484 



NA TURE 



[April 15, 1922 



that what we now call uranium consists of three radio- 

 elements ; a parent element and two isotopic products 

 all emitting a-rays {Phil. Mag., July 1920). In 1917^ 

 •A. Riccard put forward the view that the parent 

 of actinium is a third isotope of uranium not belonging 

 to the uranium family and having an atomic weight 

 of 240. This view is regarded favourably by Soddy 

 and Cranston. It clears up the difficulty respecting 

 the atomic weight of uranium and fits in with the 

 atomic weights of radium and of uranium lead. Soddy 

 and Cranston remark that in order to explain, in this 

 case, the constant ratio of actinium to uranium observed 

 in minerals we must suppose the period of uranium i 

 and of the hypothetical isotope to be the same. This 

 difficulty, however, is removed if we may assume that 

 the ratio varied over geological time. 



A somewhat similar theory to Riccard's may be 

 invoked to explain the abnormality of the Devonian 

 uranium halo. We have these facts to go on : — ^The 

 age indicated by uranium for Lower or Pre-Palaeozoic 

 rocks is about 4 times too great as compared with the 

 age indicated by thorium. We assume, therefore, 

 that three-fourths of the lead as measured in uranium 

 minerals is derived from a certain isotope. This 

 isotope, not having been detected in our time by its 

 primary a-radiation, we must suppose to be now 

 sensibly exhausted. We, therefore, have a known, 

 mass of this isotope transforming to lead in a known 

 time — 130 X 10^ years. Assuming that only i per 

 cent, of it is left we get its transformation constant 

 (3-5x10"^), and by Geiger and Nutall's relation we 

 find the corresponding range as 2'6 cms. at 0° C. ; or 

 about 2-75 cms. at 15° C. To-day the a-radiation 

 of the hypothetical body would be only yoVo ^^ ^^^ 

 due to uranium i, but during the period since the 

 Devonian there will be about 3 a-rays from the 

 short-lived isotope to i from the long-lived. The in- 

 tegral curve of ionisation as modified by these hypo- 

 thetical results would be in agreement with the 

 Devonian halo. We have to assume that the ranges of 

 the rays emitted by the successive disintegrating 

 products of the supposed isotope were such as to leave 

 the outer features of the halo sensibly undisturbed. 

 This seems not improbable. 



The salient facts which appear in the study of radio- 

 active haloes are : — firstly, that the agreement of our 

 laboratory measurements of to-day with the features 

 of the Palaeozoic thorium halo is such as to support 

 the view that the periods of the several elements con- 

 cerned in its genesis have remained unchanged over 

 130 millions of years. This fact, taken along with the 

 stability of thorium lead, seems to render its reading 

 of geological time authentic in a high degree. Its 

 indications are confirmed by the consistent testimony 

 of the denudative processes which have progressed on 

 the earth's surface. Secondly, it appears that the 

 uranium halo is not in conformity with the period we 

 ascribe to-day to uranium ; a disagreement which is 

 emphasised by the failure of uranium-time to conform 

 with the united testimony of thorium-time and denu- 

 dative-time ; as well as by much that remains un- 

 explained respecting the earlier changes in the uranium 

 family of elements. 



The complete tale is not yet told, but I think the 

 balance of probability is in favour of an age between 



NO. 2737, VOL. 109] 



150 and 200 millions of years for the earliest advent of 

 geological conditions upon the globe. 



Astronomical investigation on the subject of the 

 age of the earth deals, generally, with that greater age 

 which must be ascribed to the earth as a planet. For 

 this age vast periods have been claimed. But it is 

 possible to reconcile superior ages for the earth as a 

 planet with comparatively brief geological time. 

 And — to my mind — in doing so we proceed upon what 

 is no more than a necessary deduction based on our 

 knowledge of the radio-activity of terrestrial materials. 

 I would go further — still, as I believe, logically, — and 

 ascribe to radio-active energy an influence on planetary 

 and stellar evolution much greater than has hitherto 

 been admitted. 



The only planet we can investigate at all closely is, 

 of course, our earth. And what do we find ? In its 

 surface materials there are sufficient of the radio- 

 active elements, as Lord Rayleigh first showed, to 

 account for the observed average temperature gradient 

 if the surface conditions extend a little way, about 

 19 kilometres, inwards. It is, for many reasons, in 

 the highest degree improbable that such a definitely 

 defined radio-active layer exists. Nor is it probable 

 that the earth's interior is free from radio-active sub- 

 stances. We find both uranium and thorium in 

 meteorites containing a large percentage of iron and 

 nickel, and, although they have not as yet been found 

 in meteoric iron, we know from the mean density of 

 the earth that its interior cannot be composed of 

 pure iron. It is probable that a considerable pro- 

 portion (some 40 per cent.) of siliceous materials are 

 intermingled : and when such exists in meteorites 

 invariably we find the radio-active elements. By 

 what conceivable activity was all the uranium and 

 thorium separated out and brought to the surface ? 



The view that radio-active elements exist in the 

 earth's interior is sometimes met by a formal denial that 

 the earth can be getting hotter within. Upon what 

 evidence is this denial based ? If the central core of 

 the earth for a radial distance of 2000 kilometres — say 

 — had risen in temperature by 1000° C. over geological 

 time — and upon a low assumption of the interior 

 radio-activity it might reach this temperature in 150 

 million years — would we be aware of the fact ? Would 

 the day be appreciably lengthened ? Would there be 

 any effect at all if the outer par ts were cooling due to 

 loss of primal heat ? We have further to consider 

 that only over the short period of historical time would 

 any observations be available. The denial is quite 

 baseless so far as my estimates go. 



Well, then, if oui earth is heating up within, is there 

 not an impending termination to our geological age .? 

 Kelvin showed how complete is the thermal isolation 

 of the earth's interior, and it is certain that interior 

 heat is not now escaping. The rise of temperature 

 within must, go on till the present epoch succumbs 

 to the accumulated energy. Then must ensue a period 

 of vulcanicity which will end life upon the globe, and 

 probably reverse the chemical work stored up by ages 

 of denudative and organic activity. The whole 

 sequence of events — rapid cooling by radiation, 

 restoration of the oceans and, possibly, re-birth of life 

 and of its evolutionary history — ^would begin all over 

 again. On this view the Age we have been studying 



