central conglomeration, moves through space like a cloud of dust. In any case the connection lately noticed by Schiaparelli, between comets and meteoric
[1. "The Heavens," p. 239.
2. Note to Guillemin's "Heavens," p. 261.]
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showers, seems to necessitate the supposition that in many comets a similar aggregation of particles seems to exist."[1]
I can not better sum up the latest results of research than by giving Dr. Schellen's words in the work just cited:
"By collating these various phenomena, the conviction can scarcely be 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.
2. Ibid., p. 402.]
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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.]
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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.
But as ages go on and the process of cooling progresses, the crust reaches a density when it supports itself, like a couple of great arches; it no longer wrinkles; it no longer follows downward the receding molten mass within; mountains cease to be formed; and at length we have a red-hot ball revolving in a shell or crust, with a space between the two, like the space between the dried and shrunken kernel of the nut and the nut itself.
Volcanoes are always found on sea-shores or on islands. Why? Through breaks in the earth the sea-water finds its way occasionally down upon the breast of the molten mass; it is at once converted into gas, steam; and as it expands it blows itself out through the escape-pipe of the volcano; precisely as the gas formed by the gunpowder coming in contact with the fire of the percussion-cap, drives the ball out before it through the same passage by which it had entered. Hence, some one has said, "No water, no volcano."
While the amount of water which so enters is small because of the smallness of the cavity between the shell of the earth and the molten globe within, this process is carried on upon a comparatively small scale, and is a safe one for the earth. But suppose the process of cooling to go on uninterruptedly until a vast space exists between the
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crust and the core of the earth, and that some day a convulsion of the surface creates a great chasm in the crust, and the ocean rushes in and fills up part of the cavity; a tremendous quantity of steam is formed, too great to escape by the aperture through which it entered, an explosion takes place, and the crust of the earth is blown into a million fragments.
The great molten ball within remains intact, though sorely torn; in its center is still the force we call gravity; the fragments of the crust can not fly off into space; they are constrained to follow the master-power lodged in the ball, which now becomes the nucleus of a comet, still blazing and burning, and vomiting flames, and wearing itself away. The catastrophe has disarranged its course, but it still revolves in a prolonged orbit around the sun, carrying its broken débris in a long trail behind it.
This débris arranges itself in a regular order: the largest fragments are on or nearest the head; the smaller are farther away, diminishing in regular gradation, until the farthest extremity, the tail, consists of sand, dust, and gases. There is a continual movement of the particles of the tail, operated upon by the attraction and repulsion of the sun. The fragments collide and crash against each other; by a natural law each stone places itself so that its longest diameter coincides with the direction of the motion of the comet; hence, as they scrape against each other they mark each other with lines or striæ, lengthwise of their longest diameter. The fine dust ground out by these perpetual collisions does not go off into space, or pack around the stones, but, still governed by the attraction of the head, it falls to the rear and takes its place, like the small men of a regiment, in the farther part of the tail.
Now, all this agrees with what science tells us of the constitution of clay.
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"It is a finely levigated silico-aluminous earth--formed by the disintegration of feldspathic or granite rocks."[1]
The particles ground out of feldspar are finer than those derived from mica and hornblende, and we can readily understand how the great forces of gravity, acting upon the dust of the comet's tail, might separate one from the other; or how magnetic waves passing through the comet might arrange all the particles containing iron by themselves, and thus produce that marvelous separation of the constituents of the granite which we have found to exist in the Drift clays. If the destroyed world possessed no sedimentary rocks, then the entire