scope, he was accustomed to say to the secretary who assisted him: 'Prepare to write; nebulæ are about to arrive." " How does this fact consist with the hypothesis that nebulæ are remote galaxies? If there were but one nebula, it would be a curious coïncidence were this one nebula so placed in the distant regions of space as to agree in direction with a starless spot in our own sideral system. If there were but two nebulæ, and both were so placed, the coïncidence would be excessively strange. What then shall we say on finding that there are thousands of nebula which are so placed? Shall we believe that these far-removed galaxies, dispersed through infinite space, have in thousands of cases happened to agree in visible position with the thin places in our own galaxy? Such a belief is next to impossible. Still more manifest does the impossibility of it become when we consider the nebulæ in their general distribution. Not only does the law above specified apply to larger portions of the heavens, as seen in the fact that "the poorest regions in stars are near the richest in nebule," but it applies to the heavens as a whole. In that zone of celestial space where stars are excessively abundant, nebulæ are extremely rare; while in the two opposite celestial spaces that are furthest removed from this zone, nebulæ are extremely abundant. Scarcely any nebulæ lie near the galactic circle, (or plane of the Milky Way;) and the great mass of them lie round the galactic poles. Can this also be mere coïncidence? When to the fact that the general mass of nebule are antithetical in position to the general mass of stars, we add the fact that local regions of nebulæ are regions where stars are scarce, and the further fact that single nebulæ are habitually found in comparatively starless spots, does not the proof of a physical connection become overwhelming? Should it not require an infinity of evidence to show that nebulæ are not parts of our sideral system? Let us see whether any such infinity of evidence is assignable. Let us see whether there is even a single alleged proof which will bear examination. to reach our earth-to distances for whose mea- tence there is expressed a more or less so is the distance of our Sun from Sirius an inappreciable point when compared If one of these supposed galaxies is so knew it because he could see the bees as "As seen through colossal telescopes," says Humboldt, "the contemplation of these nebu-tance from the Earth to the Sun; and, lous masses leads us into regions from whencea according to the hypothesis, the distance ray of light, according to an assumption not of a nebula is something like a million wholly improbable, requires millions of years times the distance of Sirius. Now, our larger ones are the nearer ones, and the smaller the more distant. Mark, now, the necessary inference respecting their resolvability. It must follow that, in the great majority of cases, the largest or nearest nebula will be most easily resolved into stars; that the successively smaller will be successively more difficult of own "starry island, or nebula," as Hum- Another incongruity, equally insurmountable, meets us when we contrast the relative sizes and resolvabilities of the nebulæ. It is an assumption habitually made with respect to the stars, that their differences of magnitude are chiefly dependent upon differences of distance that the largest are the nearest, while the successively smaller are successively more remote; and although, as involving the supposition that all stars are actually of the same size, this is not literally true in detail, yet investigation has shown good reason for believing it true as an average fact. But the arguments which justify this asumption in the case of the stars, equally justify it in the case of the nebulæ. It is in the highest degree improbable that all the small nebulæ are relatively near, and the large ones relatively far off; or vice versa. The only warrantable supposition is, that such differences of actual size as exist among them have no relation to their distances from us; but that small and large are dispersed through space with what we may call a regular irregularity. And hence it follows that, on the average, the apparent sizes of the nebula will indicate their sizes of the nebula will indicate their distances that, speaking generally, the * Cosmos. Seventh Edition. Vol. 1 pp. 79, 80. and that the irresolvable ones will be the smallest ones. This, however, is exactly the reverse of the fact. The largest nebulæ are either wholly irresolvable, or but partially resolvable under the highest telescopic powers; while a large proportion of quite small nebulæ are easily resolved by far less powerful telescopes. The same instrument through which the great nebula in Andromeda, two and a half degrees long and one degree broad, appears merely as a diffused light, decomposes a nebula of fifteen minutes diameter into twenty thousand starry points. While the individual stars of a nebula eight minutes in diameter are so clearly seen as to allow of their number being estimated, a nebula covering an area five hundred times as great shows no stars at all. What possible explanation can be given of this on the current hypothesis? Yet another difficulty remains one which is, perhaps, still more obviously fatal than the foregoing. This difficulty is presented by the phenomena of the Magellanic clouds. Describing the larger of these, Sir John Herschel says: "The nubecula major, like the minor, consists partly of large tracts and ill-defined patches of irresolvable nebula, and of nebulosity in every stage of resolution, up to perfectly resolved stars like the Milky Way; as also of regular and clusters in every stage of resolvability, and of irregular nebulæ properly so called, of globular clustering groups sufficiently insulated and condensed to come under the designation of 'cluster of stars.'"