560 Proceedings of Royal Society of Edinburgh. [sess. 
displacements towards the violet indicate that the ejected gases 
move with enormous velocities, the greatest exceeding the critical 
velocity of a star such as our sun. Matter moving with such speeds 
is most probably for ever lost But we may admit that there 
must be a considerable range of velocities among the various 
parts of this expanding cloud. Some move quicker, others 
more slowly, and all those particles whose initial velocities were 
less than the critical will sooner or later come to a point of 
rest whence they begin their return journey towards the star. 
Obviously the slowest particles must have returned soonest ; they 
have either impinged upon the solar body long ago, or, in 
consequence of perturbations, have been drawn into elliptic 
orbits. They may have formed systems of periodic comets in the 
past, which now, through the continued disintegrating action of 
the sun and planets, have degenerated into meteoric swarms. 
Some of the present periodic swarms probably had this origin. 
If this view is correct, then the comets falling upon our system 
at the present moment must move in ellipses not distinguishable 
from parabolae, since their return points must have been at 
practically infinite distances from the sun. That the fall of 
these bodies is not central, may be explained by the doubtless 
inevitable perturbations experienced, not only during their outward 
journey, but perhaps also at the outer limit, where the cloud may 
at times have been under the gravitational influence of neigh- 
bouring stars.* Accepting this view, we understand why comets 
describe parabolic and elliptic orbits, why all inclinations are 
possible, and why there is the well-known physical resemblance 
between the members of this cosmic family. The expanding 
atmosphere of a new star would thus be a cometary cloud in statu 
nascendi. 
The assumption of previous solar condensation, which is clearly 
necessary in this theory, may appear as a disadvantage, because it 
involves the creation and expenditure of solar energy before the 
planets were formed, and thereby seems to limit the time at our 
disposal for explaining the evolution of the planetary system. But, 
on the one hand, we must keep in mind that the generation of heat 
by contraction is at first a slow process. Indeed, the amount of 
* This is also Schiaparelli’s view. 
