144 SCIENCE AND IMMORTALITY 



age of the Earth by the amount of heat it has lost, 

 but the discovery of radium in the crust of the 

 Earth has quite destroyed the value of these 

 calculations, and Lord Kelvin's estimate that it is 

 only about twenty million years since the solidifica- 

 tion of the Earth is no longer accepted. 



The discovery of radium has also greatly altered 

 the prospects of the Earth's life. The Earth must 

 depend for life on the heat of the Sun ; and on 

 looking into the question of the Sun's expenditure 

 of heat, physicists found it difficult to understand 

 how it had radiated heat so long, and came to the 

 conclusion that it could not continue to do so 

 much longer. It was calculated that even if it 

 consisted of pure carbon it would burn out in 

 four thousand years. Mayer suggested that 

 swarms of meteorites rushing into the Sun might 

 have helped to maintain the heat of the Sun in the 

 past, and might help to maintain it in the future ; 

 but further consideration of this theory showed 

 that it could not be correct, and that even if it 

 were, an incredibly huge supply of meteorites 

 would be necessary, for even if the Earth itself ran 

 into the Sun, the Sun's life would not be greatly 

 lengthened. Helmholtz made a more satisfactory 

 suggestion, namely, that the Sun's heat was 

 augmented by the continual contraction of its 

 mass ; but even so, it would have little heating 



