HYPOTHETICAL STAGES LEADING UP TO THE KNOWN ERAS. 101 



densation was probably only in part analogous to that of a gas, but 

 it is possible that an internal temperature not unlike that of a con- 

 densing gas might be developed. The young earth may, therefore, 

 have inherited a hot nucleus. 



(3) Heat from central compression. — The chief source of internal 

 heat is, however, assigned to the progressive condensation of the grow- 

 ing body as material was added to its surface. The amount of this 

 condensational heat for the full-grown earth, computed on the best 

 data now available, is ample to meet all the requirements of the known 

 geologic ages, as set forth under the subject of internal temperature 

 in Vol. I, pp. 562-563. That heat arising from condensation solely 

 would reach the melting temperature of rock in a body 1/20 of the 

 earth's mass seems more or less doubtful; but in a body 1/10 of the 

 earth's mass the required conditions would probably be reached. The 

 requisite data are too imperfect for a definite decision of this point 

 at present. If the pits of the moon (1/81 of the earth's mass) repre- 

 sent volcanic explosions, and not the infall of planetoids as Gilbert 

 suggests, 1 it is necessary to postulate in its case conditions very favor- 

 able to the generation of heat by compression, or else to assign some 

 notable portion of the requisite heat to the quasi-gaseous condensa- 

 tion of the nucleus, to the collisions of planetesimals, and to the source 

 next to be considered, all of which would necessarily contribute some- 

 thing to the sum total of internal heat. 



(4) Heat from molecular rearrangement. — Another source of heat 

 lay in the atomic and molecular rearrangement of the material after 

 it became entrapped in the growing mass. This was not simply chemi- 

 cal recombination, as usually understood, but molecular readjust- 

 ment under pressure as well. The planetesimals were aggregated, by 

 hypothesis, in a vacuum of the highest order, and with very slight 

 mutual gravity, and the mode of molecular arrangement was that 

 suited to this extremely low pressure. Under the rising pressure of 

 the earth's interior, new arrangements of the molecules into denser 

 combinations with lower specific heats are theoretically probable, if not 

 inevitable, with the freeing of heat as a consequence. In a sense, 

 this is a mode of condensation falling under the previous head, but 

 it is not identical with mere mechanical compression, and is not wholly 

 covered by computations based on compression. 



1 Bull. Phil. Soc. Washington, Vol. XII, 1892, pp. 241-292. 



