Popular Science Monthly/Volume 29/July 1886/The Origin and Structure of Meteorites
|THE ORIGIN AND STRUCTURE OF METEORITES.|
TILL the second half of this century the nature of the innumerable stars with which space is peopled was wholly a subject of imaginative speculation. Recent science has been able to substitute more exact ideas for premature hypotheses. Notwithstanding the immense distances that separate them from us, spectrum analysis has enabled us to make chemical investigations of the sun, the comets, stars, and nebulæ. It is, further, possible to reach results more precise and more complete in other respects for many extra-terrestrial bodies; that is, for those bodies, fragments of which are dropped from time to time upon our globe. Although we have no means of going to them, they come to us, real messengers from above, to satisfy our legitimate curiosity. The study of these fragments, the only cosmic bodies which it is possible for us to handle immediately, concerns one of the fundamental questions of the physical history of the universe.
The list of meteors, both in ancient and modern times, is very full. Some of the more remarkable falls were objects of extraordinary attention among the inquisitive but imperfectly informed ancients, and the stones themselves were invested with something like divine honors. But, notwithstanding the frequent and authentic testimonials that were given of the fall of meteoric bodies upon the earth in the course of more than twenty centuries, educated people were still incredulous on the subject not more than a hundred years ago. Inversely to the usual course, even the advance of knowledge furnished objections against the truth. The most natural supposition of an extra-terrestrial origin of the meteors appeared to contradict the immutable laws of the movements of the heavenly bodies; for those laws seemed to be inconsistent with the possibility of irregular phenomena. It was easier to deny the reality of such anomalies than to believe in them. But it will not be right to give too severe a condemnation to this persistent denial; for the fabulous and fanciful details with which the accounts of the phenomena were charged necessarily gave an air of incredibility to the whole. It was not till the end of the last century that conditions especially favorable to exact observations afforded the means of unanswerably demonstrating the existence of meteors. The recognition of them became general and complete after the showers that occurred at Benares, India, at eight o'clock in the evening of the 13th of December, 1798, in the presence of a large number of spectators; and was further strengthened after the fall at L'Aigle, France, at one o'clock in the afternoon of the 26th of April, 1803. Biot, acting under a commission of the Academy of Sciences, made a minute account of all the circumstances of the last fall.
Meteorites interest us not only in respect to the origin and the causes of their descent upon our planet, but also in respect to their constitution. It is to the last aspect that we shall pay particular attention, after giving a succinct account of the circumstances under which they come to us.
The phenomena that precede and accompany falls of meteorites, while they vary very much in their secondary details, nevertheless present a whole of general character, reoccurring with constancy at each apparition, and adequately proving that the origin of the bodies is foreign to our planet.
The first appearance is that of a globe of fire bright enough to set all the atmosphere aglow at night, or to be visible at high noon, if in the daytime. Its apparent diameter increases as it gets nearer. It describes a track whose incandescence makes it perceptible from a distance, and which is only slightly inclined to the horizon. The cosmic character of the bodies is indicated by their excessive velocity, which surpasses anything that we know of on the earth, and is in reality comparable to that of the planetary bodies. After a longer or shorter career, the body bursts with a noise which has been compared with that of thunder, a cannon, or musketry, according to the distance away of the observer. A single explosion is rare. There are generally two or three of them. Sometimes they are violent enough to shake houses and give the impression of an earthquake, as was the case in Iowa when the meteor of the 12th of February, 1875, fell. They are often heard over a considerable extent of country, as was the case with the Orgueil meteors, the explosions of which were heard three hundred miles away. When we reflect that these detonations take place at heights where the thinly rarefied air affords a very poor medium for the propagation of sound, we become satisfied that they must be extremely violent. Sometimes a trail of vapors is perceived in the regions of the atmosphere which the body has traversed. These phenomena are manifested in the most diverse regions of the globe, at every season and every hour, and frequently in calm and cloudless weather. Storms and whirlwinds, therefore, have nothing to do with them. Their speed as observed by us being only relative, varies according to the correspondence or non-correspondence of the direction of their path with the course which the earth is pursuing.
The outer configuration of meteorites is remarkable for its fragmentary aspect, or for its angular formations and its likeness to irregular polyhedrons, the edges of which have been blunted.
