CHAPTER V. 



RESPIRATION. 



THE maintenance of life is impossible without a supply of 

 energy, the motive power in the absence of which activities 

 must come to an end. This is more obvious in an animal 

 than in a plant, which generally is less obtrusive in its move- 

 ments and various activities and may make use of radiant 

 energy, more particularly in the production of food. 



That energy may be produced by the combustion of a suitable 

 fuel is a commonplace and is illustrated in the steam engine, the 

 boilers of which are heated by fires fed with fuel which varies 

 according to local circumstances. The heat-producing power, 

 or calorific value, of fuels varies and the most efficient material 

 in this respect is the one which produces the maximum number 

 of heat units or calories for a given weight of substance. 



In all cases the heat produced in the combustion of a fuel 

 is that due to the chemical reaction of oxidation, or, in other 

 words, the heat given out when the constituent elements of the 

 fuel severally combine with oxygen to form the corresponding 

 oxides. The heat of combustion of a compound will, therefore, 

 depend upon the heat of combustion of its constituent elements 

 and is greater the richer the compound is in elements possessing 

 a high heat of combustion. 



Now it is a principle in physical chemistry that the heat of 

 any chemical reaction depends solely upon the initial and final 

 products, and the total heat evolved is the same by whatever 

 method the final products are obtained, i.e., whether in one 

 single process or by a series of intermediate stages, and also 

 whether the reaction proceeds rapidly or so slowly that there is 

 no perceptible rise in temperature. In view of these facts it 

 will be clear that the same laws hold for the low temperature of 

 oxidation of various oxidizable substances in the living cell as 

 for the combustion of these substances in air or oxygen. 



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