66 Relative Activity of Radium and Uranium. 



disintegration. These two methods would therefore have 

 given different and not similar values if the collateral 

 series had originated at radium. 



Among other objections may be mentioned the experiment 

 made by Soddy*, who examined a specimen of radium salt 

 containing 13' 2 mgs. of radium which had been sealed for a 

 period of ten years. No evidence of the presence of actinium 

 was obtained. Paneth and Fajansf 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 experi- 

 mental evidence is all opposed to the emission of a /3-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 products, an activity of 0*46 

 for ionium, and an activity of 3*01 for radium and its pro- 

 ducts, with a total activity of 5"02. Paneth and FajansJ 

 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 pro- 

 ducts. Lacking any support, therefore, the supposition that 

 the collateral series arises at ionium is untenable at present. 



These circumstances compel a return to a consideration 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 con- 

 ditions indicated by our experiments. At first sight it 

 might seem that the conditions would be satisfied by assum- 

 ing that what we now call uranium consists of three radio- 

 elements, a parent element and two isotopic products in 

 equilibrium, all emitting a-rays. But if these are present 

 in relative amounts of the same approximate order of mag- 

 nitude (i.e., 100, 92, 92, etc.), then the a-rays emitted by at 

 least one of them would have to be of exceedingly short 

 range and small ionizing power and the rate of change of 

 this substance would be excessively slow. It is not impos- 

 sible, but it does not seem probable, that ordinary uranium 

 may consist of what we know as U I. and U II., both radio- 

 elements in the main line of descent, and a third isotope 



* -Nature,' xci. p. 634 (1913). 



t Wien. Ber. exxiii. Ha, p. 1627 (1914). 



% Wien. Ber. exxiii. Ha, p. 1627 (1914). 



