RADIUM, ATOMIC ENERGY, AND ATOMIC STRUCTURE 545 



depositing a solid radioactive residue on bodies in contact with it. 

 Furthermore, every known uranium ore contains radium (McCoy) 

 and radium emanation (Boltwood) in amounts proportional to 

 the uranium content. Also, after the radium has been removed, 

 the pure uranium compound gives off at first only a-particles, 

 but gradually recovers its whole radioactivity and is then found 

 to contain radium emanation once more (Soddy). It thus ap- 

 pears that uranium is the starting point, and that the disinte- 

 gration proceeds by steps, producing a number of different prod- 

 ucts. Each of these is formed from one such product and by 

 disintegration furnishes another. 



Unlike ordinary chemical change, the rate of disintegration is 

 not affected by conditions. It can neither be started nor stopped 

 at will. It is no more vigorous at 2000 than at 200. Other 

 changes occur between atoms, these within each atom. 



The law, due also to Rutherford, describing the rate at which 

 any one radioactive element disintegrates is simple. Only a 

 certain fraction of the whole of any one specimen undergoes the 

 change in unit time. Thus, as the total amount diminishes be- 

 cause of the change, the amount changing during the next unit 

 of time, being a constant fraction of the whole, must be less. 

 Hence an infinite time would be required for the complete dis- 

 integration of any one specimen. For convenience in expressing 

 the rate of disintegration, however, we calculate and tabulate the 

 average life of the element. 



Radium emits helium atoms at the rate of 3.4 X 10 10 per gram 

 per second. From this fact, we can calculate its average life to 

 be about 2400 years. Hence, if it were not continuously being 

 produced (from uranium), the whole supply would have been 

 exhausted long before the earth reached a habitable condition. 



The Uranium Group of Radioactive Elements. The 



following shows the various elements produced from uranium by 

 successive disintegrations. When a helium atom or an electron 



