ON THE SUN'S HEAT. 403 



would be augmenting, because the conduction of 

 heat outwards through the solid would be too 

 slow to compensate the augmentation of pressure 

 due to augmentation of gravity in the shrinking fluid 

 around the solid. But at a certain time in the 

 history of a wholly fluid globe, primitively rare 

 enough throughout to be gaseous, shrinking under 

 the influence of its own gravitation and its radia- 

 tion of heat outwards into cold surrounding space, 

 when the central parts have become so much 

 condensed as to resist further condensation greatly 

 more than according to the gaseous law of simple 

 proportions, it seems to me certain that the early- 

 process of becoming warmer, which has been 

 demonstrated by Lane, and Newcombe, and Ball, 

 must cease, and that the central temperature 

 must begin to diminish on account of the cool- 

 ing by radiation from the surface, and the mixing 

 of the cooled fluid throughout the interior. 



Now we come to the most interesting part of 

 our subject the early history of the Sun. Five 

 or ten million years ago he may have been about 

 double his present diameter and an eighth of his 



D D 2 



