will combine to form a single detonation unless they are separated by
perceptible intervals of time.
[Sidenote: The sounds heard after the loud detonations.]
24. After the detonation, sounds are generally heard which have been
variously likened to the flapping of the wings of wild geese, the
bellowing of oxen, Turkish music, the roaring of a fire in a chimney,
the noise of a carriage on the pavement, and the tearing of calico:
these sounds are probably due to the whirling and oscillation of the
fragments while traversing the air, with small velocity, near the
observers, and correspond to the hiss or hum observed in the case of a
projectile travelling with a velocity less than that of sound.
* * * * *
[Sidenote: The chemical elements found in meteorites.]
25. As to the _kinds of elementary matter_[13] of which meteorites are
composed, about one-third, and those the most common, of the elements
at present recognised as constituents of the earth's crust have been
met with: no new elementary body has been discovered.
The most frequent or plentiful in their occurrence are:--
Aluminium
Calcium
Carbon
Iron
Magnesium
Nickel
Oxygen
Phosphorus
Silicon
Sulphur:
while, less frequently or in smaller quantities, are found:
Antimony
Arsenic
Chlorine
Chromium
Cobalt
Copper
Hydrogen
Lithium
Manganese
Nitrogen
Potassium
Sodium
Strontium
Tin
Titanium
Vanadium.
[Sidenote: Elements present only in minute quantity.]
26. In addition to the above, the existence of minute traces of several
other elements has been announced; of these special mention may be made
of gallium, gold, iridium, lead, platinum and silver.
[Sidenote: Both simple and combined.]
27. Most of the above elements are present in the combined state;
the iron occurring chiefly in combination with nickel, and the
phosphorus almost always combined with both nickel and iron. Some of
them are found also in their elementary condition: perhaps hydrogen
and nitrogen; carbon, both as indistinctly crystallised diamond
and as graphitic carbon, the latter being generally amorphous, but
occasionally in cubic crystals (cliftonite); free phosphorus has been
found in Saline Township; free sulphur has been observed in one of the
carbonaceous meteorites, but may have been separated from the unstable
sulphides since the entry into our atmosphere.
[Sidenote: Some of the constituents are new to mineralogy.]
[Sidenote: Pane 4k.]
28. Of the constituents of meteorites, the following are by many
mineralogists regarded as being at present unrepresented among the
terrestrial minerals:--
_Cliftonite_, a cubic form of graphitic carbon,
_Phosphorus_,
_Various alloys of nickel and iron_,
_Moissanite_, silicide of carbon,
_Cohenite_, carbide of iron and nickel; corresponding
to Cementite, carbide of iron, found in artificial iron,
_Schreibersite_, phosphide of iron and nickel,
_Troilite_, proto-sulphide of iron,
_Oldhamite_, sulphide of calcium,
_Osbornite_, oxy-sulphide of calcium and titanium or zirconium,
_Daubréelite_, sulphide of iron and chromium,
_Lawrencite_, protochloride of iron,
_Asmanite_, a species of silica,
_Maskelynite_, a singly refracting mineral with the composition
of labradorite.
_Weinbergerite_, silicate intermediate in
chemical composition between pyroxene and nepheline.
[Sidenote: Nature of troilite, asmanite and maskelynite.]
Of the above, _Troilite_ is perhaps identical with some varieties of
terrestrial pyrrhotite: _Asmanite_, the form of silica obtained in 1867
by Prof. Maskelyne from the Breitenbach meteorite, was announced by him
in 1869 to be optically biaxal, and thus to belong to a crystalline
system different from the hexagonal to which both tridymite, then
just announced by Vom Rath, and quartz had been assigned. Later
investigations of tridymite have shown that its optical characters
and crystalline form are inconsistent with the hexagonal system of
crystallisation, and it is not impossible that asmanite and tridymite
may be specifically identical. It has been found that tridymite becomes
optically uniaxal at a moderate temperature, and its general characters
appear to be essentially identical with those of asmanite. According
to one view, _Maskelynite_ is the result of fusion of a plagioclastic
felspar; according to another, it is an independent species chemically
related to leucite.
