308 THE HUMAN BODY. 



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 tem- 

 perature, in the latter. In the second place, the engine dif- 

 fers from the living Body in the fact that the oxidations in it 

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

 perature 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 lib- 

 erated 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 re- 

 quires 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 converted by the living cells 

 into compounds which can be so oxidized. With some trivial 

 exceptions, all substances which fulfil these conditions are 

 complex chemical compounds, and to understand their utili- 

 zation in the Body we must extend a little the statements 

 above made as to the liberation of energy in chemical com- 

 binations. The general law maybe stated thus: Energy is 

 liberated whenever chemical union takes place: and whenever 

 more stable compounds are formed from less stable ones, in 

 which the constituent atoms were 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. Every one knows how hot 

 quicklime becomes when it is slaked; the water combining 

 strongly with the lime, and energy being liberated in the 

 form of heat during the process. Of the liberation of energy 

 by the breaking down of a complex compound, in which the 

 atoms are only feebly united, into simpler and stabler ones, 

 we get an example in alcoholic fermentation. During that 

 process grape-sugar is broken down into more stable com- 

 pounds, mainly carbon dioxide and alcohol, while oxygen is 

 at the same time taken up. To pull apart the carbon, hydro- 



