32 SOIL BACTERIOLOGY 



3. After one week in the incubator prepare agar plates 

 from the different samples. Pour from dilutions i : 1000 

 and I : 10,000. 



4. The plates must be incubated at 60^ C. 



5. Determine the number of thermophilic bacteria in 

 I gram of soil. 



6. If desirable, a study may be carried on of the bacteria 

 growing at low temperatures. 



Exercise 14 



Catalytic Power of Soils 



1. Arrange three large, heavy walled glass tubes with 5 

 grams of soil in each. 



2. Treat as follows: 



(a) Fresh soil, no treatment. 



{h) Heat in the steamer for thirty minutes. 



(c) Heat in the autoclave at 15 pounds' pressure for fifteen minutes. 



3. Set up the following apparatus (Fig. 2): Insert a 

 two-holed rubber stopper into a large test-tube. In one 

 hole fit a piece of short, straight glass tubing closed at 

 one end with a rubber tube and clamp; in the other, a right 

 angle tube of glass connected with a rubber tube 8 to 10 

 inches long. This rubber tubing should be fitted to a 

 tube of glass bent as shown in Fig. 2 and connected to the 

 gas-measuring tube. 



4. FiH the loo-c.c. gas-measuring tube with water 

 and invert mouth under water. Clamp the tube in 

 place. 



5. Then add 10 c.c. of a 1.5 per cent solution of hydrogen 



