212 CARNEGIE INSTITUTION OF WASHINGTON. 



the Stars. There is thus much of ground, therefore, for linking 

 together in genetic studies these stars, the Novce and the hehum- 

 hydrogen-nebulium nebulae and for looking upon them provision- 

 ally as primitive states. If our quest were the genesis of stars, these 

 would seem to point the way, so far as anything does at present ; 

 but our quest is the genesis of the so\Q.r family of planets, in one of 

 which our study centers, and the genesis of our earth is not neces- 

 sarily and immediately connected with the genesis of stars. Nebu- 

 lous bodies composed of helium, hydrogen, and the hypothetical 

 nebulium might, for aught we dare now affirm, remotely evolve 

 into material of the complex terrestrial type ; but the speculation 

 is rather too bold for prudent use as a basal factor in a conservative 

 hypothesis. 



The forms of the helium-hydrogeu-nebulium nebulae are scarcely 

 more promising for planetary evolution when their dynamical prop- 

 erties are considered. While observation has as j^et determined 

 almost nothing as to their internal movements, their forms do not 

 encourage the belief that they would under known laws evolve into 

 a system characterized by the peculiar distribution of mass and 

 momentum which the solar system presents. For the present, 

 therefore, these nebulae have been passed by in the search for the 

 immediate genesis of the earth. 



The other class of nebulae give continuous spectra and are con- 

 veniently styled white nebulae. The continuous spectrum is inter- 

 preted to mean that their chief luminous material is in a liquid or 

 solid state, or, perhaps better, that the molecules are in an aggregated 

 state, in distinction from the free state of the previous class. As the 

 liquid condition is limited to a rather narrow range of temperature, 

 and as this range is very different for different material, it is improb- 

 able that any large portion of a nebula is in this state, and the whole 

 may be conveniently treated as though it were formed of solid mat- 

 ter, but matter in a finely divided condition. This last qualification 

 seems necessary, for the volume of the.se nebulae is often very great. 

 and yet they appear to intercept but little light and give no signs of 

 great attractive power. 



The prevailing form of these nebulae is the spiral, as determined by 

 the late Professor Keeler, and this form particularly characterizes 

 the smaller nebulae recently brought to knowledge by improved in- 

 struments and manipulative skill. These newly discovered nebulae 

 are estimated to number at least ten times the whole number previ- 



