PLEOCHROIC HALOES 151 



is easy to show that even in the oldest haloes the 

 quantity of helium involved is so small that one 

 might say the halo-sphere was a tolerably good 

 vacuum as regards helium. There is, finally, no 

 reason to suppose that the imprisoned helium 

 would exhibit such a colouration, or, indeed, any 

 at all. 



I have already referred to the great age of the 

 halo. Haloes are not found in the younger 

 igneous rocks. It is probable that a halo less 

 than a million years old has never been seen. 

 This, prim& fade, indicates an extremely slow 

 rate of formation. And our calculations quite 

 support the conclusions that the growth of a halo, 

 if this has been uniform, proceeds at a rate of 

 almost unimaginable slowness. 



Let us calculate the number of alpha rays 

 which may have gone to form a halo in the 

 Devonian granite of Leinster. 



It is common to find haloes developed perfectly 

 in this granite, and having a nucleus of zircon 

 less than 5 x icr 4 cms. in diameter. The volume 

 of zircon is 65* io' 12 c,cs. and the mass 3 X io' 10 

 grm. ; and if there was in this zircon icr 8 grm. 

 radium per gram (a quantity about five times the 

 greatest amount measured by Strutt), the mass 

 of radium involved is 3 x icf 18 grm. From this 

 and from the fact ascertained by Rutherford that 

 the number of alpha rays expelled by a gram of 

 radium in one second is 3*4 x io 10 , we find that 

 three rays are shot from the nucleus in a year. If 



