But the most interesting inquiry into this subject is that by Professor Kapteyn of Groningen, one of the most painstaking students of the distribution of the stars. He founds his conclusions mainly on the proper motions of the stars, this being the best general indication of distance in the absence of actual determination of parallax. He made use of the proper motions and the spectra of more than two thousand stars, and he finds that a considerable body of stars having large proper motions, and also presenting the solar type of spectra, surround our sun in all directions, and show no increased density, as the more distant stars do, towards the Milky Way. He finds also that towards the centre of this cl.u.s.ter stars are far closer together than near its outer limits (he says there are ninety-eight times as many), that it is roughly spherical in shape, and that the maximum compression is, as nearly as can be ascertained, at the centre of the circle of the Milky Way, while the sun is at some distance away from this central point.[6]

It is a very suggestive fact that most of the stars belonging to this cl.u.s.ter have spectra of the solar type, which indicates that they are of the same general chemical const.i.tution as our sun, and are also at about the same stage of evolution; and this may well have arisen from their origin in a great nebulous ma.s.s situated at or near the centre of the galactic plane, and probably revolving round their common centre of gravity.

As Kapteyn's result was based on materials which were not so full or reliable as those now available, Professor S. Newcomb has examined the question himself, using two recent lists of stars, one limited to those having proper motions of 10" a century, of which there are 295, and the other of nearly 1500 stars with 'appreciable proper motions.' They are situated in two zones, each about 5 in breadth and cutting across the Milky Way in different parts of its course. They afford, therefore, a good test of the distribution of these nearer stars with regard to the Galaxy.

The result is, that on the average these stars are not more numerous in or near the Milky Way than elsewhere; and Professor Newcomb expresses himself on this point as follows:--'The conclusion is interesting and important. If we should blot out from the sky all the stars having no proper motion large enough to be detected, we should find remaining stars of all magnitudes; but they would be scattered almost uniformly over the sky, and show little or no tendency to crowd towards the Galaxy, unless, perhaps, in the region near 19h. of Right Ascension.'[7]

A little consideration will show that, as the stars of all magnitudes which are, on the average, nearest to us are spread over the sky in 'all directions' and 'almost uniformly,' this necessarily implies that they form a cl.u.s.ter or group, and that our sun is somewhere not very far from the centre of this group. Again, Professor Newcomb refers to 'the remarkable equality in the number of stars in opposite directions from us.

We do not detect any marked difference between the numbers lying round the opposite poles of the Galaxy, nor, so far as known, between the star-density in different regions at equal distances from the Milky Way'

(_The Stars_, p. 315). And again he refers to the same question at p. 317, where he says: 'So far as we can judge from the enumeration of the stars in all directions, and from the aspect of the Milky Way, our system is near the centre of the stellar universe.'

It will, I think, now be clear to my readers that the four main astronomical propositions stated in my article which appeared in the New York _Independent_ and in the _Fortnightly Review_, and which were either denied or declared to be unproved by my astronomical critics, have been shown to be supported by so many converging lines of evidence, that it is no longer possible to deny that they are, at least provisionally, fairly well established. These facts are, (1) that the stellar universe is not of infinite extent; (2) that our sun is situated in the central plane of the Milky Way; (3) that it is also situated near to the centre of that plane; (4) that we are surrounded by a group or cl.u.s.ter of stars of unknown extent, which occupy a place not far removed from the centre of the galactic plane, and therefore, near to the centre of our universe of stars.

Not only are these four propositions each supported by converging lines of evidence, including some which I believe have not before been adduced in their support, but a number of astronomers, admittedly of the first rank, have arrived at the same conclusions as to the bearing of the evidence, and have expressed their convictions in the clearest manner, as quoted by me.

