Chapter II. What Is A Comet?

CHAPTER II.

WHAT IS A COMET?

IN the first place, are comets composed of solid, liquid, or gaseous substances? Are they something, or the next thing to nothing?

It has been supposed by some that they are made of the most attenuated gases, so imponderable that if the earth were to pass through one of them we would be unconscious of the contact. Others have imagined them to be mere smoke-wreaths, faint mists, so rarefied that the substance of one a hundred million miles long could, like the genie in the Arabian story, be inclosed in one of Solomon's brass bottles.

But the results of recent researches contradict these views:

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

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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]

[1. "Edinburgh Review," October, 1874, p. 210.]

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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."[1]

There is considerable difference of opinion as to whether the bead of the comet is solid matter or inflammable gas.

"There is nearly always a point of superior brilliancy perceptible in the comet's head, which is termed its nucleus, and it is necessarily a matter of pressing interest to determine what this bright nucleus is; whether it is really a kernel of hard, solid substance, or merely a whiff of somewhat more condensed vapor. Newton, from the first, maintained that the comet is made partly of solid substance, and partly of an investment of thin, elastic vapors. If this is the case, it is manifest that the central nodule of dense substance should be capable of intercepting light when it passes in front of a more distant luminary, such as a fixed star. Comets, on this account, have been watched very narrowly whenever they have been making such a passage. On August 18, 1774, the astronomer Messier believed that he saw a second bright star burst into sight from behind the nucleus of a comet which had concealed it the instant before. Another observer, Wartmann, in the year 1828, noticed that the light of an eighth-magnitude star was temporarily quenched as the nucleus of Encke's comet passed over it."[2]

Others, again, have held that stars have been seen through the comet's nucleus.

[1. "Edinburgh Review," October, 1874, p. 207.

2. Ibid., p. 206.]

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Amédée Guillemin says:

"Comets have been observed whose heads, instead of being nebulous, have presented the appearance of stars, with which, indeed, they have been confounded."[1]

When Sir William Herschel discovered the planet Urania, he thought it was a comet.

Mr. Richard A. Proctor says:

"The spectroscopic observations made by Mr. Huggins on the light of three comets show that a certain portion, at least, of the light of these objects is inherent. . . . The nucleus gave in each case three bands of light, indicating that the substances of the nuclei consisted of glowing vapor."[2]

In one case, the comet-head seemed, as in the case of the, comet examined by Padre Secchi, to consist of pure carbon.

In the great work of Dr. H. Schellen, of Cologne, annotated by Professor Huggins, we read:

"That the nucleus of a comet can not be in itself a dark and solid body, such as the planets are, is proved by its great transparency; but this does not preclude the possibility of its consisting of innumerable solid particles separated from one another, which, when illuminated by the sun, give, by the reflection of the solar light, the impression of a homogeneous mass. It has, therefore, been concluded that comets are either composed of a substance which, like gas in a state of extreme rarefaction, is perfectly transparent, or of small solid particles individually separated by intervening spaces through which the light of a star can pass without obstruction, and which, held together by mutual attraction, as well as by gravitation toward a denser 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 material of the comet would consist of granitic stones and dust such as constitutes clays.

The stones are reduced to a small size by the constant attrition:

"The stones of the 'till' are not of the largest; indeed, bowlders above four feet in diameter are comparatively seldom met with in the till."[2]

And this theory is corroborated by the fact that the eminent German geologist, Dr. Hahn, has recently discovered an entire series of organic remains in meteoric stones, of the class called chrondites, and which he identifies as belonging to classes of sponges, corals, and crinoids. Dr. Weinland, another distinguished German, corroborates these discoveries; and he has also found fragments in these stones very much like the youngest marine chalk in the Gulf of Mexico; and he thinks he sees, under the microscope, traces of vegetable growth. Francis Birgham says:

[1. "American Cyclopædia," article "Clay."

2. "The Great Ice Age," p. 10.]

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"This entire ex-terrestrial fauna hitherto discovered, which already comprises about fifty different species, and which originates from different meteoric falls, even from some during the last century, conveys the impression that it doubtlessly once formed part of a single ex-terrestrial-celestial body with a unique creation, which in by-gone ages seems to have been overtaken by a grand catastrophe, during which it was broken up into fragments."[1]

When we remember that meteors are now generally believed to be the droppings of comets, we come very near to proof of the supposition that comets are the débris of exploded planets; for only on planets can we suppose that life existed, for there was required, for the growth of these sponges, corals, and crinoids, rocks, earth, water, seas or lakes, atmosphere, sunshine, and a range of temperature between the degree of cold where life is frozen up and the degree of heat in which it is burned up: hence, these meteors must be fragments of bodies possessing earth-like conditions.

We know that the heavenly bodies are formed of the same materials as our globe.

