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. What is the proof of this? Simply this: that it is now conceded that meteoric showers are shreds and patches of cometic matter, dropped from the tail; _and meteoric showers are stones_. "Schiaparelli considers meteors to be dispersed portions of the comet's original substance; that is, of the substance with which the comet entered the solar domain. Thus comets would come to be regarded as consisting of _a multitude of relatively minute masses_."[1] Now, what is the genesis of a comet? How did it come to be? How was it born? In the first place, there are many things which would connect them with our planets. They belong to the solar system; they revolve around the sun. Says Amédée Guillemin: "Comets form a part of our solar system. Like the. planets, they revolve about the sun, traversing with very variable velocities extremely elongated orbits."[2] We shall see reason to believe that they contain the same kinds of substances of which the planets are composed. Their orbits seem to be reminiscences of former planetary conditions: "All the comets, having a period not exceeding seven years, travel in the same direction around the sun as the planets. Among comets with periods less than eighty years long, five sixths travel in the same direction as the planets."[3] [1. "American Cyclopædia," vol. v, p. 141. 2. "The Heavens," p. 239. 3. American Cyclopedia," vol. v, p. 141.] {p. 71} It is agreed that this globe of ours was at first a gaseous mass; as it cooled it condensed like cooling steam into a liquid mass; it became in time a molten globe of red-hot matter. As it cooled still further, a crust or shell formed around it, like the shell formed on an egg, and on this crust we dwell. While the crust is still plastic it shrinks as the mass within grows smaller by further cooling, and the wrinkles so formed in the crust are the depths of the ocean and the elevations of the mountain-chains.