Existence of Uranium Y. 217 



initial slope of the soft /3-ray recovery curve of purified 

 uranium should be considerably greater than that calculated 

 for uranium X, for initially some two uranium Y atoms will 

 be disintegrating for one of uranium X. Unfortunately the 

 a-rays of uranium are not much inferior in penetrating 

 power to the soft /3-radiation. Under conditions such that 

 the a-radiation of uranium is completely absorbed, the un- 

 absorbed soft /3-radiation produces only some 10 per cent, of 

 the ionization produced by the hard /S-rays. This makes 

 the point very difficult to establish. 



The apparatus Used in the present work was designed 

 from the value of the absorption coefficient in aluminium, 

 300 (cm.)" 1 , given by Antonoff for the /3-rays of uranium Y. 

 After the experiments had been carried out a discrepancy 

 w T as discovered, for from the curve given (Phil. Mag. 1911 

 [6] xxii. p. 427) this radiation is shown to be completely 

 absorbed in aluminium equivalent to 4 cm. of air, and the 

 absorption coefficient calculated from the curve is about 

 three times greater than 300 (cm.) -1 . In my apparatus a 

 total air equivalent of 4*7 cm. at 15° intervened between the 

 preparations and the electroscope, but in spite of this dis- 

 crepancy Antonoff's decay curves were clearly reproduced 

 with the apparatus. 



The preparations to be measured were spread uniformly on 

 to the surface of circular ground-glass plates 17'7 cm. 

 diameter, 246 cm. 2 area, by moistening them with water and 

 working them over the surface with a second very small 

 ground-glass plate. The films were dried in the oven and 

 the weight of material on them found by difference. The 

 films of uranium oxide weighed usually 15 to 2 grams, or 

 from 6 to 8 mg. per cm. 2 , except in one case where only 

 0*3 g. was used. A specially wide and shallow electroscope, 

 19 cm. x 10 cm., was provided with a base of " grease-proof" 

 paper covered with aluminium leaf, weighing 5"27 mg. per 

 cm. 2 , and equivalent in stopping power to 4*3 cm. of air at 

 15°. The surface of the preparations was placed 4 mm. 

 below the paper base. The paper base and air-layer between 

 the preparations and the electroscope were thus equivalent to 

 4*7 cm. of air, and would reduce the intensity of a radiation, 

 for which /*(A1) = 300 (cm.)" 1 to 0*525, and for which 

 ^1)^500 (cm.)- 1 to 0-34 of the initial value. As screen, 

 a sheet of celluloid weighing 20'85 mg. per cm. 2 was used. 

 This cuts down the hard /3-rays of uranium X to 0'87, the 

 soft /3-rays with yi6(Al) = 300 (cm.)- 1 to 0*10, and those with 

 ^(Al^DOO^cin.) -1 to 0*02. Sometimes a thicker screen of 

 aluminium, weighing 73 mg\ per cm. 2 was employed, instead 

 of the celluloid. 



