378 



NA TURE 



[August 20, 1891 



to a succession of ignited flashes of gas from the encounters of 

 meteoric stones was suggested by Prof. Tait, and was brought to 

 the notice of this Association in 1871 by Sir William Thomson 

 in his Presidential Address. 



The spectrum of the bright-line nebuloe is certainly not such a 

 spectrum as we should expect from the flashing by collisions of 

 meteorites similar to those which have been analyzed in our 

 laboratories. The strongest lines of the substances which in the 

 case of such meteorites would first show themselves, iron, 

 sodium, magnesium, nickel, &c., are not those which distinguish 

 the nebular spectrum. On the contrary, this spectrum is chiefly 

 remarkable for a few brilliant lines, very narrow and defined, 

 upon a background of a faint continuous spectrum, which 

 contains numerous bright lines, and probably some lines of 

 absorption. 



The two most conspicuous lines have not been interpreted ; 

 for though' the second line falls near, it is not coincident with a 

 strong double line of iron. It is hardly necessary to say that 

 though the near position of the brightest line to the bright 

 double line of nilrogen, as seen in a small spectroscope in 1864, 

 naturally suggested at that early time the possibility of the pre- 

 sence of this element in the nebulae, I have been careful to point 

 out, to prevent misapprehension, that in more recent years the 

 nitrogen line and subsequently a lead line have been employed 

 by me solely as fiducial points of reference in the spectrum. 



The third line we know to be the second line of the first spec- 

 trum of hydrogen. Mr. Keeler has seen the first hydrogen line 

 in the red, and photographs show that this hydrogen spectrum 

 is probably present in its complete form, or nearly so, as we 

 first learnt to know it in the absorption spectrum of the white 

 stars. 



We are not surprised to find associated with it the line D3, 

 near the position of the absent sodium lines, probably due to the 

 atom of some unknown gas, which in the sun can only show 

 itself in the outbursts of highest temperature, and for this reason 

 does not reveal itself by absorption in the solar spectrum. 



It is not unreasonable tc assume that the two brightest lines, 

 which are of the same order, are produced by substances of a 

 similar nature, in which a vibratory motion corresponding to a 

 very high temperature is also necessary. These substances, as 

 well as that represented by the line Dj, may be possibly some of 

 the unknown elements which are wanting in our terrestrial 

 chemistry between hydrogen and lithium, unless indeed Dg be 

 on the lighter side of hydrogen. 



In the laboratory we must have recourse to the electric dis- 

 charge to bring out the spectrum of hydrogen ; but in a vacuum- 

 lube, though the radiation may be great, from the relative few- 

 ness of the luminous atoms or molecules or from some other 

 cause, the temperature of the gas as a whole may be low. 



On account of the large extent of the nebulae, a comparatively 

 small number of luminous molecules or atoms would probably be 

 sufficient to make the nebula; as bright as they appear to us. On 

 such an assumption the average temperature may le low, but the 

 individual particles, which by their encounters are luminous, 

 must have motions corresponding to a very high temperature, 

 and in this sense be extremely hot. 



In such diff^use masses, from the great mean length of free 

 path, the encounters would b^ are but correspondingly violent, 

 and tend to bring about vibrations of comparatively short 

 period, as appears to be the case if we may judge by the great 

 relative brightness of the more refrangible lines of the nebular 

 spectrum. 



Such a view may perhaps reconcile the high temperature which 

 the nebular spectrum undoubtedly suggests with the much lower 

 mean temperature of the gaseous mass, which we should expect 

 at so early a stage of condensation, unless we assume a very 

 enormous mass ; or that the matter coming together had pre- 

 viously considerable motion, or considerable molecular agitation. 



The inquisitiveness of the human mind does not allow us to 

 remain content with the interpretation of the present state of the 

 cosmical masses, but suggests the question — 



" What see'st thou else 

 In the dark backward and abysm of time?" 



What was the original state of things? how has it come about 

 that by the side of ageing worlds we have nebulae in a relatively 

 younger stage ? Have any of them received their birth from 

 dark suns, which have collided into new life, and so belong to a 

 second or later generation of the heavenly bodies ? 



