494 Bolt wood — Life of Radium. 



assumed that an atom of radium on disintegrating to form an 

 atom of emanation emits a single a particle, then the number 

 of a particles expelled per second from a gram oi" radium is a 

 measure of the number of atoms undergoing transformation in 

 the same period. It can be shown that the number of atoms 

 in one gram of radium is approximately equal to 3 - 6Xl0 21 . 

 The number of atoms disintegrating per second is, from the 

 number of a particles emitted, equal to 6*2 X 10'°. The frac- 

 tion of the whole number present undergoing transformation 

 per second is accordingly 1*72 XlO" 11 , or 5'4XlO~ 4 per year. 

 It follows from this that the half-value period of radium is 

 about 1300 years. 



Not long after making the above estimate Rutherford 

 experimentally determined the velocity and the ratio of the 

 charge to the mass of the a particles from the radium products, 



and found the value of ^ to be approximately 5*1 X 10 3 . Since 



the value of ^- for the hydrogen ion in the electrolysis of water 



is nearly 10*, he concluded that of the three possibilities, viz., 

 that the a particle is (1) a molecule of hydrogen carrying the 

 ionic charge of hydrogen, (2) a helium atom carrying twice 

 the ionic charge of hydrogen, or (3) one-half of the helium 

 atom carrying a single ionic charge ; the most probable was 

 the second. He has pointed" out that if this assumption is 

 applied in the calculation of the number of a particles emitted 

 per second by one gram of radium, the value obtained is one- 

 half of that calculated above, namely 3*1 XlO 10 . This would 

 give the half-value period of radium as 2600 years instead of 

 1300 years. 



The experimental data on the velocity and the value of A 



for the a particles have also been used by Rutherford in 

 obtaining an estimate of the number of a particles from the 

 heating effect of radium bromide. It has been found that the 

 heat emitted by pure radium bromide is equivalent to about 

 100 gram calories per hour per gram of radium containing 

 equilibrium amounts of its immediate products. This is 

 mechanically equivalent to 1*16 XlO 6 ergs per second. Taking 

 the energy of the a particle from radium itself as nineteen one- 

 hundredths of the total energy due to the four a ray changes 

 (Ra, Em, Ra A and Ra C), the heating effect which can be 

 attributed to radium alone is 2 - 24xl0 5 ergs per second per 

 gram. This is on the assumption that the energy emitted as 

 heat is due to the kinetic energy of the exj)elled a parti- 

 cles. Since the energy of an a particle from radium itself is 



*Phil. Mag., xii, 348, 1906. 



