52 General Bacteriology. 
maining gas is considered as hydrogen; bring this into the open arm, remove the thumb 
and introduce alighted match. Air mixed with the hydrogen present causes a slight ex- 
plosion. Express the amount of CO, and H. in the form of a proportion. _H _ as, 
CO. 
Rererences. A. 203; McF. 54; M. & R. 86. 
SprciaL Directions. Use B. coli, also try B. subtilis. 
EXERCISE XLIX. DETECTION OF ACIDS (Wurtz). 
GENERAL DIRECTIONS. 
a. Melt a tube of lactose agar (gelatin can be used) and add enough of a sterile, blue 
litmus solution to give it a distinct color, cool to 42° C., inoculate it with an acid-pro- 
ducing organism and pour in the usual manner. 
b. When the agar has solidified invert the dish and place it in the incubator. 
REFERENCES. McF. 54. 
SprcraL DirEcTIONS. Use B. coli and incubate at 38° C. 
EXERCISE L. QUANTITATIVE DETERIIUNATION OF ACIDS. 
GENERAL DIRECTIONS. 
a. Inoculate 5 test-tubes of glucose bouillon (or milk) with an acid-producing 
organism. 
b. At periods 24 hours apart remove, with a sterile pipette, 5 ce. of the medium from 
each and titrate with a twentieth normal potassium (or sodium) hydrate solution, using 
phenolphthalien as an indicator. 
c. Plot the results, expressing the number of cc. of hydrate solution as abscissae 
and the daily intervals as ordinates. 
SprciaL Directions. Use B. coli and incubate at 38° C. 
EXERCISE LI. DETECTION OF NITRITES IN CULTURES. 
GENERAL DIRECTIONS. 
a. Make a culture of a reducing organism in a test-tube of the nitrate solution 
(XXXVII. g.). 
b. Incubate at 28° C. for 1 week, add 1 ce. of each of following solutions: 
1) Sulphanilic acid (para-amido benzenesulphonic acid) 0.5 gm. Acetic acid (sp. 
gr. 1.04) 150 ce. 
2) @-amido-naphthalene acetate. Boil 0.1 gram of solid a-amido-naphthalene in 
20 ce. of water, filter the solution through a plug of washed absorbent cotton, and mix 
the filtrate with 180 cc. of diluted acetic acid. All water and vessels used must be free 
from nitrites. (Leffman and Beam.) 
The presence of a nitrite is indicated by a pink color. 
c. A tube of the original medium should be incubated and tested as a control. 
REFERENCES. A. 215; McF. 56. 
SpeciaL Directions. Use Bacillus vulgaris. (Hauser.) Mig. 
EXERCISE LI, DETECTION OF AMMONIA. 
GENERAL DIRECTIONS. 
a. Make bouillon culture and incubate. 
