ated, changed from a huge ball of fire-mist to a
semi-solid sphere, which threw off smaller spheres (the planets) that
gradually became solid. Now, this is contrary to our knowledge of gases.
Gases may be produced from solids, but an incandescent gas will not,
through simple motion, become a solid substance. Gases may be solidified,
but only in two ways, by pressure or when greatly cooled,--when they
become ice. But they do not retain this form when the pressure or the
cooling agency is removed. Gases, as we know them, all have a tendency
to expand indefinitely. They have no tendency to solidify, as the
hypothesis presumes.

4. La Place assumed that the solar system when still in gaseous state,
began to revolve upon its axis, and that, as the gas ball continued to
revolve, it condensed. As condensation went on, the rotation became
faster, and a ring of matter was thrown off from the hardening core.
This ring again resolved itself into a rotating globe which, still in a
fluid state, threw off other balls, which revolved around their mother,
the first planet, even as the latter continued to follow an orbit around
the central body, the sun. In this way the planets of the solar system,
including the earth, (according to the theory), were evolved together
with their satellites or moons. The difficulty attending this view of
planetary evolution is found in the difference _between the movements of
a number of satellites_ around the planets. While the satellites of the
earth, of Jupiter and of Saturn revolve _from west to east,_ the moons
of Uranus and Neptune have an orbital movement _from east to west_. This
is regarded also by the friends of the Nebular Hypothesis as one of the
gravest difficulties, since no mechanical law will explain the reverse
movement of the satellites of the remotest planets when they, as well as
Jupiter, Saturn, and the rest are supposed to have been cast off by the
same central body.

5. According to the theory, the original atoms during the process of
world-making united into _molecules_. The laws according to which atoms
unite,--so that, for instance, the hydrogen atom each unites with two
atoms of oxygen, and so down the list of all known existences,--these
laws are among the assured results of scientific study. Now, the entire
science of chemistry in all its branches is built upon the axiom that
molecules are _absolutely unalterable_ and that molecules of the same
kind are always absolutely iden