DESERT DUST.
The visitor to the Egyptian Pyramids who gazes in wonder on those colossal structures which remain to attest the activity of races long since passed away, little dreams, perhaps, that in the dust which he treads beneath his feet, or which whirls in wind-tossed eddies round his head, there exist particles of so great antiquity, that the vast age of the Pyramids shrinks into littleness beside it. Such particles also may be found by the traveller in the snows which cover the higher slopes of Mont Blanc, and on other parts of the earth’s surface.
The question arises, What are these particles which thus lie unnoticed in the dust beneath our feet, and which are fraught with such interest to mankind? Dust from the Sahara Desert, or from the upper slopes of Mont Blanc, is found to contain an appreciable quantity of magnetic iron particles. Examination by the microscope reveals the fact that the greater part of these are angular in shape, and there can be no doubt that they are simply the debris of terrestrial magnetic rocks. But here and there are found mingled with the other particles small but perfect spheres of iron, their spherical condition pointing to the fact that they have at some time been in a state of fusion. In speculating concerning their origin we are at the outset reduced to three possibilities—they may be of volcanic origin, or the product of fusion in terrestrial fires, or they may have a non-terrestrial origin, and be meteoric. A comparison with dust known to be volcanic discovers that these particles have little or no affinity with volcanic ejections. But the smoke which issues from the chimneys of our manufacturing districts contains iron particles similar in appearance to these iron particles of the Sahara and Mont Blanc; and although these latter are found far from any of the terrestrial sources which could give them birth, yet these light particles may be wafted by wind-currents to such immense distances, that this argument does not come with much strength to support the contention of their non-terrestrial origin.
The most crucial test is that of comparative chemical analysis; and its application to various of these iron particles reveals the fact, that whilst those known to be of terrestrial origin contained neither nickel nor cobalt, both these metals are found present in the magnetic particles collected at the observatory of Saint Marie du Mont, on Mont Blanc; and a meteoric origin has therefore been assigned to the latter. Nor is other proof wanting to support this presumption. In addition to these particles of cosmic dust, larger masses forming meteorites are not unfrequently found. Their general appearance is that of a dull black, but occasionally shining black, irregular exterior, forming a thin crust, which is totally different from the main mass within. Examined microscopically, the crust, which is usually one-hundredth, but may occasionally rise to one-eightieth, of an inch in thickness, is found to be a true black glass, filled with small bubbles, sharply divided from the interior—facts which indicate that the crust is due to igneous action, under conditions which have little or no influence within the mass. The interior usually consists of a stony mass formed of broken or angular particles. Here we have two alternatives—either it has been formed by aqueous deposition, or it has had an igneous origin. The latter, or fiery, origin is again believed to be the true one, for the reason, that certain microscopic characters always present in water-deposited crystalline masses are not seen in these meteorites; and an igneous non-terrestrial, rather than an igneous terrestrial, origin is assigned to them, because the glassy spherical structure found in meteorites can only be produced terrestrially by a combination of conditions very rarely found co-existent. The only instance known where such a combination obtains is in the crater of Kilauea, where the volcanic production known as Pele’s Hair somewhat resembles the glassy structure of meteorites. Nor is this all; for, knowing as we do that meteors occasionally reach the earth in the form of substantial masses, the suggestion has been ventured that they fall in sufficient numbers to affect its bulk in the course of ages; and assuming, as we are entitled to assume, that these masses, to which we are unable to assign definitely a meteoric origin, are indeed meteorites, the link connecting them with cosmic (non-terrestrial) dust has then been found. M. Tissandier examining dust which he detached from the surface of a Bohemian meteorite, found its microscopic characters to resemble those of the dust-particles of Mont Blanc; and even more proof is not wanting to vindicate its non-terrestrial origin.
The connection between cosmic dust and meteors having been thus traced, we may now proceed to a brief consideration of their history ere they find a grave in the earth.
On a calm clear night, when above us gleams
The sky
With all its bright sublimity of stars,
with their eternal suggestions of peace and immortality, there comes ever and anon from out the darkness a light darting across the heavens with increasing brightness. Sometimes the meteor will traverse a large portion of the heavens, travelling perhaps the entire vault, and then disappearing, while still bright, below the horizon. Occasionally, they may be seen to fall to earth; but more commonly, after a short course, the meteoric gleam dies away, leaving us to gaze again at the calm fixed brightness of the familiar constellations. The differences between these various classes of meteors are those of degree, and not of kind. Omitting for a moment the consideration of their origin, it is obvious that these bodies, no matter how they first originate, come within the attractive force of the earth, and enter its atmosphere in obedience to that attraction. The intense rapidity with which they fall generates an ever-increasing amount of heat, under the influence of which they become luminous, and begin to be consumed. A continuance of this process gradually diminishes their bulk, the smaller ones being entirely consumed high above the earth, and constituting the shooting-stars whose passage is as evanescent as a gleam of light. It is the dust formed as they are consumed, which, slowly settling to earth, constitutes the cosmic particles to which reference has been made.
