BiCKERTON. — On the Birth of NebulcE. 199 



There are reasons to believe that this pomt of equality of temperature 

 is also a stable condition. Of course, the original suns were in a state of 

 gaseous equilibrium, and as the density of the new Sun is one-fourth of 

 each of the original, and the sm-face is just four times as great, hence the 

 surface-pressure is one-fourth, and the density one-fourth, the temperature 

 being the same, clearly, according to Harriot's law, this is a stable condition. 

 This would, doubtless, be absolutely true, were the bodies homogeneous, but, 

 as I have shown previously, there is every reason to suppose that " selective 

 escape " would ensue, and would slightly alter the final result. Thus it is 

 proved that the complete impact of equal bodies, without initial motion, 

 will not produce a nebula (disregarding " selective escape"). But a matter 

 of great importance to other parts of the theory is shown here. An impact 

 tends to lessen density, consequently the density of very large masses may 

 reasonably be supposed to be much less than that of smaller bodies. 

 "When the available energy of the visible Universe, on the supposition of its 

 formation by partial impact, comes to be considered, this may be an impor- 

 tant point. 



It is easy to show that ui the complete impact of bodies of unequal size 

 the possibility of formmg a nebula is still smaller ; hence, as it is certain 

 that the complete impact of bodies, without original proper motion, cannot 

 much more than double the diameter of a star, it is clear it cannot produce 

 a nebula, unless the mass were almost nebulous before. Of course, as the 

 final result of an immense number of complete impacts, without loss of 

 energy, a nebula might be produced, and this would be greatly aided by 

 any proper motion any of the bodies might possess. In complete impact, 

 with an original proper motion of a few hundred miles a second, two bodies 

 like oui" Sun would be converted into a nebula ; such an impact, it appears 

 to me, would produce a roughly- spherical nebula without rotation. To 

 produce rotation, it appears that the impact must be either partial, or 

 between unequal bodies. To produce a nebula of definite form, other than 

 spheroidal, appears to demand the same conditions. 



In the partial impact of two bodies having a proper motion sufficient to 

 take the two bodies away from each other after impact, nebulae of various 

 kinds may be produced, as the coalesced part struck off from the two bodies 

 may be of very small mass ; yet the velocity at which the two bodies would 

 pass each other would be very great, hence the amount of changed potential 

 energy may be enough, and more than enough, to completely diffuse the 

 coalesced mass into space as gas, and clearly intermediate conditions may 

 make nebulae of every degree of density. Having shown that partial im- 

 pact has energy enough to form diffused nebul®, the kinematic possibihties 

 will be discussed in the origin of special forms of nebulae. 



