CHANGES IN THE REACTION OF MEDIA 225 



In preparing solution a the naphthylamine is dissolved 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 mixture of 

 starch and potassium iodide, as follows: 



Starch 2.0 grams 



Potassium iodide, 0.5 gram 



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. Control-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 satis- 

 factory results are obtained by cultivating the organisms 

 in a liter of culture fluid and then distilling off portions of 

 the culture, collecting in Nessler tubes, and applying 1 c.c. 

 of the reagent to each 50 c.c. of the distillate. The presence 

 15 



