a Particles emitted by Uranium and Thorium. 695 



kindly presented to us by Professor Boltwood, and were 

 fractions of larger quantities analysed by him. We desire 

 to express our indebtedness to Professor Boltwood for the 

 use of these materials. 



(1) Uranic-uranose oxide ( U 3 8 ) prepared from uranium 

 nitrate which had been crystallized fifteen times. The least 

 soluble fraction was taken and ignited at a high heat in a 

 current of oxygen. 



(2) Uraninite — a selected sample from Joachimsthal. This 

 contained 61*7 per cent, of uranium. The mineral, when 

 finely powdered, lost 6"2 per cent, of its emanation. The 

 sample employed had been finely ground for several years, 

 and during this time the emanation had steadily escaped. 

 Under these conditions it can be simply deduced that the 

 emission of a particles from the mineral is about three per 

 cent, less than if the mineral had retained all its emanation. 

 A correction of this amount has consequently been made to 

 the counted number of a particles. 



(3) Thorium oxide prepared from thorite. This was tested 

 five weeks after its chemical separation. Since, in the 

 chemical process of purification, the mesothorium is removed 

 from the thorium, the a-ray activity of the purified thorium 

 decays with time due to the decay of its product radio- 

 thorium. Since the half period of decay of the latter is 

 about 737 days, a positive correction of about two per cent, 

 is necessary to give the correct number of « particles emitted 

 from thorium oxide in radioactive equilibrium. The activity 

 of the thorium oxide in the form of a thin film was compared 

 with that of a film of the mineral thorite of known composi- 

 tion, and gave nearly the ratio to be expected from their 

 relative content of thorium. 



The results of the observations are included in the following 

 Table (p. 696). 



Since only about 900 scintillations were counted altogether, 

 the agreement between the three uranium films is closer 

 than could be expected, considering the possible errors in 

 the experiment. In the case of the mineral films 2000 scin- 

 tillations were counted in all, and about an equal number for 

 the thorium films. Before and after each set of observations 

 the screen was carefully tested to determine the number of 

 scintillations observed when the active material was removed. 

 The correction for the screen employed was small, and 

 usually corresponded to one scintillation in three or four 

 minutes. All the counting experiments were checked among 

 themselves by measuring the activity of the films in an a-ray 

 electroscope. The activity measured in this way was found 



