66 Ashman — Radio- Activity of Thorium. 



Ramsay, and the demonstration that the extracted radiothorium 

 produced thorium-X and the subsequent products left no 

 doubt that radiothorium was produced by the disintegration of 

 thorium, but the question still remained whether this disintegra- 

 tion of thorium itself was accompanied by a-rays ; that is, 

 whether thorium itself was radio-active. McCoy and Ross 

 found that all of the activity of a thorium mineral, not due to 

 uranium, remained in the pure thorium dioxide, separated by 

 Neisli's method : the measurements were made at the end of a 

 month to allow the accumulation of the maximum amounts of 

 thorium-X and subsequent products. Pure thorium dioxide 

 made from commercial samples of thorium nitrate had about 

 half the normal activity. This was at first thought to be due 

 to the removal of part of the radiothorium in the technical 

 process of purification. McCoy and Ross attempted to separate 

 radiothorium completely from thorium, but found it apparently 

 impossible to do so. Hahn found that the activity of com- 

 mercial samples varied with the age of the material ; the 

 activity was smallest for samples three to nine years old. A 

 sample twelve years old had greater activity. To explain these 

 results, Hahn* suggested that there is a rayless intermediate 

 product, mesothorium, having a period of about seven years, 

 between thorium and radiothorium ; that this is separated in 

 the process of extraction of thorium from minerals, and that the 

 radiothorium, which has a period of two years, decays with time, 

 causing the observed fall of activity of commercial samples. 

 Some observations by Boltwoodf tended to confirm this view. 

 Later quantitative experiments by McCoy and Ross showed 

 that the hypothesis was in good agreement with the facts and 

 that the period of mesothorium was five and one-half years ; 

 this value has since been confirmed by Hahn;); by new quan- 

 titative measurements. Hahn also compared the activities of 

 old and new thorium by means of the a-rays, the /3-rays, and 

 the emanation ; the variation with age in activity of the emana- 

 tion is greater than the variation in the a-ray activity. The 

 removal of thorium-X caused a greater proportional decrease 

 in the activity of new than in old thoria. The activity of 

 radiothorium, when freed from thorium-X, was decreased to a 

 smaller fraction of its original value than that of any ordinary 

 thorium preparation similarly treated. These facts led Hahn 

 to conclude that thorium itself has a typical a-ray activity ; 

 but he gave no estimate of its intensity. 



Rutherford and Soddy§ had observed that in the case of 

 thorium precipitated with ammonia, its final or maximum 



*Phys. Zeitschr., viii, 277, 1907. 

 f This Journal, xxiv, 93, 1907. 

 JPhys. Zeitschr., ix, 246, 1908. 

 §Phil. Mag.,iv, 378, 1902. 



