The Sun 



also suggested a hypothesis, which is no less 

 conjectural than the others; the presence of 

 two grammes of radium per ton of solar sub- 

 stance would be sufficient to account for quanti- 

 tatively the energy emitted by our central orb 

 as well as the remarkable constancy of its 

 emission. 



But more important for us than the amount 

 of the sun's total radiation is the determina- 

 tion of its temperature. This is an essential 

 datum for ascertaining what materials compose 

 it. Physicists, as a rule, determine the tempera- 

 ture of a body by the amount and nature of its 

 radiation. The sun, however, presents con- 

 ditions which would render this solution very 

 unreliable. It would be dangerous to apply 

 the laws governing our small, dim fires to this 

 powerful and radiant furnace. The absorption 

 of the sun's rays by the two atmospheres sur- 

 rounding the sun and the earth must also be 

 taken into account, and this presents consider- 

 able difficulty. These two causes account for 

 the surprising differences shown by the various 

 evaluations of the solar temperature. Water- 

 ston estimated it at 10,000,000 centigrade; 



157 



