34 /A T STARRY REALMS. 



suppose that the earth and the other planets were solid 

 bodies deeply buried in the vast bulk of the sun. It seems 

 evident that the planets were gaseous masses in those 

 ancient days and undistinguishable from the sun, which 

 gave them birth. 



We are now able to make an attempt to trace the his- 

 tory of the solar system, and to indicate the share which 

 Darwin has had in the solution of the noble problem. We 

 do not inquire how the original nebula came into being ; 

 our history must commence with the actual existence of 

 this nebula. There is, let it be confessed, a great deal of 

 obscurity still clinging to the subject. Though we may 

 be sure that the great nebula once existed, we cannot with 

 much confidence trace out the method by which the 

 planets were actually formed. It seems to be generally 

 thought that the nebula must have been originally endowed 

 with a certain rotation. This may be regarded as certain ; 

 indeed, it would be infinitely improbable that the nebula 

 should not have had some rotation. As the nebula began 

 to radiate heat, so it must have begun to contract ; and as 

 it began to contract, it began to rotate more rapidly. 

 This is only the consequence of a well-known dynamical 

 principle. But as the nebula spins more and more rapidly, 

 the cohesion of its parts is lessened by centrifugal force. 

 The moment at length arrives when the centrifugal force 

 detaches a fragment of the nebula. The process of con- 

 densation still continues both in the fragment and in the 

 central mass ; the fragment changes from the gaseous 

 state to the liquid, perhaps even from the liquid to the 

 solid, and thus becomes a planet. Still the central mass 

 condenses, and spins more and more rapi/lly, until a 



