METEORITES
Meteorites, the name for meteors which have actually gone all the way
through our atmosphere, are never regular in form or spherical. As a
rule the iron meteorites are covered with pittings or thumb marks, due
probably to the resistance and impact of the little columns of air which
impede its progress, together with the unequal condition and fusibility
of their surface material. The work done by the atmosphere in suddenly
checking the meteor's velocity appears in considerable part as heat,
fusing the exterior to incandescence. This thin liquid shell is quickly
brushed off, making oftentimes a luminous train.
But notwithstanding the exceedingly high temperature of the exterior,
enforced upon it for the brief time of transit through the atmosphere,
it is probable that all large meteorites, if they could be reached at
once on striking the earth, would be found to be cold, because the
smooth, black, varnishlike crust which always incases them as a result
of intense heat is never thick. On one occasion a meteor which was seen
to fall in India was dug out of the ground as quickly as possible, and
found to be, not hot as was expected, but coated thickly over with ice
frozen on it from the moisture in the surrounding soil.
As to the composition of shooting stars, and their probable mass, and
its effect upon the earth, our data are quite insufficient. The lines of
sodium and magnesium have been hurriedly caught in the spectroscope,
and, estimating on the basis of the light emitted by them, the largest
meteors must weigh ounces rather than pounds. Nevertheless, it is
interesting to inquire what addition the continual fall of many millions
daily upon the earth makes to its weight: somewhere between thirty and
fifty thousand tons annually is perhaps a conservative estimate, but
even this would not accumulate a layer one inch in thickness over the
entire surface of the earth in less than a thousand million years.
Many hundreds of the meteors actually seen to fall, together with those
picked up accidentally, are recovered and prized as specimens of great
value in our collections, the richest of which are now in New York,
Paris, and London. The detailed investigation of them is rather the
province of the chemist, the crystallographer and the mineralogist than
of the astronomer whose interest is more keen in their life history
before they reach the earth. To distinguish a stony meteorite from
terrestrial rock substances is not always easy, but there is usually
little difficulty in pronouncing upon an iron meteorite. These are most
frequently found in deserts, because the dryness of the climate renders
their oxidation and gradual disappearance very slow.
The surface of a suspected iron meteorite is polished to a high luster
and nitric acid is poured upon it. If it quickly becomes etched with a
characteristic series of lines, or a sort of cross-hatching, it is
almost certain to be a meteorite. Occasionally carbon has been found in
meteorites, and the existence of diamond has been suspected. The
minerals composing meteorites are not unlike terrestrial materials of
volcanic origin, though many of them are peculiar to meteorites only.
More than one-third of all the known chemical elements have been found
by analysis in meteorites, but not any new ones.
Meteoric iron is a rich alloy containing about ten per cent of nickel,
also cobalt, tin, and copper in much smaller amount. Calcium, chlorine,
sodium, and sulphur likewise are found in meteoric irons. At very high
temperatures iron will absorb gases and retain them until again heated
to red heat. Carbonic oxide, helium, hydrogen, and nitrogen are thus
imprisoned, or occluded, in meteoric irons in very small quantities; and
in 1867, during a London lecture by Graham, a room in the Royal
Institution was for a brief space illuminated by gas brought to earth in
a meteorite from interplanetary space. Meteorites, too, have been most
critically investigated by the biologist, but no trace of germs of
organic life of any type has so far been found. Farrington of Chicago
has published a full descriptive catalogue of all the North American
meteorites.
Recent investigations of the radioactivity of meteorites show that the
average stone meteorite is much less radioactive than the average rock,
and probably less than one-fourth as radioactive as in average granite.
The metallic meteorites examined were found about wholly free from
radioactivity.
From shooting stars, perhaps the chips of the celestial workshop, or
more possibly related to the planetesimals which the processes of growth
of the universe have swept up into the vastly greater bodies of the
universe, transition is natural to the stars themselves, the most
numerous of the heavenly bodies, all shining by their own light, and all
inconceivably remote from the solar system, which nevertheless appears
to be not far removed from the center of the stellar universe.