496 



Professor John Joly 



[Feb. 24, 



I shall, however, now turn to the evidence of the pleochroic halo 

 on this matter. 



The hnlo affords a means of investigating certain facts respecting* 

 the break-up of the radio-active elements in the remote past. For 

 the dimensions of the halo — minute though they be— can be deter- 

 mined with considerable accuracy, and these dimensions are condi- 

 tioned by the added effects of the several a-rays emitted by the 

 transmuting elements. Bragg and Kleeman observed and measured 

 just such integral ionisation effects in air. In the rocks the ionisa- 

 tion curves, owing to the great stopping power of minerals, are on a 

 scale 2000 times as small. They are very faithful hieroglyphics, 

 however, and carry back our knowledge over an appalling vista of 

 time. 



One single a-ray produces a well-known curve of ionisation deter- 

 mined by Geiger. The range of the rays does not affect the general 

 nature of the curve. If we imagine uranium or thorium as parent 

 elements contained in a minute crystal — of zircon, for instance — we 



must picture the various a-rays affect- 

 ing the surrounding substance— mica, 

 we may suppose — in such a way as to 

 build up concentric spherical shells 

 more or less overlapping and corre- 

 sponding to the radial distances at 

 which the ionisation of the several 

 rays is at a maximum. As seen in 

 section upon cleaved flakes of the 

 mica, we find concentric coloured rings 

 representing the ionisation due to the 

 rays. 



In order to arrive at the theoretical 

 location of these rings we must add up 

 the several ionisation effects as observed 

 in air. This involves assigning a Geiger curve to each ray according* 

 to its range and adding up the ordinates. 



Let us consider first the case of the thorium halo. Fig. 1 is a 

 curve arrived at in the manner I have just described. Its ordinates 

 are proportional to the integral ionisation effects of those radio-active 

 elements in the thorium series which emit a-rays. And above it I 

 have marked, calculated into the range in air, the positions of the 

 coloured rings which in biotite we observe encircling a minute 

 mineral particle containing thorium and all the successive products 

 of its transmutation. This, of course, necessitates magnifying the 

 halo enormously — rather more than 2000 diameters. You perceive 

 that the halo very faithfully conforms to the features of the air-curve. 

 It may be of interest to mention that the finding of the third ring 

 led to the discovery of the prominence on the curve which accounts 

 for it. This part of the curve had originally been plotted from an 



Fig. 1. 



