18 Johnston and Boltwood — Relative Activity 



Gleditsch depended on the production of radinm fromthe 

 ionium in equilibrium with a known amount of radium 

 and was measured in terms of the fraction of the total 

 equilibrium amount which was produced in a known 

 period of time. This would have given the true value 

 for the disintegration constant irrespective of the mode 

 of disintegration. These two methods would therefore 

 have given different and not similar values if the col- 

 lateral series had originated at radium. 



Among other objections may be mentioned the experi- 

 ment made by Soddy 25 who examined a specimen of 

 radium salt containing 13-2 mgs. of radium which had 

 been sealed for a period of 10 years. No evidence of 

 the presence of actinium was obtained. Paneth and 

 Fajans 26 examined a specimen containing 180 mgs. of 

 radium which had been sealed for six years, but were 

 unable to detect the presence of any actinium products. 

 We may therefore dismiss the possibility of the side 

 chain splitting off at radium as highly improbable in the 

 light of our present knowledge. 



The possibility that the collateral series originates at 

 ionium may also be considered. The fact that the ex- 

 perimental evidence is all opposed to the emission of a 

 ^-radiation by ionium is in itself a decided objection to 

 this view. Moreover, it would require (uranium taken 

 as unity) an activity of 0-56 for the actinium prod- 

 ucts, an activity of 046 for ionium, and an activity 

 of 3-01 for radium and its products, with a total 

 activity of 5-02. Paneth and Fajans 27 have directly 

 attacked this problem by seeking for the presence of 

 actinium in a strong preparation of ionium-thorium 

 which had been undisturbed for four years. They were 

 unable to discover the presence of any actinium prod- 

 ucts. Lacking any support, therefore, the supposition 

 that the collateral series arises at ionium is untenable at 

 present. 



These circumstances compel a return to a considera- 

 tion of the earlier members of the series, to U I and U II, 

 in the hope of being able to find there an explanation of 

 the conditions indicated by our experiments. At first 

 sight it might seem that the conditions would be satisfied 



"Nature, 91, 634, 1913. 



20 Wien. Ber., 123, Ha, 1627, 1914. 



27 Wien. Ber., 123, Ila, 1627, 1914. 



