Prof. J. Joly on Pleochroic Halos. 381 



soil ; and the results so far obtained confirm my former con- 

 clusions as to radiothorium transformation products being, at 

 least in these regions, a most important agent of atmospheric 

 radioactivity. 



As I have already mentioned, Eve's estimate of the quantity 

 of radium contained in the earth's crust, based on the obser- 

 vation of the penetrating radiation present in the atmosphere, 

 is four times greater than the average amount found bv Strutr 

 in rocks. Xow it is clear that the experimental method used 

 by Strutt could only reveal the presence of radium in the 

 materials tested by him. It is therefore not impossible that 

 the reason of such discrepancy should be ascribed to the 

 fact that part of the effects observed by Eve are due to 

 the presence of the product thorium B in the soil and 

 atmosphere. 



As I have already said. I shall soon be able to give a 

 detailed account of my researches, which will, I hope, enable 

 me to make an estimate of the amount of radiothorium present 

 in the earth's crust, at least as far as concerns the Roman 

 region. 



Rome, Istituto Fisico della E. Universita. 



XXIX. Pleochroic Halos. By J. Joly, F.R.S* 

 [Plate X. f] 



CRYSTALS of zircon occurring in certain anisotropic 

 coloured minerals, more especially in biotite, are 

 invariably seen to be surrounded by an area showing strong 

 pleochroism. Thus in biotite. when the section under obser- 

 vation is transverse to the principal cleavage, the darkening 

 of the halo cccurs when the vibrations are in the direction 

 of cleavage. The absorption is then often so complete as to 

 produce opacity. When the vibrations are at right angle- 

 to this direction the halo almost disappears. The zircon then 

 appears simply as a minute highly retracting speck marking 

 the centre of the halo. In short, the area referred to appears 

 as an intensification of the general pleochroism of the mica. 

 As the term halo is misleading to those unacquainted with 

 the phenomenon referred to, it is well to mention that the 

 darkening takes place uniformly over the circular area around 

 the zircon. 



The cause of this curious appearance has long been matter 

 of speculation. It has been assigned to organic (?) pigments, 

 mainly on the strength of the fact that a very high tempe- 

 rature will cause its dissipation. The following considerations 



* Communicated "by the Author. 



t [The plate illustrating this paper will appear in the April numbeiv 



