16 MICROBES AND TOXINS 



cellars is nitrate of lime which has risen from the soil through 

 the walls by capillarity and has undergone evaporation. 



Long ago Miintz prepared artificial nitre beds which furnished 

 eight grams of nitrate daily ; in recent experiments he and 

 Laind have obtained such a yield that they find themselves 

 capable of preparing by means of nitrifying organisms all the 

 saltpetre for the powder necessary to defend the nation in war. 

 Distributed over beds of peat of two metres thickness on the 

 top of a layer of clinker, the nitrifying bacteria are capable of 

 producing, for each 25 acres of area, 1,500 tons of nitrate per 

 day, after a starting period of one month at most to get the 

 beds into working order; in five days, that is, 7,500 tons, or 

 the annual requirement of powder for the army. It is easy to 

 calculate what 750,000 acres of peat bogs in France could 

 produce if necessary. 



The purification of sewage is one of the greatest tasks which 

 burden the hygienists of large towns. Broad irrigation followed 

 by cultivation demands much land and is not quite safe except 

 when employed solely for the growth of forage, not for market- 

 gardening. Hence it is gradually giving place to the biological, 

 method of purification, an intensive process carried on in 

 small space, and here again by bacteria. 



We shall not enter here upon the details of its application. 

 In principle, complete biological purification goes on in two 

 phases : a phase of anaerobic fermentation in the septic tank 

 and a phase ot aerobic fermentation in the bacterial beds. 



In the septic tank, into which the sewage must be run 

 gently — so as not to carry in the air which would favour 

 aerobic fermentation — in which it must be allowed to circulate 

 quietly, and to stay at least twenty-four hours, the disintegration 

 takes place both of hydrocarbons and proteins under the 

 agency of legions of bacteria which secrete all sorts of diastases. 

 The sludge dissolves and the resultant product can be sub- 

 mitted to the action of the nitrifying agents. 



The experimental control of the ferment activity in the 

 septic tank can be carried out by comparing the action on 

 coagulated egg-white, meat, raw or cooked, fats, paper, &c. 



