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: 