It is _their_ conclusions which I appeal to and adopt; yet my two chief astronomical critics positively deny that there is any valid evidence of the finiteness of the stellar universe, which one of them terms 'a myth,'

and he even accuses _me_ of having started it. Both of them, however, agree in stating very strongly one objection to my main thesis--that our central position (not necessarily at the precise centre) in the stellar universe has a meaning and a purpose, in connection with the development of life and of man upon this earth, and, _so far as we know_, here only. With this one objection, the only one that in my opinion has the slightest weight, I will now proceed to deal.

THE SUN'S MOTION THROUGH s.p.a.cE

The two astronomers who did me the honour to criticise my original article laid the greatest stress on the fact, that even if I had proved that the sun now occupied a nearly central position in the great star-system, it was really of no importance whatever, because, at the rate the sun was travelling, 'five million years ago we were deep in the actual stream of the Milky Way; five million years hence we shall have completely crossed the gulf which it encircles, and again be a member of one of its const.i.tuent groups, but on the opposite side. And ten million years are regarded by geologists and biologists as but a trifle on account to meet their demands upon the bank of Time.' Thus speaks one of my critics. The other is equally crus.h.i.+ng. He says:--'If there is a centre to the visible universe, and if we occupy it to-day, we certainly did not do so yesterday, and shall not do so to-morrow. The Solar System is known to be moving among the stars with a velocity which would carry us to Sirius within 100,000 years, if we happened to be travelling in his direction, as we are not. In the 50 or 100 million years during which, according to geologists, this earth has been a habitable globe, we must have pa.s.sed by thousands of stars on the right hand and on the left.... In his eagerness to limit the universe in s.p.a.ce, Dr. Wallace has surely forgotten that it is equally important, for his purpose, to limit it in time; but incomparably more difficult in the face of ascertained facts.... Indeed, so far from our having tranquilly enjoyed a central position in unbroken continuity for scores or perhaps hundreds of millions of years, we should in that time have traversed the universe from boundary to boundary.'[8]

Now the average reader of these two criticisms, taking account of the high official position of both writers, would accept their statements of the case as being demonstrated facts, requiring no qualification whatever, and would conclude that my whole argument had been thereby rendered worthless, and all that I founded upon it a fantastic dream. But if, on the other hand, I can show that their stated facts as to the sun's motion are by no means demonstrated, because founded upon a.s.sumptions which may be quite erroneous; and further, that if the facts should turn out to be substantially correct, they have both omitted to state well-known and admitted qualifications which render the conclusions they derive from the facts very doubtful, then the average reader will learn the valuable lesson that official advocacy, whether in medicine, law, or science is never to be accepted till the other side of the case has been heard. Let us see, therefore, what the facts really are.

Professor Simon Newcomb calculates that, if there are one hundred million stars in the stellar universe each five times the ma.s.s of our sun, and spread over a s.p.a.ce which light would require thirty thousand years to cross, then any ma.s.s traversing such a system with a velocity of more than twenty-five miles a second, would fly off into infinite s.p.a.ce never to return. Now as there are many stars which have, apparently, very much more than this velocity, it would follow that the visible universe is unstable.

It also implies that these great velocities were not acquired in the system itself, but that the bodies which possess them must have entered it from without, thus requiring other universes as the feeders of our universe.

