VI.] THE MOTIVE POWERS OF THE UNIVERSE. 57 



that can occur, in any natural or artificial combination, 

 only a portion of any mass of heat can become motive 

 power. 



The whole of what I may call a mass of energy of 

 motion is capable of being transformed into heat. For 

 instance, when a railway train is stopped by the action of 

 its break, the energy of its motion is all transformed into 

 heat. This transformation is constantly and everywhere 

 going on throughout the world; it almost always takes 

 place when motion ceases. But, as we have seen in 

 speaking of the steam-engine, the whole of what I may 

 in like manner call a mass of heat is not capable of being 

 transformed into energy of motion,^ or into any form of 

 potential energy. 



Thus any mass of energy of motion can be all trans- 

 formed into heat, but the heat cannot be all transformed 

 back into energy of motion. It follows from this, that 

 though both of these transformations are constantly going Transfor- 

 on, more energy of motion is transformed into heat than ^er^ ° 

 can be transformed back again ; and, as these are by far ?f motion 

 the most abundant forms of actual energy, the energy of the pre- ' 



motion in the universe is being gradually transformed into J^^Y^S 



o o ./ tendency. 



heat. Of course we do not assert that all the energy of 

 motion in the universe will ever be thus transformed. We 

 do not assert it of the planetary motions. But it is certain 

 that such is the tendency of all change. 



We thus see that motive power is constantly trans- Conse- 

 forming itself into heat, and that heat is constantly attain- straction 

 ing equilibrium : and heat in equilibrium, as we have of motive 

 seen, is not motive power. It follows from this that a^°^^^^* 

 constant destruction of motive power is going on through- 

 out the universe ; there is less motive power in existence 

 at present than there was in the past, and there will 



1 The relation between heat and electricity is in this respect the same 

 as that between heat and energy of motion. Heat can be transformed into 

 electricity only in the act of attaining equilibrium : when heat is in equi- 

 librium, it cannot be transformed into electricity, or into any other form 

 of energy whatever. The whole of a mass of electricity can be transformed 

 into heat, but the whole of a mass of heat cannot be transformed into 

 electricity. 



