54 General Bacteriology. 
b. Place in neck of tube a piece of filter paper which has been dipped in Nessler’s 
reagent (for formula see works on water analysis). A yellow to reddish brown color 
indicates the presence of ammonia. 
REFERENCES. -L. & N. 141. 
SPECIAL DirEcTIONS. Use sewage to inoculate medium. 
EXERCISE LIII. DETECTION OF SULPHURETTED HYDROGEN. 
GENERAL DIRECTIONS. 
a. Make a culture in a test-tube, or better, a flask of bouillon and incubate at 
38° C. 
b. Twenty-four hours later fasten in the flask, by means of the cotton plug, a strip 
of filter paper moistened with lead acetate. 
c. The presence of sulphuretted hydrogen is indicated by change of color from 
brownish to blue. The color change is often slight and can be best detected by frequent 
observations. 
REFERENCES. L. & N. 188. 
SpeciaL Directions. Use B. colt or sewage. 
EXERCISE LIV. DETECTION OF INDOL. 
GENERAL DIRECTIONS. 
a. Make a culture in a tube of glucose-free broth* (or Dunham’s solution). 
b. 24 hours to 1 week later add a few drops of concentrated sulphuric acid and 1 ec. 
of sodinm nitrite solution. (Sodium nitrite, 0.02 gms. Distilled water, 100 gms.) 
The presence of indol is indicated by the production of a deep red color. 
Rererences. L. & N. 142; McF. 56; M. & R. 87. 
SpeciaL Directions. Use B. colt. 
EXERCISE LY. DETERMINATION OF CHEMICAL ENZYMES IN CULTURES. 
GENERAL DIRECTIONS. 
a. Make two gelatin stab cultures of a rapidly liquefying organism and incubate 
several days or until the gelatin has all been liquefied. 
b. Pour one into a tube of gelatin to which carbolic acid (zy ce. of a5% sol. per 
ec. of medium) has previously been added. Mark the line which separates the liquid and 
solid gelatin. 
c. Add the other tube of liquefied gelatin to a tube of carbolized milk. 
d. Make control cultures in the carbolic media with a pure culture of the organism 
used above to show that the acid inhibits the growth and that the changes are not due 
to the living organism. 
REFERENCES. McF. 53. 
’ SpectaL Directions. Use B. subtilis. 
EXERCISE LVI. VARIATION IN ENZYME PRODUCTION. 
Make stab cultures of Pseudomonas aeruginosa (SCHROETER) Mia. (B. pyocyaneus), 
or any slow liquefier, in ordinary neutral gelatin and also glucose ais Compare 
rate of liquefaction in each. 
*This is prepared from beef by inoculating the meat infusion with an organism capable of fer- 
menting sugar, such as B. coli, and allowing it to stand several hours at 38° C. The meat is then 
strained and the bouillon prepared in the usual manner. This is recommended for testing for indol. 
