the magnetic permeability of the Sacramento meteoric iron with changing
temperature, Mr. S. W. J. Smith[20] has been led to infer that the
magnetic behaviour can only be explained by imagining the meteorite
to consist largely of plates of nickel-iron, containing about 7 per
cent. of nickel (kamacite), separated from each other by thin plates of
a nickel-iron constituent (tænite), containing about 27 per cent. of
nickel and having different thermo-magnetic characters from those of
kamacite; he suggests, however, that tænite is not a definite chemical
compound, but is itself a eutectic (or eutropic) mixture, and consists
of kamacite and a nickel-iron compound containing not less than 37 per
cent. of nickel. And he points out that, while the tænite mechanically
isolated from meteorites for analysis has approximately the lower
percentage (27 per cent.), the tænite chemically isolated through the
prolonged action of dilute acid (which would remove much of the admixed
kamacite) has a higher percentage, which in several cases approximates
to 40 per cent.
[Sidenote: Few siderites have been seen to fall.]
43. The Siderites _actually observed to fall_, or found soon after a
luminous meteor had been seen, or a detonation heard, by people in
the neighbourhood, reach only the small number of nine; they are,
Agram, Charlotte, Braunau, Victoria West, Nedagolla, Rowton, Mazapil,
Cabin Creek, and N'Goureyma. The remaining specimens in collections
of Siderites are presumed to be of meteoric origin by reason of the
peculiarity of their appearance and chemical composition, and of the
characters of the material in which they have been found (Art. 7).
[Sidenote: Siderites of large size.]
The large Cranbourne meteorite, mounted in a special case in the
Pavilion, before rusting weighed 3-1/2 tons. The two largest known
were found in Western Greenland and Mexico, respectively, and are both
of very irregular shape. The Greenland mass is 11 feet long, 7-1/2
feet wide, and 6 feet thick, and its weight, which had been variously
estimated at from 50 to 100 tons, has been determined to be 36-1/2
tons; the mass had long been known to the Eskimos, and was inquired
after by Captain John Ross in 1818; it was shown by a native to
Lieutenant Peary in 1894, who afterwards transported it from Melville
Bay to New York; it is now preserved in the American Museum of Natural
History in that city. The Mexican mass is 13 feet long, 6 feet wide,
and 5 feet thick, and has an estimated weight of 50 tons; it is the
property of the Mexican Government, and is still lying at El Ranchito,
near Bacubirito, Province of Sinaloa.
[Sidenote: The iron found at Ovifak is probably of terrestrial origin.]
44. The difficulty of distinguishing an iron of terrestrial from one of
meteoric origin was rendered very evident by the prolonged controversy
as to the origin of the large masses of iron, containing one or two
per cent. of nickel, and weighing 9,000, 20,000, and 50,000 lbs.,
respectively, found in 1870 by Baron N. A. E. Nordenskiöld on the beach
at Ovifak, Disko Island, Western Greenland.
A careful examination of the rocks of the neighbourhood shows that
the basalt contains nickeliferous iron disseminated through it, and
that the large masses of iron, [Sidenote: Pane 4m.] at first thought
to be meteorites, are very probably of terrestrial origin, and have
been left exposed upon the seashore through the weathering of the
rock which originally enclosed them. Some of the malleable metallic
nodules extracted from the basalt were found to contain as much as
6·5 per cent. of nickel. In 1880 Professor K. J. V. Steenstrup[21]
found ferriferous basalt _in situ_ in three different parts of the
island. At Assuk (Asuk) the enclosed balls of iron reach a diameter of
nearly three-quarters of an inch. Some assert that the basalt and the
nickel-iron have been expelled together from great depths below the
earth's surface, while others consider that the nickel-iron is due to
the reduction of the iron-compounds in the basalt by the passage of the
lava through the beds of lignite and other vegetable matter found in
the vicinity.
[Sidenote: Other terrestrial irons.]
[Sidenote: Pane 4m.]
45. With the Ovifak iron in the case are shown other specimens of iron
which have been brought by various explorers from West Greenland, and
were formerly thought to have had a meteoric origin. The discovery of
ferriferous basalt, not only _in situ_ in several places, but also
deposited in a Greenlander's grave (1879) along with knives (similar
to those given to Captain John Ross in 1818) and the usual stone
tools, renders it clear that the Eskimos were not dependent solely on
meteorites for their metallic iron, as had long been supposed.
