Theories of the Earth's Origin — Upham. 209 
The distal portions of the protuberances would obviously be 
formed from the superficial portions of the sun, while the later por- 
tions 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 extraordinary light- 
ness 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 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. 
It is thus conceived that a spiral nebula, having two domi- 
nant arms, opposite one another, each knotty from irregular pul- 
sations, and rotatory, the knots probably also rotatory, and at- 
tended 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 nebulae is confessedly 
a puzzling one. There is little ground for assigning general in- 
candescence to matter so obviously scattered and tenuous and 
possessed of such an enormous radiating surface. The assign- 
ment of the light to the collision of . meteorites, as done by 
Lockyer, encounters both dynamic and spectroscopic difliculties. 
The recent discoveries 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 maJ^ 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 
organization on the ground of low temperature do not seem to me 
to have much force. * * * 
In attempting to follow the probable evolution of such a spiral 
nebula, three elements stand out conspicuously: (1) The central 
mass, obviously to become the sun; (2) the knots on the arms 
that are assumed to be the nuclei of the future planets and per- 
haps satellites; and (3) the diffuse nebulous matter to be added 
to the nuclei as material of growth. In the particular case of the 
