366 POPULAR LECTURES AND ADDRESSES. 



generating, as they must do according to the great 

 law demonstrated by Joule, an exact equivalent 

 of heat for the motion lost in collision. 



That some form of the meteoric theory is cer- 

 tainly the true and complete explanation of solar 

 heat can scarely be doubted, when the following 

 reasons are considered : 



(i). No other natural explanation, except by 

 chemical action, can be conceived. 



(2). The chemical theory is quite insufficient, 

 because the most energetic chemical action we 

 know, taking place between substances amounting 

 to the whole sun's mass, would only generate about 

 3,000 years' heat. 1 



(3). There is no difficulty in accounting for 

 20,000,000 years' heat by the meteoric theory. 



It would extend this article to too great a 

 length, and would require something of mathe- 

 matical calculation, to explain fully the principles 

 on which this last estimate is founded. It is 

 enough to say that bodies, all much smaller than 

 the sun, falling together from a state of relative rest, 

 1 " Mechanical Energies of the Solar System." See note p. 351. 





