carried, by the cc and (3 Rays of Radium. 205 



readily be calculated that one gram of radium in radioactive 

 equilibrium emits 7*6 x 10 10 /3 particles per second. 



A second series of experiments made under different con- 

 ditions gave a value of 7'0 x 10 10 . 



Taking the mean of these two values, we may thus conclude 

 that the total number of (3 particles expelled from one gram 

 of radium per second is 7*3 X 10 10 . 



The total number of a. particles expelled per second from 

 one gram of radium at its minimum activity has been shown 

 to be 6"2 x 10 10 . The approximate agreement between these 

 numbers is a strong indication of the correctness of the 

 theoretical views, previously discussed. Tt is to be expected 

 that the number of /3 particles, deduced in this way, should 

 be somewhat greater than the number of a, particles, for the 

 J3 particles give rise to a secondary radiation, consisting also 

 of negatively charged particles moving at a high speed. 

 These secondary (3 particles, arising from the impact of the 

 ft particles on the lead, will pass through the aluminium 

 screen, and add their effect to those directly shot out. For 

 this reason, probably, the experimental number is somewhat 

 too large. The results, however, indicate that four a particles 

 are expelled from radium in radioactive equilibrium for each 

 j3 particle, and thus confirm the theory of successive changes. 



Deductions from the Results. 



The determination of the total number of « particles ex- 

 pelled per second from one gram of radium is of great value, for 

 by its means we are enabled to directly deduce the magnitude 

 of other important physical constants in radioactivity. The 

 methods of calculation of some of these quantities will now 

 be briefly given. 



Life of Radium. — We have seen that one gram of radium 

 at its minimum activity expels 6'2 x 10 10 a. particles per second. 

 It seems probable that only one a particle is expelled during 

 the disintegration of the radium atom, so that 6*2 x 10 10 atoms 

 of radium break np per second per gram. The number per 

 gram per year is 1*95 x 10 16 . It has been experimentally 

 deduced that one cubic centimetre of hydrogen at standard 

 pressure and temperature contains 3*6 x 10 19 molecules. 

 Taking the atomic weight of radium as 225, it follows that 

 one gram of radium contains 3'6 x 10 21 atoms of radium. 

 The fraction X of the radium atoms which break up per year 

 is thus 



1-95 x 10 16 



3-6 xlO 21 



or 5*4: x 10" 



