xii CONTENTS. 



PAGE 



The Evidence of the Meteorites Themselves 22 



Theories of the origin of meteorites, 23. The char- 

 acters of meteorites, 23. The bearing of these char- 

 acters, 24. Possible derivation from atmosphereless 

 bodies, 31. Hypotheses of disruption and dispersion, 

 34. Relations of meteorites to comets, 36. 



III. The Planetesimal Hypothesis 38 



Sub-Varieties of the Hypothesis 38 



Planetesimal condition arising from a gaseous 

 spheroid, 39. Planetesimal condition arising from the 

 mcteoritic, 40. Planetesimal condition arising from 

 original nebular dispersion, 40. 

 The Characteristics of Present Nebulce 41 



1. Free-molecular nebulae, 41. 2. Aggregate-mo- 

 lecular (planetesimal?) nebulae, 42. 

 The Hypothetical Origin of the Solar Nebula 51 



An implied ancestral system, 51. The question of 

 adequacy of energy, 51. The contingencies of stellar 

 collision, 53. The contingencies of close approach, 53. 

 The special consequences of close approach, 55. Excep- 

 tional protuberances developed on opposite sides by 

 close approach, 56. The demands on protuberant ac- 

 tion not severe, 56. The acquisition of rotatory motion, 

 56. The distribution of specific gravities, 58. Suc- 

 cessive partial outbursts, 58. The result a spiral nebula, 

 58. Theoretical diagram of the solar nebula, 58. The 

 passage of the molecules from the free to the aggregated 

 state, 58. Nebular luminescence, 59. The assigned 

 nebular origin not vital, 60. 

 The Evolution of the Nebula into Planets 60 



The three essential factors, 60. The attractive 

 powers of the knots and their limitations, 61. The 

 knots the planetary nuclei, 61. 

 The Mode of Accretion 64 



The nature of the planetesimal motions, 64. The 

 part played by ellipticity of planetesimal orbits, 65. 

 The evolution of circularity, 67. The time involved, 67. 

 The bearings of the rate of infall on temperature, 68. 

 The collisions all overtakes, 69. The bearing of the 

 mode of accretion on the direction of planetary rota- 

 tion, 70. The varying effects of the impacts on 



