THE PHYSICAL BASIS OF LIFE 15 



2. Metabolism 



We have emphasized that living matter is continually 

 changing, and this fundamental fact is reflected in nearly all 

 attempts to define life. Aristotle described life as "the 

 assemblage of operations of nutrition, growth, and destruc- 

 tion"; deBlainville, as a "twofold internal movement of 

 composition and decomposition"; and Spencer, as "the 

 continuous adjustment of internal relations to external 

 relations." 



This interaction consists of chemical and physical pro- 

 cesses in which combustion or oxidation plays the chief role. 

 Lavoisier and LaplajEgJn 1780 showed that animal heat results 

 from a slow burning of the materials of the body, involv- 

 ing the consumption of oxygen and the liberation of carbon 

 dioxide; and further, that for a given consumption of oxygen 

 and liberation of carbon dioxide, about the same amount of 

 heat is produced by an animal as by a burning candle. This 

 was an important discovery, because it went far toward 

 establishing the fact that at least certain characteristic vital 

 phenomena are amenable to the laws which hold in the non- 

 living world. 



But the processes involved in life are not so simple as 

 perhaps might be imagined from the results just mentioned. 

 Heat represents but one of the many energy transformations 

 within the organism. Indeed the living organism, like a 

 steam engine, is a machine for transforming energy trans- 

 forming the potential energy stored in chemical complexes 

 of its own substance into the various vital processes of living 

 into work performed. In these processes many complex 

 substances rich in potential energy, which have entered 

 as food and have in whole or part added to the protoplasmic 

 complex, are reduced to simpler and simpler conditions and 



