No new element has been found up to the present in me teorites, and, on the other hand, some common elements, includ ing the strongly radioactive ones, have not yet been detected in them. The elements of which they are mainly composed are Chose of low atomic weight such as occur commonly in the earth's crust, viz. :—in approximately their order of abundance (accord ing to W. A. Wahl) iron, oxygen, silicon, magnesium, aluminium, calcium, nickel, sodium and sulphur. In smaller amounts occur potassium, cobalt, phosphorus, carbon, hydrogen, chlorine, chrom ium, manganese, titanium, nitrogen, platinum metals and copper; while traces of iodine, bromine, argon, helium and radium, and doubtfully gold, tin and vanadium have been recorded. Carbon in meteorites is mostly in the form of graphite, but from the Canon Diablo iron and the Novo-Urei stone chips having the hardness of diamond are recorded to have been isolated, and in the Youndegin iron was found a cubic form of graphite which has been called cliftonite. A few meteoric stones, including those of the small shower which fell at Cold Bokkeveld, South Africa, on October 13, 1838, are remarkable as containing small amounts of solid hydrocarbons which can be extracted from them by alcohol and ether. According to the relative amounts of nickel iferous iron and stony matter, meteorites have been grouped into three main divisions, viz:—meteoric irons or siderites, con sisting almost wholly of nickeliferous iron ; meteoric stony-irons or siderolites, of metal and stony matter in about equal amounts; and meteoric stones or aerolites, of stony matter, usually with nickeliferous iron scattered through it in small grains.
The metallic constituent of meteorites (both irons and stones) is not uniform in composition, for the percentage of nickel in it varies widely (from about 3 to 40) in different falls. In by far the greatest number of meteoric irons, however, the per centage of nickel lies between about 7 and 15. These irons are made up mainly of two different alloys of iron and nickel, and are characterized by a peculiar structure which is revealed upon polished surfaces by the etching action of dilute nitric acid or bromine water. These structures, which are known as Widman statten figures from their discoverer, consist of bands of a nickel poor alloy, called kamacite and containing about 7% of nickel, bordered by narrower and more brilliantly reflecting bands of another alloy called taenite which is much richer in nickel (from 14% to nearly so%) and less soluble than kamacite. These bands are the edges of plates which are arranged parallel to the faces of an octahedron. Hence these irons are known as octahedrites and where the bands are "medium" and "fine," there is usually a third material called plessite which is probably an intimate mixture of the same two alloys.
Meteoric irons often contain nodules and crystals of various minerals scattered through them. Of most common occurrence is troilite, the monosulphide of iron similar to, if not identical with, the terrestrial pyrrhotite. Frequently associated with the troilite nodules are inclusions of schreibersite and of graphite. Other minerals less frequently found in irons are daubreelite, a sulphide of iron and chromium, the sulphur analogue of chromite; cohenite, a carbide of iron and nickel similar to the cemenoite of steel; and moissanite, a silicide of carbon similar to the artificial carborundum. Chromite is present in small amount in most irons, while in some the presence of the deliquescent chloride of iron called lawrencite is the cause of their liability to quick rusting and disintegration.
Gases included in small amount in meteorites have been found to consist mainly of hydrogen, carbon monoxide, carbon dioxide. nitrogen and marsh gas; carbon dioxide predominating in the stones, and hydrogen and carbon monoxide in the irons.
The stony-irons, of which about 3o are known, form a dis tinct group and are only intermediate between irons and stones as regards the amount of nickel-iron. In many of them (palla sites) the stony matter consists of olivine alone as rounded or fragmental crystals. In most of the other stony-irons (mesa siderites) the stony matter consists mainly of pyroxene with fragmental anorthite-felspar and only little olivine. Of excep tional composition is the stony-iron which was found at Stein bach in Saxony in which the stony matter consists of bronzite and tridymite (asmanite), for it is the only meteorite known containing an appreciable amount of free silica.
The great majority (about go of meteoric stones are known as cliondrites since they consist largely of curious rounded bodies (chondrules) which are embedded in a fragmental groundmass made up of irregular grains of pyroxene and olivine with scat tered particles of nickel-iron and troilite. Chondrules are mostly .3f the size of millet seeds though occasionally as large as a wal nut. They are formed of the same minerals, pyroxene and olivine with sometimes felspar and glass, as the rest of the stone. Some are of pyroxene alone in fibrous form, with the fibres often radiat ing from the edge and not from the centre.
The above tabular classification gives the names and mineral composition of the different kinds of meteorites. This classifica tion is based on the hypothesis that meteorites have been derived from a single magma which has passed through successive stages of progressive oxidation.
The following is a list of the meteorites known to have fallen in the British Isles:— Origin.—Little can be said of a definite character concerning this. The prevailing and orthodox view which dates from the time of Chladni is that the detonating meteoritic fireball and the ordinary shooting star or meteor (q.v.) are only variations of one phenomenon. According to this theory meteorites are large enough to survive their fiery transit through the air, meteors are not ; and moreover, since the orbits of some comets have been shown to be the same as those of some star-showers, me teorites are supposed to be identical with the material of comets and in that case to be moving through space in swarms having definite orbits. It has been objected to this theory that meteorites, with one exception, have not been known to fall during displays of star-showers and do not fall most frequently during that period of the year when shooting stars are prevalent. The gen eral similarity of meteorites in chemical composition, and the evidence they afford of consanguinity or derivation by progres sive oxidation from a common type, certainly indicate that they belonged originally to a single celestial body. Such a body may have suffered some catastrophic disruption, and if so, it has been argued, this must have taken place in fairly recent geological time since, with the one exception of a meteoric iron found in Pliocene gravels in Klondike, "fossil" meteorites are unknown.
No meteorite which has been seen to fall approaches in chem ical composition the acid granitic rocks which occur so plenti fully on the earth's surface. In various parts of the world, how ever, namely in Bohemia and Moravia, the East Indies and Australia, have been found peculiar small glassy bodies resem bling obsidian and containing as high a percentage of silica. As these curious bodies, which have been called tektites, occur in no obvious connection with recent volcanoes or with ancient volcanic rock, a meteoritic origin has been suggested.
The most important collections of meteorites are in the Natural History Museums of Chicago, London, Vienna and Paris. The number of falls represented in the British Museum (Natural History) collection is about 71o, and the weight of the specimens amounts to just over 6 tons.
F. F. Chladni, Ueber Feuer-Meteore, and fiber die mit denselben herabgefallenen Massen (Vienna, 1819) ; G. Tscher mak, 'Die Mikroskopische Beschaffenheit der Meteoriten (Stuttgart, 1883-85) ; A. Brezina u. E. Cohen, Die Structure and die 2usammen setzung der Meteoriten (Stuttgart, 1886-87) ; E. Cohen, Meteoriten kunde (Stuttgart, 1894-1903) ; E. A. Wulfing, Die Meteoriten in Sammlungen and ihre Literatur (Tubingen, 1897) ; L. Fletcher, An Introduction to the Study of Meteorites, it ed. (1914) ; 0. C. Far rington, Meteorites (Chicago, 1915) and Catalogue of the Meteor ites of N. America (Washington, 1915) ; G. P. Merrill, Handbook and Descriptive Catalogue of the Meteorite Collections in the United States National Museum (Washington, 1916) ; G. T. Prior, Catalogue of Meteorites (1923) and Appendix (1927), and Guide to the Collection of Meteorites in the British Museum (1926). (G. T. P.)