AËROLITES.
BY TOWNSHEND M. HALL, F.G.S.
Meteoric stones, or aërolites, as they are generally called (from two Greek words, aer and lithos, signifying "air-stones"), may be defined as solid masses consisting principally of pure iron, nickel, and several other metals, sometimes containing also an admixture of augite, olivine, and hornblende, which, from time to time, at irregular intervals, have fallen upon the surface of the earth from above.
Other designations, such as "fire-balls and thunder-bolts," have been popularly applied to these celestial masses, the former denoting their usual fiery appearance, whilst the latter has reference to the extreme suddenness of their descent.
Shooting stars also, although they are not accompanied by the fall of any solid matter upon the earth, are generally placed in the same category, since they are supposed to be aërolites which pass (comparatively speaking) very near our earth, and are visible from it by night; at the same time their distance from us, varying as it does from four to two hundred and forty miles and upward, is in most instances too great to allow of their being drawn down by the attractive power possessed by the earth. Like comets and eclipses, these celestial phenomena in former times were universally regarded with feelings of the greatest awe and superstition; and in Eastern countries especially, where the fall of a meteoric stone was supposed to be the immediate precursor of some important public event, or national calamity, the precise date of each descent was carefully recorded. In China, for example, such reports reach back to the year 644 before our era; and M. Biot has found in the astronomical section of some of the most ancient annals of that empire sixteen falls of aërolites recorded as having taken place between the years 644 B.C. and 333 after Christ, whilst the Greek and Roman authors mention only four such occurrences during the same period. Even now, in this age of science and universal knowledge, aërolites can scarcely be regarded without a certain degree of dread. Indeed, four or five cases have occurred in which persons have been killed by them; in another instance, several villages in India were set on fire by the fall of a meteoric stone; and it was by no means a pleasant subject for reflection that such a catastrophe might happen anywhere and at any moment, especially when we remember that these stones, although not quite incandescent, are always, more or less, in a heated state; and sometimes so hot that even after the lapse of six hours they could not be touched with impunity.
The first fall of meteoric stones on record appears to have taken place about the year 654: B.C., when, according to a passage in Livy, a shower of stones fell on the Alban Hill, not far distant from Rome. The next in chronological order is mentioned by several writers, such as Diogenes of Apollonia, Plutarch, and Pliny, and described by them as a great stone, the size of two millstones, and equal in weight to a full wagon-load. It fell about the year 467 B.C., at AEgos Potamos, on the Hellespont, and even up to the days of Pliny, four centuries after its fall, it continued to be an object of curiosity and speculation. [{537}] After the close of the first century we fail to obtain any account or notice of this stone; but although it has been lost sight of for upward of eighteen hundred years, the eminent Humboldt says, in one of his works, that notwithstanding all previous failures to rediscover it, he does not wholly relinquish the hope that even after such a considerable lapse of time, this Thracian meteoric mass, which it would be so difficult to destroy, may be found again, especially since the region in which it fell has now become so easy to access to European travellers.
The next descent of any particular importance took place at Ensisheim in Alsace, where an aërolite fell on November 7th, 1492, just at the time when the Emperor Maximilian, then king of the Romans, happened to be on the point of engaging with the French army. It was preserved as a relic in the cathedral at Ensisheim, until the beginning of the French revolution, when it was conveyed to the Public Library of Colmar, and it is still preserved there among the treasures.
In later years the shower of aërolites which fell in April, 1803, at L'Aigle, in Normandy, may well rank as the most extraordinary descent upon record. A large fire-ball had been observed a few moments previously, in the neighborhood of Caen and Alençon, where the sky was perfectly clear and cloudless. At L'Aigle no appearance of light was visible, and the fire-ball assumed instead the form of a small black cloud, consisting of vapor, which suddenly broke up with a violent explosion, followed several times by a peculiar rattling noise. The stones at the time of their descent were hot, but not red, and smoked visibly. The number which were afterward collected within an elliptical area measuring from six to seven miles in length by three in breadth, has been variously estimated at from two to three thousand. They ranged in weight from two drachms up to seventeen and half pounds. The French government immediately deputed M. Biot, the celebrated naturalist and philosopher, to proceed to the spot, for the express purpose of collecting the authentic facts concerning a phenomenon which, until that time, had almost universally been treated as an instance of popular superstition and credulity. His conclusive report was the means of putting an end to all scepticism on the subject, and since that date the reality—not merely the possibility—of such occurrences has no longer been contested.
