82 SOIL BACTERIOLOGY 



10, The amount of hydrogen sulphid may be determined 

 quantitatively by titrating with iodin and sodium thio- 

 sulphate (see page 154). 



Exercise 2 

 Isolation of Hydrogen Sulphid Organisms 



1. Prepare four tubes of sulphate-reducing gelatin 

 (m. 56). 



2. Inoculate shake cultures of the gelatin with various 

 dilutions of the impure cultures from the previous exercise. 



3. Harden the gelatin cultures in cold water and incubate 

 at room temperature four to seven days in an anaerobic 

 jar (see page 133). 



4. In case black colonies appear in the tubes, attempt to 

 isolate the organisms by making subinoculations into the 

 sulphate gelatin. 



Exercise 3 

 Hydrogen Sulphid from Protein and Sulphur (Optional) 



1. Prepare two small bottles of culture solutions (i) 

 and (2) (m. 59). 



2. Treat as follows: 



(a) Solution i, uninoculated. 



(&) Solution I, I gram garden soil. 



(c) Solution 2, uninoculated 



id) Solution 2, I gram garden soil. 



3. Compare the changes that take place in solutions 

 '{i) and (2). 