[Sidenote: Compounds identical with terrestrial minerals.]
[Sidenote: Pane 4k.]
29. Other compounds are present, corresponding to the following
terrestrial minerals:--
Olivine and forsterite,
Enstatite and bronzite,
Diopside and augite,
Anorthite, labradorite and oligoclase,
Leucite,
Magnetite and chromite,
Pyrites,
Pyrrhotite,
Breunnerite.
Further, from one of the Lancé stones, chloride of sodium, and from the
carbonaceous meteorites, sulphates of sodium, calcium and magnesium,
have been extracted by means of water.
In addition to the above, there are several compounds or mixtures of
which the nature has not yet been satisfactorily ascertained.
[Sidenote: The rarity of quartz.]
30. Quartz, the most common of terrestrial minerals, is absent from the
stony meteorites; but in the undissolved residue of the Toluca iron
microscopic crystals have been found, some of which have important
characters identical with those of quartz, while others resemble
zircon. As mentioned above, free silica is present in the Breitenbach
meteorite as asmanite.
[Sidenote: The conditions under which these compounds can have been
formed.]
31. As to the _conditions_[14] under which such compounds can have been
formed, we may assert that they must have been very different from
those which at present obtain near the earth's surface: in fact, it is
impossible to imagine that phosphorus, the metallic nickel-iron and
the unstable sulphides can either have been formed, or have remained
unaltered, under circumstances in which water and atmospheric air have
played any prominent part. Still, what little we do know of the inner
part of our globe does not shut out the possibility of the existence
of similar elementary and compound bodies at great depths below the
surface. Daubrée,[15] after experiment, inclines to the belief that the
iron is due, in many cases at least, to reduction from an olivine rich
in diferrous silicates, and this view perhaps acquires some additional
probability from the fact that hydrogen and carbonic oxide are given
off when meteoric iron is heated: the existence, however, of such
siderolites as that of Krasnojarsk, which is rich both in metallic iron
and in orthosilicate of iron and magnesium (olivine), and yet presents
no traces of the intermediate metasilicate of iron and magnesium
(bronzite), offers a weighty objection to the general application of
this view.
[Sidenote: Classification.]
32. Meteorites may be conveniently arranged in three classes, which
pass more or less gradually into each other: the first includes all
those which consist mainly of iron, and have, therefore, been called
by Prof. Maskelyne aero-siderites (_aer_, air, and _sideros_, iron),
or, more shortly, _Siderites;_ the second is formed by those which are
composed chiefly of iron and stone, both in large proportion, and are
called aero-siderolites, or, shortly, _Siderolites;_ while those of
the third class, being almost wholly of stone, are called _Aerolites_
(_aer_, air, and _lithos_, stone).
[Sidenote: The siderites.]
33. In the Siderites the iron generally varies from 80 to 95 per
cent., and the nickel from 6 to 10 per cent.; in the Santa Catharina
siderite (of which the meteoric origin is somewhat doubtful) 34, and
in that of Oktibbeha County 60, per cent. of nickel have been found:
the nickel is alloyed with the iron, and several of the alloys have
been distinguished by special names. Owing to the presence of the
nickel, meteoric iron is often so white on a fractured surface as to
be mistaken for silver by its finder; it is also less liable to rust
than ordinary iron is. Troilite is frequently present as plates, veins
or large nodules, sometimes surrounded by graphite; schreibersite is
almost always found, and occasionally also daubréelite.
[Sidenote: Evolution of gases on heating.]
Further, various chemists have proved that hydrogen, nitrogen, marsh
gas, and the carbonic oxides are evolved when meteoric iron or stone
is heated; in one case a trace of helium was detected. Probably the
gases were not present in the occluded state, but resulted from the
decomposition or interaction of non-gaseous constituents during the
experiments.
[Sidenote: Figures produced by action of acids or bromine.]
[Sidenote: Pane 4l.]
