resisted that the nuclei of comets not only emit their own light,
which is that of a glowing gas, but also, together with the coma and
the tail, reflect the light of the sun. There seems nothing,
therefore, to contradict the theory that the mass of a comet may be
composed of _minute solid bodies_, kept apart one from another in the
same way as the infinitesimal particles forming a cloud of dust or
smoke are held loosely together, and that, as the comet approaches
the sun, the most easily fusible constituents of these small bodies
become wholly or partially vaporized, and in a condition of _white
heat_ overtake the remaining solid particles, and surround the
nucleus in a self-luminous cloud of glowing vapor."[2]

Here, then, we have the comet:

First, a more or less solid nucleus, on fire, blazing, glowing.

Second, vast masses of gas heated to a white heat and enveloping the
nucleus, and constituting the luminous head, which was in one case
fifty times as large as the moon.

Third, solid materials, constituting the tail (possibly the nucleus
also), which are ponderable, which reflect the sun's light, and are
carried along under the influence of the nucleus of the comet.

Fourth, possibly in the rear of all these, attenuated volumes of gas,
prolonging the tail for great distances.

What are these solid materials?

[1. "Spectrum Analysis," 1872.