442 Prof. Rutherford and Mr. Soddy on 



The case of uranium presents some very interesting features 

 for. unlike thorium, this substance produces neither an ema- 

 nation nor excited activity, and the experimental analysis of 

 the processes that occur is in consequence extremely simple. 

 It will be recalled that the main difficulty in the case of 

 thorium was to separate and distinguish between the various 

 changes that are occurring simultaneously, and to eliminate 

 'the effects of the production of the emanation and excited 

 activity from the effects of the primary change which gives 

 rise to the production of Thorium X. These difficulties are 

 absent in the case of uranium. 



It was shown in 1900 by Sir William Crookes (Proc. Poy. 

 Soc., 1900, vol. lxvi. p. 409) that the activity of uranium to 

 a photographic plate is caused by the presence of a minute 

 amount of a foreign substance to which he gave the name 

 Uranium X. This substance, like uranium, is precipitated 

 from its aqueous solution by means of ammonium carbonate, 

 but, unlike uranium, is not redissolved by an excess of the 

 same reagent, being left behind with the iron and similar 

 impurities present as a minute insoluble precipitate. The 

 uranium obtained from the solution is completely inactive to 

 the photographic plate. The uranium X left behind in the 

 precipitate is intensively active, and may be obtained, weight 

 for weight, many hundred times as active as the original 

 uranium. It, in fact, possesses in concentrated form all the 

 activity that the uranium has lost. 



Becquerel, independently (ComptesRendus, 1900, vol. cxxxi. 

 p. 137) reduced the activity of uranium by successive pre- 

 cipitations in its solution of small quantities of barium sul- 

 phate. After several precipitations the activity of the 

 uranium was found to be much enfeebled, whereas the first 

 barium sulphate precipitates were more active weight for 

 weight than the original uranium. 



It has been shown by one of us (Soddy, Journ. Chem. 

 Soc. Trans. 1902, p. 860) that in the interpretation of these 

 results it of great importance to distinguish between the 

 photographic and electrical effects. For the uranium radia- 

 tion consists of two types : — (1) the a, or easily absorbed rays; 

 (2) the ft or penetrating rays which are readily deviable in 

 a magnetic field. The former contribute by far the greater 

 part of the electrical effect, the latter practically all the 

 photographic effect. Uranium freed from uranium X by 

 the methods of Crookes and Becquerel, although inactive to 

 the photographic plate, possesses a nearly normal activity 

 when examined by the electrical method. The uranium X, on 

 the other hand, although intensely active photographically, 