34. The want of homogeneity and the structure of meteoric iron are
beautifully shown by the figures generally called into existence when
a polished surface is exposed to the action of acids or bromine; they
are due to the inequality of the action on thick or thin plates of
various constituents, [Sidenote: Etched figures.] the plates being
composed chiefly of two nickel-iron materials termed kamacite and
tænite. A third nickel-iron material, filling up the spaces formed
by the intersection of these plates of kamacite and tænite, is
termed plessite; it is probably not an independent substance but an
intergrowth of the first two kinds.
In the Agram iron, investigated by Widmanstätten in 1808, the plates
are parallel to the faces of the regular octahedron; such figures
are well shown by the exhibited slice of the Toluca iron; different
degrees of distinctness of such "Widmanstätten" figures are illustrated
by specimens of Seneca River, Zacatecas, Charcas, Burlington, Jewell
Hill, [Sidenote: Pane 4l.] Lagrange, Victoria West, Nelson County, and
Seeläsgen. The large Otumpa specimen, mounted on a separate pedestal,
furnishes a good example of the less distinct, and more or less
damascene, appearance presented by the etched surface of some meteoric
irons of octahedral structure.
The Braunau iron gives no "Widmanstätten" figures, but has cleavages
parallel to the faces of a cube; on etching it yields linear furrows
which were found (1848) by Neumann to have directions such as would
result from twinning of the cube about an octahedral face; as
illustrations of the "Neumann lines," etched specimens of Braunau and
Salt River are exhibited. [Sidenote: Pane 4l.]
For meteoric irons of cubic structure the percentage of nickel is lower
than 6 or 7; for those of octahedral structure it is higher than 6 or
7, and the plates of kamacite are thinner, and the structure therefore
finer, the higher the percentage of that metal. A considerable
number of meteoric irons, however, show no crystalline structure at
all, and have percentages of nickel both below and above 7; it has
been suggested that these masses have been metamorphosed, and that
crystalline structure was once present, but has disappeared as a result
of the meteorites having been heated, not merely superficially during
their passage through the earth's atmosphere, but throughout their mass
while travelling in outer space.
[Sidenote: Cooling of fused mixtures and of solutions.]
35. Though meteoric iron has been at some time, presumably, in a state
of fusion, and its present structure is a result of the particular
circumstances of the cooling of the liquid and afterwards solid
material, attempts to produce such structures by the cooling of fused
meteoric iron or artificial mixtures of nickel and iron have not yet
been successful. It will be useful, therefore, to consider briefly
some of the manifold changes which are found to take place during the
passage of fused mixtures and of solutions to the solid state, and
during the cooling of such solids to ordinary temperatures.
If a fused mixture of antimony and bismuth is allowed to cool,
the solid which first separates is neither pure antimony nor pure
bismuth, but a material which has a percentage composition depending
on, though not identical with, that of the original mixture. The
temperature for the beginning of the solidification is different for
different proportions of the two metals, and is intermediate between
622° and 268°, the solidifying temperatures of antimony and bismuth,
respectively; it approaches the latter more and more closely as the
percentage of the bismuth is increased. The solid first separated
is somewhat richer in antimony than the original mixture; the still
fused part, therefore, is somewhat richer in bismuth than before,
and does not begin to solidify till a lower temperature is reached;
the temperature thus gradually falls, instead of remaining constant,
during the solidification. In the cooling of such fused mixtures the
changing composition of the part still fused has for effect a changing
composition of the solid already separated; whence the slower the
cooling of the fused material, the greater is the homogeneity of the
final solid.
[Sidenote: Eutectic mixtures.]
A fused mixture of silver and copper behaves in a different way. When
the percentage weight of the silver is 72, and that of the copper,
therefore, is 28, solidification begins, not at a temperature between
960° and 1083°, the solidifying temperatures of silver and copper,
respectively, but at a temperature below both, namely, 770°. The
solid which first separates has the same percentage composition as
the original mixture; the part still fused has thus itself the same
percentage composition as before, and continues to solidify at the
same temperature, and in the same way, until the solidification is
complete. Such a mixture, having a definite composition and a definite
temperature of solidification, was for a time regarded as a definite
chemical compound with a complex chemical formula, but on microscopic
examination the resultant solid is found to be heterogeneous; minute
particles of the silver and copper are seen to lie side by side,
the particles being granular or lamellar in form according to the
circumstances of the cooling. If the percentage of silver is different
from 72, whether it be higher or lower, the solidification begins at
a higher temperature than 770°; whence the mixture containing 72 per
cent. of silver has been conveniently termed _eutectic_ (i.e. very
fusible); the term was suggested by Prof. F. Guthrie,[16] to whom our
knowledge of the existence of such mixtures is due.
