NITRIFICATION 135 



limits of temperature between which the organisms 

 carried on their work, and also the conditions of aeration 

 and other factors which influenced the resulting nitrifica- 

 tion. At 50 C. the process is scarcely recognisable, 

 and at 55 C. it is destroyed : the optimum temperature 

 lies near 37 C. Warington, at a later date, showed that 

 nitrification usually takes place in solutions of ammonium 

 salts, with the formation of varying amounts of nitrites 

 and nitrates, and found that in some instances the oxida- 

 tion of ammonium compounds may proceed as far as the 

 formation of nitrites and there stop, no nitrates being 

 produced. He, moreover, proved that in the nitrification 

 of organic materials, such as milk, urine, and asparagin, 

 the formation of ammonium compounds first takes place. 

 Although it was found an easy matter to set up 

 nitrification in ammoniacal solutions by the addition of a 

 small amount of garden soil, all the early attempts to 

 isolate and prepare pure cultures of organisms capable 

 of oxidizing ammonium compounds led to unsatisfactory 

 results until 1890, when Frankland and Winogradsky 

 succeeded in the task. They found that the real agents 

 do not grow in bouillon and similar media containing 

 soluble organic substances, and stubbornly refuse to 

 develop on the ordinary gelatine and agar plates. 

 Winogradsky cultivated the organisms in a solution 

 containing ammonium sulphate, potassium phosphate, 

 and a small amount of basic magnesium carbonate. 

 From a medium of this kind devoid of all organic com- 

 pounds, and undergoing active nitrification, he inoculated 

 plates of gelatinous silica, impregnated with the inorganic 

 salts mentioned, and succeeded in isolating the two 

 kinds of bacteria now known to be the active agents of 

 the nitrification process. 



