instead of having a
diameter of eight hundred and eighty thousand miles, it filled the
space now occupied by the entire solar system. Here is where the two
theories start. According to the first, the revolving nebulous mass
threw off a ring that became the planet Neptune, afterwards another
that contained the material for Uranus, and so on, the lightest
substance in the sun being thrown off first, by which they accounted
for the lightness of the four great planets, and finally Mars, the
earth, and the small dense planets near the sun. The advocates of this
theory pointed to Saturn's rings as an illustration of the birth of a
planet, or, rather, in that case a satellite. According to this, the
major planets have had a far longer separate existence than the minor,
which would account for their being so advanced notwithstanding their
size. This theory may again come into general acceptance, but for the
present it has been discredited by the nebulous. According to this
second theory, at the time the sun filled all the space inside of
Neptune's, orbit, or extended even farther, several centres of
condensation were formed within the nebulous, gaseous mass. The
greatest centre became the sun, and the others, large and small, the
planets, which--as a result of the spiral motion of the whole, such as
is now going on before our eyes in the great nebulae of fifty-one M.
Canuin venaticorum, and many others--began to revolve about the
greatest central body of gas. As the separate masses cooled, they
shrank, and their surfaces or extreme edges, which at first were
contiguous, began to recede, which recession is still going on with
some rapidity on the part of the sun, for we may be sure its diameter
diminishes as its density increases. According to either theory, as I
see it, the major planets, on account of their distance from the
central mass, have had longer separate existences than the minor, and
are therefore more advanced than they would be had all been formed at
the same time.

"This theory explains the practical uniformity in the chemical
composition of all members of this system by assuming that they were
all once a part of the same body, and you may say brothers and sisters
of the sun, instead of its offspring. It also makes size the only
factor determining temperature and density, but of course modified by
age, since otherwise Jupiter would have a far less developed crust than
that with which we find it. I hav