152 AGEICULTUEAL AND INDUSTRIAL BACTERIOLOGY 



give 3.6 large calories of heat. The energy secured by the 

 transformation of the glucose into lactic acid is repre- 

 sented by the difference between these two or .16 calorie. 

 In other words more than twenty-three times as much 

 energy may be secured by the complete oxidation of a 

 given amount of glucose than is secured by the changing 

 of this amount of glucose into lactic acid. 



A gram molecule of ethyl alcohol yields 325.7 large 

 calories. A gram molecule of acetic acid yields 209.4 

 large calories. By the transformation of a gram molecule 

 of ethyl alcohol into acetic acid there is released the dif- 

 ference, or 116.3 large calories. 



Cycles of the Elements 



The fact that many organisms are constantly oxidizing 

 certain compounds and reducing others, or oxidizing com- 

 pounds containing one element under certain conditions 

 and reducing under others, makes possible the study of a 

 cycle of changes which these elements undergo. Three 

 of the elements are of sufficient importance from an agri- 

 cultural point of view, and are sufficiently changed by 

 microorganisms, as to warrant their discussion. These 

 elements are nitrogen, carbon, and sulphur. 



The Nitrogen Cycle. — The cycle of changes brought 

 about by microorganisms in nitrogen compounds in nature 

 can best be followed by reference to the diagram on 

 page 240. Complex nitrogen compounds are constantly 

 being broken down by a process of hydrolysis into am- 

 monia, the ammonia oxidized to nitrates, the nitrates 

 taken up by higher plants, converted into amino acids, 

 and built up into proteins, thus completing the cycle. 

 Microorganisms bringing about changes of economic sig- 

 nificance will be discussed in much greater detail under 

 later chapters. 



Proteolysis and Ammonification. — It is perhaps most con- 



