QUALITATIVE BACTERIOLOGICAL ANALYSIS. 85 



sodium carbonate in the saline solution. This salt rather 

 destroyed the transparency of the medium but drew attention to 

 the colonies which attacked the magnesium carbonate and then 

 seemed to be surrounded by a limpid zone. The colonies in the 

 depth of the jelly appeared to the naked eye as small white 

 points. 



Duclaux was the first to suggest that the process of 

 nitrification occurred in two stages, the first being the oxidation 

 of ammonia to nitrites, and the second the conversion of nitrite 

 into nitrate. Warington's experiments led him to entertain 

 the same idea, but he failed to isolate separate nitrous and 

 nitric organisms. Winogradsky, however, succeeded in cultivat- 

 ing the organisms which produced these chemical changes. He 

 divided the organisms which act in the first stage into two 

 classes i.e., the Nitrosococcus and Nitrosomonas. The Nitroso- 

 coccus was found in earth obtained from Europe, Africa, and 

 Japan ; it appeared as a non-motile spherical cell, about 3 fj. in 

 diameter. The Nitrosomonas was isolated from earth found in 

 Java ; it was motile and had a short ellipsoidal form. The 

 organism of the second stage, or Nitrobacter, was found to be a 

 small non-motile rod, about 0*5 m. long and 0'25 /n broad. It 

 grew readily in the saline fluid containing potassium nitrate 

 instead of an ammonium salt, and also on silica jelly plates. 

 Burri and Stutzer have also isolated a nitrate producing 

 organism from earth by means of silica jelly plates. It is larger 

 than Winogradsky's Nitrobacter, and is motile. It can be 

 cultivated in broth and gelatine, but then loses its nitrifying 

 action. Winogradsky found that, as a rule, the oxidation of 

 nitrates did not commence until all the ammonia had dis- 

 appeared. Occasionally oxidation of ammonia and nitrites 

 were found at the same time, but the oxidation of the nitrites 

 always commenced later than the oxidation of the ammonia. 

 The nitric ferment had no action on ammonia, and in the 

 absence of the nitrous ferment nitrification did not take place. 

 Nitrification in earth was then compared with the same process in 

 fluid media. It was found that normally only nitrates were pro- 

 duced in soil, nitrites being rarely detected. It was also noticed 

 that the oxidation of nitrites took place immediately after their 

 formation, even in the presence of large quantities of ammonia. 



