212 PHYSIOLOGY OF NUTRITION 



The circulation of energy is quite different from that of matter. The avail- 

 able supply of energy upon the earth is inadequate for a long continuation of 

 plant and animal life, which would soon cease were it not for the continuous 

 influx of energy from the sun. From the law of the conservation of energy it is 

 clear that the solar energy stored up in potential form by the photosynthetic 

 process in green plants must be completely liberated by the reverse process (the 

 formation of carbon dioxide and water), as this occurs in combustion or in plant 

 and animal respiration. The carbon dioxide thus produced can, of course, enter 

 again into organic compounds, but that portion of the energy liberated by respir-l 

 ation and fermentation that takes the form of heat is almost entirely lost from 

 the organism and does not again become available for organic synthesis; it 

 becomes dissipated into space and is gone forever from the parth. Thus vital 

 activity upon our planet is directly dependent upon the sun, from which new 

 supplies of energy must continually come if life is to be long continued. 



This process of energy dissipation may be illustrated somewhat as foUows. 

 If a small beaker of hot water is poured into a large tank of cold water, the cold 

 water is warmed but little; supposing the original temperatures to be 95° and 5°, 

 respectively, the temperature of the tank may perhaps rise to 6° when the hot 

 water is added. At first the heat energy is concentrated, or intensive, in the 

 beaker; later it is dissipated, or extensive, in the tank. The coefficient of 

 energy dissipation, that fraction of the original energy that can no more be con- 

 verted into mechanical work, is termed entropy, and entropy always tends 

 toward a maximum. In this process of the dissipation of the intensive energy 

 of our solar system, plants play a direct r61e. 



