ON THE DISSIPATION OF ENERGY. 473 



were a globe of gunpowder or guncotton burning 

 from its outward surface inwards that is to say, the 

 work done by the potential energy of the chemical 

 affinity between uncombined oxygen, and carbon 

 and hydrocarbons, attractive forces as truly forces, 

 and subject to dynamic law, as is the force of 

 gravity itself, is absolutely infinitesimal in compari- 

 son with the work done by the gravitational 

 attraction on the shrinking mass adduced by 

 Helmholtz as the real source of the sun's 

 heat. 



The whole store of energy now in the sun, 

 whether of actual heat, corresponding to the sun's 

 high temperature, or of potential energy (as of the 

 not run-down weight of the clockwork) potential 

 energy of gravitation depending on the extent of 

 future shrinkage which the sun is destined to 

 experience, is essentially finite ; and there is much 

 less of it now than there was three hundred thousand 

 years ago. Similar considerations of action on a 

 vastly smaller scale are of course applicable to 

 terrestrial plutonic energy, and thoroughly dispose 

 of the terrestrial " perpetual motion " by which 