Mr. Skey announced in 1885 the discovery of terrestrial nickel-iron
in New Zealand. Grains of the alloy (Awaruite), containing as much as
67·6 per cent. of nickel, are found in the sand of the rivers flowing
from a range of mountains composed of olivine-enstatite rocks, in
places altered to serpentine: similar particles have been found in the
serpentine itself. Similarly, in the sand of the stream Elvo, near
Biella, in Piedmont, and of the river Fraser, British Columbia, grains
of nickel-iron containing 75 or 76 per cent. of nickel have been found:
and in the placer gravel of a stream in Josephine and Jackson Counties,
Oregon, U.S.A., large quantities of waterworn pebbles, which enclose
an alloy (Josephinite) of nickel and iron containing 72 per cent. of
the former metal, have been met with. Professor Andrews many years ago
established the presence of minute particles of metallic iron in some
basalts; Dr. Sauer has lately found a single nodule of malleable iron
of the size of a walnut in the basalt of Ascherhübel, in Saxony; Dr.
Hornstein has described large nodules of (nickel-free) iron found in
basalt in a quarry at Weimar, near Cassel; Dr. Beckenkamp has described
nodules of metallic iron found in clay at Dettelbach, near Würzburg;
and Dr. Johnston-Lavis has announced the find of an enclosure of
metallic iron in a leucitic lava of Monte Somma; Dr. Hoffmann has noted
the occurrence of minute spherules of brittle iron both in perthite
and quartzite in Ontario; Dr. Hussak has recorded the discovery of
metallic iron in an alluvium of Brazil, and Dr. Högbom has found it
associated with topaz, quartz, felspar, and other minerals, in limonite
from an unspecified place in South America; two minute grains of iron
were found by Mr. Osaka in the débris of an agglomerate at Nishinotake,
Japan.
[Sidenote: The stony matter of meteorites.]
46. The stony part of the siderolites and aerolites is almost entirely
crystalline, and in most cases presents a peculiar "chondritic" or
granular structure, the loosely coherent grains being composed of
minerals similar to those which enclose them, and containing in most
cases minute particles of iron and troilite disseminated through them:
glass-inclusions are found to be present. The minerals mentioned above
as occurring in meteorites are such as are very characteristic of the
more basic terrestrial rocks, such as dunite, lherzolite and basalt,
which have been expelled from considerable depths below the earth's
surface.
47. Several attempts to classify aerolites according to their
mineralogical constitution have been made, but it cannot be said that
any of them is very satisfactory; seeing that even in the same stone
there may be much difference in its parts, a perfect classification on
such a basis is scarcely to be hoped for.
[Sidenote: Chondritic aerolites.]
About eleven out of every twelve of the stony meteorites belong to a
division to which Rose[22] gave the name of _chondritic_ (_chondros_,
a grain): they present a very fine-grained but crystalline matrix or
paste, consisting of olivine and enstatite or bronzite, with more or
less nickel-iron, troilite, chromite, augite and anorthic felspar;
through this paste are disseminated round chondrules of various sizes
(up to that of a walnut) and with the same mineral composition as the
matrix; in some cases the chondrules consist wholly or in great part
of glass.[23] In mineral composition chondritic aerolites approximate
more or less to terrestrial lherzolites. Some meteorites consist
almost solely of chondrules, others contain only few; in some cases
the chondrules are easy separable from the surrounding material. Of
the chondritic division Knyahinya, Pegu, Muddoor, [Sidenote: Pane 4n.]
Seres, Judesegeri, Khiragurh, Utrecht and Nellore (pane 4p) afford good
illustrations.
[Sidenote: A carbonaceous group.]
A few meteorites belonging to this division are remarkable as
containing carbon in combination with hydrogen and oxygen. Of these
the Alais and Cold Bokkeveld meteorites [Sidenote: Pane 4n.] are good
examples: the former has a bituminous smell; it yields sulphates of
magnesium, calcium, sodium and potassium, if steeped in water.
