208 BACTERIOLOGY 



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 dissolved. 



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 sulphiu-ic 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 litre 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 

 of ammonia in the distillate is shown by the yellow coloration 

 resulting from the addition of the reagent. 



The direct application of the reagent to the culture will 

 give satisfactory results if a great deal of ammonia has been 

 formed. In this instance the mercury in the reagent will 

 be precipitated as mercurous oxide. Another rough test for 

 the formation of ammonia is to place a strip of filter-paper — 

 moistened with the Nessler reagent — over the mouth of a 

 test-tube containing the culture, and then gently heating 

 the culture. As the ammonia is driven off by the heat, it 

 will react on the reagent on the strip of paper. 



