42 GEOLOGY. 



extremely low molecular weights of their constitutent molecules and 

 their apparently elementary condition. It is a further suggestive 

 fact that the early spectra of these new stars and the green nebulae 

 are closely similar to those of the " helium stars" and the " hydrogen 

 stars", which astronomers usually place in the first or ' earliest" group 

 in evolutionary classifications of the stars. There is thus much of 

 ground, therefore, for linking together in genetic studies these stars, 

 the Novas j and the helium-hydrogen-nebulium nebulas, and for looking 

 upon them provisionally, 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 solar family of 

 planets, in one of which our study centers; and the genesis of our earth 

 is not necessarily and immediately connected with the genesis of stars. 

 Nebulous bodies composed of helium, hydrogen, and the hypothetical 

 nebulium might, for aught science 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-hydrogen-nebulium nebulae 1 are scarcely 

 more promising for planetary evolution, when their dynamical proper- 

 ties are considered. While observation has, as yet, 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 may be passed by in a search for the immediate genesis of the 

 earth. 



2. Aggregate -molecular (planetesimal ?) nebulae. — The other class 

 of nebulae give continuous spectra, and are conveniently styled white 

 nebulae. The continuous spectrum is interpreted 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 differ- 

 ent for different materials, it is improbable that any large portion of a 

 nebula is in this state, and the whole may be conveniently treated as 

 though solid, but in a finely divided condition. This last qualification 

 seems necessary, for the volume of these nebulas is often very great, 



