THE ORIGIN OF THE EARTH. 39 



growth of a gaseous spheroid of the Laplacian type, or it might pos- 

 sibly be developed from a meteoritic state under certain conditions, 

 or it might arise from the mode of dispersion to which the nebula itself 

 was due. This last is taken as the type case. 



Planetesimal condition arising from a gaseous spheroid. — If the 

 parent nebula were a gaseous spheroid, as assumed by Laplace, and 

 were to detach material from its equatorial belt, molecule by molecule, 

 rather than by rings, these molecules would probably become planetesi- 

 mals instead of members of a true gaseous body. It is not the thought 

 that these molecules would be thrown off directly into planetesimal 

 orbits, because their initial paths would probably be ellipses that 

 would bring them back to the point of departure; but that, by cer- 

 tain classes of collisions while in these elliptical orbits, they would be 

 diverted into orbits that would not bring them again into collision 

 with the parent spheroid. It is not of sufficient importance to this 

 discussion to explain in detail the process by which this would be 

 accomplished, but the ingenious student will readily work it out for 

 himself. There is reason to believe that this method would really 

 be almost the only systematic one by which a gaseous spheroid of 

 the Laplacian type would detach material from its equatorial belt. 



But granting that this be not true, and that an earth-moon gase- 

 ous ring was formed, as assumed in the Laplacian hypothesis, com- 

 putation shows that its attractive power at any one point on its sur- 

 face would be very low. If the present earth were converted into a 

 solid ring, occupying its present orbit, it would have a diameter of 

 about twenty-five miles, with its present average density. Compu- 

 tation is scarcely necessary to show that the gravity of this ring, at 

 any point on its surface, would be very feeble; and it is obvious that 

 this gravity must be greater than the gravity on the surface of the same 

 matter when dispersed in the form of a gas by intense heat. Now 

 the application of the kinetic theory of gases to such a Ting, under 

 the postulated temperature, forces the conviction that the molecules 

 would have been so driven apart by mutual collision that they would 

 have become essentially independent of one another, each revolving 

 in its individual orbit, with only rare and incidental collisions. In 

 other words, they would have become planetesimals controlled by 

 the central mass, and not a gaseous aggregate controlled by its own 

 gravity. They would, therefore, not have been concentrated by direct 



