THOUGHTS ON NATURAL PHILOSOPHY. 23 



ago the sun must have had a diameter 40 miles greater 

 than at present, ten thousand years ago that diameter 

 must have been 400 miles more than it is now, and so 

 on. We cannot perhaps assert that the same rate is 

 to be continued for very many centuries, but it is plain 

 that the further we look back into past time the 

 greater must the sun have been. 



Dealing then simply with the laws of nature as we 

 know them, we can see no boundary to the growth of 

 the sun as we look back. We must conceive a time 

 when the sun was swollen to such an extent that it 

 filled up the entire space girdled by the orbit of 

 Mercury. Earlier still the sun must have reached to 

 the Earth. Earlier still the sun must have reached 

 to where Neptune now revolves on the confines of our 

 system, but the mass of the sun could not undergo an 

 expansion so prodigious without being made vastly 

 more rarefied than at present, and hence we are led 

 by this mode of reasoning to the conception of 

 the primaeval nebula from which our system has 

 originated. 



Considering that our sun is but a star, or but one 

 of the millions of stars, it becomes a question of great 

 interest to see whether any other systems present 

 indication of a nebulous origin analogous to that 

 which Laplace proposed for the solar system. In 

 one of his most memorable papers, Sir W. Herschel 

 marshals the evidence which can be collected on this 

 point. He arranges in this paper a selection from 

 his observations on the nebula in such a way as to 

 give great plausibility to his view of the gradual 

 transmutation of nebulas into stars. Herschel begins 



