286 THE HUMAN BODY. 



and not upon any peculiarity in the energy utilized or in 

 its source. The Body is, however, a far more economical 

 machine than any steam-engine; of all the energy liberated 

 in the latter only a small fraction, about one eighth, is use- 

 fully employed, while our Bodies can utilize for the perform- 

 ance of muscular work alone one fifth of the whole energy 

 supplied to them; leaving out of account altogether the 

 nutritive and other work carried on in them, and the heat 

 lost from them. 



The Conditions of Oxidation in the Living Body. Al- 

 though the general principles applied in the Body and 

 the steam-engine for getting available energy are the 

 same, in minor points obvious differences are found be- 

 tween the two. In the first place the coals of an engine 

 are oxidized only at a very high temperature, one which 

 would be instantly fatal to our Bodies which, although 

 "warm when compared w r ith the bulk of inanimate objects, 

 are very slow fires when compared with a furnace. Chem- 

 istry and physics, however, teach us that this difference is 

 quite unimportant so far as concerns the amount of energy 

 liberated. If magnesium wire be ignited in the air it will 

 become white-hot, flame, and leave at the end of a few 

 seconds only a certain amount of incombustible rust or 

 magnesia, which consists of the metal combined with 

 oxygen. The heat and light evolved in the process repre- 

 sent of course the energy which, in a potential form, was 

 associated with the magnesium and oxygen before their 

 combination. We can, however, oxidize the metal in a differ- 

 ent way, attended with no evolution of light and no very per- 

 ceptible rise of temperature. If, for instance, we leave it in 

 wet air it will become gradually turned into magnesia with- 

 out having ever been hot to the touch or luminous to the 

 eye. The process will, however, take days or weeks; and 

 while in this slow oxidation just as much energy is liberated 

 as in the former case it now all takes the form of heat; and 

 instead of being liberated in a short time is spread over a 

 much longer one, as the gradual chemical combination takes 

 place. The slowly oxidizing magnesium is, therefore, at no 

 moment noticeably hot since it loses its heat to surrounding 



