NATURE 



[July 20, 191 1 



which arc always progressing at all points along the path 

 of the ray :is shown in the Bragg curve. 



We should expect, however, to meet cases where, either 

 from the smallness of the quantity of radio-active material 

 or from the recentness of the formation of the rock, there 

 is a proper or correct exposure, so that the successive 

 shells of ionisation, which we may picture to ourselves as 

 surrounding a particle of pitchblende in air, would, as 

 developed in the mica, 

 be made visible to the 

 eye. In this anticipa- 

 tion we assume that 

 Bragg 's laws apply to 

 the ionisation of a 

 solid. 



Now we do indeed 

 find the several spheres 

 of ionisation — or at 

 least many of them — 

 beautifully depicted in 

 certain minerals, and 

 thus we, at one and 

 the same time, find 

 additional, indeed over- 

 whelming, evidence 

 that the haloes are due 

 ' to a rays, and also, 

 what would be hard to 

 establish experiment- 

 ally, that Bragg's laws 

 govern the effects in 

 the solid medium. 

 Here is a group of well-exposed haloes in the biotite of 

 co. Carlow. You see the outer ring due to radium C, and 

 the gap of feebler ionisation between it and the shell 

 due to radium A. We even find some which are actually 

 " under-exposed." These often have got no further than 

 the record made by the intense triple effect due to uranium 

 and ionium. I show you this photograph again, but this 

 time with an engraved scale of hundredths of a milli- 

 metre, which was photographed without disturbing the 

 microscope ; so that it is possible for you to verify the 



- 





1 



Fig. 



radium and emanation, and the outermost sphere, for 

 some unexplained reason, often becomes conspicuous before 

 radium A has produced much effect. The effects of the 

 latter rays sometimes appear as a distinct ring. 



We find a striking comment on the immense age of the 

 haloes and of the containing rocks by a study of these 

 objects, for it is easy to show that the growing haCoes we 

 have now been looking at are the accumulated effects of 

 ionisation acting with extreme slowness. It is calculable 

 directly that, even if we supposed the minute nuclei of 

 some of these haloes to consist, not of zircon, but of the 

 most radio-active ore known, pitchblende, the rate of 

 expulsion of the a rays has, owing to the smallness of the 

 quantities of radio-active substances involved, been fewer 

 than eighty in a year. But this is not all. Some of the 

 nuclei are identified with certainty as zircons. If we 

 ascribe to these, a radio-activity even greater than Strutt 

 found in his highest measurements, one or more years 

 would have elapsed between one expulsion of consecutive 

 helium atoms and another. But geological time is long; 

 and we may still recognise in the feeblest haloes the work 

 of many millions of atoms of radiant matter, each exert- 

 ing its own small effect, but these effects carefully pre- 

 served and accumulated. In short, we recognise the halo 

 and detect its nature and origin on the same principles as we 

 recognise by their light-effects accumulated upon the photo- 

 graphic plate the presence of stars invisible to the eye 



We find, then, in the rocks a record of the laws of 

 radiant matter in the handwriting of the radiant matter 

 itself — a record which took many millions of years to 

 inscribe. Haloes are not found in the younger rocks. 

 We must clearly recognise the halo as the result of the 

 integration of effects of unimaginable feebleness ; and as 

 we see them in the Archaean granites, they probably date 

 their beginnings from times long antecedent to the appear- 

 ance of life upon the globe, not fewer than ioo million 

 years ago. 



They assure us, therefore, of the remote antiquity of 

 the atomic instability which calls radiant matter into 

 existence. But even more they tell us of the enduring 

 stability of the ordinary elements. If the common and 

 abundant elements which occur in and around the mica 



1 low ) : 



erlapping haloes ; 

 nder-exposed hale 



vage plate of bioi 

 about 76 di: 

 re present, as 



;. 5.— A radium halo (lower right-hand 

 part of the field) and a thorium halo 

 (upper left-hand part) in brown mica 





The 



the cleavage. Enlargement about i 

 diameters. The thorium halo sho; 



an inner sphere due to the th trn 



The ratio of the diameters of inner ai 

 outer spheres will be found to be 



le radium halo from the Carlow 

 s enlarged toabout 5oodiameters. 

 dark disc is due to emanation. 

 1 A sphere succeeds and appears 

 jvetoped than that of radium C. 

 :leavage. 



fact that the dimensions of the fully formed haloes an' all 

 over the plate alike, and just that which the radiant matter 

 from the uranium series of elements would account for. 



It is possible to trace the development of haloes by 

 observation of those arising from a feebler and Fi 

 central radiation. A succession of photographs taken to 

 the same enlargement reveals that the innermost sphere is 

 first formed. Then this widens under the rays from 

 NO. 2177, VOL. 87] 



emitted radiant matter, even at the slowest rates, the clear 

 transparency of the mica must long ago have vanished, and 

 the whole become obscured under the effects accumulated 

 during the ages which have elapsed since the formation of 

 the rocks. 



Wi seem entitled to conclude that the atomic stability 

 and instability which we observe to-day have prevailed 

 during geological time. 