[Sidenote: Aerolites without chondrules.]
[Sidenote: Pane 4o.]
48. The remaining aerolites are not chondritic, and they contain little
or no nickel-iron; of these we may specially mention for their mineral
composition the following:--
_Juvinas_ and _Stannern_, consisting essentially of anorthite and
augite.
_Petersburg_, consisting of anorthite, augite and olivine, with a
little chromite and nickel-iron: both Juvinas and Petersburg may be
compared to terrestrial basalt.
_Sherghotty_, consisting chiefly of augite and maskelynite.
_Angra dos Reis_, consisting almost wholly of augite; olivine is
present in small proportion.
_Bustee_, of diopside, enstatite and a little anorthic felspar, with
some nickel-iron, oldhamite and osbornite.
_Bishopville_, of enstatite and anorthic felspar, with occasional
augite, nickel-iron, troilite and chromite.
_Roda_, of olivine and bronzite.
_Chassigny_, consisting of olivine with enclosed chromite, and thus
mineralogically similar to a terrestrial dunite.
[Sidenote: Is there a periodic recurrence?]
49. The importance of the examination and classification of
meteorites, with a view to a possible recognition of _periodicity_
of fall of specimens presenting the same characters, need only be
mentioned to be appreciated: such a determination is, however, rendered
very difficult by the close similarity of structure and composition
presented by the great majority of the aerolites of the large
chondritic division.
[Sidenote: Few aerolites are known which have not been seen to fall.]
50. Attention has been already directed to the fact that although
many masses of meteoric iron, some of them like that of El Ranchito,
near Bacubirito, in Mexico, weighing very many tons, have been found
at various parts of the earth's surface, very few of them have been
actually observed to fall: in the case of the stony meteorites just
the opposite holds good, for they are never very large, and few are
known which have not an authenticated date of fall. This may be due to
the fact that a meteoric stone is less easily distinguished than is a
meteoric iron from ordinary terrestrial bodies, and will thus in most
cases remain unnoticed unless its fall has been actually observed;
while, further, a quick decomposition and disintegration must set in on
exposure to atmospheric influences. The smaller size of the meteoric
stones may be due to the greater ease with which they break up on the
sudden increase of temperature of their outer surface, consequent
on their entry into the earth's atmosphere. The largest meteoric
stone preserved in a Museum is one which fell as part of a shower at
Knyahinya, Hungary, in 1866: it weighs 647 lbs. and is at Vienna. A
larger stone (723 lbs.) fell at Tabory, Russia, in 1887, but was broken
to pieces by the impact on the earth; fragments of a still larger
single stone, weighing at least 1244 lbs., were found near together at
Long Island, Kansas, U.S.A., but the fall was not observed.
[Sidenote: The chondrules and their matrix.]
51. If we now examine minutely the structure of the meteoric stones,
it will be seen that almost all of them appear to be made up chiefly
of irregular angular fragments, and that some of them bear a close
resemblance to volcanic tuffs. In the large group of chondritic
aerolites, chondrules or spherules, some of which can only be seen
under the microscope while others reach the size of a walnut, are
embedded in a matrix, apparently made up of minute splinters such as
might result from the fracture of the chondrules themselves. In fact,
until recently, it was thought by some[24] that the chondrules owe
their form, not to crystallisation, but to friction, and that the
matrix was actually produced by the wearing down of the chondrules
through collision with each other either as oscillating components of
a comet or during repeated ejection from a volcanic vent of some small
celestial body. Chondrules have been observed, however, presenting
forms and crystalline surfaces incompatible with such a mode of
formation, and others have been described which exhibit features
resulting from mutual interference during their growth.
The crystallisation of the chondrules is independent of their form,
and must have started, not at the centre, but at various places
on their surfaces; Dr. Sorby[25] argued that some at least of the
chondrules must once have fallen as drops of fiery rain, and have
assumed their shape in an atmosphere heated to nearly their own
temperature. The chondritic structure is different from anything
which has been observed in terrestrial rocks, and the chondrules are
distinct in character from those observed in perlite and obsidian.
After much study, Dr. Brezina[26] lends his weighty support to the
hypothesis that the structural features of meteorites are the result
of a hurried crystallisation: and Prof. Wadsworth[27] accepts the same
interpretation.
