8o NebulcB and Nebular Hypotheses, [Sess. 



that now our system has little or no nebulous matter scattered 

 throughout its extent, and the coils of the one-time spiral have 

 quite disappeared. It follows that the ejected masses were 

 much smaller at the time of their disruption than they are 

 now, since they have incorporated so much additional material ; 

 and the bearing of this fact upon the evolution of our earth is 

 so curious and revolutionary, that I should like to explain it. 



Like the other masses, that which went to form our earth 

 was very hot at the time of its ejection, but was so small that 

 it speedily cooled to solidity. It grew to its present size solely 

 by the accretion of the small solid particles (meteorites) which 

 surrounded it, and which it encountered in its revolutions 

 round the solar nucleus. After a time it attained in this 

 manner to such dimensions that its interior became subjected 

 to immense pressure from the accumulating overlying layers, 

 and this pressure caused the material in the interior to con- 

 tract. In consequence of such shrinkage, heat was developed 

 in the interior — ^just as it is in the sun ; and computation 

 shows that if the earth shrank so that its density increased 

 from that of meteorites (3*5) to its present amount (5*5), 

 enough heat would be generated to increase its temperature 

 more than ten thousand degrees Fahr. This heat would be 

 conducted to the surface and lost very slowly, and it is much 

 more than sufficient to account for all the known igneous 

 action which has taken place on the earth. 



According to the Laplacian doctrine, our " world was once 

 a fluid haze of light," and the heat still to be found within 

 it is the residue of its original birthright, claimed when it 

 was cast off by the parent mass. It is supposed to have 

 cooled very slowly, to have consequently dwindled in size, 

 and to have solidified from the outside inwards by the forma- 

 tion of an ever-thickening surface crust. On the new hypo- 

 thesis, the earth has been solid practically from the beginning 

 of its history, and has grown to its present size, instead of 

 decreasing to it. Further, the internal heat which is still 

 oozing slowly to the surface is entirely the result of its 

 growth, and is in no sense aboriginal. 



I might give you many other details of this theory, but 

 time, and, I fear, your endurance, forbid. Let me just warn 

 you not to accept it as final. There are many points where 



