow and multiply. Scientists 

 who have studied germs (bacteriologists) tell us that the bacterium 



