154 y-Rai/s of Thorium and Actinium. 



the 7-ray and the /3-ray which accompanies it. It is possible, 

 from the results that have been given, to make a rule con- 

 necting the penetrability of the 7-ray with the period of the 

 change in which it occurs, analogous to Rutherford's well- 

 known rule for the range of the a-particle. Actinium is 

 an exception throughout, and has been omitted from the 

 comparison. In ascending order the penetrabilities of the 

 7-rays are : — 



Uranium X, Mesothorium 2, Radium C 1? Thorium D. 



This is also the descending order of their periods, and also 

 of their probable atomic weights. 



If the conclusions of Halm and Meitner (Pliys. Zeit. x. 

 p. 697, 1909) are correct, the j3- and 7-rays of radium C 

 come from radium C L (half-period 19 minutes), while the 

 a-rays come from a succeeding product, radium C 2 (half- 

 period, about 2 minutes). This accords with Rutherford's 

 rule, as the still more penetrating a-ray of thorium C is, in 

 all probability, derived from a change even more rapid than 

 that of radium C 2 . The above order of the penetrabilities of 

 the 7-rays is also that of the a-rays which precede and follow 

 them, whereas the order of the penetrabilities of the /3-rays 

 is quite different. The figures in brackets denote ranges of 

 the a-rays in centimetres of air. 



y-rays 



Uranium X 



Mesothorium 2 



Kadium C x 



Thorium D 



a-rays 

 preceding : — 



Uranium (2'7) 



Thorium (3-5 ?) 



Radium A (4'83) 



Thorium C (8*6) 



a-rays 

 following : — 



Ionium (2"8) 



Radiothorium (3"9) 



Radium C 2 (7'06) 



— 



{3-rays 



Mesothorium 2 



Thorium D 



UraDium X 



Radium C x 



Neither in penetrability, relative intensity nor homogeneity 

 are the /3-rays obviously connected with the 7-rays, whereas 

 there is a certain connexion between the 7-rays and the 

 a-rays. 



Physical Chemistry Laboratory, 

 Glasgow University. 



