Solar and Planetary Evolution. 



57 



and gravitation, those arch-mechanics, which without the 

 aid of saw or hammer have builded the universe, contended 

 for mastery in the evolution of the earth just as they did in 

 our puff of steam. Gravitation has always prevailed in the 

 end, because gravitation cannot leak away like heat. The 

 earth was first a nebula — perhaps an invisible gas — then 

 in the state of a liquid, and finally it became, as we see it, 

 solid, cool and habitable. It is only a question of the 



^~~ : 



Fig. 2,— representing the ring broken into several segments. 



amount and intensity of heat. Given heat enough and it 

 would be as easy to turn the earth into a nebulous cloud as 

 to melt a snow-flake. 



Laplace gives us one hypothesis of the origin of the so- 

 lar system, which I will endeavor to illustrate. (At this 

 point the lights in the church were extinguished, and a pic- 

 ture of a revolving nebula was thrown upon the screen, 



