FOOTNOTES:

FOOTNOTES:[62]Lorentz,Proceedings Amsterdam Academy, 1900, p. 565.[63]Sir W. Thomson,Philosophical Magazine, vol. xlv., fourth series, 1873. Many hints have been taken, in what is above written, from this valuable paper.[64]Sir W. Thomson (Lord Kelvin),loc. cit., p. 323.[65]Electricity and Matter, p. 159.[66]Loc. cit., p. 331.[67]Traité de Mécanique Céleste, tome. iv., p. 495.[68]Electricity and Matter, p. 160.[69]Proceedings Amsterdam Academy of Sciences, 1900, p. 559.[70]Monthly Notices, vol. lxii., p. 619; vol. lxiii., p. 258.[71]Monthly Notices, vol. lxiii., p. 424.[72]J. J. Thomson,Electricity and Matter, p. 88.[73]Ibid., p. 47.

[62]Lorentz,Proceedings Amsterdam Academy, 1900, p. 565.

[63]Sir W. Thomson,Philosophical Magazine, vol. xlv., fourth series, 1873. Many hints have been taken, in what is above written, from this valuable paper.

[64]Sir W. Thomson (Lord Kelvin),loc. cit., p. 323.

[65]Electricity and Matter, p. 159.

[66]Loc. cit., p. 331.

[67]Traité de Mécanique Céleste, tome. iv., p. 495.

[68]Electricity and Matter, p. 160.

[69]Proceedings Amsterdam Academy of Sciences, 1900, p. 559.

[70]Monthly Notices, vol. lxii., p. 619; vol. lxiii., p. 258.

[71]Monthly Notices, vol. lxiii., p. 424.

[72]J. J. Thomson,Electricity and Matter, p. 88.

[73]Ibid., p. 47.

THE INEVITABLE ETHER

Etheris the fundamental postulate of physics. Its existence, nowise apparent, is in all manner of ways implied. The properties that must be assigned to it are certainly arduous of conception. We need the aid of forced analogies to enable us to realize, even imperfectly and indistinctly, the mode in which it discharges functions obviously somehow discharged. But in the last resort everything is obscure; if our thought-borings go deep enough, they always reach the incomprehensible.

The original ether was the 'quintessence' of the ancients—a kind of matter vaguely imagined as pure and incorruptible enough to serve for the raw material of the heavenly bodies, the four common elements being reserved exclusively for sublunary use. Thedistinction, however, eventually broke down. All the spheres, from theprimum mobileto the very surface of our low earth, are pervaded by a subtle mode of action, demanding for its transmission machinery of a finer kind than could be constructed out of gross everyday matter. The phenomena of light, when they came to be attentively studied, imperatively required a medium, universally diffused, evasive to sense, accessible only by processes of reasoning. Hooke and Newton accordingly brought the ether through the Horn Gate of dreamland into a region of reality, where it became a subject of legitimate speculation to men of science. The task, nevertheless, of definitely specifying its qualities was not taken seriously in hand until the beginning of the nineteenth century, when the establishment of the undulatory theory of light supplied tangible holding-ground for ideas regarding the vehicle of propagation, and rendered the ether a fixture of thought.

A great deal is demanded from it. We cannot afford to set up an establishment of ethers; one factotum must suffice. Incongruous offices are devolved upon it. It has to be Atlas andMercury in one. It is the universal supporter and the universal messenger. Whatever kind of influence or form of energy can pass from world to world is conveyed by its means. If 'action at a distance' be inadmissible (as Newton himself held it to be), the pull of gravity must be exerted through a medium; and common-sense insists upon its identification with the transmitting medium of light, as well as upon the identification of that with the transmitting medium of electricity. A genuine conformity to these demands of reason is vouched for, not only by Hertz's discovery that an electrical explosion starts an undulatory disturbance indistinguishable, except in scale, from luminous waves; but also by Dr. Lorentz's indicated conclusion that strains of the same ethereal essence bear the all-pervading mandates of gravity. The unity of the medium may, then, be regarded as finally ascertained; the complex interactions of sundry different 'fluids' need no longer be taken into account. To provide one with the capabilities implied by the services we perceive it to render is, indeed, a sufficiently formidable task.

