6?>6 



Prof. J. Jolv and Mr. A. L. Fletcher : 



Containing 



R. 



r. 



Radioactive 



Rock. 



mineral. 



mm. 



mm. 



substance. 





Bictite 



0-023 

 0039 





Ra 



Th 



Durbachite, Sehwartzwald. 







0-039 



0023 



Th 



Dioiite, Redwitz. 



)• 



0032 





Ra 



j» 





0033 



0021 



Ra 



Granite, Freiberg. 



,, 



00393 



00244 



Th 



„ Ochsenkopf. 



;) 



0-0319 



00191 



Ra 



m >> 



„ 



0033 



0022 



Ra 



,, Leinster. 



Zinnwaldite.. 



0-0312 





Ra 



Greisen. Altenberg. 



,, 



0-0305 



0-0199 



Ra 



> i >> 



„ 



0-0321 



0-0211 



Ra 





,, 



0-0323 



0-0202 



Ra 



„ ,, Corona detached. 



„ 



0-0326 





Ra 



,, 



,, 



0-0317 



0-0198 



Ra 



,, ,, Corona detached. 





00301 



00191 



Ra 



)> >j >» 



,, 



00316 



00190 



Ra 



M >» >> »' 



Lepidolite ... 



0-028 





Ra 



Lourdalite, Langesund fiord. 



Cordierite ... 



0-031 





Ka 





Hornblende.. 



0040 



0-028 



Th 



Syenite, Ivnorre. 



„ 



033 





Ri 



„ St. Maurice. 



One o£ the thorium halos in the durbachite is that of which 

 we have just given details, and is illustrated in the paper 

 referred to. We reproduce here (PI. VIII.) a photograph of 

 a thorium and a uranium halo (contained in the one crystal 

 of mica from a granite of Ochsenkopf in the Fichtelgebirge) 

 the dimensions of which are given in the table. The uranium 

 halo (in lower part of figure) is too dense to permit of clear 

 photographic reproduction of the pupil; but the latter can be 

 seen and measured in the microscope. The corona of the 

 thorium halo is not strongly developed, but is quite distinct. 

 The nuclei in the case of both these halos are small and 

 practically negligible. 



From the measurements given in the table it will be seen 

 that halos developed in biotite are somewhat greater in radius 

 than those in zinnwaldite or lepidolite. This agrees with the 

 theoretic radii in these minerals as already deduced. The 

 fact that the radius of the pupil is not in every case quite up 

 to the theoretic size is, we think, explicable on the view that 

 these halos are not always fully developed. It will be seen 

 further on that the corona appears before the pupil reaches 

 its limits. 



Thorium halos are not so frequently met with as uranium 

 halos. The thorium halo observed in the syenitic hornblende 

 of Knorre is a very beautifully coloured object when seen 

 between crossed nicols. The nucleus in this case is pre- 

 sumably thorite. It is black and nearly opaque and almost 



