THE STORY OF NINETEENTH-CENTURY SCIENCE 



became possible to calculate the limits of time within 

 which this culmination would probably occur. It was 

 only necessary to calculate the total amount of heat 

 which could be generated by the total mass of our solar 

 system in falling together to the sun's centre from " in- 

 finity" to find the total heat-supply to be drawn upon. 

 Assuming, then, that the present observed rate of heat- 

 giving has been the average maintained in the past, a 

 simple division gives the number of years for which the 

 original supply is adequate. The supply will be ex- 

 hausted, it will be observed, when the mass comes into 

 stable equilibrium as a solid body, no longer subject to 

 contraction, about the sun's centre such a body, in 

 short, as our earth is at present. 



This calculation was made by Lord Kelvin, Professor 

 Tait, and others, and the result was one of the most truly 

 dynamitic surprises of the century. For it transpired 

 that, according to mathematics, the entire limit of the 

 sun's heat-giving life could not exceed something like 

 twenty-five millions of years. The publication of that 

 estimate, with the appearance of authority, brought a 

 veritable storm about the heads of the physicists. The 

 entire geological and biological worlds were up in arms 

 in a trice. Two or three generations before, they hurled 

 brickbats at any one who even hinted that the solar sys- 

 tem might be more than six thousand years old ; now 

 they jeered in derision at the attempt to limit the life- 

 bearing period of our globe to a paltry fifteen or twenty 

 millions. 



The controversy as to solar time thus raised proved 

 one of the most curious and interesting scientific dispu- 

 tations of the century. The scene soon shifted from the 

 sun to the earth ; for a little reflection made it clear 



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