The number of stones brought down in a single meteoric shower is extremely variable. Sometimes only one is found; sometimes many; and, in rare cases, hundreds and thousands. At the instant the stones reach us their velocity is small, compared with that which the body of which they are fragments had previous to the explosion. If they are of considerable size, they will perhaps bury themselves at a slight depth under a yielding soil, and remain there unperceived. After all the light they give and the noise they make in their flight, the minuteness of the masses which we find upon the surface of the ground is sometimes surprising. The largest one ever found—at St. Catherine, Brazil, 1875—weighed 25,000 kilogrammes; stones of more than 300 kilogrammes, like the one that fell at New Concord on the 1st of May, 1860, are rare, while the weight of 50 kilogrammes is seldom exceeded. Often whole meteorites weigh only a few grammes, or are of the size of a hen's egg, a walnut, or a hazel-nut; and masses of still smaller ones have been observed when they fell upon a bed of snow, as at Hersle, near Upsala, Sweden, in 1869, when many of the stones weighed only a few decigrammes, and one of them as little as six centigrammes. These little grains, it should be remarked, were not fragments broken off by the shock of larger pieces against the ground; but each one was a complete meteorite, enveloped in a crust of half-melted matter. That so small meteors had not been noticed before is explained by the difficulty of distinguishing them from the particles composing the general surface, among which they are lost.
When the meteors of the same shower are numerous, they are generally distributed at various points within an elongated oval area, the axis of which corresponds with the direction of the trajectory, and within which they appear to have been sifted by the resistance offered by the atmosphere, in the order of their magnitude.
Meteors are not incandescent when they reach the ground, but are still too hot to handle. Sometimes the high temperature is limited to their surface, while within they are extraordinarily cold. The spectators of a meteoric fall at Dhurmsalla, India, on the 14th of July, 1860, eagerly broke up the stones, still burning hot on the outside, and were greatly surprised to find that it was impossible to handle the inside parts on account of their extreme coldness. A similar observation was made on the 16th of May, 1883, at Alfianello, near Brescia. This contrast between the central part, still retaining the intense cold of the planetary spaces, and the outside, which only a few moments before had been red-hot, may be easily understood when we reflect on the feebleness of the conducting powers of stony substances, and the very short time that they had been heated.
One effect of this heat persists, and is obvious at first sight as a general characteristic of meteorites, in the shape of a black crust, entirely covering them. It is not a millimetre thick, and is generally dead, but forms in some especially fusible types a glossy enamel. The same effect, of vitrification, is produced by lightning on rocks which are struck by it. The incandescence of which this is the effect, and which had been observed in the meteor flying in the distance, is the result of the extreme speed with which the body penetrates the atmosphere.
It is, unfortunately, very rarely possible to find the fragments projected by meteors; and it is only under quite exceptional circumstances, even in populous countries, that one is discovered among the clods and under the vegetation by which they are commonly concealed. The observer enjoys the illusion of supposing he sees them fall at no great distance from him; but he will hardly ever find one if he looks for it. Probably three quarters of them are swallowed up by the sea.
Supposing there are three meteoric showers a year in Europe—and this is the mean of what has been observed there—and that that part of the earth is not exceptionally favored by them, we have one hundred and eighty a year for the whole surface of the globe. But, as many of the showers are not perceived, we may safely triple the figure, or even suppose there are six hundred, and still underestimate the reality. We are dealing, therefore, with a daily phenomenon.
We do not know in what regions of space meteors originate, nor what courses they follow before they come within the sphere of the earth's attraction. They have been supposed to be ejections from volcanoes in the moon. If this were the case, they would have to be supposed to have been ejected by the eruption with velocity enough to pass the neutral point, or the point where a body is equally attracted by the moon and the earth. That velocity should be at least 2,270 metres a second, or about five times that of a cannon-ball; if it were less, the mass would fall back to the moon. Another more probable supposition is that they come from a group of minute asteroids which revolve in the space between Mars and Jupiter, whose orbits cross those of the large planets, ours included, and are occasionally met by the earth in its course. There is nothing else, since the beautiful researches of Schiaparelli have connected the periodical swarms of shooting-stars with comets, to assure us that they do not come from still more distant parts of the sky, or even from without the solar system.
Shooting-stars come to us by millions at regular periods; and the number of those which are directed toward our globe in a single year is estimated at many milliards. They somewhat resemble meteorites in the abruptness of their appearance in our atmosphere and the excessive rapidity of their motion, but they differ from them in an important characteristic. None of them ever reach the ground. They appear to share in the properties of comets, from which they may have been dismembered and told off by perturbing actions; while meteorites seem to be related to the planets. The difference between them is like the difference between gases or vapors and solid bodies.
