TRANSFORMATIONS OF CARBON AND NITROGEN. 85 



their molecules a variety of by-products must commonly be 

 found. Part of the sulphur present in the proteids, for example, 

 makes its appearance as H 2 S, the hydrogen 

 as H 2 O, and numerous other products ap- 

 pear in small amount according to the 

 conditions. Many different species of mi- 

 croorganisms are known to have the power 

 of inducing ammoniacal fermentation. It 

 is the common character of most of the 

 putrefactive bacteria which may be found B ; cteria producing am 

 in a manure heap, and it is also charac- moniacai fermentation: A. 



B. mycoides; B, B. stutzeri. 



teristic of some molds and yeasts. Promi- 

 nent among these species two may be selected as perhaps con- 

 tributing to the result more commonly than any others, B. 

 mycoides and Proteus vulgaris, well-known species almost al- 

 ways found in soil and in decaying organic masses (Fig. 13 

 and Fig. 15). 



The universal occurrence of such a decomposition of organic 

 bodies is no new discovery. It has long been known and its ex- 

 treme significance recognized. It is the first step necessary, in 

 the proteid destruction, to bring the nitrogen locked up in the 

 proteid back again within reach of plants. Its value in produc- 

 ing what has been called the self -purification of the soil, has 

 been only recently appreciated. A little thought will show 

 us that without the existence of some such process the soil 

 would rapidly become unfit for the support of life simply by 

 becoming clogged up with the remains of past animals and 

 plants. If all the bodies of animals remained on the soil after 

 death, and if the roots and stems of plants were not disposed 

 of by some such process of fermentation and slow oxidation, 

 it is evident enough, from simple mechanical reasons, that 

 vegetation would soon cease, since there would be no room 

 left in the soil for new plants. When we realize, in addition, 

 that the very processes which purify the soil of these cumber- 



