230 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



1 litre of river water ; 



2 grains calcium tartrate ; 



0'5 gram potassium hydrogen phosphate ; 

 O'l gram potassium nitrate. 



This is sown with a httle horse dung, or straw, and in- 

 cubated at 35° C. The general reaction taking place may 

 be expressed by the following equation : — 



4KNO3 + 5C + 2H2O = 4KHCO3 + 2Nj + CO2 



It will be remembered that when the decomposition of 

 cellulose under aerobic conditions was being considered, a 

 mixture was made of a similar character to the solution just 

 described, the carbon being represented by the carbon of 

 cellulose ; the importance of de-nitrification as a natural 

 phenomenon is thus seen. On the one hand, we have the 

 nitrifying organisms oxidising ammonia to nitrite and nitrate, 

 while on the other hand the de-nitrifying organisms make use 

 of the nitrate thus formed, to oxidise organic matter. 



De-nitrification is by no means so restricted a phenomenon 

 as nitrification, and quite a large number of organisms have 

 been found which are capable of bringing about de-nitri&cation 

 to a greater or less degree. Broadly speaking, these may be 

 classified into two classes, true de-nitrifying organisms which 

 are capable of pushing the reaction to its final hmit and 

 producing free nitrogen ; and indirect de-nitrifying organisms, 

 which only reduce nitrates to nitrites, when, through the inter- 

 action of nitrites with amido compounds in acid solution, as 

 in the case of urea, we have : — 



C0{NH2)2 + 2HNO2 = 2N2 -f CO2 -t- 3H2O 



Urea Kitrous acid 



Or to take an analogous, but more comphcated, instance, 

 asparagin may be converted into mafic acid, thus : — 



