Popular Science Monthly/Volume 37/July 1890/A Talk About Meteorites
|A TALK ABOUT METEORITES.|
INSTRUCTOR IN MINERALOGY AND CHEMISTRY, HARVARD UNIVERSITY.
METEORITES are particularly interesting because they comprise the only material coming to us from outer space. In consequence of the striking phenomena resulting from their rapid passage through our atmosphere, making them appear like balls of fire visible at great distances, sometimes exploding with such violence as to be taken for earthquakes, their falls have been noticed and recorded since the earliest times. The accounts, however, were so imbued with superstitions, and so distorted by the terrified condition of the narrators, that in most cases the witnesses of the event were laughed at for their supposed delusions, and it was not till the beginning of the present century that men of science and people in general began to give credit to such reports.
The earliest authentic records of stones falling from the sky are to be found in the Chinese annals, which go back to 644 b. c., and between that time and 333 a. d. Biot has traced sixteen distinct occurrences. In Europe, a meteorite is said to have fallen in Crete as far back as 1478 b. c., but Greek history can not be depended upon for events earlier than 700 b. c. A more probable fall, in 705 b. c., is mentioned by Plutarch; while Livy, in his History of Rome, gives an account of a shower of stones which fell on the Alban Mount about 652 b. c., and which so impressed the senate that they decreed a nine days' solemn festival. Again, in 466 b. c., a stone fell at Ægospotamos, in Thrace, which is mentioned in the Parian Marbles, and also by Plutarch and Pliny, which is said to have been of the size of two mill-stones, and equal in weight to a full wagon-load. Still more famous was the meteorite which fell 204 b. c. in Phrygia, described as conical in shape, of a deep-brown color, and looking like a piece of lava, and so pointed at the top that it was called the "needle" of Cybele. This stone was believed to have fallen from heaven, and was worshiped at Pessinus by the Phrygians and Phœnicians as the Great Mother of the Gods. At the time of the second Punic war, upon the announcement by an oracle that its possession would secure continued prosperity to the state, it was demanded from King Attalus and taken with great ceremony to Rome, where it was mounted on a silver statue of the goddess in place of the head. Signor Lanciani has traced its existence down to 1730. It was then finally lost sight of, but he thinks it may still exist, buried in the ruins of the Palace of the Cæsars. The Diana of the Ephesians, "which fell down from Jupiter," and the Image of Venus at Cyprus, are now considered to have been similar meteoric masses.
There is a stone whose history goes back at least twelve centuries, built into the northeast corner of the Kaaba at Mecca, held in great reverence by the Moslems, which is supposed to have had a similar origin.
There are also numerous accounts of meteorites having been worshiped in more modern times. One which fell about one hundred and fifty years ago was worshiped for some time in the temple of Ogi in Japan; and a stone which fell in a field near the village of Dooralla, in India, in 1815, was immediately decked with flowers, and the natives would have built a temple over it were it not for a powerful constraint which took it to the British Museum.
Undoubtedly the oldest meteorite still preserved is one now in the Harvard collection, which was found by Prof. Putnam on the altar of Mound No. 4 of the Turner Group (Little Miami Valley, Ohio). It possibly had been an object of worship to the old mound-builders during some prehistoric age, and the worship of such sky-stones is considered by many writers to have been the oldest form of idolatry. It is well known, however, that meteoric iron was used by the mound-builders for coating bronze ornaments with a white metal; and two meteoric fragments, consisting wholly of iron, were found on a neighboring altar. Many such ornaments are to be found in our museums. There is an account in Dio Cassius of an attempt, under the Emperor Severus, to coat bronze coins with silver which was said to have come down from heaven. The same mistake of taking meteoric iron for silver is frequently made in the present day, owing to an unusual whiteness of the iron and its extreme malleability.
The oldest undoubted meteorite seen to fall was, till recently, suspended by a chain from the vault of the parish church of Ensisheim, in Alsace. The following, translated from a document still preserved in the church, gives an account of its fall:
"On the 16th of November, 1492, a singular miracle happened; for, between eleven and twelve in the forenoon, with a loud crash of thunder and a prolonged noise heard afar off, there fell in the town of Ensisheim a stone weighing two hundred pounds. It was seen by a child to strike the ground in a field near the canton called Gisgaud, where it made a hole more than five feet deep. It was taken to the church as being a miraculous object. The noise was heard so distinctly at Lucerne Villing and many other places that in each of them it was thought some houses had fallen. King Maximilian, who was then at Ensisheim, had the stone carried to the castle; after breaking off two pieces, one for the Duke Sigismund of Austria and the other for himself, he forbade further damage, and ordered the stone to be suspended in the parish church."