For the accuracy of the above statement the authority of Professor Newcomb is an ample guarantee; but there may be modifications required in the data on which it is founded, and these may greatly alter the result. If I do not mistake, the estimate of a hundred million stars is founded on actual counts or estimates of stars of successive magnitudes in different parts of the heavens, and it does not include either those of the denser star cl.u.s.ters nor the countless millions just beyond the reach of telescopes in the Milky Way. Neither does it make allowance for the dark stars supposed by some astronomers to be many times more numerous than the bright ones, nor for the vast number of the nebulae, great and small, in calculating the total ma.s.s of the stellar system.[9] In his latest work Professor Newcomb says, 'The total number of stars is to be counted by hundreds of millions'; and hence the controlling power of the system on bodies within it will be many times greater than that given above, and might even be ample to retain within its bounds such a rapidly moving star as Arcturus, which is believed to be travelling at the rate of more than three hundred miles a second. But there is another very important limitation to the conclusions to be drawn from Professor Newcomb's calculation. It a.s.sumes the stars to be nearly uniformly distributed through the whole of the s.p.a.ce to which the system extends. But the facts are very different. The existence of cl.u.s.ters, some of which comprise many thousands of stars, is one example of irregularity of distribution, and anyone of these larger cl.u.s.ters would probably be able to change the course of even the swiftest stars pa.s.sing near it. The larger nebulae might have the same effect, since the late Mr. Ranyard, taking all his data so as to produce a minimum result, calculated the probable ma.s.s of the Orion nebula to be four and a half million times that of the sun, and there may be many other nebulae equally large. But far more important is the fact of the vast ring of the Milky Way, which is now universally held by astronomers to be, not only apparently but really, more densely crowded with stars and also with vast ma.s.ses of nebulous matter than any other part of the heavens, so that it may possibly comprise within itself a very large proportion of the whole of the matter of the visible universe. This is rendered more probable by the fact that the great majority of star-cl.u.s.ters lie along its course, most of the huge gaseous stars belong to it, while the occurrence there only of 'new stars' is evidence of a superabundance of matter in various forms leading to frequent heat-producing collisions, just as the frequent occurrence of meteoric showers on our earth is evidence of the superabundance of meteoric matter in the solar system.

It is recognised by mathematicians that within any great system of bodies subject to the law of gravitation there can be no such thing as motion of any of them in a straight line; neither can any amount of motion arise within such a system through the action of gravitation alone capable of carrying any of its ma.s.ses out of the system. The ultimate tendency must be towards concentration rather than towards dispersal.

It seems, therefore, only reasonable to consider whatever motions and whatever velocities we find among the stars, as having been produced by the gravitative power of the larger aggregations, modified perhaps by electrical repulsive forces, by collisions, and by the results of those collisions; and we may look to the changes now visibly going on in some of the nebulae and cl.u.s.ters as indications of the forces that have probably brought about the actual condition of the whole stellar universe.

If we examine the beautiful photographs of nebulae by Dr. Roberts and other observers, we find that they are of many forms. Some are extremely irregular and almost like patches of cirrus clouds, but a large number are either distinctly spiral in form, or show indications of becoming spiral, and this has been found to be the case even with some of the large irregular nebula. Then again we have numerous ring-formed nebulae, usually with a star involved in dense nebulosity in the centre, separated by a dark s.p.a.ce of various widths from the outer ring. All these kinds of nebulae have stars involved in them, and apparently forming part of their structure, while others which do not differ in appearance from ordinary stars are believed by Dr. Roberts to lie between us and the nebula. In the case of many of the spiral nebulae, stars are often strung along the coils of the spiral, while other curved lines of stars are seen just outside the nebula, so that it is impossible to avoid the conclusion that both are really connected with it, the outer lines of stars indicating a former greater extension of the nebula whose material has been used up in the growth of these stars. Some of these spiral nebulae show beautifully regular convolutions, and these usually have a large central star like ma.s.s, as in M. 100 Comae and I. 84 Comae, in Vol. II. Pl. 14 of Dr. Roberts's photographs. The straight white streaks across the nebula of the Pleiades and some others are believed by Dr. Roberts to be indications of spiral nebulae seen edgewise. In other cases, cl.u.s.ters of stars are more or less nebulous, and the arrangement of the stars seems to indicate their development from a spiral nebula. It is to be noted that many of the objects cla.s.sed as planetary nebulae by Sir John Herschel are shown by the best photographs to be really of the ring-type, though often with a very narrow division between the ring and the central ma.s.s. This form may therefore be of frequent occurrence.

