Relative Activity of Radium and Uranium. 51 



radioactive products in equilibrium with one another, it was 

 necessary to assume that either the uranium atom emitted 

 two a-particles simultaneously, which was improbable, or 

 that two distinct a-ray changes existed in ordinary uranium. 

 Neither of these assumptions, however, completely obviated 

 the difficulty. 



The fact that uranium actually did emit twice the number 

 of a-particles to be expected on theoretical grounds was sub- 

 sequently demonstrated by Geiger and Rutherford * and by 

 Brown f who counted the number of a-particles emitted per 

 second from a film of pure uranium oxide and a similar film 

 of uraninite of known composition. Using the scintillation 

 method, Marsden and BarrattJ made a careful examination 

 of the a-radiation from uranium and concluded as a result of 

 their experiments that ordinary uranium consists of a mix- 

 ture of two successive a-ray products in equilibrium with one 

 another. Attempts to measure the separate ranges of the 

 a--particles emitted by these two products were made by 

 Foch§ and Friedmann||. By the use of a better method of 

 measurement, in which the Bragg ionization curves for a 

 uranium film were compared with the corresponding curves 

 obtained with polonium and ionium, Geiger and Nuttall 

 calculated the ranges of the a-particles from uranium to be 

 2-5 cm. and 2*9 cm. (at 0°C). 



Numerous unsuccessful attempts have been made to reduce 

 the specific a-rav activity of uranium. In one experiment 

 conducted by the authors about two kilograms of pure 

 uranium nitrate were subjected to fractional crystallization, 

 and a least soluble "head" fraction weighing about twenty 

 grams was obtained after about forty operations. The 

 specific a-ray activity of the uranium in this material did 

 not vary by as much as one per cent, from the specific activity 

 of the uranium in the original nitrate. This shows that the 

 two components are so closely allied chemically as to be 

 inseparable, a conclusion which is supported by all the other 

 known facts at our disposal. 



We may outline, therefore, the progressive disintegration 

 of the uranium atom, considering for the present only the 

 products emitting a-ray s, as taking place in the following 

 manner: the parent element, uranium I., produces the pro- 

 duct uranium II. This in turn produces ionium, which 



* Phil. Mag. xx. p. 691 (1910). 



t Proc. Rov. Soc. A lxxxiv. p. 151 (]910). 



t Phvs. Soc. Proc. A xxiii. p. 367 (1911). 



§ Le Radium, viii. p. 101 (1911). 



jl Wien. Ber. cxx. p. 13(51 (1911). 



E 2 