But whilst on almost any clear night some few meteors may be seen to flash across the sky, observation has revealed the fact, that in certain months of the year, and on certain dates in those months, shooting-stars are much more numerous than in other months and on ordinary nights. Herr Schwabe, referring to the discovery of the sun-spot period as the result of continuous observation undertaken for the sake of recording phenomena, says: ‘I went out like Saul to find my father’s asses, and lo! I found a kingdom.’ The remark might be echoed by those who made this discovery of the periodicity of these shooting-stars, leading, as it has done, to the discovery of facts hitherto unsuspected, and pointing to a connection and commonness of origin between phenomena apparently very widely divergent. At first, the meteor-streams of August 10 and November 14 were alone recognised. Soon it was discovered that the month of April was one in which a very large number of meteors were visible, and the 20th was fixed as the date for the maximum shower. But not only was it found that these evenings were characterised by large showers of meteors, but the further fact was ascertained, that all the meteors on any given evening emanated from one quarter of the heavens. Thus, in the shower which occurs on the 20th of April—although the shower has not been very marked of late years—the radiant point for the meteors is in the constellation Lyra; hence it is termed the Lyriad shower. In like manner, the August train has its radiant point in Perseus; and that of November 14 in Leo. It was still, however, thought that the meteors of ordinary nights had no connection of this kind; but later observations revealed the fact that they also are controlled by similar laws; and the further discovery was made that some, notably the mid-November swarm, vary in intensity from year to year, in obedience to regular laws, the entire cycle in that instance requiring thirty-three and a quarter years for its completion.
But although the life-history of the cosmic dust-particles of the Sahara has thus been traced back until they are found to be component parts of meteor-swarms, whose movements are controlled and dominated by definite laws, there yet remains the question of their origin, the explanation of the annual periodicity, and why this secular cycle should exist. Meteors being thus found to occur in these streams, it became possible to calculate their orbits, and M. Schiaparelli did this with the August swarm. A connection had begun to be suspected between meteors and comets, and it was found that the orbit of the August meteors, as calculated by the Italian physicist, coincided with that of a known comet. More life being thus given to the hypothesis, the orbit of the November stream was similarly calculated. It was found to be almost identical with the independently ascertained orbit of Tempel’s comet. Other corroborative elements soon followed. The April meteors perform their journey in space along the orbit of the comet of 1861, while many other meteor-streams have been discovered to be similarly related to other comets. With the knowledge of the connection between comets and meteor-swarms, and our knowledge of the constitution of meteorites themselves, the vexed question as to the constitution of comets would seem to be rendered more easy of solution; but the subject is beset with many difficulties, and comets well situated for observation do not too often visit our skies.
Having traced back the history of the desert dust-particles until they have been found to be intimately bound up, if indeed not themselves forming bodies whose motions have laws ‘as fixed as planets have,’ it now remains to take yet another step back into the history of things, and endeavour to form some idea as to their first origin, and the part they have played, or play, in the economy of nature. Many and strange are the hypotheses which have from time to time been put forth. Some have held meteors to be the scattered remnants of an exploded planet, ‘battered by the shocks of doom.’ Other speculators have thought that this dust of space originated in ejections from volcanic vents when the volcanoes which stud the surface of our satellite were in energy. But for this to be true, it seems somewhat, though not entirely necessary that the moon’s volcanoes should yet be active; whilst the question arises as to the possibility of the eruptive forces on the moon to have expelled matter beyond the influence of its attraction; and those who give most weight to these objections have themselves been inclined to believe that the true origin of meteors is to be found in eruption from one of the minor planets whose attractive force would be less than the moon; but when it is remembered how slight would be the chance of any such matter crossing the earth’s path, such a theory loses all probability. There have not been wanting, either, those who, having in mind the brecciated structure of meteorites and the fewness of the characters in which they differ from terrestrial rocks, have boldly proclaimed for them a terrestrial origin, imagining them to have been erupted from volcanic vents at an early period of the world’s history—a view of course not open to the very serious objections which surround the minor planets’ hypothesis. Yet another class of theorists hold that the sun itself is the source of these wandering streams, they being continually sent far into space by those mighty eruptions with which we know that orb to be continually convulsed. It is, however, probable that none of these theories of an eruptive origin, whether from satellite, planet, or sun, is the true one—it being more likely that meteors are the residue of nebulous matter not gathered into planets when the different members of the solar system began to exist independently, but which each hour, day, and year is being slowly gathered in by the earth and the other planets as these bodies come within the sphere of their gravitative influences. Thus much as to the origin of these meteoric swarms.
The final question now arises as to whether they play any part in the economy of nature. The aggregate weight of these small scattered streams must be beyond comprehension, and is probably to be estimated by billions of tons. These small masses are constantly falling towards the earth, some reaching its actual surface. So it must be with the moon, and with the other planets and satellites which compose the solar system; and this continual impact of meteors, however inappreciable its influence on the earth, cannot be without its heat-producing effects on the larger bodies of our system. If this be so, how much greater must be the result produced by the enormous number of these bodies which, from a variety of causes, would be incessantly precipitated upon the sun’s surface; and the suggestion has been put forward that we may find in this a sufficient explanation of the apparently inexhaustible emission of light and heat which the sun is ever radiating into infinite space.
And if it be true that these meteors have had their origin in solar eruptions, we are brought to the strange reflection, that the matter which in the yesterday of ages was hurled with awful energy from the sun’s surface, is being partly returned to it in the present age, as the energy and matter of to-day will be partly returned to feed its fires in the ages of to-morrow. Should these speculations be correct, then our meteor-systems do indeed play an important part in the economy of nature. All forms of force on earth, the energies of man himself, have their physical source in the centre of our system; and if it be that the energy of that source is being ever renewed by the physical impact of meteoric masses, they have an equal title with the sun to be regarded as the source of energy, although it must not be forgotten that the rain of meteors on the sun’s surface is itself due to the attractive force inherent in the sun itself.
Will the continual gathering in by the sun, the earth, and other planets, gradually lead up to the time when these meteoric swarms shall have ceased to be, and the sun grow cold and dull? Who shall say? There are many causes to delay this end. As the sun, together with the solar system, sweeps through space, it will pass through regions now rich, now poor, in meteoric aggregations, and the total amount of matter which it will gather in will, therefore, vary from century to century, from epoch to epoch. Such are the thoughts up to which we are led in pursuing the history of our particles of dust. But whether or not these speculations be true, the study of this subject teaches many a theme of interest for the leisure hours of our workaday world.