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.