386 PRINCIPLES OF SOIL MICROBIOLOGY 



nates that, in all physical and chemical processes, the sum of energy 

 remains unchanged, while the form and distribution are changed. In 

 other words, according to the law of conservation of energy, any system 

 in a given condition contains a definite quantity of energy; when this 

 system undergoes change, any gain or loss in its content of energy is 

 equal to the loss or gain of the energy in the surrounding systems. 7 



According to the second law of thermodynamics, every process that 

 occurs spontaneously is capable of doing work ; to reverse any such process 

 requires the expenditure of work from outside (Lewis). While all forms 

 of energy can be completely transformed into heat, heat energy can 

 never be completely transformed into other forms of energy; this 

 leads to a decrease in the convertible or free energy. Each trans- 

 formation of energy involving heat formation leads, therefore, to a 

 dissipation of energy and to a diminution of the amount of free energy. 

 This law formulated by Carnot and Clausius is also designated as the 

 law of entropy. Under "entropy" we understand the part of heat 

 energy which, at a given temperature, cannot be further converted 

 into other forms of energy. All processes in nature lead to an increase 

 in entropy. 8 The supply of energy in nature is constantly replenished 

 by the green plants, utilizing the free energy of the sun. In the com- 

 bustion of coal, in the decomposition of complex carbon compounds, 

 heat is liberated; in the building up of organic compounds from carbon 

 dioxide, heat is stored as potential energy. In the course of life proc- 

 esses, cell growth and cell activities, a part of the energy is either used 

 up for the activities of the cell or is stored away in the process of building 

 up the more complex cell constituents; the remainder, after a series of 

 transformations (overcoming resistance, etc.), is transformed into heat 

 and dissipated into space. 



Only "free energy" may be transformed into mechanical work. 9 The 

 maximum of work which can be obtained from a given process, at 

 constant temperature and pressure, is designated as AF, F being the 

 free energy. This is related to the total external work done by a 

 system as a heat of reaction is related to the total energy content. A 

 change in the condition of the substrate can be characterized by a 

 change in free energy, or better, by the maximum of work which can 



7 Lewis, G. N. The free energy of chemical substances. Jour. Amer. Chem. 

 Soc, 35: 1-30. 1913. 



8 Tangl, F. Energie, Leben und Tod. Springer. Berlin. 1914. 



9 Lewis, G. N., and Randall, M. Thermodynamics and the free energy of 

 chemical substances. McGraw-Hill, New York. 1923. 



