the Radioactivity of Uranium. 443 



possesses comparatively little activity when tested in the 

 ordinary way by the electrical method. But its radiation, 

 when tested in a magnetic field, is found to consist almost 

 entirely of deviable rays, whereas the uranium from which 

 it has been separated gives no deviable rays at all (Rutherford 

 and Grier, Phil. Mag. 1902, iv. p. 323). The conclusion was 

 drawn that the chemical separation had effected the separation 

 of the matter causing the photographically active or ft rays 

 from the uranium, but had not all affected the easily-absorbed 

 or a rays of the latter. All attempts to alter the amount of 

 a. rays from uranium resulted in failure, and this radiation 

 appears to be derived from a specific and non-separable 

 activity of the element uranium itself. The magnetically 

 deviable and penetrating ft ray, on the other hand, which 

 causes the photographic effect, is entirely produced by a non- 

 uranium type of matter, uranium X. 



Becquerel {Comptes Rendus, 1901, vol. cxxxiii. p. 977) has 

 shown that uranium rendered photographically inactive in 

 the manner described recovers its activity with time, and 

 after the lapse of a year is again as active as at first. The 

 barium sulphate, on the other hand, which had been rendered 

 active by precipitation with uranium, completely lost its 

 activity after a year. As this behaviour is exhibited by 

 thorium and thorium X respectively, it seemed likely that the 

 phenomena would prove susceptible to the same explanation 

 in the two cases. Experiments were therefore undertaken to 

 examine the rate of loss of the activity of uranium X with 

 time, and the rate of recovery of the activity of uranium freed 

 from uranium X. The problem resolves itself into a measure- 

 ment of the amount of penetrating rays given out in the two 

 cases after regular intervals. The electrical method was 

 employed, a very sensitive electrometer being necessary for 

 the measurement of the somewhat small effects involved, and 

 the observations were continued over a period of 160 days. 



At the beginning of the period the uranium that had been 

 freed from uranium X by the various methods gave practically 

 no ft rays, although less than one per cent, of the normal 

 amount could have been detected. The activity of the 

 uranium X, on the other hand, steadily diminished, until at 

 the end of the period it possessed less than one per cent, of 

 its original activity. The result is shown graphically in the 

 figure. The initial value of the activity of the uranium X is 

 taken as 100, and the final value attained by the uranium is 

 also made equal to 100. It will be seen that the activity of 

 the uranium X decays very approximately in a G. P. with 

 the time, and is, on an average, reduced to half value in about 