Dana says:

"Meteoric stones exemplify the same chemical and crystallographic laws as the rocks of the earth, and have afforded no new element or principle of any kind."[2]

It may be presumed, therefore, that the granite crust of the exploded globe from which some comet was created was the source of the finely triturated material which we know as clay.

But the clays are of different colors--white, yellow, red, and blue.

[1. "Popular Science Monthly," November, 1881, p. 86.

2. "Manual of Geology," p. 3.]

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"The aluminous minerals contained in granite rocks are feldspar, mica, and hornblende. . . . Mica and hornblende generally contain considerable oxide of iron, while feldspar usually yields only a trace or none. Therefore clays which are derived from feldspar are light-colored or white, while those partially made up of decomposed mica or hornblende are dark, either bluish or red."[1]

The tail of the comet seems to be perpetually in motion. It is, says one writer, "continually changing and fluctuating as vaporous masses of cloud-like structure might be conceived to do, and in some instances there has been a strong appearance even of an undulating movement."[2]

The great comet of 1858, Donati's comet, which many now living will well remember, and which was of such size that when its head was near our horizon the extremity of the tail reached nearly to the zenith, illustrated this continual movement of the material of the tail; that appendage shrank and enlarged millions of miles in length.

Mr. Lockyer believed that he saw in Coggia's comet the evidences of a whirling motion--

"In which the regions of greatest brightness were caused by the different coils cutting, or appearing to cut, each other, and so in these parts leading to compression or condensation, and frequent collision of the luminous particles."

Olbers saw in a comet's tail--

"A sudden flash and pulsation of light which vibrated for several seconds through it, and the tail appeared during the continuance of the pulsations of light to be lengthened by several degrees and then again contracted."[1]

[1. "American Cyclopædia," article "Clay."

2. "Edinburgh Review," October, 1874, p. 208,

3, "Cosmos," vol. i, p. 143.]

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Now, in this perpetual motion, this conflict, these great thrills of movement, we are to find the source of the clays which cover a large part of our globe to a depth of hundreds of feet. Where are those exposures of granite on the face of the earth from which ice or water could have ground them? Granite, I repeat, comes to the surface only in limited areas. And it must be remembered that clay is the product exclusively of granite ground to powder. The clays are composed exclusively of the products of disintegrated granite. They contain but a trace of lime or magnesia or organic matters, and these can be supposed to have been infiltrated into them after their arrival on the face of the earth.[1] Other kinds of rock, ground up, form sand. Moreover, we have seen that neither glaciers nor ice-sheets now produce such clays.

We shall see, as we proceed, that the legends of mankind, in describing the comet that struck the earth, represent it as party-colored; it is "speckled" in one legend; spotted like a tiger in another; sometimes it is a white boar in the heavens; sometimes a blue snake; sometimes it is red with the blood of the millions that are to perish. Doubtless these separate formations, ground out of the granite, from the mica, hornblende, or feldspar, respectively, may, as I have said, under great laws, acted upon by magnetism or electricity, have arranged themselves in separate lines or sheets, in the tail of the comet, and hence we find that the clays of one region are of one color, while those of another are of a different hue. Again, we shall see that the legends represent the monster as "winding," undulating, writhing, twisting, fold over fold, precisely as the telescopes show us the comets do to-day.

[1. "American Cyclopædia," vol. iv, p. 650.]

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The very fact that these waves of motion run through the tail of the comet, and that it is capable of expanding and contracting on an immense scale, is conclusive proof that it is composed of small, adjustable particles. The writer from whom I have already quoted, speaking of the extraordinary comet of 1843, says:

"As the comet moves past the great luminary, it sweeps round its tail as a sword may be conceived to be held out at arm's-length, and then waved round the head, from one side to the opposite. But a sword with a blade one hundred and fifty millions of miles long must be a somewhat awkward weapon to brandish round after this fashion. Its point would have to sweep through a curve stretching out more than six hundred millions of miles; and, even with an allowance of two hours for the accomplishment of the movement, the flash of the weapon would be of such terrific velocity that it is not an easy task to conceive how any blade of connected material substance could bear the strain of the stroke. Even with a blade that possessed the coherence and tenacity of iron or steel, the case would be one that it would be difficult for molecular cohesion to deal with. But that difficulty is almost infinitely increased when it is a substance of much lower cohesive tenacity than either iron or steel that has to be subjected to the strain.

