5H. v.] PLANE TABY E VOL UTION. 397 



has been transformed into heat and radiated away, and is not 

 represented by any form of motor energy now stored ap in 

 the solar system, it follows that the sudden transformation of 

 the penultimate molar motions of the planets into heat cannot 

 result in the production of another nebula so large as the 

 one from which our present system has been evolved. In 

 seeking to trace out the implications of this conclusion, wo 

 at once arrive at an impassable barrier, which is only shifted, 

 but not overthrown, when we consider the results of the 

 probable ultimate conflict between our own system, thus dis- 

 integrated, and other sidereal systems belonging to our galaxy. 

 In order to give a complete account of the matter, we ought 

 to know what has become of all this motor energy which we 

 have been so prodigally pouring away, in the shape of radiant 

 heat, into the interstellar spaces. Is the equivalent of this 

 motor energy ever to be restored, or is the greater part of it 

 forever lost in the abysses of infinite space? Before we can 

 answer such a question, we need to know whether the inter- 

 stellar ether, which is the vehicle for the transmission of 

 molecular motion, is definitely limited in extent, or prac- 

 tically infinite; and we need to take into the account the 

 dynamic relations, not only of our entire galactic system, 

 but of other stellar systems, if such there are, beyond the 

 utmost ken of the telescope. Here science fails us. Astro- 

 nomy, the simplest and clearest of the sciences, becomes, 

 when treated on this great scale, the most difficult and 

 obscure. An infinity and an eternity confront us, the secrets 

 of which we may not hope to unravel. At the outermost 

 verge to which scientific methods can guide us, we can only 

 catch a vague glimpse of a stupendous rhythmical alterna- 

 tion between eras of Evolution and eras of Dissolution, suc- 

 ceeding each other " without vestiges of a beginning ami 

 without prospect of an end." 