In popular apprehension the ether of space figures as a finer kind of air. No idea could be more misleading. The elasticity by which air transmits the longitudinal waves of sound is totally different from the elasticity by which ether propagates the transversal waves of light. Air yields to pressure; disturbance hence produces in it undulatory condensations due to oscillations of the gaseous molecules along the line in which the audible commotion travels. Ether, on the contrary, appears to be entirely incompressible; it conveys no vibrations directed lengthwise. Now this is extremely perplexing. We have no experience of a kind of matter absolutely rigid to pressure, while yielding, albeit with intense reluctance, to distortional stresses.

A jelly-like solid makes the nearest, though a very distant, approach to fulfilling the indispensable conditions; and a solid ether was accordingly in vogue until long past the middle of the nineteenth century. For a solid it had very peculiar qualities; that, for instance, of offering no resistance to motion. It was, in truth, obviously a mere temporary expedient—a scientific fiction which might pass musteruntil replaced by something corresponding less distantly with the fundamental fact. At last, on the advent of the electro-magnetic theory of light and the modified conceptions which it brought in its train, the solid ether withdrew behind the scenes. Its properties, though inconsistent and unconvincing, had not been chosen arbitrarily; they were imposed by the necessities of the situation; and when these varied, speculators naturally had recourse to fresh inventions.

The most plausible is that of a medium neither solid, liquid, nor gaseous in the ordinary sense, but in the ideal state of a 'perfect fluid.' Out of such an ether Lord Kelvin, with exquisite ingenuity, constructed his 'vortex-atoms,' which 'had their day and ceased to be.' Other ideas now prevail. 'The present tendency of physical science,' the late Mr. Preston wrote in 1890,[74]'is to regard all the phenomena of Nature, and even matter itself, as manifestations of energy stored in the ether.' The more closely we look into the things around us, the more strongly the persuasion is forced upon us that what we call ether,electricity, and matter are really varied forms of one primal substance.

Two comprehensive schemes of molecular physics, resting upon the basis of this unifying thought, have lately been elaborated—one by Dr. Larmor, the other by Professor Osborne Reynolds. They have nothing in common beyond the largeness of their synthesis. In every respect they are radically unlike, save in regarding the intangible ether as the one material reality. Dr. Larmor, however, is not quite confidently explanatory. He presents no cut-and-dried theory of the universe; its haunting mysteries are not ignored in his efforts to rationalize them. He is vividly aware of the difficulties besetting the endowment of the ether with the type of elasticity which it is recognised to possess. He can only surmise that it results from particular modes of motion—from 'kinetic stability' ensuing upon a special dynamical state. The medium may thus be thought of as pervaded by 'a structure of tangled or interlaced vortex filaments, which might resist deformation by forming a stable configuration.'[75]But thedetails of any such scheme of action are evidently far too intricate to be easily unravelled; what concerns us here is to point out that no simple structureless fluid can avail to maintain cosmical communications.

Reduced to its lowest terms, Dr. Larmor's conception of the ether is that of a 'rotationally elastic medium.'[76]In other words, it resists being turned round an axis. The forces continually acting upon it are nevertheless of a gyratory nature; and hence arise strains, betrayed to our apprehension by electrical manifestations. Each 'electron' is held to be the nucleus of some kind of distortion or displacement,[77]and carries with it, as it moves, a field of force. Out of these 'point charges' material atoms are variously built up. They are 'structures in the ether,' encompassed by 'atmospheres of ethereal strain,' not—as they were formerly taken to be—'small bodies exerting direct action at a distance on other atoms according to extraneous laws of force.'[78]Obviously the new view brings to the frontextremely subtle questions regarding the nature of 'dynamical transmission'[79]—what the propagation of energy essentially consists in, and by what mechanism it is effected; and they are, for the present, unanswerable. Electricity is, on the theory we are attempting to sketch, positive or negative according to the direction of the originating strain. A positive electron might be imagined to resemble a spiral nebula of the right-handed sort, a negative one a left-handed spiral, orvice versâ. The analogy is, perhaps, fanciful; yet it helps towards obtaining a mental picture of objects which, insignificant and elusive though they appear, may be the initials and ultimates of this strange world.

