SOURCE OF BODILY ENERGY. 287 



objects as fast as it is generated. The oxidations occurring 

 in our Bodies are of this slow kind. An ounce of arrow- 

 root oxidized in a fire, and in the Human Body, would 

 liberate exactly as much energy in one case as the other, 

 but the oxidation would take place in a few minutes and 

 ;at a high temperature in the former, and slowly, at a lower 

 temperature, in the latter. In the second place, the engine 

 differs from the living Body in the fact that the oxidations 

 in it all take place in a small area, the furnace, and so the 

 temperature there becomes very high; while in our Bodies 

 the oxidations take place all over, in each of the living 

 cells; there is no one furnace or hearth where all the energy 

 is liberated for the whole and transferred thence in one 

 form or another to distant parts: and this is another reason 

 why no one part of the Body attains a very high temperature. 

 The Fuel of the Body. This is clearly different from 

 that of an ordinary engine: no one could live by eating 

 coals. This difference again is subsidiary; a gas-engine 

 requires different fuel from an ordinary locomotive; and 

 the Body requires a somewhat different one from either. It 

 needs as foods, substances which can, in the first place, be 

 -absorbed from the alimentary canal and carried to the 

 various tissues; and, in the second, can be oxidized at 

 a low temperature in the blood or tissues, or can be con- 

 verted by the living cells into compounds which can be 

 .so oxidized. With some trivial exceptions, all substances 

 which fulfill these conditions are complex chemical com- 

 pounds, and to understand their utilization in the Body 

 we must extend a little the statements above made as to the 

 liberation of energy in chemical combinations. The general 

 law may be stated thus Energy is liberated whenever chemi- 

 cal union takes place : and whenever more stable compounds 

 are formed from less stable ones, in which the constituent 

 atoms ivere less firmly held together. Of the liberation by 

 simple combination we have already seen an instance in the 

 oxidation of carbon in a furnace; but the union need not 

 be an oxidation. Everyone knows how hot quicklime 

 becomes when it is slaked; the water combining strongly 

 with the lime, and energy being liberated in the form of 



