2 MANUAL OF ELEMENTARY ZOOLOGY 



The body of a living man is always at work. In moving, 

 in producing the heat which usually keeps 

 The Life him warmer than the air around him, in the 



i?D?sintegi-a- manufacture of such liquids as sweat, spittle, 

 fiSm'iuti n anc * tears > * n ^ e activity of the brain which 

 of Energy. is known to accompany thinking, in a thousand 

 ways this is seen from moment to moment of 

 his life. Even in sleep the heart-beat and breathing go 

 on. Now this, like all other work, involves the using of 

 energy, and the energy of the living body is not im- 

 parted from without, as the energy which causes a kettle to 

 boil is imparted by the fire, but derived from within, as the 

 energy which drives the shot from a gun is derived from 

 the explosion of the powder within it, or a steam-engine, 

 regarded as a whole, obtains its energy from an internal 

 furnace. If, howevej, the body is to evolve energy 

 from itself, it can only obtain that energy by some re- 

 arrangement of the particles which compose its own 

 substance. Nor is it necessary to look far for evidence of 

 the occurrence of such a process or for information as to its 

 nature. Let us consider for a moment the way in which 

 the energy of the work of a steam-engine is obtained. It 

 depends upon the fact that whenever atoms unite to form 

 molecules energy is set free, and the stabler the molecules 

 formed the greater, almost invariably, is the amount of 

 energy liberated in their formation. The same amount of 

 energy must be used to break up a molecule as was set 

 free when it was formed. The substances which are used 

 as fuel consist of molecules which are complex and 

 relatively unstable, and rich in energy. The furnace of the 

 engine, receiving constantly and often under forced draught 

 fresh supplies of air, breaks down the molecules of its fuel 

 and unites the carbon and hydrogen atoms of which they 

 principally consist with the atoms of the oxygen in the air, to 

 form smaller and more stable molecules of carbon dioxide and 

 water. This process is, of course, the burning of the fuel. 

 The energy freed in the formation of the stable molecules is 

 so much greater than that which is required to break down 

 the unstable molecules that a large balance of energy is 

 freed and becomes available for driving the engine. The 

 chemical energy of the fuel and the oxygen becomes trans- 



