Predominance of atmosphere in comets–Luminous sectors; emission of vaporous envelopes from the nucleus in the comets of 1835, 1858, 1860, and 1861–Formation of envelopes in Donati's comet; progressive diminution of the velocity of expansion in emissions from the nucleus.

The planets, as seen through a telescope, are bodies of regular form and definite dimensions, probably invariable, so far as we can judge from observations made in the short period of two centuries and a half that has elapsed since telescopes have been invented. A globular mass, solid or liquid, surrounded on all sides by a light and comparatively thin aëriform envelope, is perhaps, from a physical point of view, the simplest description of a planet. The comparative stability is due, on the one hand, to the preponderance of the central globe, where general phenomena are modified only at long intervals ; and on the other to the trifling depth of the atmosphere, the portion of the planet the most subject to variation and internal change.

In comets, we have seen, this relation is reversed, and the atmosphere or nebulous envelope constitutes the entire body, or at all events greatly preponderates. At the utmost we can only conjecture that in some comets the nucleus is solid or liquid. Certainly its volume is generally but a very insignificant portion of the entire nebulosity, even if we except the tail.