THE LATEST STAGE 239 



chemical change, new substances appearing. Thus 

 radium evolves continuously a radio-active gas or 

 emanation, which itself deposits radio-active matter 

 on solid surfaces. Moreover, these changes are 

 associated with the emission of large amounts of energy, 

 which may be measured as heat, and are shown to 

 transcend by far the energy liberated by any ordinary 

 chemical action. 



These striking phenomena are explained and co- 

 ordinated by a theory propounded by Rutherford and 

 Soddy in 1903. Radio-activity is due to the explosive 

 disintegration of individual atoms, one in many millions 

 breaking up each hour. In this way it is calculated 

 that half of a given quantity of radium would disappear 

 in some two thousand years, passing successively into 

 the emanation and various kinds of solid matter, 

 while each act of disintegration involves the explosive 

 emission of one atom of helium. Radium itself is 

 probably formed by a similar even slower change from 

 a substance now called ionium, which itself is derived 

 directly or indirectly from uranium. Uranium thus 

 becomes the first known ancestor of the family of 

 radio-active elements of which radium is the most 

 distinguished representative. 



In this way the dream of the mediaeval alchemist 

 at length has come true. Chemical elements are not 

 all indestructible and eternal, though their changes 

 seem to be beyond our control. We can but watch 

 one type of matter arising from another ; no philo- 

 sophers' stone will produce the change ; even the 

 resources of a modern laboratory seem powerless to 

 hasten or delay the disintegration of a single 

 atom. 



