BiCKEKTON. — On the Genesia 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 nebula may have been produced 

 have been discussed. Thus it is suggested that the spiral nebul® may have 

 been produced by the collision of two previously existing nebulous masses, 

 otherwise it appears that the extreme pressm-e would have destroyed the 

 central x^art of the spiral. Singularly enough these nebula 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-hke nebula 

 are masses with a high resviltant velocity ; that the planetary nebula are 

 gaseous shells produced by the outrushing gas leaving the position of im- 

 pact, and travelling outwards in every direction mto space. Eeasons 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 nebulae, 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 nebula have not only 

 been found, but some have disappeared again, and many that used to exist 

 are lost. 



But such mere gas as nebulae 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 Vulcan. 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 wliui set all the 

 planets travelhng 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 coUidmg 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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