BicKERTON. — On the Birth of Nebulcp. 203 



tained until a spindle nebula of considerable dimensions is formed. It is 

 not difficult to account for the origin of cometary nebulae. 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 nebulae 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 om* most distant planets. Eecent 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 

 strikmg 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. Either 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 a 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, tha 

 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 ap^Droximately equal mole- 

 cular weights ; and I need not say such reasoning applies equally to 

 planetary nebulae ; the nucleus being in some cases the aggregated heavy 

 molecules. These bodies are doubtless dynamically in an unstable con- 



