500 



NA TURE 



[March 29, 1906 



specifically and repeatedly mentioned in connection 

 with the hypothesis, as, for instance, in the remark 

 that the general absence of bright lines of metallic 

 vapours, and of the bright lines of hydrogen in white 

 nebulae " evidently justifies the conclusion that we are 

 here in presence of those bodies in celestial space, the 

 temperature and the electrical excitation of which are 

 at a minimum, and as the continuous part of the spec- 

 trum is brought under examination further stages will 

 be recognised. . . . There can be little doubt that when 

 our instrumental appliances and observing conditions 

 become more perfect, it will be found that the so-called 

 continuous spectra will be a perfect mine of new 

 knowledge regarding the true nature of the changes 

 which occur as condensation increases " (" Meteoritic 

 Hypothesis," pp. 323, 324). 



This was written sixteen years ago ; and the work 

 that has since been done enables Miss Clerke to arrive 

 at the same conclusions, though she has forgotten the 

 observations which have changed the position formerly 

 occupied. She can now write : — 



" Gaseous nebula; are, in fact, reasonably believed 

 to be at a temperature not much above absolute zero. 

 Thev are not, then, incandescent, but rather 'lumin- 

 escent ' ; their light is independent of thermal con- 

 ditions." 



Also we find in several places the view accepted that 

 even the spectra of white nebulae are not truly con- 

 tinuous, but show 



" slight inequalities in the flow of light, indicating 

 effects of absorption, of emission, or of both com- 

 bined. " 



In the present state of knowledge of nebular spectro- 

 scopv, no opinion of substantial value can be ex- 

 pressed as to the relationship existing between nebulas 

 exhibiting the characteristic line at A 5007 alone or 

 with other lines, and nebulae with apparently con- 

 tinuous spectra, but the view that the difference is due 

 to different degrees of condensation of congeries of 

 meteoritic matter — cosmic dust — the Andromeda type 

 of nebula being in a more condensed condition than 

 an irregular nebula like that of Orion, is at least as 

 reasonable as any other. Even if this suggestion is 

 considered to be of negligible value, there remains the 

 fact that the present tendency is to regard the spectra 

 of all nebulae as being really discontinuous, as was 

 anticipated in the meteoritic hypothesis. Moreover, 

 Miss Clerke admits that, apart from the question of 

 the apparent differences between the two spectroscopic 

 types of nebulae, 



" relationship between the various orders of nebulas 

 is manifest. The tendency of all to assume spiral 

 forms demonstrates, in itself, their close affinity; 

 so that to admit some to membership of the 

 sidereal system while excluding others would be a 

 palpable absurdity. And since those of a gaseous 

 constitution must be so admitted, the rest follow in- 

 evitably." 



It is not clear from this, or from other remarks 

 in the book, how we are to regard the white 

 nebulae, if only as a working hypothesis. Most 

 spiral nebulae do not give the characteristic spectrum 

 of hydrogen and helium, yet it is agreed that all 

 spiral forms are closely related. If it is assumed that 

 NO. 19OO, VOL. 73] 



the spiral nebulae with bright line spectra are entirely 

 gaseous in constitution, it is difficult to explain the 

 existence of apparent " stars," presumably at a higher 

 temperature than that of the general mass of gas, 

 threaded upon the spirals. If, however, the view 

 is accepted that the luminous radiations of so-called 

 " gaseous " nebulae are really only the visible mani- 

 festations of electric or dynamic disturbances of a 

 mass of cosmic dust — and few would now deny the 

 existence of dark matter in nebulas — the explanation 

 of spiral nebulae is easy, and the difficulty as to the 

 relationship between these nebulas and others dis- 

 appears. The existence of streams, sheets, and shells 

 of palpable matter intersecting at various points is 

 sufficient to account for the bright portions of spiral 

 nebulas with the apparent stars arranged along the 

 spirals ; if nebulas have this meteoritic constitution 

 spiral forms ought to predominate, as they actually 

 do. Spiral nebulas like those of Andromeda and 

 Canes Venatici are certainly not purely gaseous in 

 constitution, though a few years ago it was more 

 necessary to insist upon their non-gaseous nature than 

 it is now ; but while we await crucial observations to 

 decide how nebulae of different spectroscopic types are 

 related, Miss Clerke might have shown that an ex- 

 planation has been given, though she may disapprove 

 of it. 



A knowledge of the constitution of nebulae is of 

 fundamental importance, for upon it must be based 

 any satisfactory scheme of evolution of celestial 

 species. All astronomers accept the idea of evolution 

 from nebulas, but as to the order of development, and 

 the means by w-hich it is brought about, no hypo- 

 thesis has met with general adoption, even as a work- 

 ing principle. A common ground of agreement, how- 

 ever, is that nebulas showing the characteristic trio 

 of lines- -aa 4860, 4950, and 5007 — in their spectra 

 are at a relativelv low temperature. Given the 

 cosmical amoeba in the form of a nebula, what will 

 become of it? The question cannot be answered 

 directly, because sidereal ontogeny transcends human 

 experience; but though the development of the in- 

 dividual nebula cannot be followed, a sidereal phylo- 

 geny can be based upon the spectroscopic characters 

 of celestial species. Miss Clerke does not deal with 

 nebular and stellar spectra from the point of view of 

 development; for she takes Secchi's four types of 

 spectra as her groups for discussion, and only makes 

 incidental reference to the stages occupied by par- 

 ticular stars in an evolutionary scheme. The helium- 

 stars, tvpified by certain stars in the constellation 

 Orion, are regarded as the first results of condensation 

 from nebulae ; Sirian stars represent the next stage, 

 and then the solar condition is reached. 



" In a general sense it may indeed be said that the 

 spectra of the sun and of solar stars imply a state of 

 things in their reversing layers analogous to that 

 prevailing in the arc-light, while in helium and Sirian 

 stars the conditions of the spark are more nearly 

 reproduced." 



From stars of the solar type, Miss Clerke suggests 

 that the development is toward the condition of Betel- 

 o-eux, which exhibits tin- Fraunhofer spectrum in 