The meteors coming to our earth without, excepting as to their superficial vitrification, undergoing any change, we are able, by subjecting them to analysis, to derive from them some precise facts respecting the constitution of the bodies in space. The first fact, which comes out from hundreds of analyses, is, that they have not brought a single substance which is foreign to our globe. About twenty-two elements, all known to the chemistry of the earth, have been recognized as present in them. Among these, iron, silicon, magnesium, nickel, sulphur, phosphorus, and carbon, are the most important. While they are all clad externally in a common livery, meteorites, when examined in their fractured parts, along with traits of similarity, present considerable differences. They have been classified, according to their types, into four groups, according to the proportion of iron they contained. Those of the first group are composed almost wholly of iron, which is known as meteoric iron. It is always alloyed with nickel and a few other metals, and contains carbon free or in combination, as in steel, with frequently sulphuret and phosphuret of iron in scattered globules and grains. It is always recognizable by a single peculiarity in its structure. If we moisten a polished surface of it with an acid, we shall immediately observe the appearance of numerous straight lines, as fine and as true in their parallelism as if made with an engraver's tool, and crossing one another in a net-work of regular geometrical figures. These designs, called the figures of Widmanstaetten, after the first observer of them, result from the fact that the metal is not of homogeneous constitution. It is composed of two alloys of iron and nickel, in a crystalline condition, one of which, not being affected by the acid, stands out in relief from the other, which is attacked by it. The meteorites of this group are called holosiderites, or all iron, in distinction from the others, which contain also stony matters. They are vastly more rare than those of the other groups. The stony substances of the other groups consist chiefly of silica in combination with magnesia and peroxide of iron, as peridote or pyroxene. If these silicates are in small proportion and thinly scattered through the iron, they are syssiderites; if it is the iron that is in relatively small proportion and appearing only in isolated grains, they are sporadosiderites. In other meteorites, comparatively few in number, no metallic iron can be perceived, and they are called asiderites. The most interesting specimens among them are remarked by their dull-black color, and a general appearance like that of peat or lignite. Besides stony matters, they contain carbon in combination with hydrogen and oxygen—a chemical quality which has led to their being examined for remains of organic beings. But no trace of anything of the kind has been discovered. They also have escaped all alteration by heat beyond the superficial glazing, and thus strengthen the evidence that their origin is exterior to our globe.
Among all the diversities presented by the specimens of more than four hundred meteoric showers, is the remarkable fact that meteorites which have fallen at the most distant epochs, and in countries most remote from one another, not only conform to the same type, but present so complete an identity that their respective fragments can not be distinguished even upon a close mineralogical examination. Nothing in the exterior form of meteorites is more striking than a general aspect indicating that they are parts of a broken body. When we compare hundreds or thousands of stones of the same fall, we find that they all present polyhedral forms like those of stones broken for a macadamized road, except that the angles are more or less rounded. Even meteoric iron exhibits this angular shape, showing that its malleability and extreme tenacity have not preserved it from a violent rupture. It seemed impossible that such an effect could be produced solely by the action of the air, especially in the upper regions where it is in an extremely rarefied condition. But light has been thrown upon the problem since the introduction of the new explosives, which illustrate, in their industrial applications, the prodigious force that gases are capable of exerting, even in small quantity, when they are suddenly animated by a considerable tension. The explosion of a kilogramme of dynamite will break up bars of steel which a pressure of a million kilogrammes would hardly crack. Similar conditions concur in the upper strata of the atmosphere, slight as their density may be, when a meteor moving with planetary velocity strikes upon them. The body compresses the air more rapidly than it can yield, and transmits an equivalent motion to its own molecules. Under these circumstances, in the successive detonations caused by an enforced rotation, iron and the most tenacious bodies will fly into pieces, as if they were struck with a pile-driver.
There is another no less characteristic feature of the surface of meteorites which testifies to the violence of the mechanical action produced upon them by the atmospheric rebound, exhibited by rounded cavities resembling finger-marks. They appear in the stony meteors, but are particularly characteristic of the iron masses. These marks were at one time attributed to transient explosions taking place during the course of the meteor through the air; but experiment has shown that the same appearance is produced in bodies which are acted upon by an explosion of dynamite; in the grains of coarse powder that drop, half consumed, from the mouth of a cannon when it is fired, and upon the touch-hole of the cannon. They are all due to the same cause to the erosive action of gas revolving rapidly and moving spirally and under high pressure against the projectiles, boring into them as if it were a gimlet. The mechanical action is accompanied and aided by a chemical action which is dependent upon the combustible nature of iron at high temperatures. Although these blister-holes are worked only on the face which is exposed to the direct pressure of the gas, meteorites present them on various sides, and sometimes over their whole surface. This arises from the rotatory character of the motion of the body which makes it present every side in succession to the front.
