period during which the emission will go
on, must be taken as much greater than if the Sun is supposed to be
permanently constituted of the elements now predominating in him, and to
be capable of only that degree of condensation which such composition
permits.

NOTE III. Are the internal structures of celestial bodies all the same,
or do they differ? And if they differ, can we, from the process of
nebular condensation, infer the conditions under which they assume one
or other character? In the foregoing essay as originally published,
these questions were discussed; and though the conclusions reached
cannot be sustained in the form given to them, they foreshadow
conclusions which may, perhaps, be sustained. Referring to the
conceivable causes of unlike specific gravities in the members of the
solar system, it was said that these might be--

"1. Differences between the kinds of matter or matters composing
them. 2. Differences between the quantities of matter; for, other
things equal, the mutual gravitation of atoms will make a large
mass denser than a small one. 3. Differences between the
structures: the masses being either solid or liquid throughout, or
having central cavities filled with elastic aeriform substance. Of
these three conceivable causes, that commonly assigned is the
first, more or less modified by the second."

Written as this was before spectrum-analysis had made its disclosures,
no notice could of course be taken of the way in which these conflict
with the first of the foregoing suppositions; but after pointing out
other objections to it the argument continued thus:--

"However, spite of these difficulties, the current hypothesis is,
that the Sun and planets, inclusive of the Earth, are either solid
or liquid, or have solid crusts with liquid nuclei."[23]

After saying that the familiarity of this hypothesis must not delude us
into uncritical acceptance of it, but that if any other hypothesis is
physically possible it may reasonably be entertained, it was argued that
by tracing out the process of condensation in a nebulous spheroid, we
are led to infer the eventual formation of a molten shell with a nucleus
consisting of gaseous matter at high tension. The paragraph which then
follows runs thus:--

"But what," it may be asked, "will become of this gaseous nucleus
when exposed to the enormous gravitative pressure of a shell some