The Evolution of the Sciences 



example, the external crust consists of the lighter 

 bodies, whereas at the centre the density is 

 perhaps as high as 7, which is practically the 

 density of iron. Probably similar conditions 

 prevail in the sun. 



The smallness of the sun's density is deserv- 

 ing of attention, because it allows us to infer that 

 the bulk of the sun cannot be in a liquid, or still 

 less in a solid state. On the contrary, the cal- 

 culations of Schuster show that a gas raised to 

 the high temperature probably existing in the 

 sun would, in consequence of the conditions of 

 internal pressure, present more or less the density 

 inferred by us from experience. This hypothe- 

 sis x is distinctly conjectural and must be 

 treated accordingly; it is of the nature of a 

 probability, and scientists have discovered by 

 experience that probability is a bad criterion. 

 However, we may adopt for future use the 

 fundamental idea that the solar mass is pro- 

 bably composed of gases and vapours at a high 

 temperature and subjected to enormous com- 

 pression. 



1 As recently as 1887 Lord Kelvin considered the sun to 

 be liquid. 



154 



