678 SCIENCE PROGRESS 



and plants. The recognition of their importance is entirely 

 the outcome of modern methods of investigation. This know- 

 ledge is of recent origin, and a short consideration of the main 

 changes produced in nature by bacterial activity is essential in 

 order to understand the nitrogenous cycle. 



Nitrification. — As a final product of decay of plants or 

 animals brought about by micro-organisms, the nitrogenous 

 substances furnish mainly ammonia. This is transformed in 

 the soil into nitric acid and nitrates, and enormous deposits 

 of saltpetre have been formed in this way on the earth's surface 

 under certain conditions. It was originally pointed out by 

 Sir Humphry Davy that saltpetre is formed by the help of 

 the oxygen of the air from the ammonia present in the soil, 

 but the fact that this saltpetre formation, now usually called 

 nitrification, is due to a biological process, carried out by the 

 activity of microbes, has only been demonstrated in recent 

 times. It was not until 1890 that Winogradsky succeeded in 

 producing pure cultures of these organisms. This achievement 

 offered great difficulties until it was found that these organisms 

 grow best in a completely inorganic medium. They are able 

 to obtain all their necessary carbon and nitrogen from ammonium 

 carbonate. This nitrification process is the work of several 

 kinds of microbes, one class called "nitrosomonas," oxidising 

 ammonia to nitrites, and the other class, called "nitrobacter," 

 which oxidise nitrites to nitrates. It has been shown that a 

 large amount of organic nitrogen is transformed into nitrates 

 in the soil and thus it is made available for the building up 

 of the protein material of vegetable protoplasm. This is used 

 as food for animals, and the largest amount of the protein- 

 nitrogen of food is excreted by them in the urine, as urea 

 or uric acid, to be transformed, also by bacterial activity, 

 into ammonia and finally in the soil into nitrates. 



De nitrification. — Whilst the nitrogenous cycle would thus 

 appear to be a relatively simple one, another very old 

 observation seems to break its continuity. Long ago Davy 

 discovered that free nitrogen is formed in large quantities 

 during the decomposition of organic substances. Towards the 

 end of last century it was finally established that we have 

 to deal here also with the work of microbes, which, how- 

 ever, work only in the absence of oxygen. This process 

 is called " denitrification," and the organisms which are the 



