180 The Nebular Hypothesis : Its Present Condition. 



* as indicated by Laplace. Those familiar with the extraordinary convo- 

 lutions of many of the nebulce will not fail to see how easily many such 

 appearances maybe explained, by imagining a long stream of meteoric 

 bodies in the track of a comet pouring into the nebulous matter, and 

 being retarded and absorbed in their passage through it. There is 

 good reason to suppose that the product of chemical union assumes 

 generally the liquid, rather than the gaseous form, since it has fre- 

 quently been noted that a faint, continuous spectrum maybe discerned 

 along with that of the bright, gaseous lines in many nebulse, and which 

 is not confined to any particular portion of them. 



We may now pretty clearly depict the condition of a nebula accord- 

 ing to this hypothesis, when after the lapse of many ages a large 

 numbeiof comets and accompanying meteor-streams have been absorbed 

 into its substance. It will consist of a greater or less residue of the 

 original gaseous constituents, which, for reasons well known to 

 spectroscopists, will still continue to yield most conspicuously the 

 characteristic bright line spectrum, and dispersed throughout the mass 

 an immense numl)er of liquid nuclei, all tending toward the common 

 center of gravity in spiral orbits, the center being occupied by a 

 brilliant white-hot liquid mass. The temperature of such a nebula 

 will be much higher than of (me less developed — a fact which will tend 

 to keep the density of the gaseous constituents at nearly the same level, 

 in spite of the increased gravitational tendency to condensation near 

 the center. We may even conceive that this intense heat might be 

 sufficient to render gaseous the liquid nuclei, when a numerously 

 bright-lined spectrum should make its appearance, but it would then 

 be necessary to assume that the faintness of the spectrum Avould make 

 it difficult to distinguish the difference between this and one truly con- 

 tinuous. AVe may, on the contrary, imagine the well-developed 

 nebula to become a compact, liquid mass, in this case also yielding a 

 continuous spectrum. Nor does it seem unlikely that a considerable 

 number of bodies in this latter state of existence may actually be 

 discovered in the heavens. Many nebuke, after having resisted the 

 utmost efforts of astronomers, armed witli the largest telescopes, to 

 class them among the clusters of stars, have equally refused to be 

 entered in the list of true nebulse by yielding the discontinuous spec- 

 trum, and whose real condition remains, therefore, for the present an 

 enigma. Of the two suppositions, the second appears to me to be the 

 most probable ; yet, if it be accepted, it will be necessary to recon- 

 struct the nebular hypothesis, if it can still be maintained under 

 the altered circumstances. There are, however, some facts which tend 

 strongly to show that the first suggestion is the more correct one, or, 