[Sidenote: Some meteoric materials appear to have been altered since
their consolidation.]
[Sidenote: Pane 4o.]
52. Since the time of their consolidation some meteoric stones, as
Tadjera, appear to have been heated throughout their mass to a high
temperature: and in the case of Orvinio, Chantonnay, Juvinas, and
Weston, fragments are cemented together with a material having the
same composition as the fragments themselves, thus giving rise to
a structure resembling that of a volcanic breccia. Others seem to
have experienced a chemical change, for some of the chondrules in
Knyahinya and in Mezö-Madaras, when examined with the microscope, are
found to be surrounded by spherical and concentric aggregations of
minute particles of nickel-iron, perhaps due to the reducing action
of hydrogen at a high temperature. Others, as Château-Renard, Pultusk
and Alessandria, present what in terrestrial rocks would probably be
called faults: in some cases the fissures are seen to have been filled
with a fused material after the chondrules have been broken and one
side of the fissure has glided along the other. These peculiarities of
structure suggest that the small body which reaches the earth is only
a minute fragment of a much larger mass. It has been suggested that
the chondritic structure is of metamorphic origin, and a mere result
of enormous pressure on the stony material during the passage through
the earth's atmosphere; according to still another view, the structure,
though metamorphic, is of extra-terrestrial origin, and due to the
quick cooling of a tuff-like stone which has been partially melted, for
instance, by the heat from a neighbouring new star or by traversing the
hot vapours on the limits of an old one.
[Sidenote: Do meteorites reach our atmosphere as clouds of gas or dust?]
53. The idea that meteorites arrive at our own atmosphere, not as
fragments of rock, but as mere clouds of gas or dust, has been recently
revived and again discarded. According to this hypothesis, the air,
instead of dispersing the entering cloud, acts in the contrary way,
and in a few seconds of time presses the particles together to form
solid bodies. This idea is open to various objections, and in any case
one can scarcely understand how large masses of iron, presenting a
wonderful regularity of crystalline structure, can have been the result
of so hurried a process: and if we once grant that the irons enter the
atmosphere as solid bodies, it is difficult to believe that the same is
not the case with the stones.
* * * * *
[Sidenote: Where do meteorites come from?]
54. From the above it will be evident that the old hypotheses that
meteorites are terrestrial stones which have been struck by lightning,
or carried to the sky by a whirlwind, or are concretions in the
atmosphere, or are due to the condensation of a dust-cloud coming from
some volcano, or have been shot recently from terrestrial volcanoes,
are inconsistent with later observation; it may be granted that the
bodies reach our atmosphere from outer space. From what part or parts
of space do they come? Their general similarity of structure and
chemical composition, and more especially the presence of nickeliferous
iron in almost every one, suggest that most, if not all of them, have
had a common source, and that they are chips of a single celestial body.
[Sidenote: Probably not from the sun, nor from the moon, earth, or
other planet.]
55. Dr. Sorby suggested that they are probably ejected from the sun
itself, though this is difficult to reconcile with the fact that some
of them are easily combustible. Others, among whom we may mention
Laplace, have suggested that they come from volcanoes of the moon which
are now active; but the suggestion, although mathematically sound, has
no physical basis, for, so far as one can discover, active volcanoes
do not there exist: and Sir Robert Ball[28] has virtually excluded the
lunar volcanoes, which were active in times now long past, by pointing
out that if a projectile from the moon once misses the earth, its
chance of ever reaching the earth is too small to be worthy of mention.
It has further been shown that, although the explosive force necessary
to carry a projectile so far from one of the smaller planets that it
will not return, is not very large, yet the initial velocity requisite
to carry the body as far as the earth's orbit is so considerable,
and the chance of hitting the earth so slight, that a more probable
hypothesis is, to say the least, desirable. If these bodies have been
shot from volcanoes of any planet, Sir Robert Ball is himself inclined,
upon mechanical grounds alone, to believe that the projection was from
our own in bygone ages; for as such projectiles, having once got away
from the earth, would take up paths round the sun which would intersect
the earth's orbit, every one of them would have a chance of some time
or other meeting with the earth again at the point of intersection, and
of appearing as a meteorite. The size and initial velocity requisite
for the escape of a projectile through a lofty atmosphere would be
enormous: even then the difficulty would still remain that meteorites
generally differ, both in structure and material, from anything known
to have been ejected from existing terrestrial volcanoes. To meet
these difficulties, Sir Robert has speculatively suggested that the
matter was expelled before the surface of the earth became solid, and
at a time when there was as much activity in the terrestrial planet as
there is now in the material of the sun itself.
