2S NebulcB and Nebular Hypotheses. [Sess. 



still closer the tidal strain would become too severe for the 

 maintenance of stability, and outrushes of matter would occur 

 at the ends of the elongation. This erupted matter would not 

 simply fall back again upon the star from which it had been 

 ejected : the attraction of the passing star would cause it to 

 swing round its own star in an ellipse, so that when the two 

 stars separated again, each would have very much the appear- 

 ance of a spiral nebula. 



The material ejected would presumably consist of large 

 masses, together with a vast quantity of fine matter, — ^just as 

 our volcanoes throw out both rocks and lava. Now one of the 

 most noticeable features of a spiral nebula is that knots occur 

 along the coils, and these may be taken to represent the large 

 masses ejected from the nucleus when the nebula was formed. 

 Photographs of spiral nebulse seem to show them in all stages 

 of development, and it appears clear that the knots in the 

 coils gradually gather up and incorporate the nebulous matter 

 surrounding them, and emerge at last as clear-cut stars with 

 no nebulous strand connecting them. Hence we may safely 

 conclude that in spiral nebulse, as in dumb-bell nebulse, we do 

 actually perceive the genesis of stars ; but it is important to 

 notice that while the spiral nebula is probably the ddhris of a 

 star which existed previously, we have no inkling whatever as 

 to the origin of the nebulosity in the dumb-bell. 



Of late years many astronomers have come to believe that 

 the process of evolution exemplified in a spiral nebula presents 

 to us a picture of that other process by which the planets — our 

 earth among them — were evolved from the sun. The old 

 conception as to the modus operandi by which this process 

 was carried out was that due to Laplace, — the nebular hypo- 

 thesis, strictly so called. According to it, our sun is the 

 shrivelled and shrunken nucleus of what was in primeval 

 times a vast nebula, extending far beyond the orbit of the 

 outermost planet. Laplace supposed that this nebula was 

 endowed with a slow rotatory motion, and that as it contracted 

 under the influence of gravity this rotation was accelerated. 

 Eventually the gyration became so rapid as to produce a con- 

 dition of unstable equilibrium, — like an over-driven grind- 

 stone, the nebula could no longer hold together, and to regain 

 its stability it parted with a ring extending round its circum- 



