500 Professor John Joly [Feb. 24, 



forward the view that the parent of actinium is a third isotope of 

 uranium not belonging to the uranium family and having an atomic 

 weight of 240. This view is regarded favourably by Soddy and 

 Cranston. It clears up the difficulty respecting the atomic weight of 

 uranium, and fits in with the atomic weights of radium and of 

 uranium lead. Soddy and Cranston remark chat in order to explain, 

 in this case, the constant ratio of actinium to uranium observed in 

 minerals we must suppose the period of uranium 1 and of the hypo- 

 thetical isotope to be the same. This difficulty, however, is removed 

 if we may assume that the ratio varied over geological time. 



A somewhat similar theory to Piccard's may be invoked to 

 explain the abnormality of the Devonian uranium halo. We have 

 these facts to go on :— The age indicated by uranium for Lower or 

 Pre-Palaeozoic rocks is about four times too great as compared with 

 the age indicated by thorium. We assume, therefore, that three- 

 fourths of the lead as measured in uranium minerals is derived from 

 a certain isotope. This isotope, not having been detected in our 

 time by its primary a-radiation, we must suppose to be now sensibly 

 exhausted. We, therefore, have a known mass of this isotope trans- 

 forming to lead in a known time — 180 x 10 6 years. Assuming that 

 only 1 per cent, of it is left we get its transformation constant 

 (S'SxlO -8 ), and by G-eiger and Nutall's relation we find the 

 corresponding range as 2*6 cms. at 0° C. ; or about 2*75 cms. at 

 15° C. To-day the a-radiation of the hypothetical body would be 

 oidy T oVir of that due to uranium 1, but during the period since the 

 Devonian there will be about three a- rays from the short-lived isotope 

 to one from the long-lived. The integral curve of ionisation as modi- 

 fied by these hypothetical results would be in agreement with the 

 Devonian halo. We have to assume that the ranges of the rays 

 emitted by the successive disintegrating products of the supposed 

 isotope were such as to leave the outer features of the halo sensibly 

 undisturbed. This seems not improbable. 



The salient features which appear in the study of radio-active 

 haloes are : — Firstly, that the agreement of our laboratory measure- 

 ments of to-day with the features of the Palaeozoic thorium halo is 

 such as to support the view that the periods of the several elements 

 concerned in its genesis have remained unchanged over 130 millions 

 of years. This fact, taken along with the stability of thorium lead, 

 seems to render its reading of geological time authentic in a high 

 degree. Its indications are confirmed by the consistent testimony of 

 the denudative processes which have progressed on the earth's 

 surface. Secondly, it appears that the uranium halo is not in con- 

 formity with the period we ascribe to-day to uranium — a disagree- 

 ment which is emphasised by the failure of uranium-time to conform 

 with the united testimony of thorium-time and denudative-time ; as 

 well as by much that remains unexplained respecting the earlier 

 changes in the uranium family of elements. 



