THE GROWTH OF RADIUM 95 



and the process proceeds through a long succession 

 of more or less unstable intermediate elements, until 

 the final stable product is reached. In this process 

 energy is evolved of the order of a million times 

 greater than the energy ever liberated in ordinary 

 chemical changes, in which the groups of atoms, or 

 the molecules, change, but not the constituent atoms 

 themselves. The energy evolved by an ounce of 

 radium, in the course of its life, equals that evolved 

 from the burning of ten tons of coal. The period of 

 average life in this case is about 2500 years, which 

 means that ^oth part of any quantity of radium 

 changes per annum. 



The rate at which the various radioactive products 

 change varies very widely. It may be slow or rapid, 

 a matter of seconds or even billionths of a second on 

 the one hand, or of years or centuries or aeons on the 

 other. It was reasonable to interpret what Mme. 

 Curie had done for pitchblende in exactly the same 

 way as had been done for thorium, merely extending 

 the time scale. The radium, polonium, actinium and 

 the other new intensely active radio-elements she 

 discovered in such infinitesimal amount in pitchblende 

 were in all probability the products of the change of 

 the parent element uranium. The view carries with 

 it the corollary that, if you separated uranium from 

 radium and everything else completely and left it to 

 itself, in the course of years or centuries a new crop 

 of radium would be gradually formed. The case 

 of radium is specially interesting as it has been 

 established that it is an ordinary element resem- 

 bling barium, with definite spectrum, atomic weight, 

 chemical properties and position in the Periodic 

 Table. It was one of very many startling predictions 

 of a similar character made as soon as the new point 

 of view was attained. But it has been the last to 

 receive confirmation and the difficulties have been 



