ORGANIC NITROGEN. ITS DECOMPOSITION. 51 
tion that is far from complete. It is generally produced by bacteria 
growing in the absence of oxygen, and hence by the anaerobic or 
facultative anaerobic bacteria. These break down the proteids, 
but do not carry the decomposition to its final stages, the final pro- 
duct, thus formed, being still quite complex. Many of them have 
unpleasant odors and many of them are poisonous. 
Decay. This is the type of complete decomposition that takes 
place in the presence of oxygen. It is produced by aerobic bacteria, 
and results in a very complete disintegration of the decomposing 
body. The end-products are much simpler than in the case of 
putrefaction, and the gaseous products arising have little or no odor. 
CO 2 ,N and H 2 O are among these final products, and are all odorless. 
Putrefaction and decay cannot be sharply separated from each other, 
the former being in many cases only a step toward the latter. The 
bad-smelling or poisonous products of putrefaction will, if exposed 
to the air, undergo further disintegration until the decay is complete. 
But, though not sharply distinct, the difference above noted is a 
convenient method of designating the complete decomposition, 
in the presence of air, from the incomplete decomposition in the 
absence of air. 
Of the many species of bacteria associated with putrefaction 
and decay, some are likely to be found under one set of conditions 
and others under different conditions. Some are particularly 
common in decaying vegetable substances and others in decaying 
animal tissues, while some are most characteristic in fermenting 
urea. No attempt need be made here to classify this miscellaneous 
host of putrefactive organisms. They include cocci, bacilli, and spiral 
forms as well as yeasts and higher fungi (Figs. 14, 15). Some 
of them produce their fermentation only when oxygen is present, 
while others do so in the absence of oxygen, and the by-products 
produced in the absence of oxygen are different from those produced 
in its presence, since the former are more likely to be of a poisonous 
nature. These decomposition bacteria occur practically everywhere 
in nature in the air, in all bodies of water, and in extreme abund- 
ance in the soil. They are so widely distributed and so abundant 
that they are sure to seize hold of any bit of nitrogenous organic 
