100 THE EVOLUTION OF MATTER 



product of polonium, actinium and other of the new 

 radio-elements. 



Gradually the tangled and complex succession of 

 changes being undergone by uranium and thorium 

 have been straightened out, and it is probable that 

 the work is now complete. Some of the changes 

 require millions of years, some are over in a billionth 

 of a second or less. The atom of uranium expels 

 7 a- and 5 ^-particles, in twelve successive changes, 

 one particle per atom at each change. The atom of 

 thorium expels 6 a- and 3 /3-particles. The /3-particles 

 are atoms of electricity rather than of matter, and 

 their expulsion affects the mass of the parent atom to 

 only a negligible extent. But the a-particles are 

 atoms of helium and the expulsion of each particle 

 must lower the atomic mass of the parent atom by 

 4 units. 



So long as the process of disintegration of the 

 atom is proceeding, the rays emitted and the energy 

 they possess afford the necessary evidence for their 

 experimental study. But when it is all over, how 

 are we to proceed? The final product into which 

 uranium or thorium turns, if it is the final product, 

 by hypothesis emits no rays. The quantity produced 

 from any manageable quantity of uranium or thorium 

 in a lifetime is too small to detect chemically. How 

 can we find out even what it is ? 



There is the method that already had indicated 

 helium as the element constituting the a-particle. In 

 the natural radioactive minerals one would expect 

 to find the end products of the radioactive changes in 

 greater or less relative abundance, according as the 

 mineral is geologically ancient or modern. This 

 evidence for long indicated the element lead as the 

 final product of the changes of uranium. To-day we 

 know that the radioactive minerals are in reality 

 geological clocks, and they record more accurately 



