220 CARNEGIE INSTITUTION OF WASHINGTON. 



nebula, three elements stand out conspicuously: (i) The central 

 mass, obviously to become the sun ; (2) the knots on the arms that 

 are assumed to be the nuclei of the future planets and perhaps 

 satellites ; and (3) the diffuse nebulous matter to be added to the 

 nuclei as material of growth. In the particular case of the solar 

 nebula it is assumed ( i ) that the central mass was relatively very 

 great ; (2) that the knots were very irregular in size and placed at 

 irregular distances from the center; and (3) that the nebulous portion 

 was very small relative to the central mass and probably large rela- 

 tive to the knots. 



It is assumed that the masses of matter in the knots were suffi- 

 ciently large to hold themselves together in spite of the differential 

 attraction of the central mass, otherwise they would soon have been 

 scattered. They seem to have maintained themselves successfully 

 in existing nebulse that appear to have undergone some notable 

 degree of evolution. 



On the other hand, it is presumed that the mutual attraction of 

 the more tenuous nebular matter was insufficient to aggregate itself 

 directly in the presence of the central attraction, for in the existing 

 nebulae this portion seems to show a progressive tendency to a more 

 general diffusion. The planetesimals of the diffused nebulous portion 

 are assumed to be controlled essentially b}^ the gravitation of the 

 main mass and to revolve in individual orbits about it. 



The irregularity of the forms of the knots seems to imply that their 

 organization is also planetesimal, though the larger ones may be able 

 to hold gases also. The direction of revolution of these knots is sup- 

 posed to be usually the same as that of the rotation of the nebula as 

 a whole, but subject to local and special influences that might lead 

 to important variations. 



While the knots of the solar nebula are regarded as the nuclei 

 about which gathered the planetesimals to form the future planets, 

 all such nuclei did not necessarily retain their independence and grow 

 to planets, though no planet probably developed except from such a 

 nucleus. Existing nebulae show clusters of knots and aggregates of 

 irregular form susceptible of development into complex planetary 

 systems, such as the large planets and their families of satellites. 

 The earth-moon system is assigned to a couplet of companion nuclei 

 of quite unequal sizes. 



Certain studies were made to determine the probable amount of 

 growth of the planets, as this possesses much geological interest. 

 Two considerations bear upon the size of the original nuclei. 