Nor is it probable that they are portions of a lost satellite of the
earth, or are due to a collision of two planets; for in each of these
cases we should expect to have received some of the larger fragments
which must at the same time have been produced.
Much light is thrown on the history of meteorites by the discovery of a
relationship with shooting stars and comets.
[Sidenote: Shooting or falling stars.]
56. The meteorite-yielding fireball, referred to in Art. 17, is not
the only luminous meteor, apart from lightning, with which we are
acquainted. On a clear dark night any one can see a star shoot now
and then across the firmament: it is estimated that on the average as
many as fourteen are visible to a single observer every hour. Are the
_shooting_, or, as they are often called, _falling stars_ products of
our own atmosphere, or do they, like the meteorites, come from outer
space? In 1794 Chladni, in the memoir already referred to, gave reasons
for believing that a meteoritic fireball and a shooting star are only
varieties of one phenomenon.
[Sidenote: The November star-showers.]
57. But long after the cosmic origin of meteorites had been generally
acknowledged, the atmospheric origin of the shooting stars was still
asserted, and it was not till the wondrous star-shower of November
12-13, 1833,[29] that the cosmic origin of any of the shooting stars
was finally established. During that night upwards of 200,000 shooting
stars, according to a rough estimate, were seen from a single place;
and the remarkable observation was made at various localities, widely
distributed over North America, that the apparent paths of the shooting
stars in the sky, when prolonged backwards, all passed through a point
in the constellation Leo: this point of radiation appeared to rotate
with the heavens during the eight hours for which the shower was
visible.
Hence it was manifest that the star-shower was independent of the
earth's rotation and must therefore have come from outer space; that
the radiation of the paths was only apparent and due to perspective;
and that, relatively to an observer, the flights of all the shooting
stars were really parallel to the direction of the apparent radiant
point.
On the same day of November in each of the three following years the
shower was repeated though on a less grand scale, and the constancy of
the radiant point was confirmed: similar small showers had been seen
also in 1831 and 1832 before the radiation had been noticed. Though in
the years immediately before and after 1831-6 no remarkable display
of November meteors took place, it was remembered that a similar
shower had been chronicled by Humboldt and by Ellicott, as observed by
them on November 12, 1799; and a study of ancient documents revealed
the fact that a grand star-shower had been recorded several times
in October and November since A.D. 902, the date having gradually
advanced, during that long space of time, from the middle of October
to the middle of November.[30] The only sufficient explanation of the
observed facts is that a swarm of isolated small bodies, solid and
non-luminous--meteorites in fact--is moving in an orbit round the sun,
completing the circuit in 33-1/4 years; the orbit intersects that of
the earth, and the earth meets the swarm at the place of intersection.
The isolated bodies or meteorites become luminous, as already explained
in Art. 17, after their entry into the earth's atmosphere. The swarm
can be only a few hundred thousand miles thick, for the earth,
travelling through space at the rate of 66,000 miles an hour, passes
through the densest part in 2 or 3 hours, and through the whole in
10 to 15 hours: its length, however, must be enormous, amounting to
hundreds of millions of miles; for, although the meteorites move with
a velocity of twenty miles a second, the swarm takes 5 or 6 years to
pass the place of intersection with the earth's orbit, thus causing
star-showers, more or less dense, during that number of years.
Contrary to expectation, no large November star-shower occurred either
in the year 1899 or in the years which have since elapsed.
Schiaparelli has shown that the unequal attraction of the sun for the
individuals of a swarm of meteorites moving round it would scatter
them along the orbit, and in the course of time produce a more or
less complete ring; if this intersects the earth's orbit an annual
star-shower must ensue.
[Sidenote: The August star-shower and its comet.]