I5O BACTERIA 



during the operation/' The reduction to a nitrite is a com- 

 mon property of bacteria. But only a few species have the 

 power of reducing a nitrate to gas. These few species are, 

 however, widely distributed. In 1886 Gayon and Dupetit 

 first isolated the bacteria capable of reducing nitrates to the 

 simplest element, nitrogen. They obtained their species 

 from sewage, but ten years later denitrifying bacteria were 

 isolated from manure. That soil contains a number of these 

 reducing organisms is known by introducing a particle of 

 surface soil into some broth, to which has been added one 

 per cent, of nitre. During incubation of such a tube gas is 

 produced, and the nitrate entirely disappears. 



Whenever decomposition occurs in organic substances 

 there is a reduction of compound bodies, and in such cases 

 the putrefying substances obtain their decomposing and 

 denitrifying bacteria from the air. The chief conditions 

 requisite for bringing about a loss of nitrogen by denitrifi- 

 cation are enumerated by Professor Warington as follows: 

 (i) the specific micro-organism ; (2) the presence of a nitrate 

 and suitable organic matter; (3) such a condition as to 

 aeration that the supply of atmospheric oxygen shall not 

 be in excess relatively to the supply of organic matter ; (4) 

 the usual essential conditions of bacterial growth. " Of 

 these," he says, " the supply of organic matter is by far the 

 most important in determining the extent to which denitri- 

 fication will take place." The necessarily somewhat un- 

 stable condition facilitates its being split up by means of 

 bacteria. The bacteria in their turn are ready to seize upon 

 any products of animal life which will serve as their food. 

 Thus, by reducing complex bodies to simple ones, these 

 denitrifying organisms act as the necessary link to connect 

 again the excretions of the animal body, or after death the 

 animal body itself, with the soil. 



In a book of this nature it has been deemed advisable not 

 to enter into minute description of all the species of bacteria 



