CHAPTER II 
FERMENTATION, PUTREFACTION, AND DECAY 
THE NATURE OF THE ACTIVITIES OF MICRO- 
ORGANISMS 
Everyone at all familiar with nature must realize that there is 
constantly going on, in earth, water, and air, an uninterrupted 
series of slow changes. Rocks disintegrate; fruits decay and their 
juices ferment; vegetables rot; animal bodies putrefy; milk sours; 
cheese ripens; the soil becomes contaminated by the decaying 
waste of sewage and then purifies itself; streams become foul and 
grow clear again; even tree trunks rot and disappear. These and 
hosts of other kindred phenomena are matters of such every-day 
occurrence that we scarcely ever stop to think what they mean or 
how they are brought about. Butit is with these phenomena that 
we are chiefly concerned in the study of germ life on the farm. 
These changes have one characteristic in common: they are all the 
result of chemical decomposition. Until comparatively recently 
it has been supposed that they are the result of purely chemical 
forces. ‘The chemical agency of oxidation, especially the so- 
called slow oxidation, has been supposed to account for most of 
them. 
But it has been proved by modern study that pure chemical 
forces are not able to produce these phenomena, and that many a 
process formerly called slow oxidation is not the result of chemical, 
but rather of beological forces. If microdérganisms can be kept from 
them, fruits will not decay, vegetables will not rot, and many other 
changes will fail to appear. Most of the slow changes referred to 
are the result of the action of the great class of*fungi, foremost 
among which stand the bacteria and yeasts. The reason why 
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