76 BACTERIOLOGY. 



agitation with access of air the colors may be again re- 

 stored, but at the same time, acid being formed, the 

 litmus pigment is turned red. According to Colin, the 

 property of reducing litmus belongs to all liquefying 

 bacteria, but some non-liquefying species also possess it. 



2. The reduction of nitrates to nitrites and am- 

 monia. The first of these properties seems to pertain 

 to a great many bacteria at least Petri and Maassen 

 found in six species, grown in bouillon containing 2.5 

 to 5 per cent, peptone and 0.5 per cent, nitrate, that 

 almost all produced nitrite abundantly; once only was 

 ammonia observed. In a number of bacteria studied 

 by Rubner only one failed to produce nitrite. The 

 test for nitrites is made as follows : Two bouillon tubes 

 containing nitrates are inoculated, and, along with two 

 unirioculated tubes, are allowed to remain in the incu- 

 bator for several days; then to the cultures and control 

 test is added a small quantity of colorless iodide of 

 starch solution (thin starch-paste containing 0.5 per 

 cent, potassium iodide) and a few drops of pure sul- 

 phuric acid. The control tubes remain colorless or 

 become gradually slightly blue, while if nitrites are 

 present a dark blue or brown-red coloration is produced, 



The demonstration of ammonia is made by the addi- 

 tion of Nessler's reagent to culture media free from 

 sugar. In bouillon, if ammonia be present, Nessler's 

 reagent is almost immediately reduced to black mer- 

 curous oxide. A strip of paper saturated with the 

 reagent can also be suspended over the bouillon tube, 

 or this can be distilled with the addition of magnesium 

 oxide and the distillate treated with Nessler's reagent. 

 A yellow to red coloration indicates the presence of 

 ammonia. Controls are necessary. 



