138 CONCEPTION OF THE CHEMICAL ELEMENT 



which the uranium series branches has not yet been 

 definitely ascertained, as there is a choice of alterna- 

 tives, at present experimentally indistinguishable. 

 Uranium- Y may be either the product of uranium-/ 

 or of uranium-//, and the latter alternative, which is 

 that shown in the figure, is taken for the present as 

 likely to be on the whole the more probable. The 

 point can only be settled by the determination of the 

 atomic weight of ekatantalum or actinium. 



Independently, Hahn and Meitner obtained the 

 parent of actinium from the insoluble siliceous 

 residues left after the treatment of pitchblende with 

 nitric acid by adding tantalum, and then separating 

 it and purifying it by chemical treatment. They 

 showed that it gave a-rays of range 3-314 cm. of air 

 at N. T.P.i and, from this range, estimate its period 

 to be from io 3 to 2-io 4 years. There should therefore 

 be sufficient of the element in uranium minerals to 

 enable the spectrum, atomic weight, and chemical 

 character of the pure substance to be determined in 

 the same way as for radium. Its separation on a 

 large scale will enable actinium itself to be grown in 

 a pure state, analogously to the preparation of radio- 

 thorium from mesothorium, and so should allow the 

 spectrum at least of actinium to be found. 



With regard to the period of actinium, there is at 

 present a real conflict of evidence, and so it is 

 impossible to say whether our knowledge of actinium 

 is ever likely to become as complete as that of 

 radium, or to remain, like that of polonium, confined 

 to what can be learned from infinitesimal quantities. 

 Cranston and I, on certain assumptions, concluded 

 from indirect evidence that the period of actinium 

 was 5000 years, but Hahn and Meitner, on the other 

 hand, state that they have obtained evidence con- 

 firming Mme. Curie's provisional estimate of the 

 period as about thirty years, from the direct obser- 



