FUNDAMENTAL PROBLEMS OF GKOLOGY. 209 



condition might arise revealed several possible methods. Such con- 

 dition might arise from a nebula that was originally gaseous. If, 

 for example, it be supposed that the parent nebula was a gaseous 

 spheroid, and that it detached material from its equatorial belt mole- 

 cule by molecule, rather than by rings, as postulated by Laplace, 

 these molecules would probably become planetesimals instead of 

 members of a true gaseous body. It is not the thought that these 

 molecules would be thrown off directlj^ into planetesimal orbits, be- 

 cause their initial paths would probably be ellipses that would bring 

 them back to the point of departure ; but that, bj^ certain 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. There is reason to believe that this method would really 

 be almost the onh" systematic one by which a gaseous spheroid of 

 the Laplacian type would detach material from its equatorial belt. 



But if this be not true, and if an earth-moon gaseous ring were 

 formed, as assumed in the Laplacian hypothesis, computation shows 

 that its attractive power at any one point on its surface would be 

 very low. If the present earth were converted into a solid ring, 

 occup^dng its present orbit, it would have a diameter of about 25 miles 

 with its present average density. Computation is scarcely necessary 

 to show that the gravitj^ 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 if it were 

 dispersed by intense heat into the form of a gaseous ring. The 

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

 postulated temperature, forces the conviction that the molecules 

 would have been so driven apart by mutual collision and rebound 

 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 con- 

 trolled by its own gravity. They would, therefore, not have been 

 concentrated by direct attraction on the principles controlling a 

 cooling gaseous body, but would have been subject to accretion one 

 by one in the modes presently to be described. 



Under certain circumstances meteorites might be assembled in such 

 a way that they would come to revolve in concentric orbits about 

 their common center of gravity, as previously indicated, and thus 

 assume a quasi-planetesimal condition in contradistinction to that of 

 a quasi-gaseous swarm of meteorites, in which each is habitually 



