Transformation of Thorium and Actinium Emanation. 721 



emanation using the constant-flow method, and obtained a 

 value 3*9 seconds. Two years later Hahn and Sackur * found 

 the value to be the same. In 1912 Miss Leslie f , using the 

 direct method, found a mean value of 3*92 seconds, as a 

 result of a large number of experiments, which, however, 

 showed wide variations from the mean. 



The earlier experiments of Rutherford ± on thorium ema- 

 nation gave a half -value period of about 60 seconds. 

 Rossignol and Gimingham §, by a similar method, lowered 

 the value to 51 seconds. Hahn ||, using the constant flow 

 method, obtained 53*3 seconds. Bronson's^" direct constant 

 deflexion method gave a value of 51 seconds, and largely 

 eliminated sources of error in the previous investigations. 

 Miss Leslie's** experiments gave values varying from 53*5 to 

 55*1, with a mean of 54*3 seconds. 



Before attempting some work involving the square roots of 

 these transformation periods, it seemed necessary if possible 

 to determine them to one-half per cent. 



Since the transformation constant of any radioactive sub- 

 stance is determined from the equation X= — ~— °— , where I 



is the initial activity, and I the activity after a time t, it is 

 obvious that the relative error in \ is decreased as t is 

 increased, provided I /I can be determined with the same 

 degree of accuracy in each case. In practice the difficulty 

 arises in obtaining I /I exactly when I is a very small fraction 

 of I . In the present method this difficulty has been 

 eliminated. 



The rate of movement of the leaf of an electroscope varies 

 inversely with the total capacity of the leaf system. If a 

 large amount of a rapidly decaying gas be introduced into an 

 electroscope, it is therefore possible to follow its decay by 

 noting the rates of leak, when capacities diminishing in 

 amount are successively connected in parallel with the leaf 

 as decay proceeds. This device was used in the present 

 investigation. The rates of deflexion were corrected for the 

 growth of the active deposit and the ratios between the 

 capacities were determined under experimental conditions. 



* Hahn and Sackur, Chem. Ber. xxxviii. p. 1948 (1905). 



f Miss Leslie, Phil. Mag-, xxiv. p. 037 (1W12). 



X Rutherford, Phil. Mag. xlix. p. 1 (1900). ^ 



§ Rossignol and Gimingham, Phil. Mag. viii. p. 107 (1901). 



|| Hahn, Jahrbuch d. Radioakt. ii. p. 233 (1905). 



^1 Bronson, Amer. Jourii. Sci. xix. p. 185 (1905). 



** Loc. cit. 



