Bickerton.— On the Genesis of Worlds and Systems. 193 
spread out more or less in a plane, with frequently gas at the poles, and 
that the middle body produced by the collision will generally be very hot, 
proportional to the mass. 
In the papers presented to the Institute, the possible conditions of im- 
pact under which the different kinds of nebule may have been produced 
have been discussed. Thus it is suggested that the spiral nebule may have 
been produced by the collision of two previously existing nebulous masses, 
otherwise it appears that the extreme pressure would have destroyed the 
central part of the spiral. Singularly enough these nebule are found in 
the nebulous portion of the celestial sphere. Such evidence as this gives 
great probability to this theory. It is suggested that the comet-like nebule 
are masses with a high resultant velocity; that the planetary nebule are 
gaseous shells produced by the outrushing gas leaving the position of im- 
pact, and travelling outwards in every direction into space. Reasons based 
on the dynamical theory of gases have been urged why the heavier chemical 
molecules should return, and form the star which is very often seen at the 
centre of these bodies. Anyone who has followed this speculation must see 
that if this theory does represent the birth of nebulæ, they must be changing 
their shape, and sometimes new ones will be formed and old ones die out, 
and this is really the case. They vary; many new nebule have not only 
been found, but some have disappeared again, and many that used to exist 
are lost. 
But such mere gas as nebule must not be allowed to detain us. There 
are far more solid matters to be discussed yet. Thus it has been suggested 
that the Solar System is not the kind of family Laplace has pictured it, with 
the Sun as the parent and Neptune as the eldest brother, down to the 
youngest, Mercury, or perhaps Vulean. But it implies that the whole sys- 
tem are twin brothers and sisters, all born together; a deserted family 
whose severed parents are wildly travelling space. The collision which 
gave the Sun its heat gave it its rotation, threw off the masses which 
became planets, set these spinning also, giving them their accompanying 
masses of cosmical dust we call moons. That same great whirl set all the 
planets travelling in orbits all in the same direction, and nearly in one 
plane. The theory also attempts to show how the elliptical orbit became 
nearly circular ;—how the original rotation of the two colliding masses 
would disturb the exact symmetry of the rotation of the planets. It at- 
tempts to account for many things too numerous to speak of here. But 
you will say the Solar System could not have been born in two different 
ways. Well, hardly. Then you must dispose of Laplace’s nebular rings, 
Perhaps so; but even Laplace’s theory demands a rotary nebula to start 
with, and it would therefore still seem that he needs partial impact to 
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