108 F. W. Very— A Cosmic Cycle. 



a Jielical structure, something like a variously twisted rope, 

 suggesting that it may have been produced by an explosion 

 directed, by exception, along the axis of a rotating body. 



The circular form of planetary nebulae and the presence of 

 a single star at the precise center of a circular nebulous spot 

 are sufficient indication of central forces in these objects, but 

 it has perhape been too hastily assumed that the forces are 

 always centripetal. The recent discovery of a radial structure 

 in certain planetary nebulae lends to these forms also the simili- 

 tude of an explosive phenomenon. 



The sudden dispersal of intensely heated matter through a 

 large volume may be expected to produce by expansion either 

 a mass of relatively cool gas or a clond of dust. In either 

 case, the dispersal is incomplete, because modification of 

 motions by internal collisions prevents the escape of any con- 

 siderable number of particles from the central control. 



The case is otherwise if the explosive subdivision has been 

 into a comparatively small number of distinct bodies, where 

 the central control may be insufficient to prevent the wander- 

 ing of the members to such distances that the system is even- 

 tually completely broken up. 



The existence of a few stars of exceptionally great mass 

 and late type is an objection which may be urged against the 

 proposed theory. It seems possible to meet this objection 

 as follows : In case the explosions resulting in the formation 

 of a planetary nebula have eliminated most of the especially 

 explosive material, we may surmise that a repetition of the 

 condensation of the diffused matter will give a stable star of 

 enormous size. 



It may be granted that quasi-fluidity of a meteoric swarm, 

 produced by internal collisions,* supplies a needed analogy for 

 the explanation of certain figures observed among the nebulae, 

 resembling those of the fluid masses freed from gravitational 

 control, which have been made known to us by Plateau, where 

 the interaction of pressure from fluid surface-tension and rotary 

 forces give a number of similitudes; but the existence of 

 these simple forms among the nebulae only emphasizes the 

 necessity for additional hypotheses to explain the numerous 

 departures from forms which may be regarded as cases of 

 simple rotary equilibrium ; and the radial and branching 

 features of a ^reat number of nebulae require that the added 

 force shall act radially and outward from a center. 



The hypothesis of planetary genesis by nebular annulation 

 through excessive rotary velocity is beset with difficulties. 



*See the mathematical researches of G-. H. Darwin, " On the Mechanical Con- 

 ditions of a Swarm of Meteorites, and on Theories of Cosmogony,'" Proc. R. S. 

 London, vol. xlv, p. 3, 1888. Phil. Trans. R. S, vol. clxxx. A, p. J, 1889. 