-Cape Observations, p. 146. In his Outlines of Astronomy, Sir John Herschel, after repeating this description in other words, goes on to remark that "This combination of characters, rightly considered, is in a high degree instructive, affording an insight into the probable comparative distance of stars and nebula, and the real brightness of individual stars as compared with one another. Taking the apparent semi-diaand regarding its solid form as, roughly speaking, meter of the nubecula major at three degrees, spherical, its nearest and most remote parts differ in their distance from us by a little more than a tenth part of our distance from its center. The brightness of objects situated in its nearer portions, therefore, can not be much exaggerated, nor that of its remoter much enfeebled, by their difference of distance. Yet within this globular space we have collected upwards of six hundred stars of the seventh, eight, ninth, and tenth magnitude, nearly three hundred nebula, and globular and other clusters of all degrees of resolvability, and smaller scattered stars of every inferior magnitude, from the tenth to such as by their multitude and minuteness constitute irresolvable nebulosity, extending over tracts of many square degrees. Were there but one such object, it might be maintained without utter improbability that its apparent sphericity is only an effect of foreshortening, and that in reality a much greater proportional difference of distance between its nearer and more remote parts exists. But such an adjustment, improbable enough in one case, must be rejected as too much so for fair argument in two. It must, therefore, be taken as a demonstrated fact, that stars of the seventh or eighth magnitude, and irresolvable nebula, may coexist within limits of distance not differing in proportion more than as nine to ten."-Outlines of Astronomy, pp. 614, 615. believe they will be found in entire harmony with it. Starting with the assumption of a rare and widely-diffused mass of nebulous matter, having a diameter, say as great as the distance from the sun to Sirius, let us consider the successive changes that will take place in it. Mutual gravitation will approximate its atoms; but their approximation will be opposed by atomic repulsion, the overcoming of which implies the evolution of heat. As fast as this heat partially escapes by radiation, further ap proximation will take place, attended by further evolution of heat, and so on continuously: the processes not occurring separately as we have described them, but simultaneously, uninterruptedly, and with increasing activity. Eventually this slow movement of the atoms towards their common center of gravity, will bring about an entirely new set of phenomena. Arguing from the established laws of atomic combination, it will happen that Now, we think this clearly supplies a when the nebulous mass has reached a reductio ad absurdum of the doctrine we certain stage of condensation-when its are combating. It gives us the choice of internally-situated atoms have approached two incredibilities. If we are to believe to within a certain distance, have genthat one of these nebulæ is so remote that erated a certain amount of heat, and are its hundred thousand stars look only like subject to a certain mutual pressure, (the a milky spot, invisible to the naked eye, heat and pressure both increasing as the we must, at the same time, believe that aggregation progresses,) some of them there are single stars so enormous that will suddenly enter into chemical union. though removed to this same distance they Whether the binary atoms só produced remain visible. If we accept the other be of kinds such as we know, which is alternative, and say that many nebulæ are possible, or whether they be of kinds no further off than our own stars of the simpler than any we know, which is more eighth magnitude, then it is requisite to probable, matters not to the argument. believe that at a distance not greater than It suffices that molecular combination of that at which a single star is still faintly some species will finally take place. When visible to the naked eye, there may exist it does take place, it will be accompanied a group of a hundred thousand stars which by a great and sudden disengagement of is invisible to the naked eye. Neither of heat; and until this excess of heat has esthese positions can be entertained. What, caped, the newly-formed binary atoms then, is the conclusion that remains? This will remain uniformly diffused, or, as it only that the nebulæ are not further off were, dissolved in the preexisting nebulous