The forces, at any rate, by which it is at present kept going are evokedad libitumby the pioneers of modern research from the ethereal plenum. The actualities of matter are potentialities in the ether. 'All mass,' in Professor J. J. Thomson's opinion, 'is mass of the ether, all momentum, momentum of the ether, and all kinetic energy, kinetic energy of the ether.[80]Only if this be so,' he adds, 'thedensity of the ether must be immensely greater than that of any known substance.'

The condition is startling, but in dealing with such subjects we must be prepared to meet with anomalies. They come, as the ghosts appeared to Odysseus in Hades, at first one by one, then in an awe-inspiring swarm. Yet, in spite of the perplexities they occasion, we can discern, with growing sureness of insight, the amazing reality of the universal medium. It is, in a manner, the only reality. For what is manifest to sense is subject to change. We can conceive that the visible framework of material existence might crumble and dissolve, like 'the baseless fabric of a vision,' into seeming nothingness. But a substance that is inapprehensible is, to our limited ideas, imperishable. The ether is assuredly the seat of intense activities, which lie at the root, most likely, of all the processes in Nature. An absolutely uniform medium, however, can scarcely be imagined to energize or react. Some kind of heterogeneity it must possess; and the heterogeneity produced, in Dr. Larmor's view, by strains, is associated, in Professor Reynolds's theory, with intrinsic texture.

The 'Sub-Mechanics of the Universe' are here made to depend upon the fitting together of ineffably small, ideally rigid grains. A misfit gives rise to matter, which might hence be defined as 'ether out of joint'; and the misfit can be propagated endlessly from one range of granules to the next. This propagation through the ether of an abnormal arrangement of its constituent particles, without any transference of the particles themselves, explains the phenomena of matter in motion. A concrete existence belongs to the ether alone. It is composed of round aboriginal atoms, the diameters of which measure theseven hundred thousand millionth partof the wave-length of violet light.[81]They are packed closely together, yet not so closely but that free paths are left to them averaging in length thefour hundred thousand millionth partof their diameters.

This inconceivably small relative motion suffices, nevertheless, to render the medium elastic; is, indeed, 'the only cause of elasticity in the universe, and hence is the prime cause of the elasticity of matter.' The medium soformed is ten thousand times denser than water; it exerts a mean pressure of 750,000 tons on the square inch; the coefficient of its transverse elasticity is 9 + 1024(in C.G.S. units); which gives a velocity of transmission identical with that of light for vibrations of the same type, while longitudinal waves are propagated 2·4 times more rapidly. The scheme further includes a plausible rationale of gravity and of electrical effects; so that there is much to warrant the claim of its author to have excogitated 'the one and only conceivable purely mechanical system capable of accounting for all the physical evidence, as we know it, in the universe.'

The machine, to be sure, lacks motive power; but that is a want which no human ingenuity can supply. Its source is obscured in the primal mystery of creation. And as regards the preliminary assumptions required for the constitution of an atomic ether, inclined though we might be to cavil at them, we should, perhaps, act more wisely in following Dr. Larmor's advice by abstaining from attempts to explain 'the simple group of relations which have been found to define the activity of the ether. Weshould rather rest satisfied,' he tells us, 'with having attained to their exact dynamical correlation, just as geometry explores or correlates, without explaining, the descriptive and metric properties of space.'[82]Yet one cannot help remarking that the properties of space are not ordinarily modified to suit the needs of demonstration, while those of the ethereal medium are varied at the arbitrary discretion of rival cosmogonists. In the future, when they come to be more clearly ascertained, they will, perhaps, form the basis of a genuine new science. Already, the study of ethereal physics excites profound interest and attention. Nor is it possible to ignore the gathering indications that it will impose qualifications upon principles consecrated by authority and hitherto regarded as fundamental.

