Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/123

Rh than the track of the meteor. They begin at once to broaden in the middle and to fade away at one or both ends. Presently they become curved, sometimes with two or three convolutions. The white cloud floats slowly away among the stars, coiling up more and more, and finally fades out of sight. The cause of all this seems to be as follows. The heated air charged with the debris of the meteor is by the meteor's impact driven off horizontally, causing the narrow train to spread into a cloud. The currents of air differing in direction at different altitudes twist the cloud into its varied fantastic forms. Attempts to obtain the spectrum of the trains have been made, and sodium and magnesium lines have been thought to be detected in them. The observation, however, is one that is not easy to make or confirm. The trains have often colours other than white, and in the case of the brighter meteors different colours are seen in the different parts of the train.

Magnitude.—Some computations have been made of the size of the shooting star meteoroids from the mechanical equivalent of the light developed by their disintegration. If all the energy of the meteor is changed into light, then these computations would be conclusive. But a part is spent in disintegrating and burning the stone, a part in heating the air, and a part in giving direct motion to portions of air. A computation based on the light developed gives only a lower limit to the size.

It seems probable that the larger meteors might be safely regarded as weighing on entering the air only a few hundreds or at most a few thousands of pounds. The smallest visible shooting stars may be equal in size to coarse grains of sand, and still be large enough to furnish all the light exhibited by them. The largest shooting stars furnish matter enough to fill with thin trains cubic miles of space, but this need not require a very large mass.

Meteoric Irons.—There have been found at various times on the surface of the earth masses of metallic iron combined with nickel. These have been so like the irons which have been known to fall, both in their structure and in composition, that they have been without hesitation classed among the meteoric irons. A mass of this character weighing 1635 lb, found in Texas, is in the Yale College Museum. The Charcas (Mexico) iron in the Paris museum is about the same size. A ring-shaped mass somewhat smaller, from Tuczon, is in the United States National Museum in Washington. A still larger mass is in the British Museum, and many other large masses are in public collections or private possession.

Widmannstatten Figures.—If in any of the meteoric irons, whether seen to fall or found on the earth, a section is cut and polished and then etched with acids, a series of peculiar lines are developed which are known as Widmannstatten figures. The lines of iron unattacked by the acid consist of an irregular grouping of parallel rulings often lying along the faces of a regular octahedron. The exhibition of these figures and the combination of iron with nickel have been usually considered conclusive evidence of the meteoric origin of any iron mass.

Nickel Iron of Ovifak.—In 1870 Baron Nordenskiöld, in his voyage to Greenland, found on the shore of the island of Disco fifteen iron masses, the largest of which weighed 20 tons, all in an area of half an acre. In the basaltic rocks not far distant other iron masses were found embedded in the basalt. The presence of nickel with the iron, and the development of lines like the Widmannstatten figures, were at once accepted as proof of their meteoric origin, in spite of the combination with basalt. A more complete examination has, however, established the terrestrial origin of these irons, and given reasons to hope for new discoveries of relations existing between the earth and the meteors. The additional discovery of small particles of metallic iron in certain other igneous rocks proves that the union of the Ovifak irons with basalt is not exceptional.

Chemical Constitution of the Meteorites.—No new element has been found in the meteorites. Three elements most widely distributed and most important among the meteorites iron, silicon, and oxygen are also most abundant in our earth. Daubrée gives the following lists of elements, arranged somewhat in the degree of their importance, in meteorites (Maskelyne adds lithium and antimony):—

Iron.

Magnesium.

Silicon.

Oxygen.

Nickel.

Cobalt.

Chromium.

Manganese.

Titanium.

Tin.

Copper.

Aluminium.

Potassium.

Sodium.

Calcium.

Arsenic.

Phosphorus.

Nitrogen.

Sulphur.

Chlorine.

Carbon.

Hydrogen.

