OXYGEN IN THE SUN. 23 



cesses suggested by Laplace are in accordance with the now 

 known laws of physics ? To begin with, the original nebulous 

 mass extending to a distance exceeding the earth's distance 

 from the sun more than thirty times (this being only the 

 distance of Neptune), if we assign to it a degree of compres- 

 sion making its axial diameter half its equatorial diameter, 

 would have had a volume exceeding the sun's (roughly) 

 about 120,000,000,000 times, and in this degree its mean 

 density would have been less than the sun's. This would 

 correspond to a density equal (roughly) to about one- 

 4oo,oooth part of the density of hydrogen gas at atmospheric 

 pressure. To suppose that a great mass of matter, having 

 this exceedingly small mean density, and extending to a 

 distance of three or four thousand millions of miles from its 

 centre, could under any circumstances rotate as a whole, 

 or behave in other respects after the fashion attributed to the 

 gaseous embryon of the solar system in ordinary descriptions 

 of the nebular hypothesis, is altogether inconsistent with 

 correct ideas of physical and dynamical laws. It is abso- 

 lutely a necessity of any nebular hypothesis of the solar 

 system, that from the very beginning a central nucleus and 

 subordinate nuclei should form in it, and grow according to 

 the results of the motions (at first to all intents and purposes 

 independent) of its various parts. Granting this state of 

 things, we arrive, by considering the combined effects of 

 accretion and contraction, at a process of development 

 according fully as well as that ordinarily described with the 

 general relations described by Laplace, and accounting also 

 (in a general way) for certain peculiarities which are in no 

 degree explained by the ordinary theory. Amongst these 

 may specially be noted the arrangement and distribution of 

 the masses within the solar system, and the fact that so far 

 as spectroscopic evidence enables us to judge, a general 

 similarity of structure exists throughout the whole of the 

 system. 



Inquiring as to the significance of his discovery, Dr. 

 Draper remarks that it seems rather difficult " at first sight 