The grand modern tenet of the conservation of energy, for example, may need a gloss; it may prove to be admissible only with certain restrictions. The second bulwark of the scientific edifice is even more seriously undermined. For the 'strain theory' of atomic constitution necessarily includes the conceptionof opposite distortions corresponding to positive and negative electricity. And the further inference lies close at hand that these, by combining, may neutralize one another. The coalescence, then, of a positive and negative electron should result in the smoothing out of the complementary strains they stand for; and there would ensue the annihilation of a pair of the supposed ultimates of matter. The event might be called the statical equivalent of the destruction of light through interference. That its possibility should be contemplated even by the most adventurous thinkers is a circumstance fraught with meaning as to the subversive tendencies of recent research.

Already, in May, 1902, Professor J. A. Fleming[83]pointed out that 'if the electron is a strain-centre in the ether, then corresponding to every negative electron there must be a positive one. In other words, electrons must exist in pairs of such kind that their simultaneous presence at one point would result in the annihilation of both of them.' The consequence thus viewed in the abstract findsconcrete realization, if Mr. Jeans's suggestion be adopted,[84]in the processes of radio-activity, which possibly consist 'in an increase of material energy at the expense of the destruction of a certain amount of matter. There would, therefore, be conservation neither of mass nor of material energy.'

No longer ago than at the opening of the present century such notions would have been scouted as extravagant and paradoxical; now there is no escape from giving them grave and respectful consideration. Scientific reason has ceased to be outraged by hypotheses regarding the disappearance of mass and the development of energy. Mass and energy may, after all, be interchangeable; they are, at any rate, kept less rigidly apart in our meditations than used formerly to be the case. Nor can we assert with any confidence that partial subsidences into or emergences from the surrounding medium are for either a sheer impossibility; the universal framework, on the contrary, presents itself to us in the guise of an iridescent fountain leaping upward from, and falling back towards, the ethereal reservoir.

To the very brink of that mysterious ocean the science of the twentieth century has brought us; and it is with a thrill of wondering awe that we stand at its verge and survey its illimitable expanse. The glory of the heavens is transitory, but the impalpable, invisible ether inconceivably remains. Such as it is to-day, it already was when theFiat Luxwas spoken; its beginning must have been coeval with that of time. Nothing or everything, according to the manner in which it is accounted of, it is evasive of common notice, while obtrusive to delicate scrutiny. Its negative qualities are numerous and baffling. It has no effect in impeding motion; it does not perceptibly arrest, absorb, or scatter light; it pervades, and may even share in the displacements of gross matter; yet its motion (if it do move), is without effect on the velocity of light.

Looking, however, below the surface of things, we find the semi-fabulous quintessence to be unobtrusively doing all the world's work. It embodies the energies of motion; is, perhaps, in a very real sense, the trueprimum mobile; the potencies of matter are rooted in it; thesubstance of matter is latent in it; universal intercourse is maintained by means of the ether; cosmic influences can be exerted only through its aid; unfelt, it is the source of solidity; unseen, it is the vehicle of light; itself non-phenomenal, it is the indispensable originator of phenomena. A contradiction in terms, it points the perennial moral that what eludes the senses is likely to be more permanently and intensely actual than what strikes them.

FOOTNOTES:[74]Theory of Light, second edition, p. 28.[75]Encyclopædia Britannica, vol. xxv., p. 106.[76]Report of the British Association, 1900, p. 626.[77]Æther and Matter, p. 26.[78]Nature, vol. xlii., p. 453.[79]Larmor,Report British Association, 1900, p. 625.[80]Electricity and Matter, p. 51.[81]The Structure of the Universe, Rede Lecture, June 10, 1902, p. 14.[82]Nature, June, vol. lxii., p. 451.[83]Proceedings of the Royal Institution, vol. xvii., p. 177.[84]Nature, vol. lxx., p. 101.