In 1768 a stone was seen to fall at Lucé, in France, and three French Academicians, one of whom was Lavoisier, were appointed to investigate it. As they were convinced beforehand that the stone could not have fallen from the sky, they reported to the Academy that it was an ordinary stone, which had been struck by lightning.
The German philosopher Chladni, in 1794, was the first to bring together the accounts of bodies said to have fallen from the sky, and he felt confident in his conclusion that at least two of these came from outer space. One was the now well-known Pallas meteorite, found by a Cossack, in 1749, on the top of a lofty mountain, and brought by the traveler Pallas from Krasnojarsk, Siberia, in 1772. The mass, consisting largely of iron, weighed fifteen hundred pounds, and was thought by the Tartars to be a holy thing fallen from heaven, because it differed entirely from all the rocks of the country. The second was one found, in 1783, by Indians, projecting a foot above the ground, at Otumpa, province of Tucuman, Argentine Republic. It was thought to be an iron mine, and Don Michael Rubin de Celis was sent to investigate it. He reported that it was a mass of iron weighing about thirty thousand pounds, and that there was no other iron in the neighborhood, and no stones, and no human habitations.
Chladni argued that these two masses of iron must have been formed through fire, and, as there were no signs of volcanoes in the countries where they were found, and as volcanoes had never been known to eject masses of iron, he concluded that they must have come to our earth from space.
Two months after Chladni had advanced his theory, there fell a shower of stones at Siena, in Tuscany, an account of which was given in a letter received by Sir William Hamilton from the Earl of Bristol, dated Siena, July 12, 1794:
"In the midst of a most violent thunder-storm, about a dozen stones of various weights and dimensions fell at the feet of different persons, men, women, and children. The stones are of a quality not found in any part of the Siennese territory: they fell about eighteen hours after the enormous eruption of Mount Vesuvius, which circumstance leaves a choice of difficulties in the solution of this extraordinary phenomenon. Either these stones have been generated in this igneous mass of clouds which produced such unusual thunder, or, which is equally incredible, they were thrown from Vesuvius at a distance of at least two hundred and fifty miles; judge, then, of its parabola. The philosophers here incline to the first solution. I wish much, sir, to know your sentiments. My first objection was to the fact itself, but of this there are so many eye-witnesses it seems impossible to withstand their evidence."
As the wind was from the north when the stones fell at Siena, while Vesuvius was to the south, it was suggested that the cloud from which they came had been blown all the way from Vesuvius past Siena and then back again, before it condensed.
The next meteorite seen to fall was in England itself. On December 13, 1795, a stone weighing fifty-six pounds fell at Wold Cottage, in Yorkshire, at about three o'clock in the afternoon, and several persons saw it fall. It fell on a perfectly clear day, and penetrated twelve inches of soil and six inches of chalk rock. In the neighboring villages sounds were heard which were taken for the firing of guns at sea, and in two villages there was such a distinct sound of something whizzing through the air toward the house of a Mr. Topham that several people ran there to see what had happened. When the stone was dug up it was warm and smoked. It was exhibited in London, and handbills were distributed giving an account of its descent. Such advertising, however, did not tend to make people believe in the celestial origin of the stone; and, as there were no volcanoes in England, it was thought that it might have been condensed from a cloud of ashes blown from Mount Hecla in Iceland. We do not, however, have to go back one hundred years to find wild hypotheses as to the probable origin of meteorites. Even now very little is known, and the field for speculation is nearly as unlimited as it was then, though the theories of a few centuries ago are simpler and more amusing than the recent ones. In the chronicles of the Benedictine monks a theory of the origin of meteorites is given briefly thus:
"In the year 921, in the time of Lord John X, pope, in the seventh year of his pontificate, signs were seen; for, near the city of Rome, many stones were seen to fall from the sky—such dreadful and terrible ones in the city of Narnia that people had to believe that they were brought straight from hell. The very biggest of the stones, falling into the river Narnius, can be seen to this day, projecting a cubit above the surface of the water."
A Persian philosopher, Syed Abdulla, in 1814, describing a fall of stones near Bombay, says: "The causes of this may be, that in the course of working (or of changes on) the ground, air being extricated, may have entered into combination, and come near elemental fire, and from this fire have received a portion of heat; that then it may have united with brimstone and terrene salt, as, for instance, saltpeter; when the mixture, from some cause, being ignited, the fire bestows its own property on the mass, and the stones which may have been above it are blown into the air—God knows the truth."
