PLANETARY EVOLUTION 



the sun, the next stage is the dissipation of the 

 whole mass into a nebula. 



If we now go back for a moment to the be- 

 ginning, and ask what antecedent form of en- 

 ergy could have generated the motion of repul- 

 sion which sustained our genetic nebula at its 

 primitive state of expansion, the reply must be 

 that nothing but a rapid evolution of heat could 

 have generated such a motion of repulsion. 

 And if we ask whence came this rapid evolu- 

 tion of heat, we may now fairly surmise that it 

 was due to some previous collision of cosmical 

 bodies ; arrested molar motion being incompar- 

 ably the most prolific known source of heat. 

 Thus we get a glimpse of some preceding epoch 

 of planetary evolution, from the final catastro- 

 phe of which emerged the state of things which 

 we now witness. 



We have here reached the very limit of 

 scientific inference. For note that, since the 

 greater part of the potential energy represented 

 by the primitive expansion of our solar nebula 

 has been transformed into heat and radiated 

 away, and is not represented by any form of 

 motor energy now stored up in the solar svs- 

 tem, it follows that the sudden transformation 

 of the penultimate molar motions of the plan- 

 ets into heat cannot result in the production of 

 another nebula so large as the one from which 



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