[74]Theory of Light, second edition, p. 28.

[75]Encyclopædia Britannica, vol. xxv., p. 106.

[76]Report of the British Association, 1900, p. 626.

[77]Æther and Matter, p. 26.

[78]Nature, vol. xlii., p. 453.

[79]Larmor,Report British Association, 1900, p. 625.

[80]Electricity and Matter, p. 51.

[81]The Structure of the Universe, Rede Lecture, June 10, 1902, p. 14.

[82]Nature, June, vol. lxii., p. 451.

[83]Proceedings of the Royal Institution, vol. xvii., p. 177.

[84]Nature, vol. lxx., p. 101.

THE FORMS OF NEBULÆ

Sir William Herschel'scelestial surveys first made the classification of nebulæ practicable. Until he began grinding specula at Bath very few such objects were known, and those too imperfectly for the effectual discrimination of their differences. Arrangement presupposes comparison, and comparison some variety of specimens to be compared, which became available only through Herschel's scrutiny. The rapidity and penetrative power of his observations in this field almost passes belief. He detected with discernment. Discovery and enrolment did not satisfy him; he was, besides, keen to note analogies and contrasts, likenesses and dissimilitudes. He could not see without at the same time setting in order what he saw; and the law of order that commended itself tohim was founded on an evolutionary principle. The contents of the heavens seemed to fall spontaneously, as he regarded them, into genetic sequences; and the nebulæ with particular facility. The criterion adopted was that of progressive condensation. Development must clearly, he judged, be attended by contraction and local brightening. Diffused milky tracts represented cosmic formations in their most rudimentary form; they assumed, through the unremitting action of gravity in drawing their particles together, a more compact texture, more definite shapes, and a heightened lustre.

But things have changed somewhat in aspect during the last hundred years. Herschel's simple regulative plan, although of unquestioned validity, needs to be supplemented and controlled. Much auxiliary knowledge has been acquired since it was formulated. In attempting to estimate the comparative antiquity of nebulæ, we no longer depend exclusively upon one set of indications. The conclusions drawn from their immediate inspection can at least be checked by the study of their spectra and distribution.

The Milky Way might be figuratively described as the nursery-garden from which the parterres of the universe are stocked. A primitive condition is usually assigned, not without good reason, to any class of objects markedly tending to collect in its plane. And this is the case with gaseous or 'green' nebulæ. Moreover, their materials appear to be in a highly elementary state (if it be permissible to speak of one kind of matter as more elementary than another); their spectra including no rays due to metallic incandescence, but mainly those of nebulium, hydrogen, and helium. These substances, inconceivably attenuated, constitute the vast irregular formations placed by Herschel at, or near, the start of cosmical development. And so far he has been justified by the outcome of modern research. But he has not been justified in his description of planetary nebulæ as 'very aged, and drawing on towards a period of change or dissolution.' For, despite their determinate shape and definite boundaries, they do not appreciably differ in composition from nebulæ of the irregular class, and must be reckoned as, in a manner, coeval with them.

There is, on the whole, a concurrence of evidence that gaseous nebulæ are at a very early stage of growth. They are the least elaborated of sidereal objects; they seem, many of them, barely to have crossed the threshold of creation. Yet their mutual relations in time are by no means obvious. They cannot easily be disposed in any kind of rational sequence. Each of the great nebulæ, at any rate, exhibits features and occupies a position shared by none of its fellows. The most discerning cosmologist cannot pretend to say that the Argo nebula, say, is of greater or less antiquity than the Orion or the 'America' nebula. They are individual growths, simultaneous, not successive. The line of development suggested by their relationships is rather towards the formation of star-clusters than of diverse nebular species. Thus, the Pleiades illustrate not improbably the future condition of the Orion nebula, the contained stars having gained predominance over their misty envelopments, though fragmentary swaddling-bands, later, presumably, to be shaken off, still adhere to many of them.

