Alfred Russel Wallace 



To Prop. Barrett 



Parkstone, Dorset. February 12, 1901. 



My dear Barrett, — I shall be much obliged if you will 

 give me your opinion on a problem in physics that I cannot 

 find answered in any book. It relates to the old Nebular 

 Hypothesis, and is this : 



It is assumed that the matter of the solar system was 

 once wholly gaseous, and extended as a roughly globular 

 or lenticular mass beyond the orbit of Neptune. Sir 

 Robert Ball stated in a lecture here that even when the 

 solar nebula had shrunk to the size of the earth's orbit it 

 must have been (I think he said) hundreds of times rarer 

 than the residual gas in one of Crookes's high vacuum 

 tubes. Yet, by hypothesis, it was hot enough, even in its 

 outer portions, to retain all the solid elements in the 

 gaseous state. 



Now, admitting this to be possible at any given epoch, 

 my difficulty is this : how long could the outer parts 

 of this nebula exist, exposed to the zero temperature of 

 surrounding space, without losing the gaseous state and 

 aggregating into minute solid particles — into meteoric 

 dust, in fact ? 



Could it exist an hour ? a day ? a year ? a century ? Yet 

 the process of condensation from the Neptunian era to that 

 of Saturn or Jupiter must surely have occupied millions of 

 centuries. What kept the almost infinitely rare metallic 

 gases in the gaseous state all this time ? Is such a condi- 

 tion of things physically possible ? 



I cannot myself imagine any such condition of things 

 as the supposed primitive solar nebula as possibly coming 

 into existence under any conceivably antecedent conditions, 

 but, granted that it did come into existence, it seems to me 

 that the gaseous state must almost instantly begin changing 



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