482 



NATURE 



[April 15, 1922 



to them. Here, then, the results join issue : the 

 uranium result is just four times as great as the thoriuni. 

 We notice, too, that on the uranium-scale of time, this 

 thorite must be older than Silurian or Ordovician, 

 which have been determined by uranium lead as 430 

 millions of years ago. Probably its age dates back 

 to Cambrian or even to pre-Cambrian time. From 

 what we have already inferred we cannot regard 130 

 millions of years for early Palaeozoic time as irreconcil- 

 able with the maxima which denudative methods 

 afford. More recently, lead derived from a Norwegian 

 thorite of Langesundfiord — also of lower Palaeozoic 

 age — seems to reveal an age of 150 millions of 

 years. In this case, also, there is the added security 

 of a determination of the atomic weight of the 

 lead. 



We cannot discredit these results on the score of 

 radio-active instability of the lead. Why, then, set 

 them aside in favour of results reached on uranium 

 lead, which are in hopeless contradiction with the 

 indications of the record of the surface activities of the 

 globe ? It is, indeed, not too much to say that the 

 whole position is now reversed and that to-day 

 suspicion attaches to the uranium-lead ratio. And, 

 as we shall see, there is much unknown about the 

 earlier radio-active sequence in the uranium series ; 

 while the discovery of isotopes opens the way to 

 possibilities unthought of in the earlier days of radio- 

 active science. 



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

 pleochroic halo on this matter. 



The halo 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 determined 

 with considerable accuracy, and these dimensions are 

 conditioned 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 ionisation 

 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 determined by Geiger. The range of the 

 ray 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 affecting the sur- 

 rounding substance — mica, we may suppose — in such 

 a way as to build up concentric spherical shells more 

 or less overlapping and corresponding 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. I is a curve arrived at in the manner I have just 

 described. Its ordinates are proportional to the 



NO. 2737, VOL. 109] 



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 con- 

 taining thorium and all the successive products of its 

 transmutation. This, of course, necessitates magnify- 

 ing 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 find- 

 ing 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 

 insufficient number of fig- i- 



ordinates. This close agreement really reveals a very 

 important fact. The air-curve depends for its dimen- 

 sions on the ranges of the several a-rays as we 

 measure them to-day in the laboratory. The halo- 

 measurements refer to radio-active effects which began 

 their record in this mica in Carboniferous times — 

 possibly long before. The halo reveals no sign of 

 change in the several ranges concerned. As you are 

 aware, the rate of break up, the transformation constant 

 of the element, is related to the range. We are, there- 

 fore, in the case of the thorium family, entitled to read 

 in these minute and ancient records a guarantee that 

 the accumulation of the final product^-the thorium 

 isotopes of lead — was in the remote past effected 

 at just such a rate as we have inferred from the 

 splendid researches of our day. The thoiium halo 

 gives us this guarantee. It also tells us that it is 

 improbable that the resulting lead is unstable. For if 

 it were we must find room for rays additional to those 

 we have used in deriving the ionisation curve. True, a 

 coincidence of range might enable a ray to lie concealed 

 in the halo ; but the fit of the halo is so absolutely 

 faithful to every feature of the curve that this seems 

 improbable. 



It is also possible to observe the successive 

 stages of development in thorium haloes. The first 

 rings to appear are those corresponding to the two 

 conspicuous crests of the curve. Fig. i . If the central 

 nucleus is small or feeble, nothing more may be 

 developed. 



We now turn to the uranium curve. The eight 

 contributory ionisation curves are placed according to 

 the range of each ray, and Fig. 2 shows the curve 

 ■produced by adding up the ordinates. Above it are 

 laid out the several rings observed in the uranium 

 halo. 



Looking at these rings, we notice that the outer 

 features of the halo seem in fair agreement with the 

 present-day ranges. But the innermost ring has a 

 larger radius than would be expected from the curve. 

 Much care has been expended in verifying this point. 

 In the Devonian mica of County Carlow these haloes 

 are found in every stage of development according to 

 the size or activity of the nucleus. The uranium halo 