Leaving out, for the present, innumerable foreign instances which might be quoted, we must now glance rapidly at a few of the most noticeable examples of the fall of meteoric stones which have taken place in England. The earliest which appears on record descended in Devonshire, near Sir George Chudleigh's house at Stretchleigh, in the parish of Ermington, about twelve miles from Plymouth. The circumstance is thus related by Westcote, one of the quaint old Devonshire historians:
"In some part of this manor (Stretchleigh), there fell from above—I cannot say from heaven—a stone of twenty-three pounds weight, with a great and fearful noise in falling; first it was heard like unto thunder, or rather to be thought the report of some great ordnance, cannon, or culverin; and as it descended, so did the noise lessen, at last when it came to the earth to the height of the report of a peternel, or pistol. It was for matter like unto a stone singed, or half-burned for lime, but being larger described by a richer wit, I will forbear to enlarge on it."
The "richer wit" here alluded to was in all probability the author of a pamphlet published at the time, which further describes this aërolite as having fallen on January 10th, 1623, in an orchard, near some men who were planting trees. It was buried in the ground three feet deep, and its dimensions were three and a half feet long, two and a half wide, and one and a half thick. The pamphlet also states that pieces broken from off it were in the possession of many of the neighboring gentry. [{538}] We may here remark that no specimen of this stone is at present known to be in existence, and that although living in the county where it fell, we have hitherto failed in tracing any of the fragments here referred to. A few years later, in August, 1628, several meteoric stones, weighing from one to twenty-four pounds, fell at Hatford, in Berkshire; and in the month of May, 1680, several are said to have fallen in the neighborhood of London.
The total number of aërolitic descents which up to this present time have been observed to take place in Great Britain and Ireland is twenty, of which four occurred in Scotland, and four in Ireland. The largest and most noticeable of all these fell on December 13th, 1795, near Wold Cottage, in the parish of Thwing, East Riding of Yorkshire. Its descent was witnessed by two persons; and when the stone was dug up, it was found to have penetrated through no less than eighteen inches of soil and hard chalk. It originally weighed about fifty-six pounds, but that portion of it preserved in the British Museum is stated in the official catalogue to weigh forty-seven pounds nine ounces and fifty-three grains—just double the weight of the Devonshire aërolite.
When we come to inquire into the various opinions which have been held in different ages respecting the origin of aërolites, and the power which causes their descent, we must go back to the times of the ancient Greeks, and we find that those of their philosophers who had directed their attention to the subject had four theories to account for this singular phenomenon. Some thought that meteoric stones had a telluric origin, and resulted from exhalations ascending from the earth becoming condensed to such a degree as to render them solid. This theory was in after years revived by Kepler the astronomer, who excluded fire-balls and shooting stars from the domain of astronomy; because, according to his views, they were simply "meteors arising from the exhalations of the earth, and blending with the higher ether." Others, like Aristotle, considered that they were masses of metal raised either by hurricanes, or projected by some volcano beyond the limits of the earth's attraction, so becoming inflamed and converted, for a time, into starlike bodies. Thirdly, a solar origin; this, however, was freely derided by Pliny and several others, among whom we may mention Diogenes of Apollonia, already alluded to as one of the chroniclers of the aërolite of AEgos Potamos. He thus argues: "Stars that are invisible, and consequently have no name, move in space together with those that are visible. . . . These invisible stars frequently fall to the earth and are extinguished, as the stony star which fell burning at AEgos Potamos." This last opinion, it will be seen, coincides, as far as it goes, almost exactly with the most modern views on the subject.