from us than parts of our own sideral sys- medium. But now mark what must by tem, of which they must be considered and by happen. When by radiation the members; and that when they are re-temperature has been adequately lowered, solvable into discrete masses, these masses can not be considered as stars in any thing like the ordinary sense of that word. And now having, as we believe, disposed of this idea, rashly promulgated by sundry astronomers, that the nebulæ are extremely remote galaxies, let us see whether the various appearances they present are not reconcilable with the nebular hypothesis. Rightly interpreted, we these binary atoms will precipitate; and having precipitated, they will not remain uniformly diffused, but will aggregate into flocculi, just as water, when precipi * Any objection that may be raised to the extreme tenuity this involves, is at once met by the calculainch of air removed four thousand miles from the tion of Newton, who proved that were a spherical Earth, it would expand into a sphere more than filling the orbit of Saturn. tion to the rarer medium through which it is moving. Now, the probabilities are infinity to one against all the respective motions thus impressed on this rarer medium, exactly balancing each other. And if they do not balance each other, the inevitable result must be a rotation of the whole mass of the rarer medium in some one direction. But preponderating momentum in some one direction, having caused rotation of the medium in that direction, the rotating medium must in its turn gradually arrest such flocculi as are moving in opposition, and must impress its own motion upon them; and thus there will ultimately result a rotating medium with suspended flocculi partaking of its motion while they move in converging spirals towards the common center of gravity. . The re tated from air, collects into clouds. And, I scribing its spiral course, must give moindeed, this à priori conclusion is confirmed by the observation of those still extant portions of nebulous matter which constitute comets; for, "that the luminous part of a comet is something in the nature of a smoke, fog, or cloud, suspended in a transparent atmosphere, is evident," says Sir John Herschel. Concluding, then, as we are warranted in doing, that a nebulous mass will, in course of time, resolve itself into flocculi of precipitated denser matter, floating in the rarer medium from which they were precipitated, let us inquire what will be the mechanical results. We shall find that they will be quite different from those occurring in the original homogeneous mass, and also quite different from those which would occur among bodies dispersed through empty space. A group of bodies dispersed through empty space would move in straight lines toward their common center of gravity. So, too, with a group of bodies dispersed through a resisting medium, provided they were spherical, or of forms presenting symmetrical faces to their lines of movement. But a group of irregular bodies dispersed through a resisting medium will not move in straight lines towards their common center of gravity. A mass which presents an irregular face to its line of movement through a resisting medium, will necessarily be deflected from its original course, by the unequal reactions of the medium on its different sides. Hence, each flocculus, as, by analogy, we term one of these precipitated masses of gas or vapor, will acquire a movement, not towards the common center of gravity, but towards one or other side of it; and this more or less oblique movement, accelerated as well as changed in direction by the increasing centripetal force, but retarded by the re-gregations may have concentrated into sisting medium, will result in a spiral, ending in the common center of gravity. Observe, however, that this conclusion, valid as far as it goes, by no means proves a common spiral movement of all the flocculi; for as their forms must not only be varied in kind, but disposed in all varieties of position, it must happen that their respective movements will be deflected, not towards one side of the common center of gravity, but towards various sides. How then can there result a spiral movement common to them all? Very simply. Each flocculus, in de Before comparing these conclusions with the facts, let us pursue the reasoning a little further, and observe the subordi nate actions and the endless modifications which will result from them. spective flocculi must not only be drawn towards their common center of gravity, but also towards neighboring flocculi; and, as a result of this play of forces, the whole assemblages of flocculi will break up into subordinate groups: each group concentrating towards its local center of gravity, and in so doing acquiring a vortical movement, like that subsequently acquired by the whole nebula. Now, according to circumstances, and chiefly according to the size of the original nebulous mass, this process of local aggregation will produce various results. If the whole nebula is but small, the local groups of flocculi may be drawn into the common center of gravity before yet their constituent masses have coalesced with each other. In a larger nebula, these local ag rotating spheroids of vapor, while