34 



ORIGIN OF THE EARTH 



nebulae, at least ten times as many as were known a few years 

 ago, and that in this multitude there is one dominant form, the 

 spiral nebula (Fig. 288). The spiral nebula has a central nucleus, 

 from which two arms or sets of arms project on opposite sides, and 

 curve spirally outward. The arms of some nebulae branch, and are 

 much interrupted and knotted, and between them there is much 

 scattered hazy matter. The prevalence of this form of nebula 

 implies that it is due to some process which has been common. The 

 numerous nebulous knots on the arms, and in some cases more or less 

 outside them, are significant features. Clearly the matter of the 

 nebula is very unequally distributed, and does not conform to the 

 laws of gaseous distribution. 



Recent advances in spectroscopy throw much light on the con- 

 stitution of nebulas. As inferred from their forms, the spiral 



nebula? seem to be composed 

 of solid or liquid particles, 

 though gases may be present 

 particularly in their nuclei and 

 knots. These tiny bodies are 

 believed to revolve about the 

 center of gravity of the nebula, 

 like little planets (planetesi- 

 mals), but this has not yet been 

 proved. The planetesimal hy- 

 pothesis is based on a spiral 

 nebula of this supposed organi- 

 zation. 1 



The planetesimal hypothe- 

 sis starts with a spiral nebula 

 consisting of the following ele- 

 ments: (i) The main knots to 

 serve as nuclei for the planets, 

 (2) small scattered knots as the 

 nuclei of asteroids, (3) other 



Fig. 289. A typical spiral nebula in 

 Piscium, Messier 74, with very symmetri- 

 cal arms, pronounced nucleus and knots, 

 and a relatively limited amount of nebu- 

 lous haze. (Photo, from Lick Observa- 

 tory.) 



small knots near to the large ones and controlled by them, as the 

 nuclei of satellites, and (4) scattered matter or nebulous haze to 

 be gathered into these nuclei to give them their mature sizes, and 

 (5) the great central mass of the nebula, forming the nucleus of 



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

 work on Geology, Vol. II. 



