MICROORGANISMS AND HUMAN WELFARE 25 



23. To determine the best method of cleaning a room. (Optional 

 Demonstration.) 



Select three rooms with rugs or carpets as nearly as possible of the 

 same size and amount of dirt. Open Petri dish No. 15 and expose 

 its surface for five minutes at the level of the table while one of 

 the three rooms is being swept with a broom. In a similar man- 

 ner expose the surface of dish No. 16 for five minutes to the air in a 

 room that is being cleaned with a carpet sweeper, and dish No. 17 

 in the third room for five minutes while it is being cleaned with a 

 vacuum cleaner. Close each of the dishes, label, and put in a warm 

 place (90 to 100 F.) for several days. 



1. Describe the preparation of dishes 15, 16, and 17. 



2. What difference do you find in the relative number of bacteria 



colonies in the three dishes ? 



3. What do you conclude, therefore, as to the most effective method 



of removing dust and germs from a room ? 



24. Proper methods of sweeping and dusting. From 

 our experiments (13, 23) we have learned that large num- 

 bers of bacteria are present in the air of rooms where 

 dust is raised by the movement of people or by sweeping. 

 Since each colony started from a single bacterium, it is easy 

 to show the relative number of germs present in the air under 

 varying conditions (Fig. 11). 



The number of bacteria that may be found in a church, 

 schoolroom, theater, or living room has been proved by a 



that of the milk and a drop of ferric chloride (made by dissolving a 

 spoonful of ferric chloride in a quart of water) . Put both dishes of 

 milk into a dish of boiling water and stir or shake frequently for 

 five minutes. 



1. Describe the preparation of the experiment. 



2. At the end of five minutes state the color produced in the milk 



in each of the two test tubes. 



3. How, then, can you determine whether or not formalin has been 



added to milk? 



