THE ORIGIN OF THE EARTH. 41 



The characteristics of present nebulas. 



So far as their constitution is now revealed by the spectroscope, 

 nebulas fall into two general classes, the one characterized by bright 

 spectral lines, the other by a continuous spectrum. 



1. Free-molecular nebulae. — The first are usually said to be gaseous, 

 but this not a sufficiently accurate designation. The bright lines of the 

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

 is in a free-molecular condition. They do not certainly indicate whether 

 (1) the molecules are in the collisional relations of gaseous molecules, 

 or (2) are scattered widely, like the stars of heaven, so that collisions 

 are rare and incidental, or (3) are moving on radiating or on parallel 

 lines, or (4) are pursuing concentric orbits and are thus planetesimal in 

 dynamic character. For the purposes of this discussion, where dynamic 

 distinctions are important, these nebulas may be designated, with due 

 reserve, simply as free-molecular nebula. They often have a greenish 

 cast from the predominance of green lines in their spectra, and are 

 conveniently styled green nebula?. The bright spectral lines indicate 

 the dominance of hydrogen, helium, and an otherwise unknown element, 

 or elements, provisionally called nebulium. There are occasionally a 

 few other non-metallic elements, but metals have not been detected. 

 Their constitution, as now determined, does not, therefore, fit them 

 for the parentage of our earth, in which metals abound and in which 

 hydrogen and helium are subordinate elements, while nebulium is 

 unknown. The possibility of transmutation into suitable elements can- 

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

 coveries. The class includes the " planetary, " the "stellar," the 

 "ring," and most of the irregular nebula?. 



Almost identical with the spectra of these nebulas are the spectra 

 developed in an early 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 " Novce" is unknown, the con- 

 jecture that they are due to collision or to explosion has been enter- 

 tained, and this conception has also been extended to the free-molecular 

 class of nebula?. By remote and uncertain speculation under the 

 stimulus of recent revolutionary discoveries in dissociation, the vio- 

 lence of their assigned origin may be in some way correlated with the 



