The doctrine of a Nebular Cosmogony was first suggested by some observations of the elder Herschell on those cloud-like appearances which may be discerned in various parts of the heavens by the aid of the telescope, or even, in some cases, by the naked eye. It assumed a more definite form in the hands of La Place, although even by him it was offered, not as an ascertained discovery of science, but simply as a hypothetical explanation of the way in which the production of the planets and their satellites _might_ possibly be accounted for _by_ the operation of the known laws of Nature. The explanation of the whole theory may be best understood by dividing it into two parts: the _first_ being that which attempts to account for the formation of planets and satellites, _on the assumption of the existence of a central sun_, and _of certain other specified conditions_; the _second_ being that which undertakes to account for the formation of the sun itself, on the assumption of the existence of _a diffused nebulous matter_ in space, or, as it has been aptly called, "a universal Fire-Mist."[28] When the theory is limited to the explanation of the origin of the planets and their satellites, the original condition of our solar system is assumed to have been widely different from what it now is; the sun is supposed to have existed for a time alone, to have revolved upon his axis, and to have been surrounded with an atmosphere expanded by intense heat, and extending far beyond the limits of our system as it now exists. This solar atmosphere revolved, like the sun itself, around its axis; but its heat, constantly radiated into sidereal space, gradually diminished, and the atmosphere being contracted in proportion as it cooled, the rapidity of its rotation was accelerated, until it reached the point at which the central attraction was overcome by the centrifugal force, and then a zone of vapor would be detached or thrown off, which might either retain its form as a nebulous ring, like the ring of Saturn, or first breaking into fragments, from some want of continuity in its structure, and afterwards coalescing into one mass, might be condensed into a planet as the vapor continued to cool. These rings or planets, thus detached from the central atmospheric mass, would continue to revolve, in virtue of the force originally impressed upon them, and their motion would be nearly circular, in the same plane and in the same direction with that of the sun. The first planet, so formed, must have been that at the extreme limit of our solar system; the second the next in point of remoteness from the centre, and so on; each resulting from the operation of the same natural laws, and emerging into distinct existence at that precise point in the gradual cooling and contraction of the atmosphere at which the centrifugal became stronger than the centripetal force. But each planet might also be subjected to