12 Introductory Chapter [CH. i 



pointed out, if such a ring agglomerated into a planet, the resulting planet 

 ought to coincide with the centre of gravity of the ring and not with a point 

 on its perimeter. The stability of Saturn's rings was of course explained by 

 Maxwell in his celebrated Adams Prize Essay of 1857*. 



The theory of Laplace, like that of Kant, contained little but pure specu- 

 lation. It is however obvious that these theories admit of mathematical 

 verification or disproof. With the exception of an investigation by Roche f, 

 in which the primitive nebula is represented by a heavy nucleus surrounded 

 by an atmosphere of infinitesimal density, almost all attempts to treat the 

 question mathematically have represented the matter of the supposed nebula 

 by a homogeneous incompressible fluid. This differs by so much from the 

 tenuous gas postulated by Laplace, that we cannot with any certainty regard 

 his theory as being either vindicated or condemned by such researches, what- 

 ever their result. It may, however, be remarked that the results obtained 

 from such researches have led to a continual modification of the theory until 

 in its present form it contains little in the way of detail that would be recog- 

 nised as his own by Laplace, and perhaps nothing that would be recognised 

 by Kant. But two outstanding features of the theory have survived, namely 

 (i) the supposition that our solar system originated out of a nebulous 

 mass of gas, 



(ii) the supposition that the change from the primitive stage to the 

 present stage has been produced mainly by the agency of in- 

 creasing rotation. 



Of these two suppositions, the former receives almost universal acceptance, 

 at any rate as a provisional hypothesis, while the latter can probably claim 

 more adherents than any other theory of planetary origin. In order to avoid 

 the very ambiguous term "Nebular Hypothesis," which in view of the 

 innumerable modifications the hypothesis has undergone might mean almost 

 anything, it will be convenient to refer to these two essential parts of the 

 hypothesis as the "Theory of Nebulous Origin" and the "Rotational Theory." 

 These two theories contain about all of the original " Nebular Hypothesis " 

 which can survive serious criticism ; we shall now consider these theories 

 in turn in the light of modern astronomical knowledge. 



The Theory of Nebulous Origin 



12. The general belief in the theory of nebulous origin is based mainly 

 upon direct observation of the sky ; a reasoned defence of it might rest mainly 

 upon a consideration of the classification of stars according to spectral type ; 

 those stars which are believed to be in the earliest stages of development are 



* On the Stability of the Motion of Saturn's Rings (Cambridge, 1859). 



t "Essai sur la Constitution et 1'origine du syst&me solaire," Acad. de Montpellier, Section 

 des Sciences, vm (1873), p. 235. 



