92 GEOLOGY. 



I. The Nuclear Stage. 



Under the planetesimal hypothesis, the earth is supposed to have 

 started as a nebular knot, acting as a nucleus, and to have grown 

 gradually to its present mass by accessions. It does not seem to be 

 important, geologically, to consider the special state of the nebular 

 nucleus if it be granted, for the reasons previously assigned (p. 62), 

 that it constituted only a small fraction of the grown planet. If its 

 mass were quite small, there is reason to assume that it was composed 

 of material of high molecular weight, since light molecules would 

 probably escape by virtue of their high velocities, except as they were 

 coherent with, or occluded in, heavier material. The nucleus is supposed 

 to have been originally an assemblage of planetesimals grouped together 

 by their mutual gravity, and to have passed gradually into a solid 

 nucleus in connection with the capture of outside planetesimals. As 

 the planetesimals were solid aggregates in the main, and only partially 

 elastic, their collisions mutually destroyed their orbital motions in 

 a certain proportion of cases, and led to their collection at the center. 

 In other cases the orbital motions were increased, but any planetesimals 

 which were thus temporarily driven away were subject to subsequent 

 capture. 



II. A Possible Atmosphereless Stage. 



As the solid nucleus thus formed may not have been massive enough 

 to control a gaseous envelope in its earlier stages, a possible atmos- 

 phereless stage is to be recognized. Just how massive a planetary 

 body must be to hold permanently an appreciable atmosphere is not 

 accurately computable at present, because of the uncertain value 

 of some of the factors involved. 1 A fairly safe conclusion may per- 



1 The following papers bear upon this subject: G. Johnstone Stoney, On the Cause 

 of the Absence of Hydrogen from the Earth's Atmosphere, and of Air and Water 

 from the Moon, Roy. Dublin Soc, 1892. G. Johnstone Stoney, On Atmospheres 

 upon Plants and Satellites, Trans. Roy, Dublin Soc, 2 series, 6, 1897; Ibid., 1898, 

 p. 305. T. C. Chamberlin, A Group of Hypotheses bearing on Climatic Changes, Jour, 

 of Geol., Vol. V, 1897, p. 653. G. Johnstone Stoney, On the Presence of Helium in 

 the Earth's Atmosphere, and its Relation to the Kinetic Theory of Gas, Astrophys. 

 Jour., Vol. VIII, Dec. 1898, p. 316. S. R. Cook, On the Escape of Gases from Planetary 

 Atmospheres according to the Kinetic Theory, Astrophys. Jour., Vol. XI, Jan. 1900, 

 p. 36. G. Johnstone Stoney, On the Escape of Gases from Planetary Atmospheres 

 According to the Kinetic Theory, No. I, Astrophys. Jour., Vol. XI, May 1900, p. 