As some of the Greeks derived the origin of meteorites from the sun (probably from the fact of their sometimes falling during bright sunshine), so we find, at the end of the seventeenth century, it was believed by a great many that they fell from the moon. This conjecture appears to have been first hazarded by an Italian philosopher, meeting Paolo Maria Terzago, whose attention was specially directed to this subject on the occasion of a meteoric stone falling at Milan in 1660, and killing a Franciscan monk. Olbers, however, was the first to treat this theory in a scientific manner, and soon after about fall of an aërolite at Siena, in the year 1794, he began to examine the question by the aid of the most abstruse mathematics, and after several years' labor he succeeded in showing that, in order to reach our earth, a stone would require to start from the moon at an initial velocity 8,292 feet per second; then proceeding downward with increasing speed, it would arrive on the earth with a [{539}] of 35,000 feet per second. But frequent measurements have shown that the actual rate of aërolites averages 114,000 feet, or about twenty-one miles and a half per second, they were approved by these curious and most elaborate calculations to have come from a fire greater distance than that of our satellite. It is but fair to add that the question of initial velocity, on which the whole value value of this so-called "ballistic problem" depends, was investigative by three other eminent geometricians, Biot, Laplace, and Poisson, who during ten or twelve years were independently engaged is calculation. Biot's estimate was 8,282 feet in the second; Laplace, 7,862; and Poisson, 7,585—results all approximating very closely with those stated by Olbers.
We have already observed, at the beginning of this paper, that meteoric stones may fall at any moment, but observations, extending over many years, have sometimes been brought forward to show that, as far as locality is concerned, all countries are not equally liable to these visitations. In other words, the large number of aërolites which have been known to fall within a certain limited area has been contrasted with the apparent rarity of such occurrences beyond these limits. If it could be proved that the earth possessed more attractive power in some places than in others, this circumstance might be satisfactorily explained, but in default of any such evidence, the advocates of this theory must rely solely upon statistics, which from their very nature require to be taken with a certain amount of reserve. Professor Shepard, in Silliman's American Journal, has remarked that "fall of aërolites is confined principally to two zones; the one belonging to America is bounded by 33° and 44° north latitude, and is about 25° in length. Its direction is more or less from north-east to south-west, following the general line of the Atlantic Coast. Of all known occurrences of this phenomenon during the last fifty years, 92.8 per cent, have taken place within these limits, and mostly in the neighborhood of the sea. The zone of the eastern continent—with the exception that it extends ten degrees more to the north—lies between the same degrees of latitude, and follows a similar north-east direction, but is more than twice the length of the American zone. Of all the observed falls of aërolites, 90.9 per cent, have taken place within this area, and were also concentrated in that half of the zone which extends along the Atlantic."
On reference to a map, it will be seen that in the western continent the so-called zone is simply confined to the United States—the most densely inhabited portion of America. In like manner the eastern zone leaves out the whole of desert Africa, Lapland, Finland, the chief part of Russia, with an average of thirty-two inhabitants to each square mile; Sweden and Norway, with only seventeen per mile; whilst it embraces all the well-peopled districts of central Europe, most of which, like England, are able to count between three and four hundred persons to every mile of their territory. In fact, Professor Shepard's statement may almost be resolved into a plain question of population, for were an aërolite to fall in the midst of a desert, or in a thinly peopled district, it is needless to point out how few the chances are of its descent being ever noticed or recorded. That innumerable aërolites do fall without attracting any attention, is clearly proved by the number of discoveries continually taking place of metallic masses which, from their locality and peculiar chemical composition, could only be derived from some extra-terrestrial source. The great size also of many of these masses entirely precludes the possibility of their having been placed by human agency in the positions they have been found to occupy—sometimes on the surface of the earth, but just as frequently buried a few feet in the ground.
