3. According to the theory matter was first in _"nebular" (gas) form,_ and that the gases existing diffused through space were, through the motion which originated, 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.