1922] 



on The Age of the Earth 



497 



insufficient number of ordinates. This close agreement really reveals 

 a very important facfc. The air-curve depends for its dimensions 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, therefore, in the case of the thorium family, entitled to read 

 in these minute and ancient records a guarantee that the accumula- 

 tion 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 thorium 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 addi- 

 tional to those we have used in deriving 

 the ionisation curve. True, a coincidence 

 of range might enable a ray to lie con- 

 cealed 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. 1. 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 pro- 

 duced by adding up the ordinate?. 

 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 begins as a single delicate ring surrounding the 

 minute central nucleus. It can be measured from a stage bordering 

 on invisibility to a stage when its central area is beginning to darken 

 up and the first shadowy signs of the outermost ring of all — that due 

 solely to radium C — appear. A large number of readings on these 

 embryonic haloes, made recently by various observers, confirm the 



2x2 



Fig. 2. 



