nd here we are met by a
difficulty at the outset; for the existence of comets with their very
eccentric orbits is wholly irreconcilable with the theory. At their
perihelion, many of these bodies pass within the orbit of Mercury, while
the aphelion of some lies without the path of Uranus. Where were they,
when the body of the sun filled up the whole of the vast sphere
circumscribed by the orbit of the remotest planet? If we suppose that
they are late comers, after the rest of our system was perfected,--that
they were generated by themselves in distant regions of space, and,
having strayed about, orphan-like, for a while, they accidentally
crossed our track, and were taken as adopted children into our family,
another question remains to be answered. Why did they not remain in
their first position, absorb their full share of nebulous matter, beget
a respectable family of planets, and take rank as chiefs of their own
clan? These comparatively anomalous bodies are great stumbling-blocks
for the _soi-disant_ historians of creation.

Again, if an immense orb be formed, the parts of which cohere strongly
enough for the whole to turn upon its axis as one body, the process of
cooling can go on only from the surface. A crust may finally be formed
there; but we see not how the refrigeration and shrinking of the
interior parts can then go on separately, until the mass in the centre
finally becomes detached from its envelope, like a shrivelled nut from
its shell. Our earth is cooling down at this moment, unless the warmth
which it receives from the sun exactly counterbalances the loss by
radiation of internal heat. But the exterior and interior do not cool by
different radiations, nor is there, so far as we know, the least
tendency in the central mass to shrink separately, so as to detach
itself from the surrounding crust. As deep as we can penetrate towards
the centre, we find the heat regularly increase,--just as we might
expect, if the only absolute loss of heat be from the surface.

If the matter now concentrated in the sun, and that which forms the
several planets with their secondaries, were all moulded into one mass,
and then dilated so as to fill the vast sphere of which the orbit of
Uranus forms a circumference, the substance would evidently be in a
state of extreme tenuity and diffusion. Immense as the mass of the sun
now is, it is but a mere nut at the centre of the grand globe which we
are now considering. Expanded to su