ADVANCE OF ASTRONOMY. 207 



is coal enough in the State of Pennsylvania to sup- 

 ply the wants of the United States for many centuries 

 to come, yet the heat which could be generated by 

 the combustion of all the coal in Pennsylvania would 

 not be sufficient to supply the sun's radiation for the 

 thousandth part of a single second." * 



From experiments on the intensity of the radi- 

 ation emitted by an incandescent body, Le Chatelier 

 has argued (1892) that the temperature of the sun 

 cannot be less than 7,600C., and probably much 

 more. These and similar figures convey little mean- 

 ing in themselves, but they are significant in rela- 

 tion to the problem of how the supply of energy is 

 sustained. 



Maintenance of Solar Energy. Especially after 

 the formulation of the doctrine of the conservation 

 of energy (about 1843), the problem of the main- 

 tenance of the sun's heat urgently claimed atten- 

 tion. It soon became evident that it is impossible 

 to think of the sun as like an enormous fire giving 

 out heat by combustion. " Massive as the sun is, 

 if its materials had consisted even of the very best 

 materials for giving out heat by what we understand 

 on the terrestrial surface as combustion, that enor- 

 mous mass of some 400,000 miles in radius could 

 have supplied us with only about 5000 years of the 

 present radiation." f From what we know of the 

 sun's age and the amount of its radiation, it is cer- 

 tain that its heat cannot be mainly due to chemical 

 processes at present known to us. 



Setting aside the chemical solution of what Sir 

 John Herschel called " the great secret," we find two 



* Sir Robert Ball, The Story of the Sun, 1893, p. 265. 

 tP. G. Tait, Recent Advances, 1876, p. 151. 



