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.
2. Ibid., p. 402.]
{p. 70}
Stones, and sand, the finely comminuted particles of stones ground off by ceaseless attrition.