WORLD-MAKING 15 



part of the history of the earth and of the system to which it 

 belongs, nor does it enter at all into the more recondite 

 problems involved ; still it forms, I believe, some necessary 

 preparation at least to the comprehension of these. If we are 

 to go farther back, we must accept the guidance of physicists 

 rather than of geologists, and I must say that in this physical 

 cosmology both geologists and general readers are likely to 

 find themselves perplexed with the vagaries in which the most 

 sober mathematicians may indulge. We are told that the 

 original condition of the solar system was that of a vaporous 

 and nebulous cloud intensely heated and whirling rapidly 

 round, that it probably came into this condition by the impact 

 of two dark solid bodies striking each other so violently, that 

 they became intensely heated and resolved into the smallest 

 possible fragments. Lord Kelvin attributes this impact to 

 their being attracted together by gravitative force. Croll 1 

 argues that in addition to gravitation these bodies must have 

 had a proper motion of great velocity, which Lord Kelvin 

 thinks " enormously " improbable, as it would require the 

 solid bodies to be shot against each other with a marvellously 

 true aim, and this not in the case of the sun only, but of all 

 the stars. It is rather more improbable than it would be to 

 affirm that in the artillery practice of two opposing armies, 

 cannon balls have thousands of times struck and shattered 

 each other midway between the hostile batteries. The ques- 

 tion, we are told, is one of great moment to geologists, since 

 on the one hypothesis the duration of our system has amounted 

 to only about twenty millions of years ; on the other, it may 

 have lasted ten times that number. 2 In any case it seems a 

 strange way of making systems of worlds, that they should 

 result from the chance collision of multitudes of solid bodies 



1 " Stellar Evolution." 



2 Other facts favour the shorter time (Clarence King, Am.fl. of Science, 

 vol. xlv., 3rd series). 



