410 



3Ir. Frederirlc Soddy 



[March 15, 



ionium. (As was shown with the machine, with all the nuts free- 

 wheeling to imitate radio -elements with periods infinitely long 

 compared with the time of the experiment, when no intermediate 

 substance intervenes, the growth is a straight line as in Figs. 3 and 9, 

 when one intervenes the curves rise at a rate proportional to the square, 

 when two intervene, according to the cube of the time.) Hence, if 

 uranium is the primary jjarent of radium, it is to be expected that the 

 rate of growth of radium from the preparations will increase as time goes 

 on according to some power of the time higher than unity. As Fig. 10 

 shows there is still no evidence of this increase of slope in any of the 



Fig. 11. 



preparations. This indicates, either that the period of ionium must 

 be enormously long, or that several intermediate long-lived members 

 intervene. If ionium is the only intervening member a minimum 

 possible limit to its period may be arrived at by applying the above 

 e(]uation to the results. If it is assumed that the growth observed is 

 due to uranium and that no ionium was initially present,the mini- 

 mum periods calculated in the several experiments are as follow : 

 No. I, 28,000 years ; No. II, 41,400 years ; No. Ill, <SO,000 years ; 

 and No. IV, 69,200 years. Since, in all, certainly some of the 



