228 THE NEW KNOWLEDGE. 



Next, it \vas supposed that the meteorites falling into the 

 sun could generate heat sufficient to maintain its energy. 

 This also was disproved. Finally then came Helmholtz's 

 theory, based upon the nebular hypothesis, that the heat 

 of the sun might be maintained by its own contraction 

 from a nebular condition. It is not too much to say that 

 in recent years this has been the accepted theory of science. 

 It has always been burdened, however, by the fact that, on 

 this basis, the sun could not have maintained its energy and 

 have illuminated the earth in the past for a time sufficient 

 to account for the observed geological changes. Professor 

 Young says in his " General Astronomy/' " No conclusion of 

 geometry is more certain than this that the contraction of 

 the sun to its present size, from a diameter even many times 

 greater than Neptune's orbit cannot have been emitting 

 heat at its present rate for more than 18,000,OCO years, if 

 its heat has really been generated in this manner." 



Finally, Lord Kelvin has calculated the energy lost in 

 the concentration of the sun from a condition of infinite 

 dispersion with the conclusion that it is " on the whole 

 probable that the sun has not illuminated the earth for 

 100,000,000 years and almost certain that he has not done 

 so for 500,000,000 years. As for the future we may say 

 with equal certainty that inhabitants of the earth cannot 

 continue to enjoy the light and heat essential to their life 

 for many million years longer, unless sources now unknown 

 to us are prepared in the great store-houses of creation." 



We shall see that in radio-activity we have, probably, 

 an additional store of energy. We know that there exists 

 in the sun enormous quantities of the element helium. We 

 know, also, that helium is a decomposition product from 

 radio-active substances, and finally we know that radio- 

 active substances generate enormous quantities of heat. It 



