Boltwood — Thorium Minerals and Salts. 421 



plate before they have reached the ends of their free paths in 

 air, as a result of which the maximum ionization is not 

 obtained. This is shown by the value of the ratio of the 

 activities of the films in the smaller electroscope to the activi- 

 ties of the same iilms in the larger electroscope, which for the 

 thorium minerals was equal to approximately 1*8, while the 

 same ratio for a film of pure uranium oxide was only 1'6. 

 This indicates that the average range of the a-particles from 

 thorium is greater than the average range of the a-particles 

 from uranium. 



Radio-activity of Thorium Salts. 



In order to determine whether the activity of thorium salts 

 which had been separated from various minerals by different 

 chemical methods was the same per gram of thorium as the 

 specific activity of the thorium contained in the minerals, the 

 following salts were examined : 



1. Portion of about 18 grams of pure thorium oxide which 

 had been obtained by working up a quantity of old worn-out 

 incandescent gas mantels obtained from various sources. The 

 mantel dust had been treated with sulphuric acid to convert 

 the earths into sulphates, and the sulphates in dilute solution 

 had been precipitated as oxalates with oxalic acid. The 

 thorium in the oxalates was then extracted by boiling with a 

 strong solution of ammonium oxalate, the solution was greatly 

 diluted and was then allowed to stand in the cold for 48 hours, 

 when the insoluble oxalates were filtered off. The thorium 

 was precipitated as oxalate from the filtrate by the addition 

 of hydrochloric acid to the boiling solution, and the oxalate 

 was converted into oxide by intense ignition over the blast 

 lamp. The oxide thus obtained was pure white. 



2. Portion of a quantity (1 kilo) of thorium nitrate obtained 

 from the Welsbach Light Co. A few grams of this nitrate 

 were dissolved in water and the thorium was precipitated as 

 hydroxide, which was then converted into the oxide by intense 

 ignition over the blast lamp. The nitrate had been prepared 

 by the Welsbach Co., by their customary process, from JSTorth 

 Carolina monazite sand. 



3. Thorium oxide prepared from the same material as ISTo. 

 2. A portion of the nitrate was converted into the hydrated, 

 crystalline sulphate [Th(SO,) 2 . 9H 2 0]. A few small, per- 

 fectly clear crystals were selected and converted into the 

 oxide by intense ignition over the blast lamp. 



4. A portion of the nitrate mentioned under JNo. 2 was con- 

 verted into the anhydrous sulphate. The solution of the sul- 

 phate in cold water was heated and a quantity of the so-called 



