A HISTORY OF SCIENCE 



time, but for a limited time only; destined ultimately 

 to become liquid, solid ; to cool below the temperature 

 of incandescence to die. Not only so, but it 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 sys- 

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

 ity" 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 gen- 

 erations before, they hurled brickbats at any one who 

 even hinted that the solar system 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. 



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