232 BACTERIOLOGY. 



In preparing solution a the naphthylamine is dis- 

 solved in 20 c.c. of boiling water, filtered, allowed to 

 cool, and mixed with the dilute acetic acid. Solutions 

 a and b are then mixed. It is best prepared as needed, 

 though it may be preserved for some time in a glass- 

 stoppered bottle. 



In testing for nitrites the reagent is added in the 

 proportion of one volume of reagent to five volumes 

 of culture. When nitrites have been formed a deep- 

 red color appears in a few seconds. If no nitrites have 

 been formed the culture remains colorless. In testing 

 cultures it is always necessary to control the results by 

 blank tests on a portion of the same medium that had 

 not been inoculated, as some of the ingredients of the 

 medium may have contained nitrites. 



Another test for the formation of nitrites is a mixt- 

 ure of starch and potassium iodide, as follows : 



Starch, 2.0 grammes. 



Potassium iodide, 0.5 " 



Water, 100.0 c.c. 



Warm the mixture until the starch is completely dis- 

 solved. 



In testing for nitrites add 0.5 c.c. of the reagent to a 

 tube of culture, and follow this by the addition of 2 or 

 3 drops of pure sulphuric acid. If nitrites have been 

 formed, a dark-blue or purple color will appear. Con- 

 trol-tubes of the medium show no color reaction, or 

 merely a trace of blue coloration. 



Test for Ammonia. The formation of ammonia may 

 be detected by testing with Nessler's reagent. The 

 most satisfactory results are obtained by cultivating 



