BicKERTON. — On the Birth, of Nebidm. 201 



powers to collect up the slow heavy molecules, but not the lighter ones, and 

 again the lightest molecules may have velocity enough to gradually escape 

 the mass. Thus, the gas left would only be the intermediate molecular weights. 

 It is a peculiar coincidence that many nebulaa give only a few spectral 

 lines as though of a single gas, and that gas in some cases nitrogen, an 

 intermediate weight. It is somewhat singular that lithium and rhodium, 

 should not be seen in these nebulas, unless these elements should be cos- 

 mically rare. In some cases the temperature may, perhaps, be below that 

 of dissociation of the compounds of these elements, and they may be chemi- 

 cally combined, and their compound molecules may be heavy enough to be 

 picked up by attraction. The star of 1866, before it faded, gave a feeble 

 continuous spectrum with apparently the lines of nitrogen, as though 

 selective escape had acted in such a way that the heavy molecules had 

 become dense enough to produce the continuous spectrum, all the hydrogen 

 had escaped, and the nitrogen was forming a nebula. Thus it appears, 

 that in a large diffused nebulous mass we may have aggregation by original 

 irregularities ; and also by bodies passing into the mass and being volatil- 

 ized, then gradually aggregating the denser molecules arround it. Finally, 

 these may become stars, the hydrogen may escape, and the nitrogen may 

 be the only part left in a su.fficiently gaseous state to give bright lines. 



It does not appear impossible, that a mass of gas may be at too high a 

 temperature for dense aggregation, and may be orbitally connected, and 

 the free molecules may be revolving around the central mass, the nucleus 

 gradually picking it up as its velocity was lessened by loss of heat by radia- 

 tion, or the velocity diminished by impact, or its direction so changed as to 

 impinge upon the central mass. 



Having thus glanced at the various generic modes of the origin of nebulse, 

 I wiU shortly discuss their spherical forms and the mode in which they may 

 have originated ; but I shall not enter too much into detail, or give any 

 lengthy demonstrations, until I have laid the whole of the broader generaliza- 

 tions before the Institute. 



Nebul® are roughly divided into resolvable and irresolvable nebulse, 

 according as to whether the telescope can resolve them into stars or not. 

 They are distinguished by their forms into — 



Nebulous stars 



Spherical nebulae 



Spindle ,, 



Sph-al ,, 



Cometary ,, 



Planetary , , 



,, „ with nucleus. 



Annular ,, 19 



