Masterpieces of Science 



that of the duration of the universe in time. 

 The modern discovery of the conservation of 

 energy has raised the question of the period dur- 

 ing which our sun has existed and may continue 

 in the future to give us light and heat. Modern 

 science tells us that the quantity of light and heat 

 which can be stored in it is necessarily limited, 

 and that, when radiated as the sun radiates, the 

 supply must in time be exhausted. A very 

 simple calculation shows that were there no 

 source of supply the sun would be cooled off in 

 three or four thousand years. Whence, then, 

 comes the supply ? During the past thirty years 

 the source has been sought for in a hypothetical 

 contraction of the sun itself. True, this con- 

 traction is too small to be observed; several 

 thousand years must elapse before it can be 

 measurable with our instruments. Granting that 

 this is and always has been the sole source of 

 supply, a simple calculation shows that the sun 

 could scarcely have been giving its present 

 amount of heat for more than twenty or thirty 

 millions of years. Before that time the earth 

 and the sun must have formed one body, a great 

 nebula, by the condensation of which both are 

 supposed to have been formed. But the geolo- 

 gists tell us that the age of the earth is to be 

 reckoned by hundreds of millions of years. Thus 

 arises a question to which physical science has 

 not been able to give an answer. 



The problems of which I have so far spoken 

 are those of what may be called the older astron- 

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