Relative Activities of Radio-Products of Thorium, 355 



•decrease o£ velocity of the alpha particles within the barium 

 sulphate film. Although the latter was very thin, we 

 estimate that its stopping povver was equivalent to about 

 0'2 cm. o£ air. The ranges then become 4' 14, 4'88, and 

 5*42 cm. for air at standard temperature and pressure. The 



Fio-. 1. 



P/f£SSURE /N CM. Of MCffCUf?) 



last two figures are close to the values found by Marsden and 

 Barratt for Em and A ; namely 5*0 and 5*4 cm. respectively. 

 Our lowest value must therefore represent the range of ThX. 

 Three additional series of measurements gave results which 

 agreed well with those represented by fig. 1. They all con- 

 firm Marsden and Barratt's values for Em and A and show, 

 further, that the range of ThX is very close to 4*1 cm. at 

 0° and 76 cm. pressure. This value agrees with that cal- 

 culated from the period of ThX and is also in good accord 

 with that expected from its activity, as is shown in the 

 following section. 



VI. Relation of Activity to Range. 



After the ranges of a number of the radio-products had 

 been measured by Bragg, Boltwood* suggested that the 

 ionization produced by equal numbers of alpha particles of 

 various substances was proportional to their ranges. He 

 found that the activity of radium plus the rapidly changing 

 products of radium was 5*64 times the activity of radium 

 alone, while the sum of the ranges was 5*60 times the 

 range of the alpha particles of radium. Attempts to establish 



* Phys. ZeitscJu: vii. p. 489 (1906). 