Planetary nebulæ have much more incommon than irregular nebulæ, and their minor varieties might, with some plausibility, be associated with differences in relative age. They are marked chiefly by the character of the nuclear star which, in nearly all such objects, appears to act as the pivot of the surrounding vaporous structure. The supposition lies close at hand that it is designed as a provision for the nourishment of the star—that the star gains in mass and light at the expense of the nebula, which it is eventually destined wholly to absorb and supersede. On this view, planetaries like the green glow-lamp at the pole of the ecliptic (N.G.C. 6,543) should be regarded as the most advanced, while Webb's planetary in Cygnus (N.G.C. 7,027) would exemplify an inchoate condition. In the former the central star is of 9·6 magnitude, and sharply stellar; in the latter it is double and diffuse,[85]perhaps a wide binary system in embryo.

The question is, however, still open as to the real nature of the connection between planetaries and their central stars. The pabulum theory is a promising conjecture; but no factswith which we are acquainted stringently enforce it. Ideas on the subject will need complete revision if the traces of spirality noted from time to time in some of these peculiar objects prove to be of radical significance. Theoculi, distinctive of the 'Owl nebula' (N.G.C. 3,587), as originally shown by the Parsonstown reflector, consisted of luminous traceries coiled roundtwointerior stars,[86]but the appearance was either due to illusion, or became effaced by change, since the camera has refused to endorse it as genuine. The 'helical' planetary in Draco[87]is doubtless essentially a spiral conformation;[88]and Professor Schaeberle, by means of exposures with a 13-inch reflector of 20 inches focus, has compelled, not only the Ring nebula in Lyra,[89]but the Dumb-bell in Vulpecula, to betray the surprising secret of their whorled structure. Both these nebulæ give a spectrum of bright lines, and inventiveness is at a loss to devise means for building up gaseous materials into edifices of strongly characterized architectural forms. The materials, indeed, may not be wholly gaseous;[90]or we possibly see (as Professor Darwin long ago suggested) merely illuminated stream-lines of motion furrowing an obscure mass. But if this be indeed so, there is the further question to be asked: What direction does the motion take? Do the tides set inward or outward?

Our spontaneous impressions are all in favour of concentrative tendencies. We cannot easily shake off centripetal prejudices. Our lives are passed under a regimen of central attraction, and we naturally incline to universalize our experience. Herschel's scheme of sidereal evolution invites accordingly at first sight ready acceptance. Stars seem as if they could not act otherwise than as foci of condensation in nebulæ; the lucid stuff involving them must, apparently, with the lapse of ages, settle down towards their surfaces, and become absorbed into their substance. Such processes, indeed, belong, unless counteracted by differentmodes of action, to the inevitable order of nature; but these may, and probably do, exist. From sundry quarters the conviction is pressed upon us that cosmic bodies can drive out matter as well as draw it in. Repulsive forces insist upon recognition, and their effects become more palpable the more attentively they are considered. Under certain conditions they get the better of gravity; and stars may possibly, like cocoon-spinning insects, expend their organic energies in weaving themselves faintly lucent envelopes, the products of subtle and unaccountable activities.

The example of Nova Persei is fresh in every mind, but we make no pretension to decide the controversy it raised. A dogmatic pronouncement is unadvisable where the unknown elements of the question obscure and outweigh those that are known. A less slippery foundation for reasoning is afforded by the permanently visible spiral nebulæ, and features charged with an emphatic meaning have been revealed in them by photographic means.

Looking at the entire contents of the nebular heavens, we find the spiral type very largely predominant. It claims more specimens, andemerges more distinctly with each development of delineative power. Its chief prevalence is among 'white' nebulæ, showing continuous spectra.

