IIG Radioactive Haloes [May 11, 



But in order to account for the mode of development of the halo, 

 i.e. the sequence in which its several features appear, it is further 

 necessary to assume that some cause exists which favours the 

 development of the outer features, or in other words counteracts the 

 weakening of the ionization arising from the spreading of the rays 

 from the radioactive nucleus of the halo. In it is found that the 

 inner ring which marks the first beginning of the halo may be 

 attended by the appearance of the outermost ring due to the alpha- 

 ray of greatest range ; an effect which should not occur if the 

 ionization intensity fell off with the divergence of the rays, as might 

 be expected. The explanation offered is that the halo-genesis follows 

 similar laws to those governing photographic effects, so that repetition 

 of stimuli leads to reversal of the earlier effects. Reversal will 

 therefore be less active as the rays diverge outwards, and lierein is 

 found a reason for the relative accentuation of the ionization due to 

 the rays of longest range. Haloes presenting all the appearance of 

 more complete reversal have been observed. 



The foregoing principles satisfactorily explain the features, 

 dimensions, and order of formation of haloes formed around radio- 

 active centres. 



A third type of halo is observed which is referable to the 

 emanation of radium as parent substance. This halo appears 

 generally in connexion with conduits in which there is plain evidence 

 that radioactive gas or liquid has at one time circulated. The 

 nucleus of the halo has apparently absorbed the emanation, and the 

 further changes of this substance, giving rise to four characteristic 

 alpha-rays, have sufficed to form a halo which finds, in its every 

 feature, explanation according to the principles outlined above. 



There is some degree of misfit in the primal rings of both the 

 thorium and the uranium halo. In the case of the thorium halo this 

 misfit is very minute, but is sufficient to suggest that the range 

 generally accepted for the thorium alpha-ray is a little excessive. 

 This agrees with the indication of the Geiger-Xutall curve, connect- 

 ing range and period of transformation. In the case of the primal 

 ring of the uranium halo — which is largely due to the ionization 

 arising from Uj and Uo — the misfit is more conspicuous, and is in 

 the opposite sense to the misfit of the thorium ray. The observed 

 halo-ring indicates a former range greater than what is now observed. 



Until tliis misfit is shown to be confined to halves of Palaeozoic or 

 Arch^an ages it seems premature to advance any explanation of it. It 

 is worth noting, however, that a former longer range of the ray of Uj, 

 and hence a more rapid rate of decay of uranium in early times, 

 would explain the disagreement of the lead-ratio of the uranium series 

 with that of the thorium series, and concurrently would reconcile 

 radioactive methods of determining the earth's age with those based 

 on the indications of denudative effects. 



[J. J.] 



