ELISHA MITCHELL SCIENTIFIC JOURNAL 61 



It was quite important to secure Thoria from several min- 

 erals containing no Uranium, as from Norwegian gadolinite, 

 orthite and yttrotitanite. Thorium separated from these 

 minerals is inactive upon the plate and the electroscope. Mr. 

 Curie was kind enough to make a control determination of the 

 activity of one of these preparations, and he also found it 

 practically inactive or if possessing- any activity at all, its 

 value is only one seventh of the activity ever observed. 



The inactive Thorium shows all chemical properties of 

 ordinary Thorium. To prove that it was really Thorium I 

 made an approximate determination of its atomic weight, and 

 found 222-7. This figure is of value mainly, as it indicates an 

 element with an atomic weight above 200. 



It is most probable that the Thorium itself -does not possess 

 any primary activity at all, but that it receives its activity 

 from other primarily active bodies like Uranium or Radium, or 

 even from the air, which, according to the investigations of 

 Elster and Geitel contains always and everywhere, more or less 

 radioactive particles. We succeeded in securing a further 

 very important proof of our opinion, by producing artificially 

 active Thorium from an inactive one, and by showing that 

 this artificially active Thorium behaves in every way like the 

 natural active Thorium. 



The experiments were made as follows: to a solution of 

 inactive Thorium nitrate a ten to twenty-fold excess of Uran- 

 ium nitrate in solution was adder. ; the mixture was then kept 

 about two weeks, and the thoria precipitated with oxalic acid. 

 The well washed and burned Thorium preparation behaved 

 then exactly like the thoria of broggerite and cleveite. It 

 showed a very considerable amount of a- and /3-activity. But 

 after a year the /3-radiation disappeared nearby altogether, 

 and the a-radiation recovered gradually a small, but finally 

 constant value. In a similar way as with Uranium, we ren- 

 dered thoria active by means of Polonium. 



Although -per se inactive Thorim can be obtained of some 

 minerals, and although, on the other hand, the at first 



