THE ORGANISM AS A MECHANISM 79 



the indirect cycle. The heat taken from the source 

 and given to the refrigerator in the direct cycle is 

 equal to the heat taken from the refrigerator and given 

 to the source in the indirect cycle. But it is a purely 

 imaginary mechanism, and all experience shows not 

 only that it has not been realised in practice, but that 

 it cannot so be realised. If it could be realised, we 

 should show that the second law of thermo-dynamics 

 is not physically true. 



Do the energy processes of life realise such a 

 perfectly reversible cycle of operations ? In order to 

 answer this question we must consider the fate of the 

 energy which is absorbed in the plant metabolic cycle, 

 and that which is given out in the animal one. Does 

 all the energy of solar radiation which is absorbed by 

 the plant pass into the form of the potential chemical 

 energy of the carbohydrates and other substances 

 manufactured ? Does any of the energy of the animal 

 which results from the metabolism of its body pass 

 into the unavailable form — that is, into a form in which 

 it cannot be utilised by other organisms ? That is to 

 say, is energy dissipated by the organism ? 



Undoubtedly it is to some extent, but to a far less 

 extent than in the inorganic train of processes. Some 

 of the energy of solar radiation absorbed by the plant 

 must become transformed, by the friction of whatever 

 movements occur, into low-temperature heat, and 

 some quantity of heat, however small, is generated by 

 the metabolism of the plant. Again, some of the heat 

 of the warm-blooded animal must be radiated into 

 space, or conducted away from its body ; and this 

 energy becomes dissipated — let us assume, at least, 

 that it is so dissipated in the physical sense. Probably 

 also some quantity of heat is generated by the meta- 

 bolism of the cold-blooded animal, though this must 