With these mechanical phenomena of meteorites is connected the coming to the earth of dusts of celestial origin. In examining these, we must first be careful to separate the earthly dusts, with which the air is more or less loaded, of every kind, natural and artificial, from volcanoes and from waste tracts of the earth's surface, mineral, vegetable, and animal. These are recognizable by careful examination; and, after they are all detected, there remain still other dusts, which incontestably come to us from regions foreign to our globe. The carbonaceous meteorites of Orgueil furnish us a very interesting prime document respecting them. These bodies are so friable that they are reduced to powder under the slightest pressure of the fingers, and they would probably have been pulverized in their course through the air if they had not been protected by their heat-formed crust. Further, when aëroliths of this species are moistened with a little water, they are completely disaggregated and reduced to extremely fine particles in consequence of the solution of the alkaline salts which perform to them the part of a cement. Under this property, if it had been raining when the Orgueil meteorites fell, on the 14th of May, 1864, or if they had had to pass through a stratum of cloud, they would have been dissolved in their course, and all we should have found of them would have been a little black slime on the ground.
Extra-terrestrial dusts usually reach us under quite different circumstances, and without the intervention of water. The meteoric stones as they pass through the air are followed by a tail which, at first bright, soon blends itself with the darkness, like the smoke of a piece of fire-works. It keeps the direction of the path of the meteor for a longer or shorter time, and is undoubtedly composed of particles which have become detached from the body, and remain suspended in the atmosphere till they are scattered by the winds. The mode of the action by which this dust is rubbed or blown off is explained by the experiments I have made with explosive gases in the investigation of the origin of the blister-holes of meteorites. Masses of gas at enormous pressures almost instantaneously pulverize whatever bodies they strike, and this is precisely what happens to the meteoric stones as they pass through the air. Judging from the thickness of the clouds following the bodies and the space they occupy, we conclude that they furnish considerable quantities of metallic and rock-dusts to our atmosphere.
A careful investigation of the dusts which may be supposed to be of cosmic origin is very desirable, as also is a systematic examination of the atmosphere by all the means in our possession, after every explosion of a meteor, for that which they may have left. Something has been done in this direction by Mr. Phipson, M. Nordenskiöld, and M. Gaston Tissandier. Doubtless the shooting-stars, extreme as their tenuity may be, also bring down ponderable substances in minutely divided condition. The spectroscopic examination of these asteroids by Mr. Alexander Herschel has revealed the presence in them of sodium, magnesium, carbon, and other bodies. The fact is confirmed by the formation, in connection with the extraordinary meteoric shower of the 27th of November last, of a cloud of vapors which obscured all the stars except those of the first three magnitudes, and was shortly afterward dissipated.
The question, whether gaseous and invisible substances may not also be introduced to the earth from the realms of space, can not yet be answered from observation.
The most interesting resemblances, and even identities, are occasionally revealed between the meteorites and some of the deeper rocks of our planet.
Volcanoes bring up daily, besides prodigious quantities of vapor of water and gaseous products, melted, intensely hot stony matters, which spread upon their flanks and are known as lavas. During the ancient periods, there came out also, from the depths below the granite, rocks of a nature very different from that of the stratified rocks, and presenting an analogy with the lavas. They occur on the surface in various forms of sheets, cones, and irregular masses. Below it, they constitute, in the thickness of the incasing rocks, a kind of columns, which are connected with the extremely deep reservoirs from which they have been thrown up; they have in fact been thrust here and there in consequence of eruptions, through the superposed masses, far from their original bed. Like the lavas, they are composed chiefly of silicates.
We observe, in the first place, that most of the eruptive rocks differ considerably from meteorites. The most important point of contrast is that the latter contain nothing resembling the arenaceous or fossiliferous matters of which the stratified beds are constituted—that is, nothing suggesting the action and movement of an ocean or the presence of life. A great difference also appears in comparison with the masses on which the sedimentary beds immediately rest. Thus, we never find granite, or any of the minerals associated in it, in meteorites. The analogies of meteorites must be sought in the silicate rocks, which originate in deep regions, below the granite.
