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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