ORGANIC NITROGEN—ITS DECOMPOSITION. 57 
position of organic substances under the influence of bacteria. 
There is a distinction between them, however, which may be 
properly drawn. 
Putrefaction.—This is the name given to a partial decomposi- 
tion that is far from complete. It is generally produced by bac- 
teria 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 product, 
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 Itis 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. COs, N and H2O 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 putrefac- 
tion 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. Noattempt need be made here to classify this miscellaneous 
host of putrefactive organisms. They include cocct, bacillt, thread 
and spiral forms as well as yeasts and higher funge (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- 