A striking example of this similitude is afforded by the recent lavas, which are formed from the association of two silicates, pyroxene and anorthite feldspar, and which correspond exactly with the meteorite picked up at Jonzac (Charente-Inférieure) on the 15th of June, 1819, and with that one which fell at Juvinas, in the department of Ardèche, on the 13th of June, 1821. Peridote, which is remarkably constant in meteorites, also occurs in the eruptive masses, often abundantly.
An equally remarkable fact is the absence from meteorites of the whole series of rocks which form an important part of the crust of the globe. It may be explained by supposing that the meteoric stones that reach us come exclusively from the internal parts of planetary bodies constituted as our globe is, or that those bodies are destitute of quartziferous silicates, like granite, as well as of the stratified beds. In the latter case, those stars have suffered less complete revolutions than our planet, and exhibit no traces of the co-operation of an ocean by which most of the crust of the earth above the internal masses has received its shaping.
A recent unexpected discovery, made by M. Nordenskiöld in Greenland, has shown the resemblances we have just described to be closer and more complete. It is worthy of remark that, notwithstanding the abundance with which iron is diffused in all parts of the crust of the earth, that metal has never been found in the native state. However pure and rich may be the mineral, some kind of a process is necessary to extract the metal contained in it. This peculiarity is due to the sensitiveness of iron in the presence of chemical agents, particularly of oxygen. Sir John Ross brought back from his arctic voyage, in 1818, some knives with blades formed of pieces of iron which the Esquimaux said came from scattered blocks not far from Cape York. The analysis of this iron showing the presence of nickel, a meteoric origin was attributed to it. Other samples of iron, offering similar characteristics, were brought down from the North by other explorers. M. Nordenskiöld's attention was attracted to some of these specimens, which had been deposited in the museum at him to endeavor to ascertain their origin during his voyage to Northern Greenland in 1870. After much searching, with the aid of what directions the natives could give him, he at last found the object of his investigations in the hill of Blaafjeld, or Ovifak, near Disco Island. Blocks of iron were lying on the shore at the foot of a high cliff composed of basalt and conglomerates of the same rock in alternation; and more than twenty masses, containing not less than twenty-one thousand kilogrammes of metallic iron, were collected within a small space. They were at first supposed to be of meteoric origin, because they contained nickel, and exhibited figures which had been regarded as peculiar to meteoric iron. But this view was proved to be incorrect when M. Steenstrup, under a commission from the Danish Government to investigate the conditions under which the iron occurred, found, at one point on the coast, native iron actually imbedded in the basaltic rocks, the appearance of the larger grains of which was precisely similar to that of the scattered blocks previously found. The presence, in the eruptive rocks of the earth, of iron alloyed with nickel, similar to meteoric iron, and having the crystalline texture which had previously appeared to be an exclusive characteristic of the latter, has therefore become incontestable. It is proper to add that the metal in this condition is not a fortuitous and isolated accident in Greenland, but that it is found in many places and over considerable districts., and prompted
The geological structure of the northern part of that country is especially distinguished by the development of eruptive rocks of a relatively very recent age. It is one of the largest masses of basalt with which we are acquainted. It begins at the sixty-ninth degree of latitude, and disappears near the seventy-sixth degree, under the vast continental glacier which prevents all further exploration of the surface. It is reasonable to suppose that the eruptions of which these rocks are the result brought up metallic iron, of which they seem to indicate the existence of large masses in the deep interior. This fact has also to be taken account of in the theory of terrestrial magnetism.
After having sketched, twenty years ago, the numerous features of resemblance between the meteorites and the deep terrestrial rocks, and having shown how some of them can be imitated by a partial deoxidation of those rocks, I added: "There is nothing to prove that beneath those aluminiferous masses which have furnished, in Iceland, for example, lavas analogous to the meteorites of Juvinas, that beneath our peridotic rocks which the meteorite of Chassigny closely resembles, there may not be found masses in which native iron begins to appear, or resembling meteorites of the common type; then, below these, types richer and richer in iron, of which the meteorites offer a series of increasing density, from those in which iron represents nearly half the weight of the rock to massive iron." Five years after these lines were written, the great masses of native iron alloyed with nickel, of which we have just spoken, were discovered by M. Nordenskiöld. The discussions upon their origin, which we hesitated at first to regard as terrestrial, sufficed to bring out the close analogies between them and the meteorites. The study of the latter bodies has, then, permitted us to penetrate by induction into the internal constitution of our globe, as if by a side-look into depths wholly inaccessible to direct observation. The last demarkation has thus been effaced, and a most intimate connection has been established between the masses thrown up from the interior of our planet and the celestial masses of which the meteorites bring us the fragments.
