CHAPTER IV 



LIBERATION OF ENERGY 



(2) THE ANIMAL AS MACHINE 



" The living and the dead, things animate and inanimate, we dwellers in this 

 world, and this world wherein we dwell, are bound alike by physical and mathe- 

 matical law." THOMPSON. 



WE have just seen that : 



(1) Some of the radiant energy of the sun is stored by plant 

 agency, and is ingested by the animal as -food ; and (2) the sum 

 total of the energy taken in by the organism in this way can be 

 accounted for. There is neither gain nor loss of energy in the living 

 animal : the physical law of conservation of energy holds good. 



We must consider the physics underlying the liberation of this 

 energy. Does it follow any of the methods well known to us ? 

 Can we compare the foodstuffs to fuel and the body to a heat 

 engine ? In other words, what intermediate stage, if any, does 

 the potential energy of, say, starch, reach before becoming 

 apparent as animal energy ? The physiological text-books are 

 so full of references to combustion, fuel value, burning of food- 

 stuffs, that it is natural for the student to look upon the life 

 processes as somewhat similar to those of a steam engine. In 

 spite of this it can be definitely said that the animal body bears 

 little resemblance to any form of heat engine. The intermediate 

 stage between potential and free energy is not the wasteful one 

 of heat. In order that heat may be converted into work there 

 must be a certain allowance for " spillage." There must also 

 be a certain gradient of potential. In other words, unless 

 there is a certain minimum difference in temperature between the 

 source of heat and the sink (or heat condenser), the machine will 

 not work. (Principle of Carnot.) In 1824 Carnot determined, 

 theoretically, the percentage of heat that any heat engine could 

 convert into work. A theoretically perfect engine, working 



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