60 THE HUMAN MOTOR 



dynamics. In extending them to all the forms of energy, a 

 complete science 'has been constituted called energetics. And it 

 is seen that, every time a cycle of operations is completed the 

 law of equivalence applies, and whenever a completed cycle is 

 reversible, the principle of Carnot operates. Nevertheless, use 

 is made of the law of equivalence even when a complete cycle 

 is not performed, we say : 



U = T Q. 



Experiment justifies this equation. In addition it is used in 

 irreversible transformations. Nature evolves in an irreversible 

 manner. She follows one direction and does . not reverse her 

 progress. Life develops in the direction of age, but never in that 

 of rejuvenation. 



37. Comparison o! the Forms of Energy. To compare energies 

 quantitatively there must be common measure. It has been 

 agreed to adopt the calorie. Mechanical energy is measured at 

 425 kilogrammetres per great calorie. Solar energy, received 

 by a thermo-pile, is measured in calories. Similarly electric 

 energy can be transformed into heat. The common unit is there- 

 fore the calorie, but it does not follow that the nature of energy 

 is the same in all its aspects and that it can always be converted 

 into heat. From the qualitative point of view, the various forms 

 of energy form a hierarchy. Some (mechanical, elastic, electric) 

 are interchangeable without taking a calorific path in that 

 exchange. That is an advantage, because heat is never an 

 efficient intermediary ; it causes a depreciation or degradation 

 of the energies mentioned. 



Radiant energy, however, in its various forms, is always trans- 

 formed into heat ; it degrades to exhaustion. A steam engine 

 only performs work by degrading a quantity of heat, rejected 

 unprofitably to the condenser. It is an important fact in the 

 doctrine of energetics that the energies have a natural tendency 

 to degrade into heat, an inevitable tendency in some, but able 

 in others, to be moderated. As foi chemical energy, it only 

 degrades in part in its transformations into work or into electric 

 current. It would degrade entirely if facilities were not offered 

 to it by which it could change into work or electricity. That is 

 why it is convenient to consider both " free " energy, capable 

 of useful transformation and " bound " energy J 1 ) of degrada- 

 tion (heat). All spontaneous chemical reaction will plainly tend 

 to produce the maximum of heat (principle of maximum energy, 

 of Berthelot) (*). 



(*) The distinction between free and bound energy belongs to Helmholtz 

 Journal de Physique, 1884, p. 408 seq. 



( a ) It is improperly called the principle of maximum work, 



