'34 



SCIENCE, 



[Vol. XVIII. No. 455 



corresponding' to a very high temperature is also necessary. 

 These substances, as wtU as that represented by the line Dg, 

 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 hydro- 

 gen. 



In the laboratory we must have recourss to the electric 

 discharge to bring out the spectrum of hydrogen; but in a 

 vacuum-tube, though the radiation may be great, from the 

 relative fewness 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 compara- 

 tively small number of luminous molecules or atoms would 

 probably be sufficient to make the nebulae as bright as they 

 appear to us. On such an assumption the average tempera- 

 ture may be low, but the individual particles, which by 

 their encounters are luminous, must have motions corre- 

 sponding to a very high temperature, and in this sense be 

 extremely hot. 



In such diffuse masses, from the great mean length of free 

 path, the encounters would be rare but correspondingly vio- 

 lent, 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 previously 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 heav- 

 enly bodies? 



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

 of such an event; still it would seem to be only through the 

 collision of dark suns, of which the number must be increas- 

 ing, that a temporary rejuvenescence of the heavens is pos- 

 sible, and by such ebbings and Sowings of stellar life that 

 the inevitable end to which evolution in its apparently un- 

 compensated 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 heav- 

 ens 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. In- 

 deed, 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 tliese nebulaj group themselves 

 about the Milky Way, vehere we find a preponderance of the 

 white-star type of stars, and almost exclusively the bright- 

 liue stars which Pickering associates with the planetary 

 nebulas. Further, Dr. Gill concludes, from the rapidity 

 with which they impress themselves upon the plate, 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 may 

 be before or behind the Milky Way, and not physically 

 connected with it. 



If light matter be suggested by the spectrum of these neb- 

 ulae, 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 with- 

 out some entanglement 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 re- 

 garded 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 nebulee, as a rule, there is a sensible in- 

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

 thickening of the bright lines towards the centre of the neb- 

 ula, appearances which are in favor of the view that these 

 bodies are condensing gaseous masses. 



Professor George 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 in- 

 creasing 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 ac- 

 count he terogeneity 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 

 nebulfe, and to have given birth to a large number 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 Cheir orbits brought about by tidal friction, which would 

 play a more important part in the evolution of such sys- 

 tems. 



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 bj' conditions which are exceptional. 

 It may well be that in the very early stages condensing 

 masses are subject to very different conditions, and that con- 

 densation 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. 



(To be continued.) 



