DIASTROPHISM AND THE FORMATIVE PROCESSES 153 



between their ability to hold all sorts of molecules as well as pick 

 them up, they came to have a much larger proportion of Hght 

 molecules than the smaller bodies which could only hold the heavier 

 ones, and so they now have relatively low specific gravities. Their 

 great size helped them to remain hot, which also contributed to 

 their low specific gravities. The largest of these is now more than 

 three hundred times as massive as the largest of the terrestrial 

 group; the mean mass of the giant planets is more than two 

 hundred times the mean mass of the terrestrial planets. Our 

 problem then is with a group of distinctly small bodies in which 

 the balance between holding and non-holding power was more 

 critical, and we need to enter somewhat more into detail. 



1. As the material was shot forth from the sun, it was an 

 intimate mixture of solar molecules of various kinds in a very hot 

 gaseous form, and the molecules were interacting upon one another 

 at speeds inversely proportional to the square roots of their molec- 

 ular weights. In the process of forming definite knots under 

 self-control out of the less-defined solar outbursts, of which a 

 considerable part was dissipated into planetesimals or fell back to 

 the sun, the Ughter molecules of high speed would be more likely 

 to be dissipated than the heavy ones of lesser speed, but we need 

 not insist on that, as the dispersing danger came from the projectile 

 force and probably was not very selective. 



2. But when that contingency was passed and each knot began 

 its own independent evolution, there arose a very definite selective 

 process within the knot itself. We assume that for a short time 

 the knots would still be hot and diftuse, and that during this stage 

 there would be larger chances for molecules to escape from the con- 

 trol of the knot than later {a) because their velocities were highest 

 on account of temperature, and {h) because their deployment was 

 relatively open so that the molecules were less in one another's way 

 when they happened to accumulate velocity enough to escape from 

 control. We assume that this was the most crucial stage of the 

 knot, and that selective loss was then its greatest danger. The 

 lightest molecules, because they had the highest mean speeds and 

 most frequently encountered and divided energies with other 

 molecules, were those that most often acquired cumulative speeds 