But if this annular form with some kind of central nucleus, often very large, is produced under certain conditions by the action of the ordinary laws of motion upon more or less extensive ma.s.ses of discrete matter, why may not the same laws acting upon similar matter once dispersed over the whole extent of the existing stellar universe, or even beyond what are now its farthest limits, have led to the aggregation of the vast annular formation of the Milky Way, with all the subordinate centres of concentration or dispersal to be found within or around it? And if this is a reasonable conception, may we not hope that by a concentration of attention upon a few of the best marked and most favourably situated annular and spiral systems, sufficient knowledge of their internal motions may be obtained which may serve as a guide to the kind of motion we may expect to find in the great galactic ring and its subordinate stars? We may then perhaps discover which now seem so erratic, are really all parts of a series of orbital movements limited and controlled by the forces of the great system to which they belong, so that, if not mathematically stable, they may yet be sufficiently so to endure for some thousand millions of years.

It is a suggestive fact that the calculated position of the 'solar apex'--the point towards which our sun appears to move--is now found to be much more nearly in the plane of the Milky Way than the position first a.s.signed to it, and Professor Newcomb adopts, as most likely to be accurate, a point near the bright star Vega in the constellation Lyra.

Other calculators have placed it still farther east, while Rancken and Otto Stumpe a.s.sign it a position actually in the Milky Way; and Mr. G.C. Bompas concludes that the sun's plane of motion nearly coincides with that of the Galaxy. M. Rancken found that 106 stars near the Milky Way showed, in their very small proper motions, a drift along it in a direction from Ca.s.siopeiae towards Orion, and this, it is supposed, may be partly due to our sun's motion in an opposite direction.

In many other parts of the heavens there are groups of stars which have almost identical proper motions--a phenomenon which the late R.A. Proctor termed 'star-drift'; and he especially pointed out that five of the stars of the Great Bear were all drifting in the same direction; and although this has been denied by later writers, Professor Newcomb, in his recent book on _The Stars_, declares that Proctor was right, and explains that the error of his critics was due to not making allowance for the divergence of the circles of right ascension. The Pleiades are another group, the stars of which drift in the same direction, and it is a most suggestive fact that photographs now show this cl.u.s.ter to be embedded in a vast nebula, which, therefore, has also a proper motion; but some of the smaller stars do not partake of it. Three stars in Ca.s.siopeiae also move together, and no doubt many other similarly connected groups remain to be discovered.

These facts have a very important bearing on the question of the motion of our sun in s.p.a.ce. For this motion has been determined by comparing the motions of large numbers of stars which are a.s.sumed to be wholly independent of each other, and to move, as it were, at random. Miss A.M.

Clerke, in her _System of the Stars_, puts this point very clearly, as follows: 'For the a.s.sumption that the absolute movements of the stars have no preference for one direction over another, forms the basis of all investigations. .h.i.therto conducted into the translatory advance of the solar system. The little fabric of laboriously acquired knowledge regarding it at once crumbles if that basis has to be removed. In all investigations of the sun's movement, the movements of the stars have been regarded as casual irregularities; should they prove to be in any visible degree systematic, the mode of treatment adopted (and there is no other at present open to us) becomes invalid, and its results null and void. The point is then of singular interest, and the evidence bearing upon it deserves our utmost attention.'

Mr. W.H.S. Monck, a well-known astronomer, takes the same view. He says: 'The proof of this motion rests on the a.s.sumption that if we take a sufficient number of stars, their real motions in all directions will be equal, and that therefore the apparent preponderances which we observe in particular directions result from the real motion of the sun. But there is no impossibility in a systematic motion of the majority of the stars used in these researches which might reconcile the observed facts with a motionless sun. And, in the second place, if the sun is not in the exact centre of gravity of the universe, we might expect him to be moving in an orbit around this centre of gravity, and our observations on his actual motion are not sufficiently numerous or accurate to enable us to affirm that he is moving in a right line rather than such an orbit.'

