LABORATORY AND FIELD WORK 395 



10. The four dishes now accounted for should be numbered and 

 kept on a shelf or other convenient place in the room, not in the 

 direct sunlight, where they may be examined from day to day. 



11. Inoculate the three remaining slices from some fresh milk. 

 Place one of these in a window where it will receive as much direct 

 sunlight as possible, one in a dark closet or other dark room, and 

 the third in an icebox. 



12. Examine all the cultures carefully, without removing the 

 covers, once a day for a week; make a full record of all that you 

 observe in each culture with respect to the following points : 



a. The appearance of spots (bacterial colonies) on the surface of the 



potato. Compare with Figure 10, page 24. 



b. The color of the spots. 



c. Their shape, and growth from day to day. Some of the spots 



may develop a fuzzy appearance. Examine these with a hand 

 lens. If a spot contains fine threads, it is not composed of 

 bacteria, but of a mold, belonging to a higher order of plants. 



d. Changes in the appearance of the potato due to the action of the 



bacteria. 



13. What do you conclude from these experiments as to : 



a. The presence of bacteria in the air ? In the dust on the floor ? 



In milk ? 



b. The rate of multiplication of bacteria ? 



c. The effect of sunlight on the development of bacteria? Of 



darkness ? Of heat ? Of cold ? 



14. Examine under the microscope bacteria from your different 

 cultures. Note the different shapes and sizes that you find. 



15. Why are the care and inspection of city water supplies ex- 

 tremely important? 



16. What serious disease is especially likely to be carried by 

 impure drinking water? 



17. How may impure water be treated to make it safe for drink- 

 ing? 



18. Why should living and sleeping rooms have abundant sun- 

 light and fresh air? 



19. Why are tuberculous patients kept in the open air as much 

 as possible? 



