50 POPULAR LECTURES AND ADDRESSES. 



having properties such as bodies we know have. 

 And this is also worthy of attention : naturalists 

 affirm that every body the earth has ever met in 

 its course through the universe, has, when 

 examined, been proved to contain only known 

 elements chemical substances such as we know 

 and have previously met on the earth's surface. 

 If we could get from the sun a piece of its 

 substance cooled, we should find it to consist of 

 stone or slag, or metal, or crystallised rock, or 

 something that would not astonish us. So we 

 must reason on the sun according to properties of 

 matter known to us here. 



24. In 1854, I advocated the hypothesis that 

 the energy continually emitted as light (or radiant 

 heat) might be replenished constantly by meteors 

 falling into the sun from year to year ; but very 

 strong reasons have induced me to leave that part 

 of the theory then advocated by me which asserted 

 that the energy radiating out from year to year is 

 supplied from year to year ; and to adopt Helm- 

 holtz's theory, that the sun's heat was generated 

 in ancient times by the work of mutual gravity 



