Bicxerton.—On the Birth of Nebula. 208 
tained until a spindle nebula of considerable dimensions is formed. It is 
not difficult to account for the origin of cometary nebule. These have 
donbtless been formed by an impact in which a want of balance in the 
momentum left a considerable residual velocity in the nebula, and that 
as it travels it becomes smaller, both by losing the hotter and more volatile 
portions, and by its own condensation. 
The planetary nebule have always been considered the most wonderful 
objects in the entire heavens. The Herschels devoted much time to their 
discussion, and in my opinion conclusively proved them to be self-luminous 
hollow spheres of most stupendous diameter, several of them being many 
times larger than the most of our most distant planets. Recent spectro- 
scopic observations have proved them to consist of gas; so that the problem 
before us is—to account for gigantic slightly-luminous hollow spheres of gas 
many thousand millions of miles in diameter. 
Supposing we have an immense crowd collected in one spot, and that 
each one begins to move on indiscriminately in a straight line, and each if * 
striking against anyone goes on again in the direction the blow has started _ 
him, and all continue to move straight on indefinitely : it is certain that in 
a few days the spot where the crowd was will be clear, and an immense 
irregular circle of people will exist, and will constantly be extending it- 
self. This I believe to be the condition of a planetary nebula; an impact 
has taken place; on grounds of probability it was most likely partial; 
but the physical conditions would be nearly the same were it a com- 
plete impact of bodies with a stupendous original proper motion. As I 
have already stated, such an event appears to me, however, to be of 
amazing improbability. Hither of these two suppositions will supply us with 
a gaseous body of such a high temperature compared with its mass, that 
every molecule will have sufficient velocity to escape the gravitating 
influence of the mass and travel straight on into space. For example: 
If a particle of gas at the surface of the Sun had a velocity of four hundred 
miles à second, such a particle would pass out of our system; and it is 
almost certain that had every particle this velocity—that is to say, tho 
necessary temperature—the Sun would become a planetary nebula or a 
hollow shell of luminous gas. It is a remarkable confirmation of this 
theory, that Lord Lindsay has reported that the temporary star of 1877 
has become a planetary nebula or a hollow shell of luminous gas. I have 
already shown how selective escape may have produced a nebula con- 
sisting of intermediate, or in fact any group of approximately equal mole- 
cular weights; and I need not say such reasoning applies equally to 
planetary nebule; the nucleus being in some cases the aggregated heavy 
molecules, These bodies are doubtless dynamically in an unstable con- 
