278 THE NEBULAE HYPOTHESIS. 



We find, then, that besides those most conspicuous pe- 

 culiarities of the Solar System, which first suggested the 

 theory of its evolution, there are many minor ones point' 

 ing in the same direction. Were there no other evidence, 

 these mechanical arrangements would, considered in their 

 totality, go far to estahlish the Nebular Hypothesis. 



From the mechanical arrangements of the Solar Sys- 

 tem, turn we now to its physical characters ; and, first, let 

 us consider the inferences deducible from relative specific 

 gravities. 



The fact that, speaking generally, the denser planets are 

 the nearer to the Sun, is by some considered as adding 

 another to the many indications of nebular origin. Legiti- 

 mately assuming that the outermost parts of a rotating 

 nebulous spheroid, in its earlier stages of concentration, 

 will be comparatively rare ; and that the increasing density 

 which the whole mass acquires as it contracts, must hold 

 of the outermost parts as well as the rest ; it is argued 

 that the rings successively detached will be more and more 

 dense, and will form planets of higher and higher specific 

 gravities* But passing over other objections, this explana- 

 tion is quite inadequate to account for the facts. Using 

 the Earth as a standard of comparison, the relative densi- 

 ties run thus : — 



Neptune. Uranus. Saturn. Jupiter. Mars. Earth. Venus. Mercury. Sun. 

 . 0-14 0-24 0-14 0-24 0-95 1-00 0*92 1-12 0-25 



Two seemingly insurmountable objections are presented 

 by this series. The first is, that the progression is but a 

 broken one. Neptune is as dense as Saturn, which, by the 

 hypothesis, it ought not to be. Uranus is as dense as Ju- 

 piter, which it ought not to be. Uranus is denser than 

 Saturn, and the Earth is denser than Venus — facts which 

 not only give no countenance to, but directly contradict, 

 the alleged explanation. The second objection, still more 



