GEOLOGY. 369 



THE BEARINGS OF NUCLEAR CONCENTRATION ON THE PROBLEM OF 



SHRINKAGE. 



Though the evolution of the planets, along the familiar gaseous Unes 

 commonly accepted during the last century, seemed excluded, it 

 appeared advisable to reconsider such an evolution under the specific 

 conditions that affected the bodies under comparison. In this re- 

 consideration of a possible derivation from a rotating gaseous nebula 

 by centrifugal action, the part played by disruptive action within the 

 Roche limit was made the key to the inquiry. Studied from this 

 criticial point of view, it seemed clear that the separation of matter 

 from the equator of the nebula would necessarily take place, not only 

 molecule by molecule, but that these molecules would ultimately be 

 thrown into orbits, so that aggregation from these would follow the 

 methods of planetesimals. Within the Roche limit aggregation 

 beyond minute sizes would be inhibited, and outside that limit for some 

 distance the conditions would be distinctly adverse to any massive 

 assemblage. Aggregation of the planetary order, if it could take 

 place at all under these conditions, would require an extremely long 

 period with corresponding physical states; precipitate condensation 

 would be quite out of the question. Generalized, it appears that the 

 only tenable way in which planets of the terrestrial order could have 

 descended along gaseous lines was through gases definitely bunched 

 in some form and sufficiently dense to permit self-control; otherwise 

 the gases must have been dissipated into planetesimals or their djmamic 

 equivalents. 



The aggregation of planetary nuclei under the planetesimal hypothe- 

 sis is of the gaseous type, in their early stages at least. This seems to 

 be the only line postulated by a tenable hypothesis that offers any 

 probability of a continuous gaseous descent of the minor planetary 

 bodies, attended by such physical states as are derivatives from the 

 gaseous condition. The concentration of a planetary nucleus of this 

 type, however, is by no means the simple process which gaseous con- 

 densation has commonly been thought to be. There are several 

 accompanying agencies which rather radically modify the process. 

 Four of these were studied in some detail. 



I. The Influence of Inherited Motions. 



The planetary nuclei postulated by the planetesimal hypothesis 

 consisted of such of the central parts of the solar outbursts as had 

 sufficient self-attraction to remain under self-control after they had 

 been projected into planetaiy space. At the distance of the earth, 

 matter in spherical form having a mean density of 0.0011 + , atmo- 

 spheric standard, would control itself against the differential attraction 

 of the sun, if affected merely by motions common to the whole mass, 

 but it would be easily dissipated by differential motions of any moment 

 within its own mass, the dispersed matter in the main becoming plan- 



