152 BACTERIA 



lows: free nitrogen, carbonic acid, gas and water, ammonia 

 bodies, and sometimes nitrites. The nitrogen passes into 

 the atmosphere, and is " lost " ; the carbonic acid and water 

 return to nature and are at once used by vegetation. The 

 ammonia and nitrites await further changes. These further 

 changes become necessary on account of the fact, already 

 discussed, that plants require their nitrogen to be in the 

 form of nitrates in order to use it. Nitrates obviously con- 

 tain a considerable amount of oxygen, but ammonia contains 

 no oxygen, and nitrites very much less than nitrates. Hence 

 a process of oxidation is required to change the ammonia 

 into nitrites and the nitrites into nitrates. 



2. This oxidation is performed by the nitrifying micro- 

 organisms, and the process is known as nitrification. It 

 should be clearly understood that the process of nitrifaction 

 may, so to speak, dovetail with the process of denitrification. 

 No exact dividing line can be drawn between the two, al- 

 though they are definite and different processes. In a 

 carcass, for example, both processes may be going on con- 

 comitantly; so also in manure. There is no hard and fast 

 line to be drawn in the present state of our knowledge. 

 Other organisms beside the true nitrification bacteria may 

 be playing a part, and it is impossible exactly to measure 

 the action of the latter, where they began and where the 

 preliminary attack upon the nitrogenous compounds term- 

 inated. In all cases, however, according to Professor War- 

 ington, the formation of ammonia has been found to precede 

 the formation of nitrous or nitric acid. 



It was Pasteur who (in 1862) first suggested that the pro- 

 duction of nitric acid in soil might be due to the agency of 

 germs, and it is to Schlosing and Miintz that the credit be- 

 longs for first demonstrating (in 1877) that the true nature 

 of nitrification depended upon the activity of a living micro- 

 organism. Partly by Schlosing and Miintz and partly by 

 Warington (who was then engaged in similar work at Roth- 



