86 Scientific Proceedings, Royal Dublin Society. 



hundredths of a millimetre which was photographed without disturbing 

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

 dimensions of the fully formed haloes are all over the plate alike, and just 

 tliat which the radiant matter from the uranium series of elements would 

 account for. 



It is possible to trace the development of lialoes by observation of those 

 arising from a feebler and feebler 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 radium and 

 emanation ; and the outermost sphere, for some unexplained reason, often 

 becomes conspicuous before RaAhas 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 haloes we have now been looking at are the accumulated effects of 

 ionization 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 radioactive ore known, pitchblende, the rate of 

 expulsion of the a rays has, owing to the smalluess of the quantities of 

 radioactive substances involved, been less than eighty in a year. But this 

 is not all. Some of the nuclei are identified witli certainty as zircons. If 

 we ascribe to these a radioactivity 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 recognize in the feeblest haloes the work of 

 many millions of atoms of radiant matter, each exerting its own small 

 effect, but these effects carefully preserved and accumulated. In short, 

 we recognize the halo and detect its nature and origin on the same 

 principles as we recognize by their light-effeels accumulated upon the 

 photographic plate the presence of stars invisible to the eye. 



We find, then, in the rooks 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 recognize the halo as tlie result of the integration of effects of 

 unimaginable feebleness ; and as we see them in the archsean granites, they 

 probably date their beginnings from times long antecedent to the appearance 

 of life upon the globe, not less than 100 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 



