TRANSFORMATIONS OF CARBON AND NITROGEN. 83 



they pass into the soil, perhaps to be drained away in the 

 drainage water. The insoluble bodies are also sure to be in- 

 corporated into the soil, becoming eventually mixed with the 

 solid masses of the earth. 



The list of the by-products of such decompositions is a long 

 one. Already a large number of products have been isolated 

 by chemists as occurring in putrefying bodies. A few of these 

 are as follows : 



Carbon dioxide, hydrogen sulphide, marsh gas, hydrogen, 

 nitrogen, calcium carbonate, propionic acid, valerianic acid, 

 acetic and lactic acids, alcohol, succinic acid, phenol, indol, 

 leusin, tyrosin, skatol, etc. 



This list is far from complete. It does not include all the 

 products already known and beyond question there are numer- 

 ous bodies, formed as by-products or excretions, which still re- 

 main to be discovered. The actual products which appear will 

 depend upon three factors : (i) The substance which is decay- 

 ing ; (2) the species of bacteria which produces the decay ; (3) 

 the conditions under which the decay occurs. 



The Ammoniacal Fermentation. Leaving out of consider- 

 ation these miscellaneous products, many of which are prob- 

 ably of no great significance in the general process, since they 

 are present in small amount only and are certainly not yet 

 understood, we may confine our attention to a few of the chief 

 products of nitrogen decomposition. It is these nitrogen prod- 

 ucts which are most intimately associated with soil fertility. 

 One of the most common and almost universal types of fer- 

 mentation is that which produces ammonia. The nitrogenous 

 body first to undergo an ammoniacal fermentation is urea. 

 Urea is always contaminated with certain species of bacteria in 

 the ducts from the bladder, and these organisms speedily in- 

 duce an ammoniacal fermentation. Several species of bacteria 

 have been found especially associated with this process. The 

 one first described was named J/. urece, but recent study has 