Minerals in Meteorites.—Among the minerals in the meteorites there are several which occur in the rocks on the earth. Among these are cited by Daubrée peridote, pyroxene, enstatite, triclinic felspar, chromite, magnetic pyrites, iron oxide, graphite, and probably water. Several minerals, however, are found which, so far as now known, are peculiar to the meteorites:—metallic nickel-iron, phosphide of iron and nickel (schreibersite), sesquisulphide of chromium and iron (daubréelite), sulphide of calcium (oldhamite), and chloride of iron (lawrencite).

Meteorites of different falls are in general unlike; but there are many instances in which the stones of two falls are so similarly constituted that it is not easy to distinguish them.

In four falls (Alais, Cold Bokkeweld, Kaba, and Orgeuil) the stones contain little or no iron. In these carbon appears not as graphite but in union with hydrogen and oxygen, and also with soluble and even deliquescent saline matters. The combinations are such as to suggest the existence of humus and organic remains. But after careful search nothing of this kind has been detected in them. In general the meteorites show no resemblance in their mechanical or mineralogical structure to the granitic and surface rocks on the earth. One condition was certainly necessary in their formation, viz., the absence of free oxygen and of enough water to oxidize the iron and other elements. Perhaps it is to this fact that are due the resemblances between these minerals and those of the deep-seated rocks of the earth in the formation of which free oxygen and water were also not present.

Gases in Meteorites.—The meteoric stones and irons when reduced to fine particles and placed in the vacuum of a Sprengel air-pump give off small quantities of gases which may be reasonably presumed to have been occluded by the irons at some time in their earlier history. Professor Graham found hydrogen in meteoric irons. Professor Wright has shown that a moderate heat drives off from the stony meteorites carbonic acid and carbonic oxide with a small amount of hydrogen. As the heat increases the proportion of hydrogen (and even some nitrogen) increases till at a full red heat the hydrogen given off is by far the largest portion. From the irons similar gases are given off, but the carbon compounds are not so large a component as hydrogen. The spectra seen in the tails of comets are not strikingly like those of any of these gases. But it is impossible to reproduce in the laboratory the conditions under which the matter of comet's tails is giving off its light. We cannot therefore say that these gases may not be the important parts of the cometic coma and tails.

Meteoroid as Part of a Comet.—Assuming that the meteorite and meteoroid once formed an integral part of a comet, not a little information is given us of the nature of this mysterious body. There is room also for speculation.

First, the comet may be a single hard body which comes from the cold of space into the heat of the sun, and there has fragments broken off, just as a stone is shattered in a hot fire. The nucleus of some of the comets must be very small because invisible in the telescope, and an impulse that would raise a stone on the earth only a few inches would send it permanently away from such a comet. The exposure of new surfaces to the heat of the sun might give occasion for the development of gas to form the comet's tail.

Or, secondly, the comet may be a tolerably compact aggregation of small bodies not in contact, each one being of the size of a meteoroid, and kept near to the rest, not by cohesion, but by their combined attraction. The total mass being small, some members of the group near the comet's perihelion passage can be by the sun's perturbing action thrown out into orbits quite independent of the comet itself, and yet such as relative to the sun shall resemble that of the main group. Perturbations resembling tidal waves might be preparing other members to be cast off at the next perihelion passage of the comet.

In either case, if we suppose, as seems probable, that the comet came from outside the solar system, and that a disturbance by a large planet changed the original hyperbolic orbit into an ellipse, the comet must have passed that planet as a very compact group, if not in a single mass, else the disturbance that changed the orbit would have scattered the group beyond the power of a future recognition of the common origin of the fragments.

Meteoroids as Fuel of the Sun.—The idea has been held by distinguished physicists that the meteoroids in falling into the sun furnish by their concussion a supply for the heat which the sun is constantly sending off into space,—that they are in fact the fuel of the sun. Such a view, however, receives but little support from facts which we know about meteors. The meteoroids of the August and two November periods are evidently permanent