78 GENERAL BACTERIOLOGY 



washed absorbent cotton, and mix the filtrate with 180 ec. of diluted 

 aeetie acid. All water and vessels used must be free from nitrites. 

 (Leffmann 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. 



Refeeences. a. 226 ; McP. 53. 



Special Directions. Use sewage. 



exercise 52. detection of ammonia. 



General Directions. 



a. Make bouillon culture and incubate 24 to 48 hours. 



h. Place in neck of tube a piece of filter paper which has been 

 dipped in Nessler's reagent (for formula see works on water analy- 

 sis). A yellow to reddish brown color indicates the presence of 

 ammonia. ^ 



Reference. L. & N. 78. 



Special Directions. Use sewage to inoculate medium. 



exercise 53. detection of sulphuretted hydrogen. 



General Directions. 



a. Make a culture in a test-tube, or better, in a flask of bouillon, 

 and incubate at 38° C. 



6. 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. 



Reference. L. & N. 76. 



Special Directions. Use B. coli or sewage. 



exercise 54. detection of indol. 



General Directions. 



a. Make two cultures in tubes of sugar-free broth (or Dunham's 

 solution). 



h. Five days later add a few drops of concentrated sulphuric 

 acid. The appearance of a pink color indicates that nitroso-indol 

 has been formed (cholera-red reaction). If the pink or deep red 

 color does not appear, add 1 cc. of sodium nitrite solution (sodium 



