58 HABIT AND INTELLIGENCE. [cHAP. 



be less motive power in tlie future than there is in the 

 present. 

 Dissipa- This is called the law of the dissi2')ation of energy. For 

 ^j^^° my present purpose I prefer to call it the law of the 

 destruction of motive ijoioer. 



It foUows from this law, that, as I have stated at the 

 commencement of this chapter, the changes of the uni- 

 verse do not come back to the point from which they 

 began : the diminishing quantity of motive power makes 

 a difference. 



Consequences of the greatest importance to science 



follow from the two closely united truths, that energy is 



invariable in quantity, and that motive power is constantly 



diminishing. 



Earth's Among other inferences may be mentioned that as the 



internal earth is. constantly though slowly parting with internal 



constantly heat by volcanic eruptions, by hot springs, and by slow 



eing OS . conduction outward through the strata ; and as all volcanic 



action depends on heat — not eruptive action only, but all 



subterranean action whatever ; it follows, as I believe, 



that the intensity of volcanic action has been constantly 



diminishing ever since the formation of the earth ; and 



Geological consequently, that geological changes went on in past 



Quence times with greater rapidity than they do at present.^ I 



admit that geological evidence of this is scarcely to be 



hoped for, but I regard it as an a 'priori certainty. 



Sun's heat. What is true of the heat of the earth is also true of the 



heat of the sun. The sun is radiating away his heat, and it 



must be exhausted if fresh supplies are not brought in from 



without. But our increasing knowledge of the physics of 



the universe has suggested a mode in wliich such supplies 



may probably be brought in.^ It has been proved beyond 



Meteoric all reasonable doubt, that the meteors which flash across 



°°^^' our sky are small planets that fall from without into our 



^ I do not believe there is any evidence in favour of the chemical theory 

 of volcanic action. But should that theory prove true, the conclusion in 

 the text will not be affected ; for energy from a chemical source obeys 

 exactly the same laws as energy from a mechanical source. 



^ See Professor (now Sir William) Thomson's article on the subject in 

 the Philosophical Transactions for 1854. 



