THE NITRIC ORGANISM 129 



eluded that the oxidation of nitrites to nitrates was brought about by 

 a specific organism independently of the nitrous organism. He suc- 

 cessfully sub-cultured it from his inorganic medium on to silica jelly 

 and also on to purified agar. He believes the organism, like its com- 

 panion, derives its nutriment solely from inorganic matter, but this 

 is not finally established. 



The form of the nitric organism (or niiromonas, as it was once 

 termed) is allied to the nitrous organism. The cells are elongated, 

 rarely oval, but sometimes pear-shaped. They are more than half a 

 micromillimetre in length, and somewhat less in thickness. The 

 cells have a gelatinous membrane. Like the other nitrifying bacteria, 

 its development and action are favoured by the presence of the acid 

 carbonates of calcium and sodium. Of the latter, six grams per litre 

 or even a smaller quantity gives good results. The sulphate of 

 calcium can be used, but the organism prefers the carbonates. The 

 differences between these two bacteria are small, with the exception 

 of their chemical action. The nitric organism has no action upon 

 ammonia, and its presence in very small amount (five parts per 

 million) hinders its development, and in sixty-four parts per million 

 prevents its action on a nitrite.* 



We may here summarise the general facts respecting nitrification. 



Winogradsky proposes to term the group nitroso-iacteria, and to 



classify thus: — 



NitrosomonaSj containing at least two 



vr.. . species, viz., the European and the 



Nitrous organisms = -^ t 



° Java. 



Nitrosococcus. 



Nitric organism = Nitrobacter. 



Mtrification occurs in two stages, each stage performed by a 

 distinct organism. By one (nitrosomonas) ammonia is converted into 

 nitrite; by the other (nitrobacter) the nitrite is converted into 

 nitrataf Both organisms are widely and abundantly distributed in 



* The course of nitrification may be followed by means of chemical tests. 1. 

 The disappearance of ammonia. 2. The appearance of nitrite. 3. Its disappear- 

 ance. 4. Appearance of nitrate. 



t Professor Warington, in Report IV. (p. 526) of his admirable series of papers 

 on the subject, draws attention to Muntz's criticism that the nitrifying organisms 

 only oxidise from nitrogenous matter to nitrites, and not from nitrites to nitrates. 

 Muntz held that the conversion of nitrite into nitrate is brought about by the joint 

 action of carbonic acid and oxygen. Professor Warington 's experiments, however, 

 clearly illustrate that the production of nitrates from nitrites in an ammoniacal solu- 

 tion can be determined by the character of the bacterial culture with which the 

 solution is seeded, and that in a solution of potassium nitrite conversion into 

 nitrate can be determined by the introduction of the nitric organism. Professor 

 Warington still adheres to the opinion, in favour of which he has produced so much 

 evidence, that the formation of nitrates in the soil is due to the nitric organism 

 which soil always contains. 



I 



