Boltwood — Radio-activity of Uranium Minerals. 279 



mineral one important point must be taken into consideration. 

 This is the spontaneous loss of radium emanation from the 

 powdered material, to which attention was first called by the 

 writer.* It will be shown later that the activity due to the 

 radium emanation and its immediate disintegration products is 

 a considerable factor in the total activity of the mineral. In 

 order to obtain a true measure of the activity of a mineral a 

 determination must be made of the emanating power of the 

 mineral in the form used in the film, and if the proportion of 

 emanation lost is appreciable a correction must be made in the 

 activity as measured. f 



The proportion of the total emanation lost by the powdered 

 minerals was determined in the manner previously described,;}: 

 and the material used was the finely, powdered mineral left 

 after the evaporation of the chloroform with which it had 

 been mixed in the operation of grinding. The necessity of 

 any very appreciable correction for the emanation lost was 

 avoided in at least one instance, by the use of a specimen of a very 

 dense and pure uraninite (the Branchville material), in which the 

 emanation lost by the powder was only 1*4 per cent of the total 

 amount present. The data from which the correction for the 

 emanation lost was calculated will be given later (p. 283). 



The correction to be applied for the thorium contained in 

 the mineral was determined in the following manner. A film 

 was prepared from a specimen of thorite containing 52*0 per cent 

 of thorium oxide and 0*37 per cent of uranium. The weight 

 of mineral in the film was 0*00955 gram and its activity was 

 0*704 divisions per minute. This was equivalent to 73*7 div. 

 per min. per gram of mineral. The activity due to the uranium 

 would be equal to 2*2 div. per gram of mineral (576 X "0037). 

 One gram of mineral contained 0*520 gram of thorium oxide, 

 and the activity per gram of thorium oxide was therefore 

 71*5 ~- 0-52 = 137 divisions per minute in the electroscope.§ 



The following minerals were used in the experiments : — 



No. 1. Uraninite from Branchville, Conn. This specimen 

 was a portion of some very fine material which was most kindly 

 presented to me by the late Professor S. L. Penfield, Curator 

 of the Brush collection. It consisted of small, imperfect crystals 



*PM1. Mag., ix, 603, 1905. 



f This point was neglected by McCoy in his earlier experiments, hut the 

 method of determining and applying the correction was called to his atten- 

 tion by the writer. The correction has been applied in the paper which he 

 recently published (Jour. Am. Chem. Soc, December, 1907). 



JPhil. Mag., ix, 603, 1905. 



§ This is equivalent to 156 div. per min. per gram of thorium. The activity 

 of one gram of uranium was 124 div. per min. The ratio of these numbers 

 is 1*26; viz., the specific activity of thorium containing equilibrium 

 amounts of products is 1*26 times that of uranium. The value found by 

 McCoy and Ross for this ratio is 1"27 (Jour. Am. Chem. Soc, xxix, 1709, 

 1907). 



