Relative Activities of Radio-Products of Thorium. 341 



series *. The results in the thorium series are much less 

 complete. Rutherford and Soddy were led to conclude that 

 thorium, ThX, Em, and the excited activity contributed 

 about equal shares of the observed activity o£ thorium. Since 

 this early work was done, our knowledge of the series has 

 been greatly extended. The complete series as we now know 

 it is as follows : — 



Thorium 



Ms, 



Ms 2 



Et 



ThX 



Em 



A 



B 



G 



D 



a 



... 



ft? 



a 



a 



a 



a 



IS 



a 



0,7 



The product C is complex, the components being known 

 as Cj and C 2 . The activity ascribed by Rutherford and 

 Soddy to thorium alone is due to thorium plus an uncertain 

 fraction of the equilibrium amount of Rt ; while that of the 

 excited activity is due essentially to C. Rutherford and 

 Soddy f found that the activity of the products now known 

 as B, C, and D was about 44 per cent, of that due to ThX 

 and Em and (the then unknown) A (of period 0*145 sec). 

 Ashman J, working in this laboratory, found the following- 

 results: — Thorium free from Rt contributes 11 per cent, of 

 the activity of thorium in equilibrium with its products ; 

 Rt, 20 per cent. ; ThX and its products, GO per cent. More 

 recently Miss Leslie § has given the following figures as the 

 relative activities of the various products : — Thorium, 5*5 ; 

 Rt, 9-4 ; ThX, 12-2 ; Em, 48"6 ; C,, 12*2 ; C 2 , 12*2. She 

 found the ratio of the activity of B, C, and D to that of 

 ThX, Em, and A to be between (>33 and 042. 



In addition to the ratios determined by Rutherford and 

 Soddy and by Ashman, it is possible to find by experiment 

 the relative activity of Rt as compared with the subsequent 

 products. We have made what we think to be quite accurate 

 determinations of two of these ratios — that of B, C, and D 

 to ThX, Em, and A, which ratio we will call x ; and that of 

 the activity of the radio-products of Rt to the activity of Rt 

 alone, which we will call y. The principle of the determi- 

 nation of the ratio as is the same as in Rutherford and Soddy's 

 experiment, although the details necessary to secure accuracy 

 are different. ThX is prepared in radioactively pure form, 

 and its activity is measured at known intervals from the time 

 (time zero) at which the accumulation of the subsequent 

 products begins. It is of fundamental importance that the 



* McCoy & Ross, J. Am. Chem. Soc. xxi. p. 1705 (1907). Boltwood, 

 Am. J.-Sci. [4] xxi. p. 409 (1906). 



f Rutherford, l Radioactivity,' 2nd ed., p. 360. 

 + Am. J. Sci. [4] xxvii. p. 65 (1909). 

 § Le Radium, viii. p. 356 (1911). 



