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. 



(6) 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. Fill 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 



