24 REPORT — 1891. 



lided 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 such an 

 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 uncompen- 

 sated 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 nebulaa 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 condensa- 

 tion. 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 the 

 lateness of their 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 

 associates with the planetary nebulas. Further, Dr. Grill concludes, from 

 the rapidity with which they impress themselves ujDon 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 nebulae, it may 

 be asked further, as a pure speculation, whether in them we are witness- 

 ing 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 entanglement of the lighter substances. The wide extent and great 

 diflfuseness of this bright-line nebulosity over a large part of the con- 

 stellation 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 shght 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. 



