670 T. C. CHAMBERLIN 
of rational hypothesis seems therefore to be wide. It is herein 
urged that it is wholesome at present to recognize this wide 
range in its fullest amplitude. 
But if we question current conceptions we should present 
alternatives which account for the atmosphere and for internal 
heat. Let us therefore hastily follow the hypothetical growth 
of a planet built up by the slow aggregation of small bodies 
which join it at low velocities and develop a minimum heat. 
Let the case be purposely made rather extreme to develop — 
sharply the difficulties springing from it. Let the infalling par- 
ticles be small and their rate such as not to generate a high 
surface temperature. The growth of such a body up to the size of 
the moon may be taken as an hypothesis of lunar history and 
the phenomena of the moon may serve as a check upon it. The 
moon may, however, have originated by fission even though the 
earth were built up by accretions. In the early stages of growth 
the gravity being low the aggregation may be supposed to have 
remained uncondensed. Volcanic aggregations of bombs, cinders 
and ashes are perhaps the nearest terrestrial analogues. The 
ingathering particles obviously carried with them so much of 
the atmospheric material as was entrapped or occluded within 
them in their solidification, or was absorbed into their pores or 
adhered to their surfaces. Judging from meteorites the amount 
of this might have been large. Gaseous molecules moving as 
independent bodies may have joined the aggregation and become 
absorbed in its porous body, but they would not have been col- 
lected into an appreciable atmospheric envelope until the body 
passed the size of the moon if the molecular considerations 
urged earlier in this paper hold good, though an atmospheric 
envelope would not have been entirely absent. As the mass 
grew the central pressure increased and condensation produced 
heat at the center proportional to the work done. I find the 
explanation of internal heat chiefly in this self-condensation, it 
being essentially the application of the Helmholtz solar theory 
to a solid body. Tidal kneading and chemical action doubtless 
added their contributions. When the growing mass reached the 