yet they have made but little approach towards the general focus of the system. In a still larger nebula, where the local aggregations are both greater and more remote from the common center of gravity, complete concentration into rotating masses of molten matter may have arisen before the general distribution of them has greatly altered. In short, as the conditions in each case determine, the discrete masses produced may vary indefinitely in number, in size, in density, in motion, in distribution. And now let us return to the visible | luminous matter which they exhibit, are characters of the nebulæ, as observed through modern telescopes. Take first the description of those nebula which, by the hypothesis, must be in an early stage of evolution : not such as would be described by more or less discrete masses starting from a state of rest, and moving through a resisting medium to a common center of gravity; but they are such as would be described by masses having their movements modified by the rotation of the medium. 66 Among the irregular nebulæ," says Sir John Herschel, may be comprehended all which, to a want of complete, and in most inIn the center of a spiral nebula is seen stances, even of partial resolvability by the a mass both more luminous and more repower of the twenty-feet reflector, unite such a solvable than the rest. Assume that, in deviation from the circular or elliptic form, or process of time, all the spiral streaks of such a want of symmetry (with that form) as luminous matter which converge to this preclude their being placed in Class 1, or that center are drawn into it, as they must be; of regular nebulæ, This second class comprises many of the most remarkable and inter-discrete bodies constituting these lumiassume further, that the flocculi or other esting objects in the heavens, as well as the most extensive in respect of the area they occupy." And, referring to this same order of objects, M. Arago says: "The forms of very large diffuse nebulae do not appear to admit of definition; they have no regular outline." Now, the fact that the largest nebulæ are either irresolvable or very difficult to resolve, might have been inferred à priori; seeing that irresolvability, implying that the aggregation of precipitated matter has gone on to a small extent, will be found in nebule of wide diffusion. Again, the irregularity of these large irresolvable nebulæ might also have been expected; seeing that their outlines, compared by Arago to "the fantastic figures which characterize clouds carried away and tossed about by violent and often contrary winds," are similarly characteristic of a mass not yet gathered together by the natural attraction of its parts. And once more, the fact that these larger, irregular, irresolvable nebulæ have indefinite outlines-outlines that fade off insensibly into surrounding darkness-is one of like meaning. Speaking generally, (and of course differences of distance negative any thing beyond an average statement,) the spiral nebulæ are smaller than the irregular nebulæ, and more resolvable; at the same time that they are not so small as the regular nebulæ, and not so resolvable. This is as, according to the hypothesis, it should be. The degree of condensation causing spiral movement, is a degree of condensation also implying masses of flocculi that are larger, and therefore more visible, than those existing in an earlier stage. Add to which, that the forms of these spiral nebula are quite in harmony with the explanation given. The curves of nous streaks aggregate into larger masses at the same time that they approach the central group, and that the masses forming this central group also aggregate into larger masses, (both which are necessary assumptions,) and there will finally result a more or less globular group of such larger masses, which will be resolvable with comparative ease. And, as the coalescence and concentration will still go on, the constituent masses will gradually become fewer, larger, brighter, and more densely collected around the common center of gravity. See now how completely this inference agrees with observation. "The circular form is that which most commonly characterizes resolvable nebulæ," writes Arago. "Resolvable nebula," says Sir John Herschel, almost universally round or oval." Moreover, the center of each group habitually displays a closer clustering of the constituent masses about the center of gravity than elsewhere; and it is shown that, under the law of gravitation, which we know extends to the stars, this distribution is not one of equilibrium, but implies progressing concentration. While, just as we inferred that, according to circumstances, the extent to which aggregation has been carried must vary; so we find that, in fact, there are regular nebula of all degrees of resolvability, from those consisting of innumerable minute discrete masses, to those in which there are a few large bodies worthy to be called stars. are On the one hand, then, we see that the notion, of late years idly repeated and uncritically received, that the nebulæ are extremely remote galaxies of stars like those which make up our own milky-way, is totally irreconcileable with the facts involves us in sundry absurdities. On CTO DO |