684 T. C. CHAMBERLIN 
mean value of about 18 miles per second. Thus sustained, they 
only joined the collecting nucleus as variations in their orbits 
brought them into conjunction with it—a slow process, occupying 
perhaps two or three billion years.t The intervals between the 
infalls of the planetesimals were, therefore, such that nearly all 
the heat of their impact with the atmosphere and with the earth’s 
surface was lost before they were buried by added material. The 
growth of the earth was thus made by the slow accumulation of 
essentially cold, solid particles mixed at random. 
On the other hand, if the earth-forming nebula be assumed to 
have been gaseous and to have descended along the gaseous line, 
its volume was sustained by collisions and rebounds of the con- 
stituent molecules, and it contracted as fast as the loss of this 
interaction, i.e., the loss of heat, permitted. Under Lane’s law 
the maximum temperature was reached at the stage when the 
gaseous body passed into the liquid state. As radiation follows 
the law of the fourth power, the collapse was relatively rapid; 
at the most it cannot be assigned more than a few million years. 
Enormous losses of energy would be suffered in either the plane- 
tesimal or the gaseous mode of assemblage, and so we must take 
up the question of the earth’s primitive energy presently from the 
opposite point of view: What energy-values were /efét for the 
evolution after the earth was able to make a record of its own 
compression? ‘The point of most importance here is the radical 
difference in the respective factors that controlled the self- 
compression which followed the nebular concentration. It is 
obvious that the gaseous descent was controlled by heat and that 
this remained the master factor in the shrinkage of the earth after 
it became a white-hot molten globe. In the self-compression 
of the earth built of solid planetesimals, or planetesimal dust, 
solidity was the primary resisting-factor that held the compression 
in check. The energy-factors in this case were those to which the 
solidity was due. These are herein interpreted as revolutional 
phases of energy together with their derivatives. Heat in one 
case and solidity in the other were then the master factors in the 
t See ‘‘The Rates of Planetesimal Infall,”’ Article XIII, Jour. Geol., Vol. XXVIII 
(1920), pp. 677 ff. 
