Radioactive Properties of Uranium. 47 



plates decayed in the first few days after crystallization to 

 about half of its original value, reaching a minimum after 

 four or five days, and then increased slowly for a very long- 

 time. The time in which the minimum was reached and the 

 initial form of the curve were both dependent on the thickness 

 of the plate. As regards the meaning of this phenomenon, 

 the authors suggest two possibilities : either that there is a 

 change in the activity itself, or that the absorption of the rays 

 is modified by the physical alteration of the crystallized 

 plates *. 



Prof. Rutherford kindly suggested to me, that I should 

 make some investigations to explain these phenomena. 



3. The Separation of UrX from Uranium by means 

 of Fractional Crystallization. 



The experiments were first made in order to find out the 

 conditions under which this first rapid decay of the (3 activity 

 of uranium is obtained. 



As in the experiments of Meyer and von Schweidler, equal 

 weights of uranium nitrate | and water were taken, and this 

 solution was shaken with an equal weight of ethyl ether. 

 The ether solution was then carefully separated from the water 

 solution, and both were evaporated to dryness. In the case 

 of water solution, the evaporation was continued until even 

 the water of crystallization was driven off. 



The oc and /3 activity of both portions were then measured. 



The j3 activity was measured by means of an electroscope J 

 of the type of 0. T. E. Wilson ; the bottom of the electroscope 

 was removed and replaced by aluminium foil 0*08 mm. thick, 

 which absorbed all the a rays. 



The measurements showed that uranium nitrate from the 

 aqueous solution which contained the excess of UrX, derived 

 from the ether solution, lost the corresponding excess of its 

 activity according to an exponential law with the time, falling 

 to half value in 22 days. The ether portion, which was 

 at first almost completely inactive, when measured by /3 



* "Beziiglichder Deutung dieses Verlialtens istzunachst dieMoglichkeit 

 gegeben, dass es sicli um Anderungen der Aktivitat selbst handelt, oder 

 dass durcli physikalische Zustandesanderungen der Kristallplatten ilir 

 Absorptionsvermogen beeinflusst wird. Eine definitive Entscheidung zu 

 gebeu ware verfriiht." Meyer imd v. Schweidler, loc. cit. p. 107-3. 



t The uranium nitrate under the experiments was obtained from Merck 

 in Darmstadt and was labelled "extra pure." 



\ See Rutherford, ' Radioactivity,' p. 71 & fig. 11. 



