194 LECTURE X. 



carbonate. 1 Nitrification is not a simple process. It requires the simul- 

 taneous activity of several varieties of bacteria. One oxidizes the ammonia 

 to nitrite, and another converts the nitrite into nitrate. The nitrifying 

 bacteria are found everywhere. They play an extremely important part 

 in the economy of nature. They effect the nitrogen cycle. 



Even the nitrogen which the animal organism utilizes for its nutrition, 

 is finally returned to the ground again as ammonia. We shall see later 

 that the largest part of albuminous nitrogen reappears in the form of urea 

 in the urine of mammals. Under the action of specific bacteria this is 

 broken down into ammonia, which is then converted into nitrates. The 

 plants utilize this anew for the synthesis of albumins, and the nitrogen 

 completes its cycle of usefulness, first, in the form of inorganic, and then as 

 organic compounds. This process is not as simple in practice as the 

 statement indicates. A large amount of free nitrogen is produced simul- 

 taneously with the combined nitrogen. When nitrogenous organic mate- 

 rial undergoes combustion, free nitrogen is obtained as well as ammonia. 

 The amount of the former may be very considerable under favorable 

 conditions. This is the case when the combustion is carried out at a high 

 temperature with a liberal supply of air. Nitrogen is also liberated in 

 large quantity by the explosion of gunpowder. It is liberated not only by 

 artificial processes, but also through the intermediacy of organisms occur- 

 ring in nature. To be sure the assumption that free nitrogen is liberated 

 in the metabolism of higher plants has been disproved by exact investi- 

 gations, just as the oft-repeated question as to whether nitrogen is elim- 

 inated as such, from albuminous material in the animal organism, has 

 been answered in the negative. We are acquainted on the other hand 

 with a large number of organisms of common occurrence which are incapable 

 of liberating nitrogen from organic compounds, but can do so from nitrates. 

 This process, also called denitrification, has been known for a long time. 



1 It may be interesting to note that there are also other organisms capable of utilizing 

 inorganic instead of organic materials. The group of sulphur bacteria is best known. 

 Kernels of sulphur are found in their cell-bodies. They thrive in sulphur springs and 

 produce therein a characteristic flora. They constitute the group of Beggiatoa, and are 

 aerobic. The Beggiatoa are capable of oxidizing sulphuretted hydrogen, when in the 

 presence of oxygen, to sulphur. The stored-up sulphur is then further utilized in the 

 cells, sulphuric acid being formed, which seems to be their characteristic product. 

 They need only small amounts of organic material. During the oxidation of a gram- 

 molecule sulphuretted hydrogen to sulphuric acid, 62.4 cal. of heat energy are obtained 

 by the bacteria. 



Many thread-bacteria, especially Leptothrix ochracea, form other examples. They 

 oxidize ferrous carbonate into a ferric salt, which is decomposed with the formation of 

 ferric hydroxide. Winogradsky, to whom we owe our knowledge of these sulphur and 

 iron bacteria, suggests the possibility of the latter participating in the formation of 

 bog-ore deposits. 



