FUNDAMENTAL PROBLEMS OF GEOLOGY. 211 



Present spectroscopic data relative to the constitution of the nebulae, 

 considered in relation to the question in hand, reveal two general 

 classes of nebulae, the one characterized by bright spectral lines, the 

 other by a continuous spectrum. 



The first are usually said to be gaseous, but this designation is 

 not sufficiently accurate for our present purpose. The bright lines 

 of the spectrum can only be affirmed to indicate that the matter of 

 these nebulae is in a free-molecular condition. They do not cer- 

 tainl}' indicate whether (i) the molecules are in the coUisional rela- 

 tions of gaseous molecules or (2) are scattered wideh', like meteor- 

 ites, so that collisions are rare and incidental, or (3) are moving on 

 radiant or on parallel lines, or (4) are pursuing concentric orbits, 

 and are thus planetesimal in dxmamic character. For the purposes 

 of this study, where dynamic distinctions are important, these neb- 

 ulae may be designated, with due reserve, simph' as free-molecular 

 nebulce. They often have a greenish cast from the predominance 

 of green lines in their spectra, and are conveniently styled green 

 nebulae. The bright spectral lines indicate the dominance of hydro- 

 gen, helium, and an otherwise unknown element or elements, pro- 

 visionally called nebulium. There are occasionally a few other 

 non-metallic elements, but metals have not been detected. Their con- 

 stitution, as now determined, does not, therefore, fit them for the 

 parentage of our earth, in which metals abound and in which hy- 

 drogen and helium are subordinate elements, while nebulium is 

 unknown. The possibilities of transmutation into suitable elements 

 can not, to be sure, be safely denied in these days of revolutionary 

 discoveries, but, on the other hand, can not very safely be made a 

 working basis. The class includes the " planetar}'," the " stellar," 

 the " ring," and most of the irregular nebulae. 



Almost identical with the spectra of these nebulae are the spectra 

 developed in an earlj^ phase of the declining stages of the new stars 

 that occasionally flash forth with sudden brilliancy and soon die 

 away to obscurity or extinction, continuous spectra sometimes 

 marking the later stages. While the origin of these "Novcs'' is 

 unknown, the conjecture that they are due to collision or to explo- 

 sion has been entertained, and this conception has also been extended 

 to the free-molecular class of nebulae. It is a further suggestive 

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

 green nebulae are closely similar to the spectra of the " helium 

 stars ' ' and the ' ' hydrogen stars, ' ' which astronomers usually place 

 in the first or "earliest" group in evolutionary classifications of 



