94 Boltwood — Radio-Activity of Thorium Salts. 



obtained. It was found that, within the limits of experi- 

 mental error, the activity corresponding to one gram of thorium 

 in a mineral was the same for the different minerals examined, 

 which indicated that the activity of one gram of thorium with 

 its equilibrium amounts of disintegration products — the normal 

 specific activity of the thorium series, as it can be called — was 

 a constant. 



Measurements were also made of the activities of a number 

 of specimens of thorium oxide which had been separated by 

 chemical methods from the minerals and from certain thorium 

 salts prepared by the Welsbach Company. The relative 

 ionization produced by a known weight of each of these oxides 

 in the form of a thin film was determined in the electroscope. 

 The specific activity of the oxides prepared directly from the 

 minerals was found to correspond to the normal specific activ- 

 ity of the thorium series found in the minerals. The activity 

 of the oxides prepared from the Welsbach salt was, however, 

 found to be much lower than the normal. These results led 

 to the conclusion that the chemical process employed by the 

 Welsbach Company was in some way peculiar since it appar- 

 ently resulted in the separation of over one-half of the radio- 

 thorium corresponding to the thorium present. 



The question of the radio-activity of thorium in minerals 

 and salts has been examined also by Dr. Dadourian,* who 

 employed a method based upon the measurement of the 

 activity of the deposit obtained by exposing a negatively 

 charged plate to the emanation evolved by a solution of the 

 thorium salt or mineral. The results obtained by Dadourian 

 and the writer were in close agreement and led to similar con- 

 clusions. Results of a similar character were also obtained by 

 McCoy and Kossf and by Eve4 



Among the salts examined by Dadourian were two specimens 

 of thorium nitrate, the one prepared from North Carolina 

 monazite and the other from Brazilian monazite, which had 

 been supplied by the Welsbach Company to the writer nearly 

 two years before. Mr. H. S. Miner, the chemist of the Wels- 

 bach Company, stated that the former salt was about two 

 years old and the latter at least one year and a half old at the 

 time they were sent to me. A third salt examine^ by 

 Dadourian was a specimen of thorium nitrate which had been 

 purchased from Eimer & Amend about three years before 

 the time at which he tested it. It had, however, been used in 

 the meantime by the writer for the preparation of thorium-X, 

 that is, the original salt had been dissolved in water, the 

 thorium had been precipitated as hydroxide with ammonia, 



*This Journal, xxi, 427, 1906. frbid., xxi, 433, 1906. 



J Ibid., xxii, 477, 1906. 



