34 The Physiology of Plants BOOK in 



10 per cent, of which both organisms can thrive. The 

 growth of the nitrous forms is inhibited in culture liquids 

 containing 20-40 per cent., and that of the nitric organisms 

 in liquids containing 60 per cent, of this preparation. The 

 effect of acetate and butyrate of soda is considerable, but 

 both organisms can withstand smaller quantities than -5 per 

 cent. The nitric microbe is impeded by as little as -0005 

 per cent, of ammonia and its growth is stopped by -015 

 per cent. The authors regarded these organic substances 

 as antiseptics towards the organisms. 



The question of the simplification of nitrogenous com- 

 pounds, with the ultimate liberation of ammonia and even 

 of free nitrogen, received considerable attention after 

 1860. About that time there was a certain controversy 

 as to whether free nitrogen is one of the products of the 

 putrefaction of manure, which later was settled in the 

 negative. The main interest of subsequent discussion turned 

 on its liberation by processes of reduction from nitrates. 

 Goppelsroder first pointed out in 1862 that such processes 

 occur in arable soil, and they were first associated with 

 bacteria by Mensel in 1875. Apart from soil action we 

 have the observations of Schloesing in 1868 to the effect 

 that during the fermentation of putrefying urine or of 

 tobacco juice nitrates disappear, nitrous and nitric oxides 

 and free nitrogen being produced. The same decomposi- 

 tion takes place during the lactic fermentation of sugar. 

 In 1873 he ascertained that a similar course of events can 

 be observed to take place in moist vegetable soil when no 

 change of atmosphere is allowed. In 1881 Warington 

 observed that soil can reduce nitrates to nitrites and finally 

 destroy the latter, and in the next year Deherain and 

 Maquenne confirmed Mensel's observation that the decom- 

 position is a bacterial one. Pure cultures of organisms 

 capable of bringing it about were made by Gayon and 

 Dupetit in 1882, by Heraeus in 1886, by Warington in 



