22 THOUGHTS ON NATURAL PHILOSOPHY. 



can be supplied to the sun, and therefore we must 

 reject this source as also inadequate. 



The truth about the sun's heat appears to be that 

 the sun is really an incandescent body losing heat, but 

 that the operation of cooling is immensely retarded 

 owing to a curious circumstance due jointly to the 

 stupendous mass of the sun and to a remarkable law 

 of heat. It is of course well known that if energy 

 disappears in one form it reappears in another, and 

 this principle applied to the sun will explain the 

 famous difficulty. 



As the sun loses heat it contracts, and every pair of 

 particles in the sun are nearer to each other after the 

 contraction than they w r ere before. The energy due 

 to their separation is thus less in the contracted state 

 than in the original state, and as that energy cannot 

 be lost it must reappear in heat. The sun is thus 

 slowly contracting ; but as it contracts it gains heat by 

 the operation of the law just referred to, and thus the 

 further cooling and further contraction of the sun 

 is protracted until the additional heat obtained is 

 radiated away. In this way we can reconcile the fact 

 that the sun is certainly losing heat with the fact that 

 the change in temperature has not been large enough 

 to be perceived within historic times. 



It can be shown that the sun is at present contract- 

 ing, so that its diameter diminishes four miles ever} 7 

 century. This is of course an inappreciable distance 

 when compared with the diameter of the sun, which 

 is nearly a million of miles, but the significance for 

 our present purpose depends upon the fact that this 

 contraction is always taking place. A thousand years 



