ORIGIN OF THE SOLAR SYSTEM 55 



nearly all of the planetesimals must have turned to gas before 

 the nuclei could be much affected by accretion. We are thus 

 back to the heterogeneous gaseous nebula. 



If the planetesimals moved initially in nearly circular orbits 

 this objection does not arise, but it can then be shown that the 

 product of the mass and the orbital eccentricity of each nucleus 

 would diminish with the time. It can thus be seen that Jupiter 

 could never have been smaller than Uranus is now. There is 

 no obvious objection to this form of the hypothesis, but there 

 is no reason to suppose that solid planetesimals did originally 

 move in nearly circular orbits. 1 



A further hypothesis that has come to be associated with 

 the present one, although not an essential part of it, is the 

 belief that the earth has always been solid. There are many 

 serious difficulties in the way of this. 



1 . The mode of formation of the nuclei described in the 

 first part of the Planetesimal Hypothesis implies that they were 

 initially liquid or gaseous. This is not, however, a direct objec- 

 tion ; one part of the hypothesis might be true and the other 

 false, as they are not interdependent. 



2. Only one satisfactory explanation of the elevation of 

 mountains by the folding of the earth's crust has been offered ; 

 this attributes it to a horizontal compression at the surface. 

 Now, if a solid earth grew by the addition of small particles from 

 outside, these would be deposited in a layer on the surface, in a 

 perfectly unstrained condition. Thus, during the whole process 

 of growth the same surface condition would always hold, namely, 

 that there is no horizontal compression at the surface, however 

 much deformation may take place within. Hence any stresses 

 available for mountain-building must have been accumulated 

 after accretion ceased ; if the theory that the earth was 

 formerly molten should be proved to give insufficient surface 

 compression to account for known mountains, then a fortiori 

 the theory of a permanently solid earth gives insufficient 

 compression, as the available fall of temperature is less. 



3. It is by no means clear that a solid earth growing by 

 accretion would reman solid. A particle falling from an infinite 

 distance to the earth under the earth's attraction alone would 

 develop a velocity almost enough to volatilise it on impact, 

 and the actual velocities must have been considerably greater 



1 Monthly Notices o/R.A.S. vol. lxxvii. 1916. 



