FOR WAGGONER’S BIOLOGY 
43 
room immediately following sweeping; No. 4, in a quiet labora¬ 
tory; No. 5, in the same laboratory after sweeping; No. 6, in the 
corridor when unoccupied; No. 7, in the same corridor during the 
passing of classes; No. 8, outside the window of an upper story; 
No. 9, on a doorstep. Brush the hair over No. 10; cough over 
No. 11; sneeze over No. 12; lay the fingers on the agar of No. 13; 
after washing the hands thoroughly, repeat with No. 14; shake a 
wrap over No. 15. Allow these Petri dishes to remain in an oven 
held at 85° F. to 95° F.- or in a warm room. Examine after two 
or three days. Whitish patches which develop upon the agar 
medium are colonies of bacteria. How do the sizes and general 
appearance of the colonies in the various dishes compare? Explain 
the results. Record in a tabulated form as follows: 
No. OF DISH 
Place or condition 
OF EXPOSURE 
No. OF COLONIES 
RESULTING 
Explanation 
OF RESULTS 
What is your conclusion as to the relation of dust and bacteria? 
Explain the necessity for care in the preparation of the materials 
and glassware which were used in this exercise. 
EXERCISE 46 
PRESERVATIVES AND DISINFECTANTS 
Prepare some clear beef bouillon, a hay infusion, or some other 
liquid that clouds as a result of bacterial action. Number two sets 
of test tubes from one to ten each. Fill each tube one-third full of 
the medium. Another tube should be filled in the same manner 
and kept as a control.” 
I. Preservatives. Add the following substances to the first set 
of test tubes: 
