17 



nitrates with the evolution of gas in decomposing organic solutions, we 

 will proceed to take a general view of the facts connected with dcnitri- 

 fication. 



The reduction of nitrates which occurs in solutions, or soil, contain- 

 ing readily oxidisable organic matter is of several distinct kinds. The 

 reduction may be simply from nitrate to nitrite : or it may be from 

 nitrate to nitric oxide gas; or to nitrous uxide gas; or, finally, to 

 nitrogen gas. The fir<t form of reduction does not necessarily involve 

 anv loss of nitrogen ; all the others do involve a loss of nitrogen, as the 

 product escapes in the form of gas. 



The reduction of nitrates is occasioned by various species of bacteria. 

 That the action in question only occurs in the presence of these living 

 organisms was first established by Meusel (lo'7o) in the case of natural 

 waters, and afterwards shown by Deherain and Maquenne (1882) to 

 hold tru^ in the case of soils, If all living organism- are destroyed in 

 sewage or soil — if, to use modern language, these mediums are steri- 

 lised — neither nitrification nor denitrification will occur. 



Bacteria reduce nitrates bv bringing about the combustion of or- 

 ganic matter by the oxygen of the nitrate, the temperature distinctly 

 rising during the operation. When circumstances are favourable to the 

 process of reduction, the amount of nitrate reduced is determined by the 

 quantity of combustible organic matter present: The combustion of the 

 organic matter, and the reduction of the nitrate, can t ike place in the 

 absence of air in the same way as gunpowder — comi-osed of a nitrate, 

 charcoal, and sulphur — is capable of burning under water. In the pre- 

 sence of air, the oxygen which it contains, takes the place, more or less, 

 of the oxygen of the nitrate, and the reiuction of nitrate is either dimi- 

 nished, or the oxidation of the organic matter much incre i-ed. The 

 whole action is quite similar to the familiar combustion of food in the 

 animal body ; or to the combustion of organic matter which occurs in 

 plants, and notably in those which, as in the case with fungi, are desti- 

 tute of chlorophyll. 



Different species of bacteria behave very differently in a mixture of 

 a nitrate and organic matter. Many species of bacteria are incapable of 

 reducing a nitrate, though quite capable of effecting the combustion of 

 organic matter with free oxygen. The reduction of a nitrate to a nitrite, 

 but not to the state of gas, is, however, a very common propertv of bac- 

 teria. Only a few species appear to have the power of reducing a ni- 

 trate to gas, but these species are widely distributed. Dr. P. Frankland 



frew thirty-two distinct species of bacteria, obtained from atmospheric 

 ust, and from natural waters, in a very weak solution, containing pep- 

 tone, sugar, and a nitrate. One-half of the species tried reduced the ni- 

 trate to nitrate. The production of gas was apparentlv not noted, and 

 would be scarcely perceptible under the conditions of the experiment. In 

 a series oF experiments conducted by myself in the Rothamsted labora- 

 tory, thirty-seven distinct species of bacteria were grown in clear beef 

 broth containing \ per cent, of nitre, at temperatures varying fron 70 ° 

 to 95 ° Fahr. Of these, fifteen failed to reduce the nitrate ; three 

 effected only a small amount of reduction to nitrite while nineteen re- 

 duced the nitrate to nitrite with great energy, one of them producing 

 gas- 