They are vastly numerous. Gaseous nebulæ are reckoned by the score, white nebulæ by tens of thousands. Moreover, they collect near the poles of the Milky Way,[91]while the gaseous variety crowd towards its plane, both branches of the family thus manifesting galactic relationships, though of an opposite character. Now, these facts of distribution have some bearing on the question of relative age. There is, as already remarked, a consensus of opinion that objects showing a marked preference for the Milky Way are in a more primitive state than those withdrawn from it, and the inference is supported by the circumstance that nebulæ situated in high galactic latitudes shine with continuous light, those near the galactic equator with vivid lines. Yet it would be rash to assume that any individual nebulatraverses these successive stages. The series would be satisfactorily established only if we could point to a number of intermediate instances, which seem to be almost wholly lacking. We cannot trace in nebular as we can in stellar growth the insensible gradations of progressive change. They are perhaps complicated in nebulæ by influences of a different kind from those which have gained the ascendancy in stars. Diffusive effects may in them be more conspicuous than concentrative effects;[92]or a balance may be temporarily struck between antagonistic tendencies.

Spiral conformation is the real crux of nebular cosmogony. The conditions from which it arises are met with only in the sidereal heavens, but are there widely prevalent. Though remote from our experience, they are fundamental in the realms of space. If we could define and comprehend them, we should be in a better position for determining the cosmical status of nebulæ.

The choice is open between two rival theories of nebulous spirals. The first is the moreobvious, and readily falls in with admitted mechanical principles. Sir Robert Ball has adopted and ingeniously advocated this view.

A globular collection of promiscuously revolving particles inclines, if left to itself, to flatten down into a disc. The reason is this: In a system of the kind moment of momentum is invariable, while energy constantly diminishes. To render the contrast intelligible we have only to consider that moment of momentum is the algebraic sum of all the products of mass and motion in the aggregation, reduced to, or projected upon, its 'principal plane,' while energy is independent of the varied directions of velocity. Collisions consequently involve no diminution of moment of momentum, but combine with radiative waste to produce a steady loss of energy. Inevitably, then, the system will assume the form in which it possesses the minimum of energy that is consistent with the maintenance of its original momentum; and it is that of a disc extended in the principal plane. Retrograde movements will by the time this shape is definitively arrived at have become eliminated; the constituent particles circulate unanimously in one direction;and Sir Robert Ball adds that their circulation, owing to the more rapid rotation of the central mass, is along spiral paths.[93]They would accordingly present the twisted conformation so commonly observed in the heavens, and might even include subordinate centres of attraction, fitted to ripen and strengthen into a full-blown retinue of planets. Such are spiral nebulæ regarded in their direct mechanical aspect. Spherical nebulæ are their immediate progenitors; suns, with or without trains of dependent worlds, their lineal descendants.

Let us, however, consult some autographic records and weigh attentively what these peculiar objects tell us about themselves. We see at once that their curving lines, far from being laid down at the dictate of chance, follow a strictly defined plan. Spiral nebulæ are not formed like watch-springs by the windings of a single thread. They are always two-branched. From opposite extremities of an elongated nucleus issue a pair of nebulous arms, which enfold it in double convolutions. Their apparent superposition and interlacements occasion, in the Lyra nebula, the notedeffect of a fringed and ruptured annulus, and it is of profound interest to perceive that even in gaseous masses the same constructive rule prevails as in the great Whirlpool in Canes Venatici.

Yet this circumstance is well-nigh irreconcilable with the hypothesis that an influx of material is in progress.[94]Falls due to gravity could not be limited to two narrow areas on the central body. Matter ejected from it might, on the other hand, quite conceivably follow this course. Interior strain could easily be supposed to cause yielding along a given diameter, and nowhere else. Solar disturbances partially and dimly illustrate such a kind of activity. Diametrically opposite prominences are not unknown. They indicate the action of an explosive force right across the solar globe. Similarly, the formation of a spiral nebula cannot be rightly apprehended otherwise than as the outcome of long-continued, oppositely-directed eruptions.

