654 Ranges of a particles from Thorium and Actinium. 

 to, the corresponding lines for the radium and actinium 



Fur. 4. 



series. 



Logarithm of Range 



The range of the a [articles from thorium itself is some- 

 what longer than should be expected from the rule, but it must 

 be remembered that this range is by far the most difficult to 

 determine, since the activity from thorium alone is only 

 about one-fifth of that from uranium. The deviation in this 

 case may therefore be within the experimental error, and 

 the only product which is not in agreement with the range- 

 period rule is, as slated above, radioactinium, but there 

 seems to be the possibility that this product has not the 

 period which is usually attributed to it. 



On account of the uncertainty of their periods the products 

 uranium 2, ionium, radium C, and thorium C 2 are not entered 

 on fig. 4. From the straight line relation it follows that 

 uranium 2 should have a period about 2 x 10 6 years and 

 ionium a period of about 200,000 years. The latter figure 

 is of tlie same order as the approximate value given by 

 Soddy*, namely, 132,000 years. The periods of radium C 

 and thorium C 2 should be about 10" 6 second and 10~ n 

 second respectively. 



We wish to express our thanks to Prof. Eutherford for 

 his kind interest in these experiments. 

 The Victoria University, Manchester, 

 Physical Laboratories, 



* F. Sod'dy, Le Radium, vii. p. 295 (1910). 



