72 Ashman — Radio- Activity of Thorium. 



It is seen that in the case of sample " D" the observed and 

 calculated activities agree quite closely. The discrepancies are 

 probably due mainly to errors of experiment. In samples "A" 

 and " B " the calculated activity is somewhat greater than the 

 observed activity, for which there is no adequate explanation. 

 The quantitative changes with time in the activity of thorium 

 and its products would be effected to a slight extent by the 

 presence of actinium or ionium* in the thorium extracted from 

 uranium-bearing minerals ; but, as the percentage of uranium 

 in the mineral used was not determined, the magnitude of 

 this effect can not be calculated. 



The more important results of this investigation may be 

 given as follows : An expedient method for separating thorium 

 and radiothorium from the other disintegration products of 

 the element has been described. Additional evidence has been 

 obtained which supports the view that thorium on disintegrating 

 emits an a-radiation. The specific activity of thorium is about 

 119 ; this is 11 per cent of the activity of the same thorium 

 when equilibrium amounts of its disintegration products are 

 present. Of the remaining activity 20 per cent is due to 

 radiothorium and 69 per cent to Th-X, and subsequent prod- 

 ucts. The activity ascribed to thorium itself is too much in 

 excess of the activity of the ionium associated with thorium in 

 minerals to warrant the conclusion that the observed activity 

 of thorium is due wholly or largely to the presence of ionium. 

 The earlier conclusions of Hofmann, and of Baskerville and 

 Zerban, that thorium itself is inactive, were without doubt due 

 to the fact that the methods used were not sufficiently refined 

 to detect the low activity of the element free from its short-lived 

 active products, and other radio-active impurities. 



* Boltwood, this Jour., xxv, 269, 1908. 



Kent Chemical Laboratory, 

 University of Chicago, 

 October, 1908. 



