194 PHYSICAL SCIENCE 



To verify this result, Rutherford and Royds 

 repeated and improved the earlier spectroscopic 

 experiments. A quantity of radium emanation 

 was compressed into a tiny thin walled capillary 

 glass tube. The a particles, shot through the thin 

 walls, were collected in another glass tube which 

 surrounded the inner one. After a few days the 

 complete spectrum of helium was seen by sparking 

 the gas from this outer tube. The a particle has 

 an atomic mass of 4 and a charge + 2e, When its 

 velocity is destroyed by passing through matter, 

 it absorbs two negative electrons, and becomes 

 an ordinary neutral helium atom. Helium is one 

 final product of radio-active atomic disintegration. 



In the course of this investigation, we have 

 seen that the number of a particles emitted per 

 second by a gram of radium was estimated as 

 3.4 X 10^^. That result enabled Rutherford 

 incidentally to calculate the rate of decay of the 

 radio-activity of radium, and therefore the life of 

 radium itself. The activity would fall to half- 

 value in 1730 years, and we may therefore con- 

 clude that a mass of radium would disintegrate 

 at a speed which would destroy half of it in that 

 time. 



Each exploding radium atom emits an a 

 particle — a helium atom of atomic mass 4 with 

 two positive electric charges. Hence the atomic 

 weight of the residue is reduced by 4 and radium, 

 with an atomic weight of 226, passes into radium 

 emanation with an atomic weight of 222. So 

 radium A must have an atomic weight of 2 1 8, and 

 radium B of 2 14. But radium B passes into radium 

 C by ejecting p and 7 rays only, hence it suffers 



