158 BACTERIA 



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 develop- 

 ment 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 the pres- 

 ence of any considerable amount of ammonium carbonate 

 hinders its development and prevents its action on a nitrite. 1 

 We may here summarise the general facts respecting nitri- 

 fication. Winogradsky proposes to term the group nitro- 

 bacteria, and to classify thus : 



(Nitrosomonas, containing at least two 

 species, viz., the European and the 

 Java. 

 Nitrosococcus. 

 Nitric organism = Nitrobacter. 



Nitrification 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 nitrate. 2 Both organisms are widely and 



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

 i. The disappearance of ammonia. 2, The appearance of nitrite. 3. Its dis- 

 appearance. 4. Appearance of nitrate. 



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

 papers on the subject, draws attention to Mtintz's criticism that the nitrifying 

 organisms only oxidise from nitrogenous matter to nitrites, and not from nitrites 

 to nitrates. Mttntz 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 



