56 SCIENCE PROGRESS 



than this, as the planetesimals would have a velocity relative 

 to the earth before entering its sphere of influence. If, then, the 

 particles required to form the earth were all brought together at 

 once, the resulting body would be gaseous. On the other hand, 

 if the accretion were spread over a long enough time, heat 

 would be radiated away as fast as it was produced, and the 

 body would remain solid. In the absence of a criterion of the 

 rate of growth it is impossible to state whether an earth growing 

 by accretion could remain solid or not. 



Holmes * has found that the hypothesis of a cooling earth, 

 initially in a liquid state, leads to temperatures within the crust 

 capable of accounting for igneous activity, whereas the view 

 that the earth is now in a steady state, its temperature gradient 

 being maintained wholly by radio-activity, is by no means 

 certain to lead to adequate internal temperatures. Assuming 

 the former fluidity of the earth, he has developed a wonderfully 

 consistent theory of the earth's thermal state. The present 

 writer, using Holmes's data, finds 2 that the available compres- 

 sion of the crust is of the same order of magnitude as that 

 required to produce the existing mountain-ranges. 



It seems, then, that whatever we may assume about the origin 

 of the earth, the hypothesis that it has at some stage of its 

 existence been liquid or gaseous agrees best with its present 

 state. The hypothesis of Laplace, however modified, implies 

 the former fluidity of the earth, and so does the standard form 

 of the Planetesimal Hypothesis. 



The Capture Theory of See? — Like the Planetesimal Hypo- 

 thesis, this has been developed during the present century to 

 avoid the objections that have been offered to that of Laplace. 

 The main features of the two theories are very similar. Both 

 involve the idea of a system of secondary nuclei revolving in 

 independent orbits about the primitive sun, with sparsely 

 distributed small particles between them, and the impacts of 

 the small particles on the nuclei are supposed in course of time 

 to act on the orbits of the latter in the same way as a resisting 

 medium ; namely, the eccentricities of the orbits tend to 

 diminish, and satellites tend to approach their primaries. The 



1 " Radio-activity and the Earth's Thermal History," Geol. Mag February — 

 March 191 5, June 1916. 



3 Phil. Mag. vol. xxxii. December 191 6. 



3 The Capture Theory of Cosmical Evolution, by T. J. J. See. 



