58 GEOLOGY. 



observed that the curves in Fig. 22 are of the general order which the 

 case requires. 



The distribution of specific gravities. — The distal portions of the 

 protuberances would obviously be formed from the superficial por- 

 tions of the sun, while the later portions of the ejections forming the 

 proximal parts of the arms would doubtless come mainly from lower 

 depths, and hence probably contain more molecules of high specific 

 gravity. In this seems to lie a better basis for explaining the extraor- 

 dinary lightness of the outer planets, and the high specific gravities 

 of the inner ones, than in the separation, from the extreme equatorial 

 surface of a gaseous spheroid, of successive rings whose mass never 

 equaled the one-thousandth part of the original nebula. 



Successive partial outbursts. — It seems consistent with the con- 

 ditions of the case to assume that the protuberances would consist 

 of a succession of more or less irregular outbursts, as the ancestral 

 sun, in its swift whirl around the controlling star, was more and more 

 affected by the latter's differential attraction; and hence the pro- 

 tuberances would be directed in somewhat changing courses, and 

 would be pulsatory in character, resulting in rather irregular and some- 

 what divided arms, and in a knotty distribution of the ejected matter 

 along the arms. These knots must probably be more or less rotatory 

 from inequalities of projection. 



The result a spiral nebula. — It is thus conceived that a spiral nebula, 

 having two dominant arms on opposite sides, each knotty from irregu- 

 lar pulsations, and rotatory, the knots probably also rotatory, and 

 attended by subordinate knots and whirls, together with a general 

 scattering of the larger part of the mass in irregular nebulous form, 

 would arise from the simple event of a close approach. 



Theoretical diagram of the solar nebula. — On the basis of the fore- 

 going considerations, there is herewith presented a theoretical restora- 

 tion of the solar nebula (Fig. 23). The purpose of this is to give a 

 definite conception of the favored phase of the planetesimal hypoth- 

 esis — not to enforce its correctness or acceptance. On the con- 

 trary, the frank concreteness it gives to the conception facilitates 

 criticism, and invites modification by future study. 



The passage of the molecules from the free to the aggregated state. — 

 The ejected matter, at the outset, must have been in the free molecular 

 state, since by the terms of the hypothesis it arose from a gas- 



