18 MICROBES AND TOXINS 



denitrification as in cultivated soil. Nitrifying and denitrifying 

 ferments can live side by side ; the latter only interfere with 

 the object desired when there is present an excess of hydro- 

 carbons. 



A town of 500,000 inhabitants, furnishing 6 million gallons 

 of waste water, could replace the 682 acres required for broad 

 irrigation, or the 15 acres required for the bacterial beds with 

 slags and clinker, by 2*5 acres of bacterial bed built on peat 

 (Calmette). 



The biological purification returns the nitrogen to nature in 

 the form of nitrate, although a certain quantity is lost in the 

 form of gas. There are other bacteria, however, capable of 

 taking up nitrogen from the air and re-introducing it into the ' 

 cycle of animal and vegetable life. 



V. — Fixation of Atmospheric Nitrogen in the Soil. 



The life of both animal and vegetable species depends on the 

 stock of nitrogen retained by the soil. Although the earth 

 acquires nitrogen from putrefying processes it loses nitrogen 

 also, discharged in the gaseous condition ; some is lost also 

 during denitrification, and in percolating water which robs 

 uncultivated soil of as much as 40 kilos, of nitrogen per acre 

 per annum. Floods also carry off the nitrates, and after the 

 floods of 1896, Schloesing calculated that the Seine carried off 

 about 5 milligrams of nitric acid per litre, at the rate of 

 800,000 litres per second ; the total nitric acid lost amounted 

 to 350,000 kilos, per twenty-four hours, equal to 650,000 kilos, 

 of saltpetre. The rivers pour this nitrogen into the sea. 



And yet in spite of these losses the soil retains its nitrogen- 

 Nay more, it accumulates it. The soil of forests is never 

 manured and the woodcutters carry off a great quantity of 

 nitrogen with the wood ; yet the soil there remains fertile. In 

 the hill pastures, flocks are feeding all the summer and 

 furnishing us with nitrogen in the form of milk, cheese and 

 meat : and yet the soil of these natural fields contains quantities 

 of nitrogen greater than is found in soil ploughed and copiously 



