186 OUTLINES OF BACTEEIOLOGY 



This, it will be observed, is a process of oxidation, inasmuch as 

 oxygen is absorbed in order to effect the chemical change. By a 

 further oxidation the nitrous acid can be changed into nitric acid : 



HNO2 + = HNO3. 



nitrous acid oxygen nitric acid 



What applies to ammonia applies to ammonium salts, in which case 

 the oxidation results in salts of nitric acid, i.e. nitrates. 



The oxidation of ammonium compounds in the soil was formerly 

 regarded as a purely chemical process, the oxygen being supplied from 

 the atmosphere. This was apparently borne out by the fact that it 

 was not possible to secure adequate nitrification in a soil unless the 

 latter were thoroughly aerated. However, Pasteur's epoch-making 

 researches, demonstrating the intimate connection between micro- 

 organisms and many of the processes that take place in nature, 

 resulted in a closer study of nitrification from this point of view, and 

 in 1877 it was shown that if a plot of soil were sterilised by heat, or 

 by some other means, no nitrification took place. Now, if nitrification 

 were a purely chemical process, the mere sterilising of the soil should 

 not have stopped that process. Hence it was concluded that sterilisa- 

 tion of the soil had killed off the microorganisms which were 

 concerned in carrying on this process of oxidation. The next step 

 was obviously to seek out these important organisms among the 

 many thousands that are found in the soil. This proved a difiicult 

 task. Whilst a portion of soil placed in an ammoniacal nutrient 

 solution invariably oxidised the ammonia into nitrites and the latter 

 into nitrates, none of the soil bacteria that appeared on the gelatine 

 plates, used for the purpose of isolating these bacteria, were able to 

 accomplish this. It was evident, therefore, that they did not grow in 

 the ordinary nutrient media. The diflaculty was at last surmounted 

 by Winogradsky in 1889, and also independently about the same 

 time by Paul G. Frankland in England and by Jordan and Richards 

 in America. Winogradsky effected the isolation of the first of these 

 organisms hy emplotjing a nutrient medium which did not contain a particle 

 of organic matter. It was made up as follows : 



Ammonium sulphate, 1 gram 



Potassium phosphate, 1 „ 



Tap water, 1 litre. 



In addition to every 100 c.c. of the nutrient medium, from 0'5 to 

 1-0 gram of basic magnesium carbonate was added. The absence of 

 organic matter prevented all except a very small number of organisms 