Now this 'systematic motion,' which would render all calculations as to the sun's motion inaccurate or even altogether worthless, is by many astronomers held to be an observed reality. The star-drift, first pointed out by Proctor, has been shown to exist in many other groups of stars, while the curious arrangements of stars all over the heavens in straight lines, or regular curves, or spirals, strongly suggests a wide extension of the same kind of relation. But even more extensive systematic movements have been observed or suggested by astronomers. Sir D. Gill, by an extensive research, believes that he has found indications of a rotation of the brighter fixed stars as a whole in regard to the fainter fixed stars as a whole. Mr. Maxwell Hall has also found indications of a movement of a large group of stars, including our sun, around a common centre, situated in a direction towards Epsilon Andromedae, and at a distance of about 490 years of light-travel. These last two motions are not yet established; but they seem to prove two important facts--(_a_) that eminent astronomers believe that _some_ systematic motions must exist among the stars, or they would not devote so much labour to the search for them; and (_b_) that extensive systematic motions of some kind do exist, or even these results would not have been obtained.

Mr. W.W. Campbell, of the Lick Observatory, thus remarks on the uncertainty of determinations of the sun's motions: 'The motion of the solar system is a purely relative quant.i.ty. It refers to specified groups of stars. The results for various groups may differ widely, and all be correct. It would be easy to select a group of stars with reference to which the solar motion would be reversed 180 from the values a.s.signed above' (_Astrophysical Journal_, vol. xiii. p. 87. 1901).

It must be remembered that, within a uniform cl.u.s.ter of stars, each moving round the common centre of gravity of the whole cl.u.s.ter, Kepler's laws do not prevail, the law being that the angular velocities are all identical, so that the more distant stars move faster than those nearer the centre, subject to modifications, however, due to the varying density of the cl.u.s.ter. But if the cl.u.s.ter is nearly globular, there must be stars moving round the centre in every plane, and this would lead to apparent motions in many directions as viewed by us, although those which were moving in the same plane as ourselves would, when compared with remote stars outside the cl.u.s.ter, appear to be all moving in the same direction and at the same rate, forming, in fact, one of those drifting systems of stars already referred to. Again, if in the process of formation of our cl.u.s.ter, smaller aggregations already having a rotatory motion were drawn into it, this might lead to their revolving in an opposite direction to those which were formed from the original nebula, thus increasing the diversities of apparent motion.

The evidence now briefly set forth fully justifies, I submit, the remarks as to the statements of my astronomical critics at the beginning of this section. They have both given the accepted views as to direction and rate of movement of our sun without any qualification whatever, as if they were astronomical facts of the same certainty and the same degree of accuracy as the sun's distance from the earth; and they will a.s.suredly have been so understood by the great body of non-mathematical readers. It appears, however, if the authorities I have quoted are right, that the whole calculation rests upon certain a.s.sumptions, which are certainly to some extent, and may be to a very large extent, erroneous. This is my reply to one part of their criticism.

In the next place, they both a.s.sert, or imply, not only that the sun's motion is now in a straight line, but that it has been in a straight line from some enormously remote period when it first entered the stellar system on one side, and will so continue to move till it reaches the utmost bounds of that system on the other side. And this is stated by them both, not as a possibility, but as a certainty. They use such terms as 'must' and 'will be,' leaving no room for any doubt whatever. But such a result implies the abrogation of the law of gravitation, since under its action motion in a straight line in the midst of thousands or millions of suns of various sizes is an absolute impossibility; while it also implies that the sun must have been started on its course from some other system outside the Milky Way, with such a precise determination of direction as not to collide with, or even make a near approach to, any one of the suns or cl.u.s.ters of suns, or vast nebulous ma.s.ses, during its pa.s.sage through the very midst of the stellar universe.

This is my reply to the main point of their criticism, and I think I am justified in saying that nothing in my whole article is so demonstrably baseless as the statements I have now examined.

