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. But it 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 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 biological forces. If microorganisms 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 these 
organisms are so closely associated with phenomena is because they 
are capable of bringing about profound chemical changes. 
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