144 T. C. CHAMBERLIN 



part of the earth, which usually have rather low specific gravities, 

 are probably wholly dependent on the presence of the atmosphere 

 and hydrosphere at the time of their formation. And so, if the 

 moon, Mars, Venus, and the earth, could have been built up in 

 this way, the moon should have the highest percentage of heavy 

 material and the others should follow in the order of their masses, 

 atmospheres, and hydrospheres. But it was previously shown that 

 the conditions were very adverse to the building up of these four 

 bodies in this way and there is no likelihood that they had such an 

 origin. 



It is perhaps worth while to add that, even if we were wrong in 

 concluding that the planetesimal aggregates would be small, the 

 result would be little different in density or in physical state, for 

 as the aggregation of the particles in their orbits proceeded, the 

 resulting aggregates would become more widely separated and 

 further aggregation would take place only at correspondingly 

 longer intervals. There would be little change in the kind of 

 material or in the heat effects. The material would be a little 

 more bunched before infall, but the bunches would be more scat- 

 tered in space and successive infalls more distant in time. Pre- 

 cipitate aggregation is quite out of the question under these 

 conditions. 



As remarked at the beginning of this section; the material 

 discharge from the rim of a rotating spheroid of gas by centrifugal 

 action should form an ideal system of planetesimals, and so the 

 method of their growth may be taken as a type of such action 

 where the molecules are given subparallel orbits from the start 

 and there is no commanding nucleus to gather them into bodies of 

 the planetary order. 



If this analysis is correct, it will be seen that the chance of 

 developing a molten earth from a rotating gaseous nebula by 

 centrifugal separation is about as remote as could well be imagined. 



THE SELECTIVE SEGREGATION OF MATERIAL UNDER 

 THE PLANETESIMAL HYPOTHESIS 



The particular form of the planetesimal hypothesis which has 

 been most fully worked out and tested postulates that the material 

 of the planets was derived from the sun by means of its own 



