﻿the a particles expelled from Radium and Actinium. 369 



5 72 

 about ^x — = 30 per cent, of uranium. The amount of helium 

 b Z 



corresponding to one gram of uranium or its equivalent in 



the mineral is consequently 24 c.c. As before, the age of 



the mineral works out to be about 400 million years. 



Numerous other examples may be given, but these serve 



to illustrate the method of calculation from radioactive data 



of the age of some radioactive minerals, and indirectly, in 



some cases, of the geological strata in which tbey are found. 



Velocity and Energy of the a particles expelled from 

 Radium products. 



If the value of e/m is the same for the a. particle expelled 

 from the various radium products, the maximum velocity of 

 each set of a particles can be deduced from their range in 

 air, knowing the velocity of the a particles expelled from 

 radium C. The velocities so determined are probably more 

 accurate than those obtained by direct measurement under 

 difficult conditions. In the following table, the second column 

 gives the range in air of the a particles from the radium 

 products, found by Bragg and Kleeman; in the fourth column 



mu 



is given the value of , where u is the initial velocity of 



projection of the a particles. 



Product. 



Range of 

 a. particle in cms. 



Velocity in 

 cms. per sec. 



mu* 



e 





350 cms. 

 4-36 „ 

 4-83 „ 

 706 „ 

 3-86 „ 



1-56 Xl0 9 

 1-70 xlO 9 

 1-77x10° 

 2-06 xlO 9 

 1-61 xlO 9 



4-7Sxl0 u 

 5-65 xlO 1 * 

 612 xlO 14 

 8-37 XlO 14 

 5-15 xlO 1 * 





Radium A 





Radium F* 



* The range of the rays for polonium (radium F) has been recently measured 

 by Levin (Amer. Journ. Sci. July 1906). 



Disregarding radium F, the average energy of the u par- 

 ticle expelled from radium in equilibrium is 3 # llxlO u £, 

 where e is the charge carried by the a particle. Assuming 

 that the heating effect of radium is a measure of the kinetic 

 energy of the expelled a particles, we can at once deduce the 

 total number of a particles expelled per second per gram of 

 radium in equilibrium. One gram of radium in equilibrium 

 emits 100 gram-calories of heat per hour. This rate of 



Phil. Mag. S. 6. Vol. 12. No. 70. Oct. 1906. 2 B 



