COSMICAL PROBLEMS AND RADIO-ACTIVITY. 229 



fs, therefore, possible, and even likely, that there exists in 

 the sun's mass large quantities of radio-active matter, and 

 on this supposition it is easily possible to increase to an 

 enormous extent the duration of the sun's age and heat 

 in the past, and its maintenance for untold millions of years 

 in the future. 



It may be shown that the presence of 3.6 grams of ra- 

 dium in each cubic metre of the sun's mass is sufficient to 

 account for its present rate of emission of energy, or, cal- 

 culated in another way, that 2.5 parts by weight of radio- 

 active matter in a million would keep the sun going. 

 Rutherford concludes that if the energy resident in the 

 atoms of the elements is available in the sun that the time 

 during which the sun may continue to radiate at its present 

 rate may be as much as 500 times longer than the maximum 

 limit afforded by Lord Kelvin. 



We see thus that the depressing conclusion of the older 

 science that the earth must come to an end in a time 

 short in comparison with its past duration, was unwarranted. 

 It may, however, be objected to this conclusion that if the 

 sun possesses radio-activity, this radio-activity ought to be 

 perceptible on earth. But this is not so, for even the most 

 penetrating rays, the #awma-rays, would be practically 

 stopped and absorbed by the earth's atmosphere which is 

 equivalent to 30 inches of mercury. 



THE AGE OF THE EARTH. 



For the last fifty years the age of the earth as a habit- 

 able planet has been a subject of bitter debate between the 

 physicists on the one hand and the biologists and geolo- 

 gists on the other. The physicists would not grant the 

 time demanded by the geologists and biologists ; for whereas 

 the physicists would grant ten million years, the geologists 



