216 Mr. F. Soddy on the 



separating uranium X would separate uranium Y to the 

 same extent. On this view, the obtaining of the effects 

 would not depend on the use of any special chemical method, 

 as Antonoff supposed, nor upon the use of a large quantity 

 of uranium to give sufficient of the new product, supposed to 

 be present in very small quantity, to be detected, but simply 

 on the time of accumulation of the products since their 

 previous complete separation. The proportion between 

 uranium Y and uranium X would be a function only of this 

 time of accumulation and would be the greater the shorter 

 this time was made. This view is entirely at variance with 

 what one would conclude from a perusal of Anton off's 

 original paper. Antonoff very kindly sent me privately 

 further details of his chemical methods of separation, which 

 confirmed my impression that it might be the time of accu- 

 mulation rather than the method of separation which was of 

 importance. My own experiments, so far as they go, have 

 borne this out. I have not tried his particular method, but 

 have got his result always when the time of accumulation of 

 the uranium X was short, even when the uranium X was 

 separated by barium sulphate, which he states gives uranium 

 X free from uranium Y. It is therefore not altogether 

 surprising that Fleck, who was unaware of the one condition 

 necessary to obtain the effect in question, Was not successful 

 in repeating the work. 



The main interest of AntonofFs discovery, as he himself 

 points out, is that uranium Y might be the first member of 

 the actinium branch series. Hahn and Meitner have 

 suggested recently {Plujsikal. Zeitsch. 1913, xiv. p. 752) that 

 uranium X : may disintegrate dually, in both cases with 

 expulsion of /3-rays, producing therefore two isotopic " eka- 

 tantalums " in group VA, one the short-lived uranium X 2 of 

 Fajans and Gobring, and the other a still unknown long- 

 lived parent of actinium in an a-ray change, as I had 

 suggested before uranium X 2 was discovered. But if this 

 possibility of a dual disintegration, the same kind of ray 

 being expelled in both branches, is admitted, many alterna- 

 tives can be framed. Thus, if either uranium I. or II. dis- 

 integrated dually in two «-ray changes, the three products, 

 uranium X-), ionium, and the new branch product would be 

 isotopic. The third product might be AntonofFs uranium 

 Y, which then would be chemically non-separable from 

 uranium X x . 



On such a view, since the period of uranium X 2 is some 

 twenty-four times that of uranium Y, and 8 per cent, of 

 uranium atoms are supposed to follow the branch series, the 