The analogies which we have pointed out between meteorites and the profound regions of our globe testify to the identity of the chemical actions, even in the formation of stars very distant from each other. In fact, a mineral generally suggests, in a precise manner, the circumstances under which it originated. We might say that in itself it tells the story of its origin, especially when it can be reproduced experimentally. We thus perceive how reason, assisted by experiment, can give us clews to the formation of the stars of which we possess fragments. Silica or silicic acid is a chemical agent, the energy of which becomes very considerable at high temperatures; it is also the characteristic element of numerous products formed in industrial furnaces, like glass, scoriæ, and slags, and of the lavas of volcanoes. All the silicates, artificial and natural, when free from water, or in the anhydrous state, denote the dominance of a high temperature over their formation.
Suppose that silicon and the metals were not originally combined with oxygen as they are now, either because the different elements were not near enough together in the primordial chaos, or their temperature was not high enough to permit them to enter into combination. When oxygen comes into action, it unites at first with the elements for which it has a predominant affinity, primarily silicon and magnesium, then iron and nickel; and, if the gas is not in excess, it leaves a residue composed of the less oxidable bodies. Iron and nickel would in that case be left in a free state, disseminated among the stony silicates. This is exactly what is observed in the meteorites; and it is also a fact which I have confirmed by experiment. By producing the conditions that have just been mentioned, I obtained an imitation in essential points of meteorites of the common type, with the production of a silicate of magnesia and protoxide of iron, having exactly the constitution of peridote.
Furthermore, one of the best-known every-day metallurgical operations, the decarbonization of cast-iron, or its transformation into malleable iron or steel, gives an analogous reaction and ends in a result of the same kind. Whether the process be carried on in little charcoal-furnaces, as in antiquity, or in puddling-furnaces, or, as by the Bessemer process, without the addition of any combustible, it is always the oxygen of the air which burns, not only the charcoal, but also the silicon in the pig and a part of the iron. The black scoria which is formed in the process often contains a peridote with a base of iron, having the same chemical constitution and crystalline form as the magnesian peridote of the eruptive rocks of the earth and of the meteorites.
The simple oxidation of silicon develops an enormous quantity of heat, very much more than the combustion of carbon; a heat which is sufficient to refine the metal in the retorts of iron and steel works without the addition of carbon. Silicon, which in nature has passed wholly into the state of silicic acid, or been burned, must, at the moment of its combination with oxygen, have been the cause of an intense heating both in our own globe and in the other stars, which are also composed of silicates. But in the last, of which meteorites are the fragments, the temperature was not probably so high as in the metallurgical furnaces and the experiments we have cited. It is, in fact, very remarkable that, notwithstanding their tendency to a distinct crystallization, the silicate compounds of which the meteorites are constituted are only in the condition of very small and quite confused crystals, as if they had not passed through fusion. We might say that, rather than the long needles of ice which liquid water forms in freezing, their fine-grained texture resembles that of frost and snow, which is known to be due to the immediate passage of atmospheric aqueous vapor to the solid state.
In brief, the extreme tendency of the oxidation of silicon to produce the formation of peridote, daily proved in our laboratories and shops, is no less evidently manifested in the deeper rocks of our globe, on the one side, and in the distant stars from which the meteorites come, on the other side. Everywhere are observed the effects of an ancient and vast oxidation. In this we have a simple and experimental explanation of the ubiquity of peridote. It is the universal scoria.
As a forest shows at a glance the plant-life of all ages, the universe presents us stars in all the phases of their existence, from that of incandescent heat to obscurity, and an advanced cooling. We have also just seen that some of them are in demolition, and that their fragments fall upon others, to which they remain attached. The numerous falls of meteorites on our globe teach us that this fact, instead of being an exception, answers to an habitual régime. And the constitution of the meteoric masses teaches us with certainty that the celestial bodies whence they emanate have a chemical history quite similar to that of the interior regions of our planet.
So, while the exploration of the sky reveals to us millions of worlds beyond our solar system, our planet, small as it is, offers us an example of the changes which the stars have undergone, and an episode in the general history of the universe. The meteorites form a kind of line of union between the succession of the epochs of the earth, the object of geological study, and the constitution of the sky, the aim of astronomical research. These two parts of human knowledge reflect complementary lights upon one another.—Translated for the Popular Science Monthly from the Revue des Deux Mondes.