Popular Science Monthly/Volume 39/July 1891/The Meteoritic Hypothesis
|THE METEORITIC HYPOTHESIS.|
By J. ELLARD GORE, F. R. A. S.
MUCH has lately been heard about the "meteoritic theory" as an explanation of the origin and construction of the heavenly bodies. This hypothesis, now generally ascribed to Prof. Lockyer, seems to have been first suggested by the German astronomer Meyer. His theory has met with some support from Helmholtz, Proctor, Thomson, and Tait in Europe, and from Profs. Newton and Wright in America. Prof. Lockyer has recently published a full exposition of his theory in an elaborate and interesting work entitled The Meteoritic Hypothesis: a Statement of the Results of a Spectroscopic Inquiry into the Origin of Cosmical Systems. In this volume the author has worked out his hypothesis in great detail, and, as his theory has recently met with much adverse criticism, a brief review of the principal facts and arguments advanced by Lockyer, and also by his opponents, may prove of interest both to those who accept and those who reject his views.
Lockyer commences his work with an account of the falls of meteoric stones recorded in history. The earliest of these dates back so far as 1478 b. c., but, of course, with some uncertainty. Numerous well-attested falls are, however, referred to, and many of these meteorites are preserved in museums, one weighing over three tons being deposited in the British Museum. This fell at Cranbourne, Australia.
The general form of these meteoric stones is fragmentary, indicating that they are the fractured portions of larger masses, burst asunder by the force of the explosion which usually accompanies these interesting phenomena. In the case of the meteorite which fell at Butsura in 1861, pieces picked up at places three or four miles apart could be actually fitted together to form the original mass.
Meteorites are generally covered by a black crust, clearly caused by the intense heat developed by the mass in rushing through the earth's atmosphere with a planetary velocity.
Meteorites are generally composed of well-known terrestrial elements. Among these may be mentioned iron, nickel magnesium, manganese, copper, carbon, sulphur, etc. Some of them, however, contain mineral compounds which are "new to our mineralogy," such as compounds of sulphur and calcium, sulphur with iron and chromium, etc. Some meteorites contain a large quantity of hydrogen gas, which has been absorbed or "occluded"; others contain carbonic-acid gas. Some are composed chiefly of iron, others mostly of stony matter.
Prof. Lockyer made a number of careful experiments on the spectra of fragments of "undoubted meteorites" obtained from the British Museum. These were examined at various temperatures, varying from that of the "Bunsen burner" to that of the electric spark with Leyden jar. He finds that, at the lowest temperature, the most prominent line of magnesium is a fluting near the wave-length 500. I may here explain that by the term "fluting" is meant a series of bright lines, usually three, which are sharp on the side toward the red end of the spectrum, but have a hazy fringe on the blue side. These "fringes," when examined with a powerful spectroscope, are seen to be composed of a number of fine lines very close together. In the case of "iron" meteorites, the lines of manganese are the first to make their appearance, owing to its volatility being greater than that of iron.
Lockyer finds that only the lowest temperature lines of magnesium, sodium, iron, chromium, manganese, strontium, calcium, barium, potassium, bismuth,-and nickel are seen in the spectra of the meteorites.
He shows the probable identity of origin of luminous meteors and falling stars with meteorites, and also that comets are probably composed of meteoric stones. Discussing the observations of the aurora, he attempts to prove that the phenomenon is due to meteoric dust in the "higher reaches" of our atmosphere, and that the characteristic line seen in the auroral spectrum is identical with the brightest fluting of manganese. Dr. Huggins's researches, however, show that this coincidence does not exist; and some recent experiments made by Messrs. Liveing and Dewar with an electric discharge passing through dust show that the dust does not act like a gas, and does not become luminous like the aurora, but that, on the contrary, the electric current drives it out of its path.
Lockyer next proceeds to discuss the appearances presented by comets,-and the character of the spectra they show at different distances from the sun, and concludes that their spectra very much resemble the spectra of meteorites seen under similar conditions of temperature. He considers that the light of comets is chiefly due to collisions between the component meteorites, and that the observed transparency of comets may be explained by supposing the meteorites to be small, and separated by considerable intervals. A portion of the light of comets, he thinks, may be produced by collisions between the cometic swarm and other swarms existing in space; and the recorded sudden increase of light in the Pons-Brooks comet of 1883, and the Sawerthal comet of 1888, seems certainly in favor of this idea.
Lockyer holds the view that both shooting-stars and comets did not originate within the solar system, but are of cosmical origin. This view of the origin of comets was held by the famous Laplace, but Kant thought they originated in the solar system; and the terrestrial origin of meteorites was advocated by Sir Robert Ball and Tschermak.
