PUTREFACTION AND OXIDATION 121 



growths, those in the sterile earth scarcely rose above the 

 ground. Experiments have also been made as to the effect 

 of adding to soil substances which inhibit the growth of 

 bacteria, with effects precisely similar to those mentioned 

 above. It was for a long time doubted that atmospheric 

 nitrogen could be assimilated by plants ; but even though 

 the means were not understood, it seems strange that 

 scientists should have doubted that nitrogen was in some 

 way assimilated, as had it not been so, our stock of com- 

 bined nitrogen would be on the decrease, whereas the 

 reverse is rather the case. 



As an instance of the utilisation by man 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 lixiviates, 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. 



An account of the nitrifying organisms will be found in 

 the chapter on fermentation. 



