COSMOGONIES 121 



take the bread out of their mouths. According 

 to this form of the theory, the planets would be 

 heated by collision and contraction and by the 

 impact of meteoric particles. 



The theory, however, takes many forms. Sir 

 Norman Lockyer supposed that the meteoric 

 stones banging about in an atmosphere of 

 hydrogen would be volatilised by the heat of 

 their mutual impacts, and would eventually form 

 such a gaseous mass as Laplace postulated. Pro- 

 fessor Darwin, on the other hand, prefers to hold 

 that the meteoric particles behave under impact 

 like colossal molecules, and thus are not volatilised, 

 but give rise to " fluid pressure." 



One of the most striking of recent hypotheses 

 has been brought forward by Professor J. H. Jeans. 

 He postulates a hot spherical nebula of gas or 

 meteorites which revolves slowly. Such a nebula, 

 he points out, would cool unequally, since the 

 outer portion would cool more quickly than the 

 central, and this inequality of temperature, together 

 with the increasing velocity of rotation consequent 

 upon the contraction due to cooling, would produce 

 instability of the nebula and cause it to take a 

 pear-shaped form. Eventually, he suggests, the 

 pear-shaped end would be flung oflF and form a 

 planet or satellite. The impact of a meteorite, or 

 the attraction of some other body, would favour 



