382 Prof. J. Joly on Pleachrok Halos. 



appear to show that it is most probably referable to those 

 radioactive properties of zircon so clearly demonstrated in 

 the Hon. R. J. Strutfs experiments. 



The well-developed halo appears never to exceed a certain 

 maximum dimension. The radius may attain 0*05 mm. or 

 even a little more. The average of measurements made on 

 good halos in mica was 0'04 mm. Smaller halos are frequent. 

 In the smaller halos the originating enclosure is often too 

 minute to be discoverable. The halo is the section of a 

 sphere of very considerable truth, judging from its accurate 

 circularity. 



Professor Rutherford has shown that 13 layers of aluminium 

 each O00031 cm. thick, reduce the velocity of the most 

 energetic a rays from radium to a degree rendering them 

 incapable of effecting photographic action or of exciting 

 phosphorescence. When this result is translated into the 

 equivalent thickness of biotite, the active range is just 

 0*04 mm. Hence the halo attains just such dimensions as 

 would be expected if due to alterations affected by the a 

 radiation. Halos of smaller radius are probably due to a 

 source of radium so minute in amount as to limit the effect 

 to the inner region which is traversed bv ravs of every 

 velocity; or the smaller radial dimension may only be apparent, 

 the section being taken in such a plane as to exclude the 

 centre. 



Some writers refer these halos to the presence of other 

 minerals. I find sphene, apatite, and allanite mentioned as 

 being occasionally surrounded by halos. As regards the first 

 I believe I have seen associated halos. However this maybe, 

 Mr. Strutt has shown that both sphene and apatite are highly 

 radioactive. Not being able to find any determination of 

 radioactivity in the case of allanite, I examined a specimen 

 in the Museum here for radium, using the methods developed 

 by Mr. Strutt; it afforded 270 X 10~ 12 grains radium per gram. 

 This is comparable with the activity of zircon. 



One conspicuous property of these halos might long ago 

 have put any aggregation-theory out of court. The fact that 

 their sphericity is independent of the cleavage or physical 

 structure of the medium in which they are formed. Aggre- 

 gation or spreading by diffusion would most certainly not 

 take place with equal ease across and along the cleavage ; as 

 is shown in all interpositions and alterations in mica. 1 have, 

 indeed, frequently observed halos extending with perfect 

 circularity across the cleavage of two or more crystals of 

 biotite in mutually inclined positions (Plate X.*). This appears 



to me to be strong evidence for their radioactive origin. 



© © 



* [This will be published in the April number.] 



