RADIO-ACTIVITY 195 



no appreciable change in mass and the atomic 

 weight of radium D is also 214. 



Now let us consider the changes in electrical 

 charge. When radium loses an a particle, it loses 

 also two units of positive charge. We shall see 

 later that there is reason to believe that what is 

 called the atomic number of radium, a number 

 which measures the essential electric charge on the 

 atomic nucleus, is ZS. Hence the nuclear charge 

 or atomic number of the emanation is ^6, of radium 

 A is 84, and of radium B, 82. Since in each case 

 the whole atom is neutral, it has to discard also 

 two of the electrons from the outer orbits. The 

 whole change involves a complete rearrangement, 

 and consequently a new atom. 



When radium B explodes, it emits only p and 

 y rays, and in losing a /3 particle it loses a negative 

 electron with a single unit charge. The high speed 

 of the /5 particle shows that it comes from the 

 nucleus, and thus the residual nucleus, though it 

 has practically the same mass as its parent, has 

 increased its charge by one unit and become a new 

 atom with quite different properties. And so the 

 whole series of radio-active changes down to 

 radium F or polinium has been traced. 



The question of the ultimate fate of the radio- 

 active matter remains. What becomes of radium 

 F when it in turn disintegrates ? No product has 

 been detected by radio-activity, and, if the sub- 

 stance formed is not active, we can only investigate 

 it by examining minerals, where the slow accumu- 

 lation of ages has gone on. Now Boltwood pointed 

 out that in minerals from the same geological 

 formations, and therefore presumably of about the 

 same age, the contents of lead are proportional to 



