* 3 1 8, 319] The Evolution of the Solar System 407 



separation of bodies which attract one another, as well as 

 in various electrical, chemical, and other ways. With this 

 discovery was closely connected the general theory known 

 as the conservation of energy, according to which energy, 

 though capable of many transformations, can neither be 

 increased nor decreased in quantity. A body which, like 

 the sun, is giving out heat and light is accordingly thereby 

 losing energy, and is like a machine doing work ; either 

 then it is receiving energy from some other source to 

 compensate this loss or its store of energy is diminishing. 

 But a body which goes on indefinitely giving out heat and 

 light without having its store of energy replenished is 

 exactly analogous to a machine which goes on working 

 indefinitely without any motive power to drive it ; and both 

 are alike impossible. 



The results obtained by John Herschel and Pouillet in 

 l8 3 6 ( 307) called attention to the enormous expenditure 

 of the sun in the form of heat, and astronomers thus had to 

 face the problem .of explaining how the sun was able to go 

 on radiating heat and light in this way. Neither in the 

 few thousand years of the past covered by historic records, 

 nor in the enormously great periods of which geologists 

 and biologists take account, is there any evidence of any 

 important permanent alteration in the amount of heat and 

 light received annually by the earth from the sun. Any 

 theory of the sun's heat must therefore be able to account 

 for the continual expenditure of heat at something like the 

 present rate for an immense period of time. The obvious 

 explanation of the sun as a furnace deriving its heat from 

 combustion is found to be totally inadequate when put to 

 the test of figures, as the sun could in this way be kept 

 going at most for a few thousand years. The explanation 

 now generally accepted was first given by the great German 

 physicist Hermann von Helmholtz (1821-1894) in a popular 

 lecture in 1854. The sun possesses an immense store of 

 energy in the form of the mutual gravitation of its parts ; 

 if from any cause it shrinks, a certain amount of gravita- 

 tional energy is necessarily lost and takes some other form. 

 In the shrinkage of the sun we have therefore a possible 

 source of energy. The precise amount of energy liberated 

 by a definite amount of shrinkage of the sun depends upon 