Considering then the whole bearing of the evidence, I refuse to accept the unsupported dicta of those who would have us believe that our admitted position not far from the centre of the stellar universe is a mere temporary coincidence of no significance whatever; or that our sun and hosts of other similar orbs near to us have come together by an accident, and are being dispersed into surrounding s.p.a.ce, never to meet again. Until this is proved by indisputable evidence, it seems to me far more probable that we are moving in an orbit of some kind around the centre of gravity of a vast cl.u.s.ter, as determined by the investigations of Kapteyn, Newcomb, and other astronomers; and, consequently, that the nearly central position we now occupy may be a permanent one. For even if our sun's...o...b..t should have a diameter a thousand times that of Neptune, it would be but a small fraction of the diameter of the Milky Way; while so vast is the scale of our universe, that it might be even a hundred thousand times as great and still leave us deeply immersed in the solar cl.u.s.ter, and very much nearer to the dense central portion than to its more diffused outer regions.

Here the subject may be left for the present. After having studied the evidence afforded by the essential conditions of life-development on the earth, and the numerous indications that these conditions do not exist on any of the other planets of the solar system, it may be again touched upon in a general review of the conclusions arrived at.

FOOTNOTES:

[4] _Nature_, October 26, 1899.

[5] _The System of the Stars_, p. 385.

[6] This account of Professor Kapteyn's research is taken from an article by Miss A.M. Clerke in _Knowledge_, April 1893.

[7] _The Stars_, p. 256. The region here referred to is that where the Milky Way has its greatest width (though nearly as wide in the part exactly opposite), and where it may perhaps extend somewhat in our direction. Miss A.M. Clerke informs me that in April 1901 Kapteyn withdrew the conclusions arrived at in 1893, as being founded on illegitimate reasoning as to the relation of parallaxes to proper motions. But as this relation is still accepted, under certain limitations, by Professor Newcomb and other astronomers, who have arrived independently at very similar results, it seems not improbable that, after all, Professor Kapteyn's conclusions may not require very much modification. Professor Newcomb also tells us (_The Stars_, p. 214, footnote) that he has seen the latest of Professor Kapteyn's papers, down to 1901; but he does not therefore express any doubt as to his own conclusions as here referred to.

[8] See _Knowledge_ and _The Fortnightly Review_ of April 1903.

[9] Sir R. Ball in an article in _Good Words_ (April 1903) says that luminosity is an exceptional phenomenon in nature, and that luminous stars are but the glow-worms and fire-flies of the universe, as compared with the myriads of other animals.

CHAPTER IX

THE UNIFORMITY OF MATTER AND ITS LAWS THROUGHOUT THE STELLAR UNIVERSE

I have shown in the second chapter of this work that none of the previous writers on the question of the habitability of the other planets have really dealt with the subject in any adequate manner, since not only do they appear to be quite unaware of the delicate balance of conditions which alone renders organic life possible on any planet, but they have altogether omitted any reference to the fact that not only must the conditions be such as to render life possible _now_, but these conditions must have persisted during the long geological epochs needed for the slow development of life from its most rudimentary forms. It will therefore be necessary to enter into some details both as to the physical and chemical essentials for a continuous development of organic life, and also into the combination of mechanical and physical conditions which are required on any planet to render such life possible.

THE UNIFORMITY OF MATTER

One of the most important and far-reaching of the discoveries due to the spectroscope is that of the wonderful ident.i.ty of the elements and material compounds in earth and sun, stars and nebulae, and also of the ident.i.ty of the physical and chemical laws that determine the states and forms a.s.sumed by matter. More than half the total number of the known elements have been already detected in the sun, including all those which compose the bulk of the earth's solid material, with the one exception of oxygen. This is a very large proportion when we consider the very peculiar conditions which enable us to detect them. For we can only recognise an element in the sun when it exists at its surface in an incandescent state, and also above its surface in the form of a somewhat cooler gas. Many of the elements may rarely or never be brought to the surface of so vast a body, or if they do sometimes appear there, it may not be in sufficient quant.i.ty or in sufficient purity to produce any bands in the spectroscope, while the cooler gas or vapour may either not be present, or be so dispersed as not to produce sufficient absorption to render its spectral lines visible. Again, it is believed that many elements are dissociated by the intense heat of the sun, and may not be recognisable by us, or they may only exist at its surface in a compound form unknown on the earth; and in some such way those lines of the solar spectrum which remain still unrecognised may have been produced. One of these unknown lines was that of Helium, a gas found soon afterwards in the rare mineral 'Cleveite,' and since detected frequently in many stars. Some of the stars have spectra very closely resembling that of the sun. The dark lines are almost as numerous, and most of them correspond accurately with solar lines, so that we cannot doubt their having almost exactly the same chemical const.i.tution, and being also in the same condition as regards heat and stage of development. Other stars, as we have already stated, exhibit mainly lines of hydrogen, sometimes combined with fine metallic lines. Of the spectra of the nebulae comparatively little is known, but many are decidedly gaseous, while others show a continuous spectrum indicating a more complex const.i.tution.