[Sidenote: Cooling of fused mixtures and of solutions.]
36. When the silver is in excess of 72 per cent., the excess of silver
gradually collects together and solidifies at various parts of the
cooling fused mass; the still fused portion thus gradually becomes
poorer in that metal, and the temperature, instead of remaining
constant, gradually falls during the separation of the solid. At length
the percentage of silver in the fused portion falls to 72 per cent.
and the temperature to 770°; the solid which now begins to form is no
longer pure silver, but a material containing 72 per cent. of that
metal; and it continues to have the same percentage composition as the
surrounding liquid, and the temperature of solid and liquid to be 770°,
until the solidification is complete. The final solid thus consists of
blebs of silver scattered through a fine groundmass of eutectic mixture
of silver and copper. Similarly, if the copper is in excess of 28 per
cent., the final solid consists of blebs of copper scattered through a
fine groundmass of eutectic mixture of silver and copper.
If the two metals are copper and antimony, instead of copper and
silver, the results are more complicated; for the first two metals are
capable of combining together to form a definite chemical compound
represented by the formula Cu{2}Sb, and each of the metals forms
a eutectic mixture with the latter. According to the percentage
composition of the original mixture, the solid which first separates
during cooling from fusion may be either copper or antimony or the
compound Cu{2}Sb; the separation continuing, and the temperature
falling, until the first eutectic proportion and its corresponding
temperature are reached.
[Sidenote: Cooling of solutions.]
37. Analogous results are obtained during the cooling of solutions;
for instance, during the cooling of a solution of sodium chloride
(common salt) in water. A solution containing 23·5 per cent. of sodium
chloride begins to solidify at -22° C.; the separating solid is not
simple sodium chloride or simple ice, but has the same percentage
composition as the original solution, and thus the temperature remains
-22° until the whole material has become solid. On microscopic
examination the solid is seen to be heterogeneous, and to consist of
small particles of sodium chloride and ice lying side by side. If the
percentage of sodium chloride is different from 23·5, whether higher
or lower, solidification begins before the temperature has fallen to
-22°. The characters of this particular solution are thus closely
analogous to those of the eutectic mixtures described above. If the
sodium chloride exceeds 23·5 per cent., the excess of sodium chloride
begins to separate, and solidify, at various parts of the liquid, at
a temperature higher than -22°; it continues to separate, and the
temperature to fall, until the proportion of sodium chloride in the
residual liquid is reduced to 23·5 per cent. and the temperature to
-22°. Afterwards the separating solid has the same composition as
the residual liquid (23·5 per cent. of sodium chloride), and the
temperature remains constant, until the residual liquid has been wholly
transformed into a solid fine-grained mixture of sodium chloride
and ice. The final solid thus consists of large particles of sodium
chloride dispersed through a fine groundmass consisting of eutectic
mixture of sodium chloride and ice. Similarly, if the water is in
excess of 76·5 per cent., the final solid consists of large particles
of ice dispersed through a fine groundmass consisting of eutectic
mixture of sodium chloride and ice.
The results of the cooling of a solution of ferric chloride are still
more complicated; for this substance enters into chemical combination
with water, and in no fewer than four different proportions. The solid
which first separates from the cooling solution may thus, according to
the percentage of ferric chloride, be either ferric chloride or water,
or any one of the various compounds of the two; and to each pair of
compounds nearest to each other in composition corresponds a different
eutectic mixture and a different temperature for its formation.
[Sidenote: Cooling of solids.]
38. Some solid bodies, during cooling, show changes analogous to those
observed in solutions, and are therefore termed "solid solutions."