In 1818 Dr. W. G. Reynolds, in offering a theory on meteorites, says: "Dr. Blagden considers electricity as the general cause of these phenomena; Dr. Gregory and others think they depend on highly inflammable matter, as phosphorus, phosphorated hydrogen, etc., being volatilized and congregated in the upper regions of the air. Dr. Halley ascribes them to a fortuitous concourse of atoms, which the earth meets in her annual track through the ecliptic; and Sir John Pringle seems to regard them as bodies of a celestial character, revolving around centers, and intended by the Creator for wise and beneficent purposes, perhaps to our atmosphere, to free it of noxious qualities, or supply such as are salutary." Dr. Reynolds then goes on to elaborate a most complicated theory in which solid substances on the earth are changed to vapor by the sun's heat; these, rising as gases, finally give up their heat by an explosion, and the particles, having no heat to keep them apart, rush together and come down as solids.
"While the minds of the scientific men of France were in this unsettled condition, there came a report that still another shower of stones had fallen, this time in their own country, and within easy reach of Paris. To settle the matter finally, if possible, the physicist Biot, member of the French Academy, was directed by the Minister of the Interior to inquire into the event upon the spot. After a careful examination of the stones and a comparison of the statements of the villagers, Biot was convinced that—
"1. On Tuesday, April 26, 1803, about 1 p. m., there was a violent explosion in the neighborhood of L'Aigle, in the department of Orne, lasting for five or six minutes; this was heard for a distance of seventy-five miles round.
"2. Some moments before the explosion at L'Aigle, a fire-ball in quick motion was seen from several of the adjoining towns, though not from L'Aigle itself.
"3. There was absolutely no doubt that on the same day many stones fell in the neighborhood of L'Aigle.
"Biot estimated the number of the stones at two or three thousand; they fell within an ellipse of which the larger axis was 6·2 miles, and the smaller 2·5 miles; and this inequality would indicate not a single explosion but a series of them. With the exception of a few little clouds of ordinary character, the sky was quite clear.
"The exhaustive report of Biot, and the conclusive nature of his proofs, compelled the whole of the scientific world to recognize the fall of stones on the earth from outer space as an undoubted fact."
The main difficulty in forming theories at the present time is due to the fact that there are two distinct classes of meteorites, irons and stones, the characteristics of which make it difficult to assign a like origin to both. It is probable, however, that they all belong to our solar system; that they are revolving round the sun in some different plane from the ecliptic, and that the earth is constantly meeting them in its yearly journey. When they come-into contact with our atmosphere, although they are moving with planetary velocity—sometimes at the rate of forty-five miles a second, more than twice as fast as the earth moves in its orbit—their motion is rapidly reduced, owing to the resistance of the air, so that in most cases they come to the ground like a spent cannonball. Their passage through the air is only of a few seconds' duration, yet the rapid reduction of velocity determines a great heating effect, so that the meteorite, a moment before intensely cold, is immediately fused on the surface, forming a coating varying from a fiftieth to a hundredth of an inch in thickness, and this crust is one of the first characteristics by which a meteorite is recognized. Moreover, the material burns away unevenly, forming pittings or thumb-marks, resembling the marks left by the fingers on a mass of putty—a character also observed on large grains of partially burned powder picked up after the discharge of large guns. The meteorite from Cynthiana, Ky., in the Harvard collection, shows similar marks though more in furrows, made by a flow of the melted surface from the front to the back of the mass during its passage.
The unequal heating of meteorites by the atmosphere causes pieces to crack off, and sometimes the whole mass explodes. In addition, the air rushing in to fill the space behind the rapidly moving body, causes a sound variously compared to claps of thunder, firing of musketry, the tearing of calico, and the like—a noise frequently heard after the passage of the meteorite, owing to the circumstance that the sound travels so much more slowly than the mass itself. Furthermore, the high temperature of the surface causes the mass to glow with a brilliant light, making it appear like a ball of fire, and visible at distances depending on its height above the horizon, sometimes over an area of one thousand miles. Thus a meteorite was seen in 1876 to pass over the States of Kansas, Nebraska, Iowa, Missouri, Wisconsin, Illinois, Michigan, Kentucky, Indiana, Ohio, West Virginia, and Pennsylvania; and explosions were heard like cannonading even to the distance of one hundred and fifty miles from its course. Over Illinois it was seen to break in pieces like a rocket, and over Indiana and Ohio the pieces were computed to cover an area forty miles long and five miles broad. At Rochester, Fulton County, Indiana, during the meteorite's passage, a farmer heard the thud of something striking the ground near his house, and in the morning found a fragment of rock on top of the snow, which is supposed to be part of the meteorite.
