42 
LABORATORY EXERCISES 
place with reference to the kinds of organisms which are present 
in the water. Make drawings of the organisms observed each time. 
Date each set of drawings. 
(Note — This infusion may be kept as a basis for work later in 
the year. See Exercises 62 and 63.) 
EXERCISE 45 
THE GROWTH OF BACTERIA 
I. Sterilization of Utensils. Prepare a number of test tubes, a 
glass funnel, a flask, and fifteen Petri dishes. To do this, wash the 
glassware clean and sterilize it in a hot air sterilizer held at a 
temperature of 170° C. (340° F.) for one hour. (A common gas 
oven serves well for a hot air sterilizer. A large double cooker or 
steamer may be employed if hot water is to be used for steriliza¬ 
tion. (See Waggoner, Page 213.) 
II. Preparation of a Culture Medium. To prepare a medium 
for the culture of bacteria, dissolve 15 gm. of agar-agar which has 
been cut into small pieces in 1000 cc. of water. Then add to this 
5 gm. of extract of beef. Boil for one-half an hour. The mixture 
should be slightly alkaline. To make it so, add just enough baking 
soda to cause red litmus paper to turn blue. Filter the medium 
into a flask through cotton placed in the funnel. Pour about 
10 cc. of the medium into each of the prepared test tubes. (This 
should fill them about two inches deep.) Insert a stopper of cotton 
in each and steam for one-half hour in a common steamer such as 
that referred to above. Repeat this steaming twice at intervals 
of twenty-four hours. 
III. Relation of Dust to Growth of Bacteria. Number the 
sterile Petri dishes from one to fifteen. In a quiet room, pour the 
medium from the test tubes into the Petri dishes. Avoid all un¬ 
necessary handling. No. 1 is to be kept as a check or control. Why? 
For one minute each, expose the medium in the following Petri 
dishes to the air: No. 2, in a quiet class room; No. 3, in the same 
