Energy of a particles from Radioactive Substances. 115 



range of about 3*5 cms., while those of radium F have the 

 next lowest range, viz. 3'86 cms. A similar result holds in 

 the case of thorium, where the oc ray particles from the 

 product o£ longest period have the least velocity. The 

 period of: transformation of radiothorium has not yet been 

 measured, but it is apparently not less than one year, and 

 may be much longer. Ln any case, the period is much longer 

 than for any of the other a ray products of thorium. 



This connexion is more clearly brought out in the table 

 below by arranging the products of each of the radioactive 

 families in the order of increasing velocity of the x particles 

 expelled from them. 



Table III. 



Product. 



Period of Transformation. 



Uranium about 10° years. 



Eadium 1300 years. 



Badiuin F 140 days. 



Emanation 3-3 days. 



Eadium A 3 rains. 



Eadium C * 19 mins. 



Product. 



Period of 

 rp ,. , Product. 

 rranformation. 



Period of 



Transformation. 



Radiothorium ... 



Thorium B 



Emanation 



Thorium X * 

 Thorium C 



More than i year. 

 1 hour. 

 54 sees. 

 3-6 days. 

 Very short. 



Eadioactinium .. 



Actinium B 



Emanation 



Actinium X * ... 



19 5 days. 



2 mins. 



3-9 sees. 

 10-2 days. 



The only exceptions to the rule that the velocity of expul- 

 sion of the a particle progressively increases with the decrease 

 of period of transformation are the three products marked 

 with an asterisk, viz. radium C, in the case of radium, and 

 thorium X and actinium X, in the case of thorium and 

 actinium respectively. 



Assuming the above rule to be valid for all the products, 



I 2 



