Bicrertox.—On the Birth of Nebula. $01 
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 onl y betheint liat lecular weights. 
It is a peculiar coincidence that many nebule 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 nebule, 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 sufficiently 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 nebulæ, 
I will 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. 
Nebule are roughly divided into resolvable and irresolvable nebulsm, 
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 » 
Spiral F 
Cometary ,, 
Planetary ,, 
