Relation between Uranium and Radium. 273 



The data, however, on which this latter estimate was based 

 need some revision in the light of present knowledge, and 

 will be again referred to in this paper. During the progress 

 of the experiment the researches of McCoy (Ber. 1904, xxxvii. 

 p. 2641), Strutt (Proc. Roy. Soc. 1905, Ixxvi. A. p. 88), and 

 Boltwood (Phil. Mag. 1905^ [6] ix. p. 599) on the ratio between 

 the quantities of the radium and uranium co-existing in the 

 natural minerals, and the proof by the two latter that this ratio 

 is a constant for all minerals, had established also, indirectly 

 but quite conclusively, that uranium is the parent of radium. 

 This satisfactory result, which is a direct prediction from the 

 disintegration theory, can be explained in no other way, and 

 proves, whatever the exact relationship of radium to uranium 

 may ultimately turn out to be, that there can be no doubt 

 that the latter produces the former. 



In consequence of the extremely slow rate of production 

 of radium from uranium which the above experiment estab- 

 lished, the theory was put forward that one or more slow- 

 changing transition or intermediate forms existed in the 

 disintegration series between uranium and radium, and three 

 or four years ago it was suggested that actinium might prove 

 to be such a transition-form. 



On the other hand, it has been pointed out (Soddy, B. A. 

 Report, 1906, " Evolution of the Elements ") that none of 

 those hypothetical intermediate forms can exceed in period 

 of life the original parent uranium, as the constancy of ratio 

 with the product must hold for that parent, when there are 

 several successive, which has the longest life in the series, 

 and cannot hold for the original parent unless it is the slowest 

 changing of the series. Also, it is clear that the method 

 adopted in the initial purification of the uranium from radium 

 must be of primary importance, for the initial rate of forma- 

 tion of radium from any preparation of uranium will depend 

 upon whether or not the initial purification removes, besides 

 the radium, the intermediate parent of the latter, produced 

 by, and therefore in ordinary circumstances present with, the 

 uranium. Quite recently Boltwood (Am. Jour. Sci. Dec. 1906, 

 p. 537) and Rutherford ('Nature/ Jan. 17th, 1907, p. 270; 

 June 6th, p. 126) have observed a production of radium in 

 preparations of actinium : and the latter has shown that this 

 is not due to actinium itself but to a separate substance ac- 

 companying it and capable of separation. The existence of 

 intermediate forms between uranium and radium necessarily 

 modifies very considerably the problem. The old experiment 

 which gave the first indication of the production of radium 

 indicated also that uranium was certainly not the direct parent, 



