18 



Gayon and Dupotit (1886) were the first to isolate tbe species of 

 bacteria reducing nitrates to gas; two organisms having this property 

 wore separated by them from sewage. The action of the more active 

 of these organisms was most intense. Cultivated in a solution of nitre 

 containing asparagine and a citrate, the liquid evolved its own volume of 

 gasper day, while the temperature rose 18° Fahr. The action was 

 most energetic in the absence of oxygen, and ceased when the supply of 

 oxygen became abundant. The work carried out by these investigators 

 at the experiment station at Bordeaux should be studied by everyone 

 desiring to obtain a scientific view of the phenomena of denitrification. 



Quite recently Burri and Stutzer (1895) have isolated denitrfying 

 bacteria from horse dung and from straw. The^e organisms differ in 

 several ways. The one obtained from straw is capable of reducing 

 nitrates to gas in the absence of oxygen, and in the presence of much 

 oxygen its action is hindered and finally ceases. Tbe bacterium from 

 horse dung developes exclusively in the presence of oxygen, but it 

 can reduce nitrates only when associated with another organism of an- 

 aerobic character, that is, one developing in the absence of oxygen. 

 When the two organisms are associated, no reduction takes place in the 

 complete absence of oxygen, but the action becomes active as soon as a 

 little oxygen is present, and when plenty of organic matter is present 

 the process is apparently not hindered by a full supply ot that gas. 

 There is no evidence that either straw or horse dung contains only, or 

 always, the two denitrifying organisms just described. 



Soil contains an abundance of reducing organisms, including those 

 produciug nitrogen gas. When broth containing 1 per cent, of nitre is 

 infected with a particle of surface soil, and kept in a warm place a 

 quantity of gas bubbles containing nitrogen i» produced, and the nitrate 

 will entirely disappear. Soil treated with a 1 per cent, solution of sugar 

 containing nitre, rapidly reduces the latter ; the gas produced contains 

 nitrous oxide and nitrogen. 



Any kind <»f organic matter readily oxidised by bacteria may be 

 used to bring about the reduction of nitrates. In the trials made by 

 various experiments, albuminoids, asparagine, starch, sugar, humus, fats, 

 tartrates, citrates, acetates and alcohol have all proved effective for this 

 purpose. 



The presence of some nitrogenons plant food, and of phosphates, 

 potash, and the other ash constituents of plants is of course necessary for 

 the growth and activity of the reducing bacteria. 



It is clear from what has gone before that denitrification may be ex- 

 pected to occur whenever a suitable mixture of nitrate and organic 

 matter is in.ected by soil dust under the conditions favourable to the 

 action. The early observations of Angus Smith in England, and of Th. 

 Schloesing in France (1868),taught us that organic solutions, as sewage, 

 diluted blood, tobacco juice, and sugar solutions, when undergoing fer- 

 mentation or putrefaction, actively reduced nitrates, nitrous oxide and 

 nitrogen being evolved. Whence did these mixtures obtain the bacteria 

 necessary for this action to take place ? Nothing had been done to in- 



