COSMICAL PROBLEMS AND RADIO-ACTIVITY. 231 



matter is radio-active, including the soil, water, air, and in- 

 deed practically all earthly substances. It has been convin- 

 cingly shown by Rutherford that this radio-activity of ordi- 

 nary substances is in the right order of magnitude to balance 

 the loss of the earth's heat into space. It seems, then, that 

 the physicists were right only so far as they went, the trouble 

 being that they did not take into account the possibility of 

 the earth being a self-heating body. The geologists and 

 biologists were also right and they may have the thousand 

 million years for the earth as a habitable planet, if they 

 desire it. 



