100 TEE HUMAN BODY. 



seconds only a certain amount of incombustible rust or 

 magnesia, which consists of the metal combined with oxy- 

 gen ; under these circumstances it has been burnt or oxid- 

 ized quickly at a high temperature. The heat and light 

 evolved in the process represent the energy which is set 

 free by the metal and oxygen when they combine. We 

 can, however, oxidize the metal in a different way, attend- 

 ed 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 magnesia without hav- 

 ing ever been hot to the touch or luminous to the eye. 

 The process then, however, takes days or weeks ; but in 

 this slow oxidation just as much energy is liberated as in 

 the former case, although 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 combi- 

 nation takes place. (The slowly oxidizing magnesium is, in 

 consequence, at no moment noticeably hot, since it loses its 

 heat to surrounding objects almost as fast as it generates 

 it.y 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 liberate exactly as much ener- 

 gy 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. 



Oxidation in the presence of moisture. Wet wood or 

 wet coal we know will not burn, or can only be made to do 

 so with difficulty. Other kinds of burning or oxidation 

 are, however, well known, which take place in the pres- 



How does the rate of oxidation differ in the two cases ? How 

 does the oxidation of arrowroot burned in a fire differ from its oxida 

 tion in the living body? 



Can oxidations occur in the presence of moisture '? 



