ORIGIN OF THE EARTH 



37 



a profound influence upon science, philosophy, and theology, and 

 certainly many of the important phenomena of the solar system 

 are explained by it. Some serious objections to it may, however, 

 be briefly stated as follows: (1) Nearly all existing nebulas are 

 spiral and not circular; (2) spectroscopic study shows that these 

 nebulas do not consist of gas, but rather of discrete liquid or solid 

 particles; (3) the backward revolutions of certain satellites oppose 

 the hypothesis; (4) rings could not have been left off, that is there 

 could have been no intermit- 

 tent process of the sort ; and 

 (5) it is not at all clear how 

 the matter of the rings could 

 have condensed into planets. 

 Planetesimal or Spiral 

 Hypothesis. — It is a remark- 

 able fact that, although many 

 thousands of nebulas are 

 known, there are very few 

 examples of ring nebulas of 

 the Laplacian type among 

 them. Spiral forms are 

 very common, especially the 

 smaller ones. Also, as above 

 stated, spectroscopic study of 

 these nebulas shows them to 

 be made up of discrete (liquid 

 or solid) particles rather than 

 of gas. The Planetesimal 

 hypothesis, 1 formulated by 

 Chamberlin and Moulton, " postulates that the matter of which 

 the sun and the planets are composed was, at a previous stage of 

 its evolution, in the form of a great spiral swarm of discrete par- 

 ticles whose positions and motions were dependent upon their 

 mutual gravitation and their velocities" (Moulton). A nebula 

 of this sort comprised a luminous central mass (the future sun) 

 from the opposite sides of which two luminous spiral arms streamed 

 out with occasional larger masses or knots along each arm, and with 

 dark lanes between the arms (see Fig. 19). Also some nebulous 



1 An elaborate discussion of this hypothesis may be found in Chamberlin 

 and Salisbury's Geology, Vol. 2. 



Fig. 19 

 A very symmetrical spiral nebula in 

 Pisces (M. 74). Photo by Lick 

 Observatory. (From Chamberlin 

 and Salisbury's "Geology," permis- 

 sion of Henry Holt and Company.) 



