346 APPLIED BACTERIOLOGY 



organism upon the other, and that when the conditions 

 which favour the abnormal development of one class be 

 present, the destruction — to a great extent — of that class 

 is certain. This is well illustrated by the application of 

 nitrate of soda in considerable proportion ^ the nitric 

 bacteria diminish most markedly. 



The nitric bacteria appear to be incapable of working up 

 nitrogenous organic matter themselves — they depend upon 

 co-workers for the initial change, and while CaS04 can be 

 used by the nitric organism, CaCOg is much more easily 

 decomposed by them, and of course they must have the 

 other essentials, viz., P2O5, K2O, MgO, and a minute 

 proportion of NaCl. 



As an instance of the utilisation of the action of the 

 nitrifying organisms, we may cite the process of making 

 artificial nitre formerly largely practised on the Continent. 

 A large heap of earth containing old mortar, chalk, and 

 organic matter, was made, and protected from the rain 

 by a roof, but exposed to the prevailing winds. It was then 

 watered with stale urine, and dug over to expose fresh sur-, 

 faces. From time to time a portion at one end was removed 

 and mixed with wood-ashes and lixiviated, whereby a crude 

 solution of nitrate of potash was obtained. After lixiviation 

 the exhausted material was mixed with fresh organic matter 

 and returned to the heap. It was found that earth which 

 had been used for this purpose always worked better than 

 fresh earth. 



Fixation of Atmospheric Nitrogen. — For more than a 

 century the question has been debated as to whether 

 atmospheric nitrogen was available as plant food. For 

 many years the balance of opinion was against this idea, 

 although it seems strange that scientists should have 

 doubted that nitrogen was in some way assimilated, even 

 though the means were not understood. 



