CHAPTER V. 



RESPIRATION. 



The maintenance of life is impossible without a supply of 

 energy. This energy is acquired by various chemical reactions 

 of an exothermic character whereby substances of different 

 nature are converted into compounds of lower thermal energy, 

 and thus energy is released. 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 the thermal energy 

 liberated by the combustion of fuel which varies according 

 to local circumstances. The heat-producing power, or calorific 

 value, of fuels varies, the most efficient material is that which 

 produces the maximum number of heat units or calories for 

 a given weight of substance. 



The heat produced in the combustion of a fuel is 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. The most economic fuel is, there- 

 fore, that which contains a minimum of constituents which 

 cannot yield heat on combustion with oxygen ; of these water 

 is the most significant, followed by various metallic elements 

 which form the so-called ash constituents. It is obvious that 

 the more complete the combustion, the greater is the energy 



99 7* 



