Bicxerton.—On the Birth of Nebula. 199 
There are reasons to believe that this point 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 surface 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 Marriot’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 in the complete impact of bodies of unequal size 
the possibility of forming 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, & 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 our 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, nebule 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 nebule of every degree of density. Having shown that partial im- 
pact has energy enough to form diffused nebule, the kinematic possibilities 
will be discussed in the origin of special forms of nebulz. 
