256 THE NEBULAR HYPOTHESIS. 



late years uncritically received, that the nebulae 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 w^as ' evolved. For 

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

 when the substances forming the sun and planets were yet 

 unconcentrated. 



When diffused matter, precipitated ft'om a rarer 

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

 there produced small flocculi, which, either 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 w^ill, in the great majority of cases, eventually 

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

 tolerably evident that some of the remotest of these small 

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

 not coalesce with the larger internal masses, but will slowly 

 follow without 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 their progress to the com- 

 mon centre of gravity, Tvill the outermost shreds of vapour 

 be left behind by the great masses of vapour internally 