Lockyer then proceeds to discuss the probable construction of the nebula?, and concludes that they are probably swarms of meteorites; the collisions between the component meteorites producing the light emitted by these objects. He attempts to prove that the brightest line seen in the spectra of the nebulæ, "the chief nebular line" as it is called, is coincident with the edge of the magnesium fluting seen in the spectra of meteorites. The nebular line certainly lies very near this fluting, but the spectroscopic power used by Prof. Lockyer was quite insufficient to decide so delicate a question. Recent observations by Dr. Huggins, with a more powerful spectroscope, and by Mr. Keeler at the Lick Observatory, with a higher power still, have, however, shown that the chief nebular line in the spectrum of the great nebula in Orion, and in some others, does not coincide with the edge of the magnesium fluting, but falls within the fluting, toward the blue end of the spectrum.
Classifying the stars in accordance with his theory, Lockyer places some of them on the rising branch of a temperature curve, and others, including our own sun, and stars with similar spectra, on the descending or cooling branch of the curve. From an examination of the spectra he considers that the red and orange stars of Secchi's third type, which includes many variable stars, are increasing in temperature, while the still redder stars of the fourth type, of which some are variable also, are cooling bodies, and are "approaching the extinction of their light," The stars showing bright lines in their spectra, he thinks, "are nothing more than swarms of meteorites, a little more condensed than those which we know as nebulæ." He identifies some of the bright lines visible in these stars with the lines of hot carbon, but this conclusion is disputed by Dr. Huggins.
Considering the subject of the binary or revolving double stars, Lockyer considers that they are merely condensed swarms of meteorites, which had probably their origin in a single nebulous mass, or a double nebulosity. He explains the phenomena of long period variable stars by supposing one swarm to revolve round another in an elliptic orbit, the increase of light at maximum being caused by collisions between the meteorites of the swarms when they clash together at the periastron. This seems a very plausible hypothesis, and quite as probable, I think, as other theories which have been advanced to explain the phenomena presented by these interesting and mysterious objects. Bright lines have been observed by Espin in several of the most remarkable variable stars when near their maximum brilliancy, and these may very possibly be due to the heat produced by meteoric collisions.
Prof. G. H. Darwin, the eminent Cambridge professor, has proved mathematically one point in favor of Lockyer's hypothesis, He shows that the conception of fluid pressure required by Laplace's nebular hypothesis, and which is applicable to a gas, is also applicable to a swarm of meteorites. The pressure exerted by a gas against the surface of an inclosing vessel is supposed to be the result of collisions between the component molecules of the gas, and Prof. Darwin shows that, if we supposed these molecules magnified up to the size of meteorites, their collisions will still give a quasi-fluid pressure, and that the law of gases will be applicable to a swarm of meteorites. One objection may be raised to this view, namely, that the ultimate molecules of a gas are supposed to be perfectly, or at least highly, elastic, while meteoric stones have very little elasticity. Prof. Darwin, however, points out that, when the meteorites come into collision, the heat generated by the shock volatilizes a portion of each, so that the result will be like that of an explosive, and consequently there will be nearly perfect elasticity. He finds, further, that the analogy with the theory of gases will hold good for the meteoric swarms from which the solar system—on Lockyer's hypothesis—is supposed to have been evolved, a swarm extending beyond the orbit of Neptune. He also finds that the swarm when widely diffused will be subject to gaseous viscosity, and will first rotate as a solid body, but when more contracted "the central portion will revolve more rapidly than the outside."
With reference to the origin of comets, Mr. Monck inclines to the opinion that some comets, at least, originated "within the limits of the solar system," and to this class he is disposed to assign "the four comets which have been connected with meteor swarms." He argues that some meteors may be of terrestrial origin, and suggests that possibly Lockyer's experiments may have been made with some of these terrestrial meteorites.
There seems to be another weak point in Prof. Lockyer's hypothesis, and that is that it offers no explanation of how the plan' ets and satellites of the solar system were evolved. This has been pointed out by Mr. Monck. He says: "Will any advocate of the meteoric theory give us an explanation of why all the planets and asteroids and the great majority of the satellites revolve in the same direction, why the orbits of the larger bodies of the system deviate so little from the circle, and why they are so nearly in the same plane? Till this is done I think the nebular hypothesis has in this case the advantage." A violent grazing collision between two dense meteoric swarms might, however, conceivably, be supposed to produce a rotation in the swarms, which would give rise to the observed planetary motions.
Another objection raised by Mr. Monck is, that it seems difficult to understand how the requisite number of collisions in a meteoric swarm could be produced and kept up, and "that meteor clouds dense enough to produce the requisite amount of light by their collisions would also be dense enough to intercept a great part of it again on its way to the earth." Mr. Monck's papers on the subject were published in the Journal of the Liverpool Astronomical Society.
Here the matter rests at present. It will be seen that hitherto the weight of evidence seems against the truth of Lockyer's hypothesis, but further researches on the subject will be looked forward to with considerable interest. — Gentleman's Magazine.