"There would be, at least, some mitigation of this difficulty if it were lawful to assume that the substance which is subjected to this strain was not amenable to the laws of ponderable existence; if there were room for the notion that comets and their tails, which have to be brandished in such a stupendous fashion, were sky-spectres, immaterial phantoms, unreal visions of that negative shadow-kind which has been alluded to. This, however, unfortunately, is not a permissible alternative in the circumstances of the case. The great underlying and indispensable fact that the comet comes rushing up toward the sun out of space, and then shoots round that great center of attraction by the force of its own acquired and ever-increasing impetuosity; the fact that it is obedient

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through this course to the law of elliptical, or, to speak more exactly, of conic-section, movement, permits of no doubt as to the condition of materiality. The comet is obviously drawn by the influence of the sun's mass, and is subservient to that all-pervading law of sympathetic gravitation that is the sustaining bond of the material universe. It is ponderable substance beyond all question, and held by that chain of physical connection which it was the glory of Newton to discover. If the comet were not a material and ponderable substance it would not gravitate round the sun, and it would not move with increasing velocity as it neared the mighty mass until it had gathered the energy for its own escape in the enhanced and quickened momentum. In the first instance, the ready obedience to the attraction, and then the overshooting of the spot from which it is exerted, combine to establish the comet's right to stand ranked at least among the ponderable bodies of space."[1]

And it is to the comet we must look for the source of a great part of those vast deposits of gravel which go to constitute the Drift.

"They have been usually attributed to the action of waves; but the mechanical work of the ocean is mostly confined to its shores and soundings, where alone material exists in quantity within reach of the waves and currents.[2] . . . The eroding action is greatest for a short distance above the height of half-tide, and, except in violent storms, it is almost null below low-tide."[3]

But if any one will examine a sea-beach he will see, not a vast mass of pebbles perpetually rolling and grinding each other, but an expanse of sand. And this is to be expected; for as soon as a part of the pebbles is, by the attrition of the waves, reduced to sand, the sand packs around the stones and arrests their further waste. To form such a mass of gravel as is found in the Drift we

[1. "Edinburgh Review," October, 1874, p. 202.

2. Dana's "Text Book," p. 286.

3. Ibid., p. 287.]

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must conceive of some way whereby, as soon as the sand is formed, it is removed from the stones while the work of attrition goes on. This process we can conceive of in a comet, if the finer detritus is constantly carried back and arranged in the order of the size of its particles.

To illustrate my meaning: let one place any hard substance, consisting of large fragments, in a mortar, and proceed to reduce it with a pestle to a fine powder. The work proceeds rapidly at first, until a portion of the material is triturated; you then find that the pulverized part has packed around and protected the larger fragments, and the work is brought to a stand-still. You have to remove the finer material if you would crush the pieces that remain.

The sea does not separate the sand from the gravel; it places all together at elevations where the waves can not reach them:

"Waves or shallow soundings have some transporting power; and, as they always move toward the land, their action is landward. They thus beat back, little by little, any detritus in the waters, preventing that loss to continents or islands which would take place if it were carried out to sea."[1]

The pebbles and gravel are soon driven by the waves up the shore, and beyond the reach of further wear;[2] and "the rivers carry only silt to the ocean."[3]

The brooks and rivers produce much more gravel than the sea-shore:

"The detritus brought down by rivers is vastly greater in quantity than the stones, sand, or clay produced by the wear of the coasts."[4]

[1. Dana's "Text Book," p. 288.

2. Ibid., p. 291.

3. Ibid., p. 302.

4. Ibid., p. 290.]

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But it would be absurd to suppose that the beds of rivers could have furnished the immeasurable volumes of gravel found over a great part of the world in the drift-deposits.

And the drift-gravel is different from the gravel of the sea or rivers.

Geikie says, speaking of the "till":

"There is something very peculiar about the shape of the stones. They are neither round and oval, like the pebbles in river-gravel, or the shingle of the sea-shore, nor are they sharply angular like newly-fallen débris at the base of a cliff, although they more closely resemble the latter than the former. They are, indeed, angular in shape, but the sharp corners and edges have invariably been smoothed away. . . . Their shape, as will be seen, is by no means their most striking peculiarity. Each is smoothed, polished, and covered with striæ or scratches, some of which are delicate as the lines traced by an etching-needle, others deep and harsh as the scores made by the plow upon a rock. And, what is worthy of note, most of the scratches, coarse and fine together, seem to run parallel to the longer diameter of the stones, which, however, are scratched in many other directions as well."[1]

Let me again summarize:

I. Comets consist of a blazing nucleus and a mass of ponderable, separated matter, such as stones, gravel, clay-dust, and gas.

II. The nucleus gives out great heat and masses of burning gas.

III. Luminous gases surround the nucleus.

IV. The drift-clays are the result of the grinding up of granitic rocks.

V. No such deposits, of anything like equal magnitude, could have been formed on the earth.

[1. "The Great Ice Age," p. 13.]

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VI. No such clays are now being formed under glaciers or Arctic ice-sheets.

VII. These clays were ground out of the substance of the comet by the endless changes of position of the material of which it is composed as it flew through space, during its incalculable journeys in the long reaches of time.

VIII. The earth-supplies of gravel are inadequate to account for the gravel of the drift-deposits.

IX. Neither sea-beach nor rivers produce stones like those found in the Drift.

I pass now to the next question.

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