On January 9, Professor Dewar delivered the sixth and last of his series
of lectures at the Royal Institution on "The Story of a Meteorite." [For
the preceding lectures, see SUPPLEMENTS 529 and 580.] He said that
cosmic dust is found on Arctic snows and upon the bottom of the ocean;
all over the world, in fact, at some time or other, there has been a
large deposit of this meteoric dust, containing little round nodules
found also in meteorites. In Greenland some time ago numbers of what
were supposed to be meteoric stones were found; they contained iron, and
this iron, on being analyzed at Copenhagen, was found to be rich in
nickel. The Esquimaux once made knives from iron containing nickel; and
as any such alloy they must have found and not manufactured, it was
supposed to be of meteoric origin. Some young physicists visited the
basaltic coast in Greenland from which some of the supposed meteoric
stones had been brought, and in the middle of the rock large nodules
were found composed of iron and nickel; it, therefore, became evident
that the earth might produce masses not unlike such as come to us as
meteorites. The lecturer here exhibited a section of the Greenland rock
containing the iron, and nickel alloy, mixed with stony crystals, and
its resemblance to a section of a meteorite was obvious. It was 2½ times
denser than water, yet the whole earth is 5½ times denser than water, so
that if we could go deep enough, it is not improbable that our own globe
might be found to contain something like meteoric iron. He then called
attention to the following tables:
_Elementary Substances found in Meteorites_.
Hydrogen. Chromium. Arsenic.
Lithium. Manganese. Vanadium?
Sodium. Iron. Phosphorus.
Potassium. Nickel. Sulphur.
Magnesium. Cobalt. Oxygen.
Calcium. Copper. Silicon.
Aluminum. Tin. Carbon.
Titanium. Antimony. Chlorine.
_Density of Meteorites_.
Carbonaceous (Orgueil, etc.) 1.9 to 3
Aluminous (Java) 3.0 " 3.2
Peridotes (Chassigny, etc.) 3.5 " --
Ordinary type (Saint Mes) 3.1 " 3.8
Rich in iron (Sierra de Chuco) 6.5 " 7.0
Iron with stone (Krasnoyarsk) 7.1 " 7.8
True irons (Caille) 7.0 " 8.0
_Interior of the Earth_
Parts
of the
radius. Density.
0.0 11.0
0.1 10.3
0.2 9.6
0.3 8.9
0.4 8.3
0.5 7.8
0.6 7.4
0.7 7.1
0.8 6.2
0.9 5.0
1.0 2.6
[Illustration]
Twice a year, said Professor Dewar, what are called "falling stars"
maybe plentifully seen; the times of their appearance are in August and
November. Although thousands upon thousands of such small meteors have
passed through our atmosphere, there is no distinct record of one having
ever fallen to the earth during these annual displays. One was said to
have fallen recently at Naples, but on investigation it turned out to be
a myth. These annual meteors in the upper air are supposed to be only
small ones, and to be dissipated into dust and vapor at the time of
their sudden heating; so numerous are they that 40,000 have been counted
in one evening, and an exceptionally great display comes about once in
33¼ years. The inference from their periodicity is, that they are small
bodies moving round the sun in orbits of their own, and that whenever
the earth crosses their orbits, thereby getting into their path, a
splendid display of meteors results. A second display, a year later,
usually follows the exceptionally great display just mentioned,
consequently the train of meteors is of great length. Some of these
meteors just enter the atmosphere of the earth, then pass out again
forever, with their direction of motion altered by the influence of the
attraction of the earth. He here called attention to the accompanying
diagram of the orbits of meteors.
The lecturer next invited attention to a hollow globe of linen or some
light material; it was about 2 ft. or 2 ft. 6 in. in diameter, and
contained hidden within it the great electro-magnet, weighing 2 cwt., so
often used by Faraday in his experiments. He also exhibited a ball made
partly of thin iron; the globe represented the earth, for the purposes
of the experiment, and the ball a meteorite of somewhat large relative
size. The ball was then discharged at the globe from a little catapult;
sometimes the globe attracted the ball to its surface, and held it
there, sometimes it missed it, but altered its curve of motion through
the air. So was it, said the lecturer, with meteorites when they neared
the earth. Photographs from drawings, by Professor A. Herschel, of the
paths of meteors as seen by night were projected on the screen; they all
seemed to emanate from one radiant point, which, said the lecturer, is a
proof that their motions are parallel to each other; the parallel lines
seem to draw to a point at the greatest distance, for the same reason
that the rails of a straight line of railway seem to come from a distant
central point. The most interesting thing about the path of a company of
meteors is, that a comet is known to move in the same orbit; the comet
heads the procession, the meteors follow, and they are therefore, in all
probability, parts of comets, although everything about these difficult
matters cannot as yet be entirely explained; enough, however, is known
to give foundation for the assumption that meteorites and comets are not
very dissimilar.
The light of a meteorite is not seen until it enters the atmosphere of
the earth, but falling meteorites can be vaporized by electricity, and
the light emitted by their constituents be then examined with the
spectroscope. The light of comets can be directly examined, and it
reveals the presence in those bodies of sodium, carbon, and a few other
well-known substances. He would put a piece of meteorite in the electric
arc to see what light it would give; he had never tried the experiment
before. The lights of the theater were then turned down, and the
discourse was continued in darkness; among the most prominent lines
visible in the spectrum of the meteorite, Professor Dewar specified
magnesium, sodium, and lithium. "Where do meteorites come from?" said
the lecturer. It might be, he continued, that they were portions of
exploded planets, or had been ejected from planets. In this relation, he
should like to explain the modern idea of the possible method of
construction of our own earth. He then set forth the nebular hypothesis
that at some long past time our sun and all his planets existed but as a
volume of gas, which in contracting and cooling formed a hot volume of
rotating liquid, and that as this further contracted and cooled, the
planets, and moons, and planetary rings fell off from it and gradually
solidified, the sun being left as the solitary comparatively uncooled
portion of the original nebula. In partial illustration of this, he
caused a little globe of oil, suspended in an aqueous liquid of nearly
its own specific gravity, to rotate, and as it rotated it was seen, by
means of its magnified image upon the screen, to throw off from its
outer circumference rings and little globes.
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