22 EVOLUTION 



the time that has elapsed since our parent nebula began 

 its fateful process of condensation ? Here we enter 

 upon precarious ground, and it will be enough to quote 

 a few opinions. How long the formation of the crust of 

 our earth may have taken is a different matter, that we 

 will consider later. At present we have the broader 

 question that faces the astronomer and the physicist. 

 It is well known that Lord Kelvin made a careful cal- 

 culation, based on the sun's expenditure of heat, and 

 came to the conclusion that the physicist could not allow 

 more than a hundred million years at the outside for the 

 development of our solar system. Many geologists and 

 biologists stoutly maintained that this would not suffice 

 for the development even of the earth's crust and its 

 living inhabitants, and for years there was a warm con- 

 troversy. The end of it is curious, and contains a good 

 moral for controversialists on these abstruse issues. 

 While geologists and biologists have, as we shall see, 

 greatly moderated their demands, the physicist has sud- 

 denly been compelled to offer them an almost indefinite 

 period. 



The discovery of radium has entirely altered the 

 situation. Lord Kelvin proceeded on the supposition 

 that the heat of the sun was due almost entirely (allowing 

 for the infall of meteorites) to condensation, in the way 

 I described above. But the discovery that heat is pro- 

 vided out of the very interior of the atom affords a new 

 and formidable source of heat. In the atom of radium 

 about a quarter of a million electrons are organised in a 

 system of minute points (or centres of energy) whirling 

 round at an inconceivable speed. As the atom breaks 

 up, they fly off at a speed of at least 100,000 miles a 

 second, and are the most interesting element in the now 

 familiar emanation from radium (and other substances). 

 The heat evolved in this process is 3$ million times 



