Padre Secchi, of Rome, observed, in Donati’s comet, of 1858, from the 15th to the 22d of October, that the nucleus threw out intermittingly from itself appendages having the form of brilliant, coma-shaped masses of incandescent substance twisted violently backward. He accounts for these very remarkable changes of configuration by the influence first of the sun’s heat upon the comet’s substance as it approached toward perihelion, and afterward by the production in the luminous emanations thus generated of enormous tides and perturbation derangements. Some of the most conspicuous of these luminous developments occurred on October 11th, when the comet was at its nearest approach to the earth, and on
October 17th, when it was nearest to the planet Venus. He has no doubt that the close neighborhood of the earth and Venus at those times was the effective cause of the sudden changes of aspect, and that those changes of aspect may be accepted as proof that the comet’s substance consists of “really ponderable material."
Mr. Lockyer used the spectroscope to analyze the light of Coggia’s comet, and he established beyond question that—
“Some of the rays of the comet were sent either from solid particles, or from vapor in a state of very high condensation, and also that beyond doubt other portions of the comet’s light issue from the vapor shining by its own inherent light. The light coming from the more dense constituents, and therefore giving a continuous colored spectrum, was, however, deficient in blue rays, and was most probably emitted by material substance at the low red and yellow stages of incandescence.”
Padre Secchi, at Rome, believed he saw in the comet “carbon, or an oxide of carbon, as the source of the bright luminous bands,” and the Abbé Moigno asks whether this comet may not be, after all, “un gigantesque diamant volatilisé.”
“Whatever may be the answer hereafter given to that question, the verdict of the spectroscope is clearly to the effect that the comet is made up of a commingling of thin vapor and of denser particles, either compressed into the condition of solidification, or into some physical state approaching to that condition, and is therefore entirely in accordance with the notion formed on other grounds that the nucleus of the comet is a cluster of solid nodules or granules, and that the luminous coma and tail are jets and jackets of vapor, associated with the more dense ingredients, and swaying and streaming about them as heat and gravity, acting antagonistic ways, determine."
[1. “Edinburgh Review,” October, 1874, p. 210.]
If the comet shines by reflected light, it is pretty good evidence that there must be some material substance there to reflect the light.
“A considerable portion of the light of the comet is, nevertheless, borrowed from the sun, for it has one property belonging to it that only reflected light can manifest. It is capable of being polarized by prisms of double-refracting spar. Polarization of this character is only possible when the light that is operated upon has already been reflected from an imperfectly transparent medium."