276 FROM NEBULA TO NEBULA 



ranged behind them, the predominance of such lines in 

 celestial spectra should cease to surprise. Given suf- 

 ficient allowance of time in which to act, gravitation 

 would of itself dispose the elements in the same order, 

 that is, the lighter on the outside. 



The so-called " green" nebulae are therefore younger 

 than the " white", and, by the same token, naturally 

 larger, from not having been condensed by long exposure 

 to the cold of space. Their youthfulness is further at- 

 tested by the fact that less than five per cent of nebulae 

 are green, a circumstance which opens interesting ave- 

 nues to the determination of the relative ages of nebulae 

 in general. The Milky Way, being the densest portion 

 of the sidereal system, is naturally richest in all sorts 

 of nebulae, but particularly in the percentage of those 

 of this green, or ultra-gaseous, type. The difference be- 

 tween the various classes of nebulae is therefore acci- 

 dental rather than fundamental or generic. The manner 

 of their formation (that is, by explosion) naturally ar- 

 rests and masks such proper motions as their parent 

 star may have had just prior to the catastrophe, so that 

 the new-born nebula is obliged to "find itself", as it 

 were, and to acquire its new motions by a protracted pro- 

 cess of gravitational acceleration. As the "dust from 

 the explosion" settles and clears, the nuclei, which were 

 always there, but only hidden from our sight, loom 

 gradually into view, and then we perceive a spiral nebu- 

 la, evenly poised like a giant pinwheel on a center, re- 

 crudescing into a new cycle of existence. 



The double spectra of stars are never visible except 

 where two bright stars are knowingly examined at one 

 time, as in the case of binaries, or where there is evidence 

 of some special eruption in progress on the body under 

 examination. In the latter case the confusion of spectra 

 is due, I opine, to the violent geyser-like ejection of gas 

 from one section, while the other part of the surface re- 

 mains normal. A suggestive illustration of this may be 

 seen in certain comets, which now and again exhibit lum- 

 inous spurs, or jets, directed toward the sun. Moreover, 

 geyers that rise must concurrently fall back by the action 



