112 BiACTERIOLOGY. 



In the fermentation of carbohydrates, or of fats, as might 

 be expected, various organic acids form. The reaction of 

 the fermenting liquid is, therefore, acid £Lnd the odor may- 

 be considered as pleasant. Oh the other hand, in the fer- 

 mentation of proteins the ammonia and amines that form 

 may impart an alkaline reaction to the medium, and, as 

 indicated above, disagreeable odors are usually present. 



Without the agency of micro-organisms the dead ani- 

 mals and plants would accumulate upon the earth's surface. 

 The action of atmospheric oxygen, and of light, would be 

 wholly insufficient to accomplish the change of "dust to 

 dust." The removal of dead matter may be considered as 

 the special function of microscopic life. In this respect, 

 the bacteria are to be looked upon as performing a most 

 important part — that of nature's scavengers. The waste- 

 matter of the living, and the lifeless remains themselves are 

 in a short time converted into the simplest inorganic chem- 

 ical compounds. The carbon present in the starch, cellu- 

 lose, or protein molecule is returned by the agency of these 

 organisms to the inorganic world as carbonic acid. In like 

 manner the hydrogen becomes converted into water. The 

 nitrogen present in the cell protoplasm, which is by far the 

 most complex chemical substance known, is brought back 

 to its simplest form ammonia, nitrous, and nitric acids. 



Bacteria are, unquestionably, the most active agents in 

 this resolution of dead matter. Other forms of life, how- 

 ever, may also induce similar changes. The moulds and 

 yeasts, and even the infusoria, are deserving of mention in 

 this connection. 



The removal of dead matter, as indicated above, has one 

 important consequence and that is the perpetuation of life. 

 The ordinary plants derive their food from the carbonic 

 acid of the air and from the nitrates of the soil. The starch, 

 sugar, fats and protein matter which they elaborate, in turn. 



