422 GEOLOGY 



start forth on opposite sides and curve spirally outward. The 

 arms often branch, and are much interrupted and knotted, and 

 between them there is much scattered hazy matter; but even in 

 the more diffuse forms, the presence of two arms is discernible. 

 The prevalence of this form of nebula implies that it is due to some 

 process which is pervasive. The numerous nebulous knots on the 

 arms, and sometimes more or less outside them, are significant 



Fig. 335. A typical spiral nebula in Piscium, Messier 74, with very symmet- 

 rical arms, pronounced nucleus and knots, and a relatively limited amount 

 of nebulous haze. (Photo, from Lick Observatory.) 



features. Clearly the matter has a very unequal dispersal and 

 does not conform to the symmetric laws of gaseous distribution. 



Recent advances in spectroscopy throw much light on the con- 

 stitution of nebulae. As just inferred from their forms, the spiral 

 nebulae seem to be composed, not of gaseous molecules, but of solid 

 or liquid particles. These tiny bodies are believed to revolve about 

 the center of the nebula, like little planets, but this has not yet been 

 proved. If it were positively known that the particles of the spiral 

 nebula3 revolve about the centers of the nebula in elliptical orbits 

 like planets, and were thus planetesimals, these nebulae might well 

 be called planetesimal nebulce. The planetesimal hypothesis is 

 based on a spiral nebula of this supposed organization. 1 



'The manner in which it may have arisen is discussed in the authors' 

 larger work on Geology, Vol. II. 



