26 THE NATURE OF THE ACTIVITIES OF MICROORGANISMS. 
equation, C 6 H IO O S +H 2 O = C 6 H I2 O 6 , though the equation certainly 
does not represent the change that goes on. Starch is known not 
to be such a simple molecule as C 6 H 10 O S , but some multiple of that 
formula, and probably a very high one. Its decomposition into 
sugar is really a long series of steps, only the final result being 
partly represented in the equation. This fermentation occurs in 
food after it is mixed with saliva in the mouth. 
Putrefaction and Decay. If any proteid body meat, eggs, 
or the like be left for some time exposed to the air, it will give off 
unpleasant odors, for it is undergoing putrefaction and decay. 
These two processes, though frequently considered the same, are 
slightly different. Both are the result of the chemical decomposition 
of organic compounds, and the terms are commonly applied only 
to the decomposition of material that contains proteids. Both 
result in chemical decompositions which are very complete, and 
more complex and indefinite than the other two types of fermenta- 
tions. They are produced by microorganisms, chiefly bacteria, 
which feed upon the putrefying mass, taking certain atoms out of 
the organic molecules. These molecules, thus losing some of their 
atoms, change their chemical nature. The remaining atoms 
necessarily rearrange themselves to form new compounds which are 
simpler in structure. The distinction between putrefaction and 
decay consists in the fact that decay is the term applied to decompo- 
sition in the presence of oxygen, while putrefaction takes place 
in the absence of oxygen. The former is much more complete 
than the latter, resulting in the more complete destruction of the 
substance decomposed. 
Organized and Unorganized Ferments. These three ex- 
amples of fermentation are very different from one another. One 
seems to break the sugar molecule into two simple portions, carbonic 
acid and alcohol; the second simply adds a molecule of water to one 
of starch; while the third results in a complete decomposition of a 
highly complex proteid into a large number of by-products, both 
known and unknown. But in some important respects all three 
agree. Each differs from ordinary chemical processes in several 
respects and all agree in the following points: 
