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 Cohn, 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 
uninoculated 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- 
phurie 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. 