During the short historic period, indeed, there is no record of 



NO. I I 38, VOL. 44J 



such iin event ; still it would seem to be only through the collision 

 of dark suns, of which the number must be increasing, that a 

 temporary rejuvenescence of the heavens is possible, and by such 

 ebbings and flowings of stellar life that the inevitable end to 

 which evolution in its apparently uncompensated progress is 

 carrying us can, even for a little, be delayed. 



We cannot refuse to admit as possible such an origin for 

 nebulae. 



In considering, however, the formation of the existing nebulae 

 we must bear in mind that, in the part of the heavens within 

 our ken, the stars still in the early and middle stages of evolution 

 exceed greatly in number those which appear to be in an 

 advanced condition of condensation. Indeed, we find some 

 stars which may be regarded as not far advanced beyond the 

 nebular condition. 



It may be that the cosmical bodies which are still nebulous 

 owe their later development to some conditions of the part of 

 space where they occur, such as, conceivably, a greater original 

 homogeneity, in consequence of which condensation began less 

 early. In other parts of space condensation may have been still 

 further delayed, or even have not yet begun. It is worthy of 

 remark that these nebulae group themselves about the Milky Way, 

 where we find a preponderance of the white-star type of stars, 

 and almost exclusively the bright-line stars which Pickering asso- 

 ciates with the planetary nebulrc. Further, Dr. Gill concludes, 

 from the rapidity with which they impress themselves upon the 

 )ilate, that the fainter stars of the Milky Way also, to a large 

 extent, belong to this early type of stars. At the same time 

 other types of stars occur also over this region, and the red 

 hydrocarbon stars are found in certain parts ; but possibly these 

 stars maybe before or behind the Milky Way, and not physically 

 connected with it. 



If light matter be suggested by the spectrum of these nebulae, 

 it may be asked further, as a pure speculation, whether in them 

 we are witnessing possibly a later condensation of the light 

 matter which had been left behind, at least in a relatively 

 greater proportion, after the first growth of worlds into which 

 the heavier matter condensed, though not without some entangle- 

 ment of the lighter substances. The wide extent and great 

 diffuseness of this bright-line nebulosity over a large part of the 

 constellation of Orion may be regarded perhaps as pointing in 

 this direction. The diffuse nebulous matter streaming round the 

 Pleiades may possibly be another instance, though the character 

 of its spectrum has not yet been ascertained. 



In the planetary nebulae, as a rule, there is a sensible increase 

 of the faint continuous spectrum, as well as a slight thickening 

 of the bright lines towards the centre of the nebula, appearances 

 which are in favour of the view that these bodies are condensing 

 gaseous masses. 



Prof. G. Darwin, in his investigation of the equilibrium of a 

 rotating mass of fluid, found, in accordance with the independent 

 researches of Poincare, that when a portion of the central body 

 becomes detached through increasing angular velocity, the 

 portion should bear a far larger ratio to the remainder than is 

 observed in the planets and satellites of the solar system, even 

 taking into account heterogeneity from the condensation of the 

 parent mass. 



Now this state of things, in which the masses though not 

 equal are of the same order, does seem to prevail in many 

 nebulae, and to have given birth to a large class of binary stars. 

 Mr. See has recently investigated the evolution of bodies of this 

 class, and points out their radical differences from the solar 

 system in the relatively large mass-ratios of the component 

 bodies, as well as in the high eccentricities of their orbits 

 brought about by tidal friction, which would play a more im- 

 portant part in the evolution of such systems. 



Considering the large number of these bodies, he suggests 

 that the solar system should perhaps no longer be regarded as 

 representing celestial evolution in its normal form — 

 " A goodly Paterne to whose perfect mould 

 He fashioned them . . ." — 



but rather as modified by conditions which are exceptional. 



It may well be that in the very early stages condensing masses 

 are subject to very different conditions, and that condensation 

 may not always begin at one or two centres, but sometimes set 

 in at a large number of points, and proceed in the different cases 

 along very different lines of evolution. 



Besides its more direct use in the chemical analysis of the 

 heavenly bodies, the spectroscope has given to us a great and 



