THE NITROGEN CYCLE 225 



in height filled with sand or earth, and dosed them with 

 defined quantities of sewage, allowing intervals for aeration 

 between each dose. By this method he was able to purify 

 much greater quantities of sewage on a given surface area of 

 soil, than by the so-called broad irrigation processes formerly 

 in vogue. At that time, however, the true explanation of 

 the oxidation change which took place was not properly 

 understood, and it was considered to be a purely chemical 

 phenomenon. Later on the Massachusetts State Board of 

 Health took up the subject, in the Light of the researches of 

 Warington, Winogradski,. and Percy Frankland, and they 

 worked out the conditions for the successful oxidation of 

 sewage matter by percolation through sand filters. They 

 showed that the results depended essentially upon the presence 

 of oxygen, and upon the time allowed for the change to take 

 place. They confirmed Warington's conclusion that it was 

 necessary for a base of some kind to be present, to combine 

 with the nitrous and nitric acid produced by the oxidation of 

 ammonia ; all other conditions they considered were secondary 

 to these three. 



It was Stoddart who showed in 1893 that the time factor 

 could be gradually decreased, if filters of more open material 

 than sand were used, and care was taken to distribute the 

 nitrifying solution in such a way that a thin film only was 

 exposed to the action of the air. By allowing a solution of 

 ammonium carbonate (1 part N in 10,000) to drip on to a 

 column of coarsely powdered chalk properly inoculated with 

 nitrifying organisms he was able to obtain highly efficient 

 nitrification. 



This experiment of Stoddart's is really the original of the 

 modern trickling or percolating sewage filter. 



Scott-Moncrieff in 1898, by employing superimposed 

 trays of filtering medium for the final purification of sewage, 

 which had undergone preliminary ammoniacal fermentation 

 in a so-called ' cultivation tank/ obtained a high degree 



