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Proceedings of Royal Society of Edinburgh. [skss. 
accepted, are radically erroneous. It seems a necessary inference from 
the results of the discussion that the solar nebula was heterogeneous 
to a degree not heretofore considered as being probable. . . 
Now, it seems to me that some of these difficulties are avoided 
if we ascribe the formation of the planets to the rotating ring 
engendered by the collision between the solar body and a 
dense cosmic cloud. I would still assume the original solar body 
to have been formed from a nebula by the process of contraction. 
But this nebula had no inherent property of rotation. Conse- 
quently the resulting liquid body had neither a tendency to 
rotate nor was it surrounded by a revolving planetary system. 
Now let us suppose this body, on its journey through space, to 
approach a cosmic cloud of considerable density. As a consequence 
of the collision, which in all probability will be one-sided, not only 
a revolving ring of matter will become permanently attached to 
the star, but also those particles which impinge upon the body 
will impart a rotation to it in the same direction as that of the 
ring. The result is a slowly rotating central nucleus surrounded 
by a ring of quickly revolving matter. I have pointed out that 
the orbits of the ring particles, immediately after the catastrophe, 
have all possible eccentricities ranging between zero and unity, 
those near the star describing circles, those farther removed 
elongated ellipses. But this aspect will gradually change. On 
each return to periastron the particles will encounter fresh colli- 
sions, by which the major axes and the eccentricities of their 
orbits are lessened, the ring thereby becoming denser, and at the 
same time more and more circular. For we must keep in mind 
that, in consequence of .the enormous heat communicated to the 
star by the impacts, there will be a dense and extensive atmosphere 
around it, through which the ring particles have to force their way 
every time they return to periastron. The tendency would there- 
fore be to establish a circular ring. The density of matter within 
this ring may be quite heterogeneous. It is indeed to be expected 
that matter may be more concentrated in some of its parts than in 
others. From the beginning distinct nuclei may be present, 
around which matter is more or less densely grouped. These 
nuclei would form centres of attraction, and, as such, would mark 
the initial steps towards the formation of planets. From this 
