156 



of such materials as the earth is made of and as the spectro- 

 scope indicates as entering into the composition of the sun, 

 and suppose this planetary body to be in comparison Avith 

 with our globe extremely small in mass, and located at such 

 a distance from the sun as to be sensibly affected by his 

 rays, say for instance within Saturn's orbit, and sup2)0se 

 further that it is retained at that distance until such changes 

 as would be produced by the temperature to which it is 

 there subjected are fully realised. We should then have a 

 central mass of more or less solid material surrounded by 

 an attenuated atmosphere of such substances as are gaseous 

 at the particular temperature there prevailing and under 

 the particular pressure exercised by the gravitation of the 

 central mass. Now let us suppose our planetary body to 

 be moved to another position considerably nearer to the 

 sun, and so subjected more largely to the influence of his 

 rays. An augmentation of its atmosphere would imme- 

 diately be commenced. Materials non-volatilisable at its 

 previous temperature would be raised into the gaseous form. 

 The volume of its atmosphere would be increased whilst the 

 planet's coercive power over its elasticity would be dimi- 

 nished. But let us suppose our planetary body to be once 

 more replaced in its former position and subjected to the 

 lesser of the two temperatures we have been considering. 

 The solar heat will now no longer be able to maintain all 

 that matter in the gaseous form which has been evaporated at 

 the shorter of the two distances from the sun. A condensa- 

 tion will accordingly be commenced through a greater or 

 less extent of the cometary atmos2)liere, and a more or less 

 dense nebulous mass will surround the central stellar 

 point. This nebulosity will be again evaporated into 



