488 The Relation between Uranium and Radium. 



Table II., giving the later results for Preparations III. 

 and IV., shows the values for l/\ 2 obtained from the individual 

 measurements. It will be seen that, with the exception of one 

 point each in the two preparations, the individual values lie 

 within 6'5 percent, of a mean value of 100,000 years for the 

 period of average life of ionium, assuming that of radium to 

 be 21)75 years. This figure has been retained unaltered from 

 the last paper, though it is slightly in error. The half- 

 value period of 1690 years accepted by Sir Ernest Rutherford 

 as the most probable corresponds with a period of average 

 life of 2440 years. A new direct estimate of the period, by 

 the Rutherford-Boltwood method of separating all the ionium 

 from the mineral and measuring the rate of growth of radium 

 from it in terms of the equilibrium quantity of radium in the 

 mineral, has since been published *. As the mean of four 

 experiments the value 2500 years was obtained, the mean of 

 the last two on which the most reliance is placed giving 

 2393 years for the period of average life. The method 

 involves the addition of rare earths to the mineral to separate 

 the ionium, and it is unlikely that these rare earths would, 

 if prepared from any known mineral, be completely free from 

 ionium. Unfortunately the authoress does not give evidence 

 of the amount of ionium so introduced and whether it was 

 sufficient to affect the period obtained. Until fresh data are 

 available, it does not seem worth altering the figure 23' 5 

 years before taken. 



As regards Preparations Land II., the much larger initial 

 quantity of radium, relative to the uranium, makes their 

 indications of little value in assigning the period of ionium. 

 But the growth of radium from the start, assuming ionium 

 to have been absent, corresponds in Preparation I. to the 

 period of 100,500 years, and in Preparation II. to 130,500 

 years. No weight is yet to be attached to these preparations. 



Summary. 

 The growth of radium in uranium purified initially from 

 ionium and radium proceeds regularly according to the square 

 of the time with the period, 237,500,000 years as the product 

 of the periods of average life of ionium and radium. This 

 value is probably accurate to within 5 per cent., and the 

 period of radium is scarcely known to this degree of accuracy. 

 Assuming the period of average life of radium to be 2375 

 years, that of ionium is 100,000 years. It is not expected 

 that the further growth of radium from these preparations 

 will enable the period of ionium to be determined with 

 much greater certainty. 



* [Mile. Ellen Gleditsch, Am. J. Sci. xli. p. 112 (1916).] 



