498 Professor John Joly [Feb. 24, 



mean value of its radius as cited in a paper communicated to the 

 Royal Society in 1916. The discrepancy with the theoretic curve is 

 small : 10 or 12 per cent, of the external radius. The allowance for, 

 and measurement of, the nucleus is sufficiently difficult to introduce 

 some uncertainty. 



This misfit may he of considerable significance. I have already 

 reminded you that the range of the a-ray emitted by a transforming 

 element is related to its rate of break-up. The range is longer for 

 the shorter-lived elements. Now, here the first ring of the uranium 

 halo in mica shows a longer range than we would expect from the 

 air-curve as observed to-day. The agreement between the two in 

 other cases appears to show that this is not due to any unknown 

 effect influencing the retardation in mica. The location of the first 

 uranium ring is mainly referable to those short-range a-rays arising 

 from the initial transformations of the uranium series. ^Ye infer 

 that one or more of these rays must have had a longer range in past 

 times, and, of course, that the corresponding transformation periods 

 must have been shorter. A specially influential ray is that slowest 

 of all the rays — that which is emitted in the break-up of uranium 1. 

 The discrepancy might be due to this ray possessing a greater range 

 in early geological times. But, whatever the cause, the nature of 

 the misfit suggests evidently that formerly the rate of transformation 

 of uranium to lead was faster than it is to-day. 



It is with some reserve that I refer here to measurements made 

 lately on haloes of comparatively recent and of very remote geological 

 ages. I say " with reserve," for not only are the results of a nature 

 calling for very adequate confirmation, but the measurements present 

 considerable difficulty. The point at issue may be stated in a few 

 words : — Is the abnormality observed in the dimensions of the 

 uranium halo dependent in amount upon the antiquity of the rock in 

 which the halo is developed ? 



I had sought occasionally for uranium haloes in rocks younger 

 than the Leinster granite — which is of early Devonian age. The 

 granite of Mourne, which is of Eocene or early Tertiary age, for 

 long refused to reveal any haloes suitable for measurement. How- 

 ever, recently I was so fortunate as to find a few of these early halo 

 rings which I was able to measure. Further search has revealed a 

 few more ; but they are excessively scarce and rather difficult to 

 detect. The nuclei of these haloes are only rarely zircon — they 

 seem to be apatite ; possibly allanite — and their average size is 

 greater than the zircon nuclei of the Carlow mica. Both the mineral 

 nature of the Mourne nuclei and their dimensions involve, therefore, 

 a bigger subtraetive correction on the observed radius than is re- 

 quired in the case of the Carlow haloes. But in addition to this, there 

 appears to be a small difference in the external radius of the Eocene 

 halo and that of the Devonian halo. According to a large number 

 of readings by several observers, some of whom were not acquainted 