Thus the traveller Pallas found, in 1749, at Abakansk, in Siberia, the mass of meteoric iron, weighing 1,680 lb., now in the Imperial Museum at St. Petersburg. Another, lying on the plain of Tucuman, near Otumpa, in South America, has been estimated, by measurement, to weigh no less than 33,600 lb., or about fifteen tons; and one added last year to the splendid collection of meteorites in the British Museum weighs rather more than three and a half tons. It was found at Cranboume, near Melbourne, and was purchased by a Mr. Bruce, with a view to his presenting it to the British Museum, when, through some misunderstanding, it was discovered that one half of it had been already promised to the museum at Melbourne. In order, therefore, to save it from any such mutilation, the trustees of our national museum acquired and transferred to the authorities of the Melbourne collection a smaller mass which had been sent in 1862 to the International Exhibition. It weighed about 3,000 lb., and had been found near Melbourne, in the immediate vicinity of the great meteorite. The latter was then forwarded entire to London. In the British Museum may also be seen a small fragment of an aërolite, originally weighing 191 lb., which from time immemorial had been lying at Elbogen, near Carlsbad, in Bohemia, and had always borne the legendary appellation of "der verwünschte Burggraf," or the enchanted Burgrave. The remainder of this mass is preserved in the Imperial collection at Vienna. In Great Britain only two meteoric masses (not seen to fall) have hitherto been discovered; one was found about forty years ago near Leadhills, in Scotland; the other in 1861, at Newstead, in Roxburghshire.
Several instances have at different times occurred in which stones like aërolites have been found, and prized accordingly, until their real nature was demonstrated by the aid of chemical analysis. One valuable specimen, found a few years ago, was shown to have derived its origin amongst the scoriae of an iron foundry; another, picked up in the Isle of Wight, turned out to be a nodule of iron pyrites, similar in every respect to those which abound in the neighboring chalk cliffs; and lastly, some aërolites of a peculiarly glassy appearance were found shortly after, of which it may, perhaps, suffice to say that the scene of this discovery was—Birmingham.
When we come to examine the composition of meteoric stones, we find in various specimens a great diversity in their chemical structure. Iron is the metal most invariably present, usually accompanied by a consider percentage of nickel and cobalt; also five other metals, chromium, copper, molybdenum, manganese, and tin; but of all these iron is that which largely preponderates, forming sometimes as much as ninety-six parts in the hundred. Rare instances have, however, been recorded where the proportion of iron has sunk so low as to form only two percent, and the deficiency thus caused has been made up by a larger admixture of some earthy mineral, such as augite, hornblende, or olivine. Other ingredients, like carbon, sulphur, alumina, etc., are also found to enter, in different proportions, into the composition of aërolites; the total number all chemical elements observed in them up to this present date the nineteen or twenty. It has been well remarked by an able writer, that no new substance has hitherto come to us from without; and thus we find that all these nineteen or twenty elements are precisely similar to those which are distributed throughout the rocks and minerals of our earth; the essential difference between the two classes of compounds—celestial and terrestrial—being seen most clearly in the respective methods in which the component parts are admixed.
In the outward appearance aërolites there is one characteristic so constant that, out of the many hundred examples that have been recorded, one only (as far as we can ascertain) has [{541}] been wanting in it. We refer to the black fused crust or rind with which the surface of meteoric stones is covered. It usually extends not more than a few tenths of an inch into the substance of the stone, and is supposed to result from the extreme rapidity with which they descend into the oxygen of our atmosphere, causing them to undergo a slight and partial combustion, which, however, from the short time necessarily occupied in their descent, has not sufficient time to penetrate beyond the surface. On cutting and polishing the stones, if the smooth face is treated with nitric acid, it will in many cases be found to exhibit lines and angular markings, commonly known by the name of "widmannsted figures." These are tracings of imperfect crystals, while the broad intermediate spaces, preserving their polish, point out those portions of the stone which contain a larger proportion of nickel than the rest of the mass. We may here add that the noise said at times to accompany the fall of aërolites, does not appear to be a constant characteristic, nor does the cause or exact nature of it seem able to be definitely specified.
In conclusion, we cannot do better than advise those of our readers who desire further information on this subject to take the earliest opportunity—if they have not done so already—of paying a visit to the magnificent collection of meteoric stones, contained in several glass cases at the end of the mineral gallery at the British Museum. The catalogue for the year 1856 gave a list of between 70 and 80 specimens; in 1863 this number had increased to 216, mainly through the energy of the curator, Mr. Maskelyne; and since that date there have been several further additions. Chief among continental museums may be mentioned the Imperial collection at Vienna, as possessing a series of specimens remarkable alike for their size and importance.