256 THE NEBULAR HYPOTHESIS. 



late years uncritically received, that the ncbuUu are ex 

 tremely remote galaxies of stars like those which make up 

 our own Milky Way, is totally irreconcileable with the 

 facts involves us in sundry absurdities. On the other 

 hand, we see that the hypothesis of nebular condensation 

 harmonizes with the most recent results of stellar astrono 

 my : nay more that it supplies us with an explanation 

 of various appearances which in its absence would be in 

 comprehensible. 



Descending now to the Solar System, let us consider 

 first a class of phenomena in some sort transitional those 

 offered by comets. In comets we have now existing a 

 kind of matter like that out of which, according to the 

 Nebular Hypothesis, the Solar System was evolved. For 

 the explanation of them, we must hence go back to the time 

 when the substances forming the sun and planets were yet 

 unconccntrated. 



When diffused matter, precipitated from a rarer 

 medium, is aggregating, there are certain to be here and 

 there produced small ilocculi, which, cither in consequence 

 of local currents or the conflicting attractions of adjacent 

 masses, remain detached ; as do, for instance, minute 

 .shreds of cloud in a summer sky. In a concentrating 

 nebula these will, in the great majority of cases, eventually 

 coalesce with the larger ilocculi near to them. But it is 

 tolerably evident that some of the remotest of these small 

 Ilocculi, formed at the outermost parts of the nebula, will 

 *iot coalesce with the larger internal masses, but will slowly 

 follow Avithout overtaking them. The relatively greater 

 resistance of the medium necessitates this. As a single 

 feather falling to the ground will be rapidly left behind by 

 a pillow-full of feathers ; so, in thuir progress to the com 

 mon centre of gravity, will the outermost shreds of vapour 

 be left behind by the great masses of vapour internally 



