20 IN STARRY REALMS. 



demonstrated to be connected with this element. I should, 

 however, say that it has by no means been proved that 

 there may not be some elements in the sun of a different 

 kind from those we find on the earth. There are indeed 

 many lines in the solar spectrum which have not up to the 

 present been identified with those arising from any terres- 

 trial element. There are, however, so many difficulties 

 attending the identification of lines under widely varying 

 Conditions that it would be rash to pronounce emphatically 

 as to whether all the ingredients in the sun may be in some 

 shape or other known to us on the earth. The constitu- 

 tion of the great luminary does not, however, give any 

 encouragement to the suggestion that the source of his 

 heat may be attributed to combustion in the ordinary 

 meaning of the word. Indeed, the difficulties attending 

 such a view seem so overwhelming that we may withdraw 

 this doctrine from any further consideration. 



We have thus shown not only that the sun's heat would 

 be insufficiently explained by supposing the great globe to 

 be a cooling body, but also that the phenomena of combus- 

 tion offer no adequate suggestion towards the explanation 

 of the difficulty. 



Our search for the source of sun heat must therefore be 

 conducted in some other direction, and, strange to say, we 

 shall find that source to be not in any immediately avail- 

 able heat, but rather in the transformation into heat of 

 something which was originally of a mechanical nature. 



When a shooting star dashes into our atmosphere its 

 course is attended with an evolution of light and heat 

 owing to its friction through the air. We are thus able 

 to account for the enormous quantity of heat, or of what is 



