PLANETESIMAL THEORIES OF THE EARTH'S ORIGIN. 



193 



the ancestral sun. However, some considerable part of the 

 projected matter must probably have been gathered back into the 

 sun, and some part may possibly have been projected beyond the 

 control of the system. Making allowances for both these factors, 

 the proportion of the sun's mass necessarily involved in the 

 protuberances is still very small. Apparently 1 or 2 per cent, 

 of the sun's mass would amply suffice. 



" The distal portions of the protuberances would obviously be 

 formed from the superficial parts 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 would probably 

 contain more molecules of high specific gravity. In this seems to 

 lie a better basis for explaining the extraordinary 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 total mass only equaled one 

 seven-hundredth part of the original nebula. 



" It seems consistent with the conditions 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 protuberances would be 

 directed in somewhat changing courses, and would be pulsatory in 

 character, resulting in rather irregular and somewhat divided arms, 

 and in a knotty distribution of the ejected matter along the arms. 

 These knots must probably be more or less rotatory from in- 

 equalities of projection. 



" It is thus conceived that a spiral nebula, having two dominant 

 arms, opposite one another, each knotty from irregular 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 disruptive approach. . . . 



" The problem of the luminescence of nebulse is confessedly a 

 puzzling one. There is little ground for assigning general incan- 

 descence to matter so obviously scattered and tenuous, and pos- 

 sessed of such an enormous radiating surface. The assignment of 

 the light to the collision of meteorites, as done by Lockyer, encoun- 

 ters both dynamic and spectroscopic difficulties. The recent dis- 

 coveries of the luminescent properties of radio-active matter and 

 of its power to awaken luminescence in other matter offers some 

 hope of a solution. . . . 



" The solution of the problem may, however, lie along electrical 

 lines. At present it seems more probable that the luminescence 

 arises from some agency that acts at low temperatures, than that it is 

 dependent on heat, and hence objections to a planetesimal organisa- 

 tion on the ground of low temperature do not seem to me to have 

 much force. . 



