but directly contradict, the alleged explanation. The
second objection, still more manifestly fatal, is the low specific
gravity of the Sun. If, when the matter of the Sun filled the orbit of
Mercury, its state of aggregation was such that the detached ring formed
a planet having a specific gravity equal to that of iron; then the Sun
itself, now that it has concentrated, should have a specific gravity
much greater than that of iron; whereas its specific gravity is only
half as much again as that of water. Instead of being far denser than
the nearest planet, it is but one-fifth as dense.

While these anomalies render untenable the position that the relative
specific gravities of the planets are direct indications of nebular
condensation; it by no means follows that they negative it. Several
causes may be assigned for these unlikenesses:--1. Differences among the
planets in respect of the elementary substances composing them; or in
the proportions of such elementary substances, if they contain the same
kinds. 2. Differences among them in respect of the quantities of matter
they contain; for, other things equal, the mutual gravitation of
molecules will make a larger mass denser than a smaller. 3. Differences
of temperatures; for, other things equal, those having higher
temperatures will have lower specific gravities. 4. Differences of
physical states, as being gaseous, liquid, or solid; or, otherwise,
differences in the relative amounts of the solid, liquid, and gaseous
matter they contain.

It is quite possible, and we may indeed say probable, that all these
causes come into play, and that they take various shares in the
production of the several results. But difficulties stand in the way of
definite conclusions. Nevertheless, if we revert to the hypothesis of
nebular genesis, we are furnished with partial explanations if nothing
more.

In the cooling of celestial bodies several factors are concerned. The
first and simplest is the one illustrated at every fire-side by the
rapid blackening of little cinders which fall into the ashes, in
contrast with the long-continued redness of big lumps. This factor is
the relation between increase of surface and increase of content:
surfaces, in similar bodies, increasing as the squares of the dimensions
while contents increase as their cubes. Hence, on comparing the Earth
with Jupiter, whose diameter is about eleven times that of the Earth, it
results that while his surface is 125