REDUCING POWER Of BACTERIA. 203 



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 

 the organisms in a litre of culture fluid and then 

 distilling oif 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 ammo- 

 nia 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 Ness- 

 ler reagent over the mouth of a test-tube containing 

 the culture, and then gently heating the culture. As 

 the ammonia is driven oif by the heat, it will react on 

 the reagent on the strip of paper. 



EXAMINATION OF CULTURES FOR BACTERIAL TOX- 

 INS. In the systematic study of a pathogenic organism 

 it is necessary to know whether it is capable of pro- 

 ducing a soluble toxin when growing in culture-media. 

 This is done by filtering cultures of various ages and 

 testing the effect of the filtrate upon susceptable animals. 



