nitrite-producing organisms. Nitrobacter nre among the smallest of all 

 known organisms being only one-tenth of the size of an average nitrosoc- 

 occus. i hey exist as slender, pear-shaped cells, possessed of ^low powers 

 of increase in comparison with their capacity for nitrate-production. 



If cuitivated in a liquid medium they grow in the form of a thin 

 scum which adheres persistently to the sides of the vessel. 



Chemistry of Nitrification. 

 We are at present without knowledge as to the complete cycle of 

 chemical changes which are brought about in the process of the nitri- 

 fication of ammonia, and can only deal with the end-products of the 

 vital activity of the nitrifying bacteria We have already noted that 

 the first stage of nitrification is the production of a nitrite by the agency 

 of the " nitroso" organisms. So far as the mere process of oxida- 

 tion is concerned, the following equation expresses the first stage of 

 nitrification ; — 



Stage I. 



2 NH3 +3 0^ -f raC0g=Ca(N0j2+ CO2 +3 H^ 



Ammonia -[- oxygen -|- chalk =:calcium -(- carbonic -[- water. 



nitrite acid gas 



This change goes on only in the dark and is independent of all 

 organic matter. The bacteria themselves, however, contain protoplasm 

 and the usual organic constituents of li\ ing organisms, and in order to 

 obtain the carbon necessary for their growth Hnd increase they avail 

 themselves of carbonic acid ga-^. The separation of carbon from its 

 union with oxygen in this gas requires the expenditure of a considera- 

 ble amount of energy. Green plants obtain their supply of this re- 

 quisite energy from the rays of the sun, but nitrifying bacteria live and 

 work in the absence of such light and are unable to make use of this 

 source of power. Winogradsky, as also Godlewski. have shown that 

 the ' nitroso' bacteria obtain their energy for ttie elimination of carbon 

 from caibonic acid gas by the supply they derive from the oxidation of 

 ammonia to nitrite. The former experimenter concluded from his in- 

 vestigations that the 'nitroso' bacteria were able _to assimilate one 

 part of carbon for every 42 parts of ammonia oxidised to nitrite. It 

 would therefore appear that the main vital object of nitrifying or- 

 ganisms is that of the assimilation of carbon from carbonic acid gas 

 and that the nitrification of ammonia is simply a means to that end. 

 The effective character of these agents of nitrification is appare .t from 

 the enormous amount of ammonia they are forced to oxidise in order 

 to gain a small supply of vital carbon. 



The chemistry of the conversion of nitrite into nitrate by the nitro- 

 bacter organisms is of the simplest. 



Stage II. 



Ca (NO,), +0, =ra (NO^), _ 



nitrite of lime -|- oxygen = nitrate of lime. 

 It is necessary to draw attention at this point to a fallacy that is 

 very constantly repr< duced in current agricultural literature. It is 

 often stated that ammonia is oxidised to nitric ticid by nitrifying or- 

 ganisms and unless enough chalk to neutralise the acid be present, 

 the organisms are destroyed or prevented from free activity. 



