Bickerton. — On Cosmic Impact. 555 



the original axes of rotation may be at any angle, or even 

 retrograde, as, in fact, they are. 



81. As the nebula shrank within the orbits of the planets, 

 the planets would again pick up light molecules that would 

 form an atmosphere; but the temperature of the planets would 

 not allow of much hydrogen being picked up unless it were in 

 combination. 



82. The resistance and contraction of the central nebula 

 would clear space of all meteoric dust unless such were orbit- 

 ally connected with a planet. The asteroids are probably 

 parts of an exploded planet. The impact of a rapidly-moving 

 body plunging into a planet could easily blow it to pieces. It 

 has been suggested that, if so, such bodies would pass through 

 the common point of their explosion. This idea is an error. 



83. The trapping of their moons by the planets would 

 probably occur when the planets were nebulous, and before 

 the central nebula had attained to any great density. Hence 

 they would lie roughly on the planet's equatorial plane. 



84. Whilst a body of the mass of the earth could pick up 

 an atmosphere, the smaller attractive power of the moon 

 would not allow this at the temperature it would be at when its 

 nebula contracted within its orbit. The moon would probably 

 be much nearer the earth at first, but the stopping of its rota- 

 tion by tidal action would increase the distance. 



85. Many other agencies that would convert the system 

 under discussion into one similar to our own are treated of in 

 my paper on " Causes tending to lessen the Eccentricity of 

 Planetary Orbits." 



Mathematical Conditions of the Formation of Nebulae. 



86. It can be shown, if two gaseous suns impact completely, 

 having no original proper motion, that were the whole of the 

 motion converted into heat, and this heat into the potential 

 energy of expansion, then the new san would have a diameter 

 the sum of the diameters of the original suns. It can also be 

 shown that such a condition is one of stable equilibrium. 



87. Consequently the complete impact of two gaseous suns 

 not possessing much original motion, and brought together by 

 gravitation, does not make a nebula of them ; but as soon as 

 the paroxysm of the encounter is over they are of the same 

 temperature as before, having used up all their energy in 

 increasing to the sum of their original diameters. This is a 

 remarkable and unexpected result. 



88. Were there great original proper motion, they might 

 become a nebula by complete impact ; but were the original 

 velocity of the two bodies very high, and the impact of very 

 great energy, then an indefinitely-diffused nebula would result. 

 Such a nebula, if hot, would be unstable, and would indefinitely 