But we also obtain considerable knowledge of the matter of non-terrestrial bodies by the a.n.a.lysis of the numerous meteorites which fall upon the earth. Most of these belong to some of the many meteoric streams which circulate round the sun, and which may be supposed to give us samples of planetary matter. But as it is now believed that many of them are produced by the debris of comets, and the orbits of some of these indicate that they have come from stellar s.p.a.ce and have been drawn into our system by the attractive power of the larger planets, it is almost certain that the meteoric stones not infrequently bring us matter from the remoter regions of s.p.a.ce, and probably afford us samples of the solid const.i.tuents of nebula; or the cooler stars. It is, therefore, a most suggestive fact that none of these meteorites have been found to contain a single non-terrestrial element, although no less than twenty-four elements have been found in them, and it will be of interest to give the list of these, as follows:--_Oxygen_, Hydrogen, _Chlorine_, _Sulphur_, _Phosphorus_, Carbon, Silicon, Iron, Nickel, Cobalt, Magnesium, Chromium, Manganese, Copper, Tin, _Antimony_, Aluminium, Calcium, Pota.s.sium, Sodium, _Lithium_, t.i.tanium, _a.r.s.enic_, and Vanadium. Seven of the above, printed in italics, have not yet been found in the sun, such as Oxygen, Chlorine, Sulphur, and Phosphorus, which form the const.i.tuents of many widespread minerals, and they supply important gaps in the series of solar and stellar elements. It may be noted that although meteorites have supplied no new elements, they have furnished examples of some new combinations of these elements forming minerals distinct from any found in our rocks.

The fact of the occurrence in meteorites not only of minerals which are peculiar to them or are found on the earth, but also of structures resembling our breccias, veins, and even slicken-side surfaces, has been held to be opposed to the meteoritic theory of the origin of suns and planets, because meteorites seem to be thus proved to be the fragments of suns or worlds, not their primary const.i.tuents. But these cases are exceptional, and Mr. Sorby, who made a special study of meteorites, concluded that their materials have usually been in a state of fusion or even of vapour, as they now exist in the sun, and that they became condensed into minute globular particles, which afterwards collected into larger ma.s.ses, and may have been broken up by mutual impact, and again and again become aggregated together--thus presenting features which are completely in accordance with the meteoritic theory.

But, quite recently, Mr. T.C. Chamberlin has applied the theory of tidal distortion to showing how solid bodies in s.p.a.ce, without ever coming into actual contact, must sometimes be torn apart or disrupted into numerous fragments by pa.s.sing near to each other. Especially when a small body pa.s.ses near a much larger one, there is a certain distance of approach (termed the Roche limit) when the increasing differential force of gravity will be sufficient to tear asunder the smaller body and cause the fragments either to circulate around it or to be dispersed in s.p.a.ce.[10] In this way, therefore, those larger meteorites which exhibit planetary structure may have been produced. Of course they would rarely have been true planets attached to a sun, but more frequently some of the smaller dark suns, which may possess many of the physical characteristics of planets, and of which there may be myriads in the stellar s.p.a.ces.