All meteorites appear to be fragments broken off from larger bodies. Sometimes numerous fragments reach the earth, and at other times only single masses. Thus, in the fall at L'Aigle, already mentioned, about three thousand pieces were picked up, scattered over an ellipse more than six miles long. An equally large number fell at Knyahinya, June 9, 1866. Still more at Pultusk in 1868. Several thousand were also picked up after a fall at Estherville, Emmett County, Iowa, May 10, 1879. In such a rain of meteorites the fragments vary greatly in size, some weighing less than a grain, while the largest may weigh a hundred pounds or more. In most cases the peculiar characters or composition of the various specimens make it easy to recognize them as fragments of the same mass. In the case of the Estherville meteorite most of the pieces were coated with a fused crust, owing to the explosion having taken place before they had lost their great velocity.
In the case of a stone which fell at Butsura in 1861, fragments found three or four miles apart could be fitted together, and some of the pieces, though fitting perfectly, had been coated on the faces of juncture with a thin crust, showing that they had been blown apart when the meteorite was still very high in the air.
Meteorites, when not seen to fall, are easily recognized, not only by the characteristic fused crust and pittings already referred to, but by certain very marked peculiarities of structure. There are three large groups: those consisting of metallic iron; those consisting of earthy minerals containing only grains of metallic iron; and those like the Pallas, made up of a continuous network of iron inclosing stony matter. The stony meteorites are usually made up of little rounded grains imbedded in a ground-mass of fragments of the same material, a type of structure called chondritic, which in its details is so characteristic that pieces of the same mass can usually be easily identified, even though found at places or times remote from each other. The iron meteorites are still more easily recognized, although only about nine at most have been seen to fall; for, since iron has not been found in masses of any size in terrestrial rocks, unless in Greenland, these large meteoric fragments are at once noticed wherever found. Stony ones, on the contrary, are not only apt to be overlooked, but the falls of past ages must have been altered and broken up by weathering. Meteoric iron can be easily identified, because it is usually extremely malleable, but at the same time very tough, owing to its being made up of a network of crystalline plates, the plates consisting of pure iron, bounded by layers of an iron-nickel alloy and other impurities, which have separated out during a slow process of crystallization, evidently from a melted condition. This structure is best seen on a polished surface which has been subject to tempering, or else etched with acid. The acid, acting most readily upon the purest parts of the iron, develops certain figures called Widmanstättian figures if the plates are broad and well marked, and called Neumann lines where they are reduced to fine markings. Till recently, these two varieties of etched figures were supposed to indicate a difference of crystalline structure; but, by a study of the Harvard collection (American Journal of Science, third series, vol. xxxii, p. 284), it has been shown to depend on the time of crystallization—that is to say, on merely the size of the crystals, and not a difference of form. In some cases these etched figures serve conclusively to distinguish irons of different falls, but frequently they vary on the same specimen, or depend on the direction in which the surface is cut; but there are large groups of irons closely resembling each other in their etched characters. The distinction of such irons has become of great importance, since the enormous prices paid for meteorites offers a strong inducement to multiply supposed falls. Iron meteorites are often cut up and distributed by the finders before they have been fully identified, and the confusion is further increased by the natural distribution due to the explosions in the upper atmosphere. Thus a meteorite which fell in Cocke County, Tennessee, some time previous to 1840, has been turning up at various places ever since, and the numerous fragments have been described from time to time under various names as different falls.
In an attempt to prove that an iron which was found in Maverick County, Kentucky, was identical with two Mexican ones, in the Harvard collection (Proceedings of the American Academy of Arts and Sciences, vol. xxiv, p. 30), the writer found that on breaking slabs of the respective irons the two Mexican specimens, which had been generally accepted as identical, showed a marked difference of structure. One of them, known as the Butcher iron, when broken in various directions by blows of a hammer, always exhibited most brilliant and complex crystal faces, some of them half an inch in diameter, certain of the faces being most beautifully marked by a system of fine parallel lines arranged at certain fixed angles. The second iron, on the other hand, from Santa Rosa, would only break in two definite directions, exhibiting a single face with little flaky surfaces, but none of the fine lines. This last iron, if sawed to a thin edge, and then forced to break in a different direction from the two just mentioned, showed only a series of little cube faces, very different from the Butcher iron.
On a similar examination of other irons resembling the two Mexican ones in the figures brought out by etching, irons from Allen and Maverick Counties, Kentucky, and Chattooga County, Georgia, appeared to be identical with the one from Santa Rosa, though found at places so distant from each other and described as independent falls, while none showed the striking fracture of the Butcher specimens.
As the irons examined were among the most compact and malleable of any in our collections, the result suggests a new way of identifying fragments of the same original mass, where external features are not sufficiently decisive, and, moreover, shows the care that must be taken in determining supposed new falls.
- British Museum Catalogue of Meteorites.