The history of the heavens involves the law of spirality. The scope of its dominion continually widens as research becomes intensified. The Huygenian 'portent' in the Sword ofOrion now figures as merely the nucleus of the 'great winding Nebula' photographed by Professor W. H. Pickering in 1889. That the vast nebulosity encompassing the Pleiades is an analogous structure seems eminently probable, though the brilliancy of the enclosed stellar group obliterates most traces of its ground-plan. The magnitude of the mixed system, we are told by Professor Barnard,[95]who detected it in 1893 by means of a ten hours' exposure with the Willard lens, transcends our powers of realization. It covers 100 square degrees of the sky with intricate details. Again, some four minutes of arc to the north-west of the Ring in Lyra lies a small nebula discovered visually by Professor Barnard in 1893, and photographically resolved by Keeler into a delicate spiral. It is a two-branched, left-handed spiral, as the large adjacent object has also proved to be. One is, in fact, the miniature of the other, and they are now shown, by Professor Schaeberle's short-focus reflector, to be linked together by curving folds of nebulosity into a compound spiral system. The Dumb-bell is held, on thesame authority, to be similarly conditioned, and the analogy frequently noted between its aspect and that of the Ring nebula has thus become incalculably widened in scale.

The galactic relations of the Magellanic Clouds are not easily defined. They are within the Milky Way, yet not of it. Enigmatical excrescences upon the universe, they suggest an origin from gigantic eddies in the onflowing current of sidereal arrangement. Their miscellaneous contents are, to all appearance, disposed along eddying lines. Mr. H. C. Russell's photographs[96]rendered this, in 1890, to some extent manifest, and their indications were ratified by the Arequipa plates, from the study of which Professor Pickering gained the conviction that the great Looped Nebula, 30 Doradûs, is the structural nucleus of the Nubecula Major. 'It seems,' he wrote,[97]'to be the centre of a great spiral, and to bear the relation to the entire system that the nebula in Orion bears to the great spiral nebula which covers a large part of that constellation.'

On all sides, in the sidereal heavens, we candiscern the signs of the working of a law of convolution. Sometimes they are patent to view; sometimes half submerged; but they can generally, with attention, be disentangled from overlying appearances. They are exhibited by stars no less than by nebulæ, as the late Dr. Roberts pointed out from convincing photographic evidence; the 'hairy' appendages of globular clusters betray them by their curvilinear forms; they meet us in every corner of the wide nebular realm. Many investigators recognise in the Milky Way itself the stamp of spirality. Stephen Alexander, of New Jersey,[98]regarded the majestic galactic arch as a four-branched spiral, resulting from catastrophic breaches in a primitive, equatorially loaded spheroid, the currents of matter ejected by which should, owing to their lower angular rotation, lag behind as they retreated from the nucleus, and thus flow along helicoidal lines. R. A. Proctor subsequently devised convoluted galactic streams, which, however, corresponded imperfectly with what the sky showed. And Dr. Easton[99]has designed by way of simpleillustration an elaborate series of spires, originating possibly from a central galactic condensation, the projection of which upon the sphere may, he thinks, account for the known peculiarities of the Milky Way.

Our interior situation, nevertheless, makes it extremely difficult to determine the real relations in space of the star-streams circling around it. The observed facts are, perhaps, equally compatible with many other structural schemes besides those based on the idea of spirality; and it will be prudent to adopt none, for the present, with settled conviction. We can, however, gather one sufficiently definite piece of information regarding the history of the Cosmos. All the inmates of the heavens, stellar and nebular, represent quite evidently the débris of a primitive rotating spheroid. Its equator is still marked by the galactic annulus, its poles by a double canopy of white nebulæ. The gyrating movement which it once possessed as a whole doubtless survives in its parts, but ages must elapse before the fundamental sidereal drift can be elicited.

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