For instance, if a hot physically homogeneous solid obtained from the
fusion of iron with carbon is cooled, there may result a separation in
the solid of particles of either iron or cementite, the latter being
a chemical compound of iron and carbon represented by the formula
Fe{3}C; the particular substance separated depending on the percentage
composition of the original solid. This separation continues, and the
temperature falls, until the residual physically homogeneous material
contains 0·9 per cent. of carbon and the temperature is 690°; the
temperature then remains constant, although the body is surrounded by
a cooling medium, until this residual physically homogeneous material
has been wholly transformed into a fine-grained mixture of iron and
cementite, containing 0·9 per cent. of carbon. This particular kind of
mixture has been termed eutectic, though the transformation has taken
place, not by solidification from fusion, but in a body which was
already solid. Prof. Rinne has proposed for such cases the substitution
of the term _eutropic_, thus avoiding the suggestion of fusion. The
eutectic mixture of iron (or ferrite) and cementite is known as
pearlite.
[Sidenote: Overcooling.]
39. Just as water may be cooled so quietly that it is still liquid at
a temperature much below the normal freezing point, a mixture may be
cooled in such a way as to pass much below the eutectic (or eutropic)
point without the normal transformation taking place; it is then said
to be overcooled. The equilibrium, however, is very unstable, and
the transformation, once begun, takes place almost instantaneously
throughout the whole mass.
[Sidenote: Crystalline structure of artificial iron.]
40. A structure analogous to that shown by the Widmanstätten figures,
though on a finer scale, has been observed by Prof. J. O. Arnold and
Mr. A. McWilliam[17] in cast steel containing 0·4 per cent. of carbon;
the plates of iron (or ferrite) in the cast steel correspond to the
plates of kamacite in meteorites. Further, it has been found that the
plates in the cast steel disappear during the process of annealing;
similarly, there are no Widmanstätten figures, and the structure of the
material is granular, near the outer surface of an unweathered meteoric
iron; presumably as a result of the high temperature to which the outer
part of the mass has been raised during the passage of the meteorite
through the earth's atmosphere.
[Sidenote: Structure of meteoric irons.]
41. At present it is generally imagined that kamacite and tænite are
definite alloys, or perhaps solid solutions, of iron and nickel, the
former being poor in nickel (6 or 7 per cent.) and the latter rich
in that constituent (25 to 38 per cent.), that kamacite and tænite
separate in succession from the molten mass or solid solution until
the residual part is so rich in nickel that a eutectic (or eutropic)
proportion is reached; the residual material then forms plessite,
which, according to this view, is a eutectic (or eutropic) mixture of
kamacite and tænite. But it is difficult to understand how the thin
plates of tænite are deposited on the plates of kamacite, seeing that
they contain more nickel than kamacite and plessite, and yet have an
intermediate epoch of formation, prior to the epoch of formation of
that tænite which is a constituent of the plessite; one suggestion is
that the thin plates of tænite have been deposited on the plates of
kamacite owing to the temperature having fallen well below the eutectic
(or eutropic) point after the separation of the kamacite and before the
eutectic transformation of the residual material has taken place. And
Prof. Rinne[18] himself is of opinion that the Widmanstätten structure
has been wholly developed in meteoric iron after the solidification
of the mass; further, as the relations of the kamacite, tænite and
plessite to the enclosed troilite indicate that the troilite was solid
before the octahedral structure was developed, and as that mineral,
under normal circumstances, solidifies at about 950°, he infers that
the structure was developed below that temperature. In the case of the
Jewell (Duel) Hill meteorite it was discovered by Dr. Brezina that,
notwithstanding the pronounced octahedral structure, plates of troilite
are embedded, not in accidental positions nor between successive
octahedral layers, but parallel to the faces of the corresponding
cube; whence Prof. Rinne suggests that this iron, now of octahedral
structure, and possibly all others of a similar character, had a cubic
structure at the epoch when they entered upon the solid condition.
But, as both Prof. Rinne and Dr. Brezina[19] have pointed out, a fused
mixture of nickel and iron, cooling undisturbedly in outer space, may
have solidified at a temperature even below 950° and thus have been
much overcooled.
[Sidenote: Tænite possibly a eutectic mixture.]