INCOME AND EXPENDITURE OF THE BODY. 307 



the character of which depends upon the peculiarities of 

 mechanism which utilizes it in each case, 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 usefully employed, while our 

 Bodies can utilize for the performance 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 utilizable energy are the same, in 

 minor points obvious differences are found between 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 with 

 the bulk of inanimate objects, are very slow fires when com- 

 pared with a furnace. Chemistry 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 represent 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 

 different way, attended with no evolution of light and no 

 very perceptible rise of temperature. If, for instance, we 

 leave it in wet air it will become gradually turned into mag- 

 nesia without 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 liber- 

 ated 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 sur- 

 rounding objects as fast as it is generated. The oxidations 

 occurring in our Bodies are of this slow kind. An ounce of 

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



