The Evolution of the Sciences 



of melting a layer of ice, covering its surface to 

 a depth of forty metres. Starting from this 

 figure nothing could be simpler than to calculate 

 the total quantity of heat radiated into space by 

 the sun; the colossal number obtained by these 

 calculations is itself of no great interest, but it 

 suggests a question which has hitherto remained 

 unanswered. By what means has the sun, 

 during the ages, made provision for this ex- 

 penditure of energy? Its brightness, practically 

 constant during many centuries, prevents it 

 being looked on simply as a hot body in process 

 of gradual cooling. Under these conditions 

 3000 years would be sufficient to extinguish 

 it. This constancy of temperature has been 

 attributed to various causes, such as a ceaseless 

 rain of meteorites upon the surface of the sun, 

 or to chemical reactions between the elements 

 which constitute the sun, or to a progressive 

 compression of its nucleus. The last of these 

 hypotheses is the only one which is free from in- 

 superable difficulties, but, while nothing is known 

 to contradict it formally, it remains in the air 

 and unsupported by experimental confirmation. 



The discovery of the radio-active bodies has 



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