Relation hetween Uranium and Radium. 219 



compared with Preparation IV. The curve in fig. 1 for this 

 preparation is the theoretical curve drawn on the same 

 assumption as those for the curve of Preparation IV. As 

 before, taking the mean of the first two and last two obser- 

 vations in the Table (p. 215), 



l/\ 2 =7-lC)xl0- 11 xl08x(8-325 2 -l-795 2 )/18-5xl0- 12 



= 104,000 years, 



a value agreeing well with that given by Preparation IV., 

 and with 96,000 years calculated three years ago for this 

 preparation. But in this case the intermediate observations 

 lie consistently above the theoretical curve. The departure 

 amounts, as a rule, to less than 3 X 10" 12 g. of radium, and it 

 would be unwise at the present stage to lay too much stress 

 upon it. If it is real, it indicates that the true period of ionium 

 is somewhat longer than that calculated and that in both 

 Preparations III. and IV. some ionium was initially present. 

 From the results with the earlier Preparations Land II., 

 where certainly some ionium was initially present, nothing 

 can yet be deduced as to the period of ionium. It may be 

 stated in conclusion that the period of average life of ionium 

 is probably about 100,000 years on the assumption that thai 

 of radium is 2375 years. This value still partakes of the 

 nature of a minimum period, but it is unlikely that it is very 

 far from the true period. 



Summary. 



The continuation of the measurements on the growth of 

 radium from purified uranium preparations has shown an 

 unmistakable increase in the rate of growth of radium in the 



case of the preparation containing 3 kilograms of uranium. 

 The growth of radium appears to be proceeding according t<> 

 the square of the time, as theory requires if ionium is the 

 only long-lived intermediate member of the series. There is 

 thus now, for the first time, direct experimental evidence 

 that uranium is the ultimate parent of radium. The period 

 of average life of ionium calculated from this experiment is 

 about 100,000 years, assuming 2375 years as the period of 

 radium. An earlier preparation containing 408 grams of 

 uranium gives practically the same value for the period 

 of ionium, calculated on the assumption that ionium was 

 initially absent. The effect of any ionium initially present 

 in the preparations would be to lengthen the period of 

 ionium, but 100,000 years is probably not far from the actual 

 period of average life. 



