August 9, 19 17] 



NATURE 



this first ring have consistently shown a small deficit. 

 Small as it is, we should not ignore it. For there is 

 some reason to suspect that our knowledge of the 

 range of the a ray of thorium itself, which is largely 

 responsible for the position of the first maximum upon 

 the curve, is incomplete. The facts appear to show 

 that the accepted range of the ray from thorium is 

 too large. The evidence for this is both interesting 

 and important. 



Rutherford long ago pointed out that there appeared 

 to exist a connection between the range of an a ray 

 and the duration of life of the element from which it 

 originates. The speed of the a particle seemed to be 

 greater the shorter the period of transformation. 

 Geiger and Nutall re-investigated the accepted ranges 

 of the a rays of the radio-active elements, and estab- 

 lished Rutherford's inference. Plotting the logarithms 

 of the range and of the period of transformation 

 against each other, they ascertained that for each 

 family of elements there rs a straight line along which 

 the points found for the several a rays lie, and— in 

 nearly every case — lie with astonishing accuracy. 

 There is only one notable discrepancy-. That, excep- 

 tion is in the case of tHe range of the a ray from 

 thorium itself. It is a few f>er cent, too great accord- 

 ing to the observ-ations. It is also, admittedly, the 

 most difficult to measure with accuracy. 



Translated into the distances obtaining in the halo, 

 the few per cent, are almost beyond the limits of accu- 

 racy which may be fairly claimed. But the evidence 

 for the slight misfit is based on many obser\'ations and 

 may be significant. 



In the case of the uranium halo there is also a dis- 

 crepancy between the curve arid the observations as 

 regards the position of the first ring ; but the mag- 

 nitude of the discrepancy is more considerable than 

 the misfit referred to above in the case of the thorium 

 halo. And here we have no reason to throw the blame 

 on any error in the accepted value of the ranges of 

 the rays of U, and U,. The curve of ionisation due 

 to the a rays of these chemically inseparable elements 

 has been investigated by Geiger and Nutall. The 

 results obtained are explained on the assumption that 

 U, has a range of 2-q cm. and U. a range of 2q cm. 

 in air. And these determinations accurately fit the 

 logarithmic curve. The position of the maximum on 

 the halo-ionisation curve is mainly determined by these 

 results. 



Careful measurements of the first ring of the 

 uranium halo reveal this small but definite discordance 

 between the radius of the ring and the position of the 

 maximum of the curve. It. will be seen that the sec- 

 tion of the ring — the feature numbered i in Fig. 3 — 

 does not lie accurately above the centre of the maxi- 

 mum. The ring has a radius which is distinctly too 

 great. That the ring essentially corresponds with the 

 first great maximum of the curve seems beyond doubt. 

 We find no other record of this maximum. There 

 seems no apparent escape from the conclusion that the 

 ring which is so largely due to the rays from U, and 

 U, has been formed by rays of greater range than the 

 average range of the rays now emitted by these 

 elements. 



The granite in which this halo-ring has been 

 measured is ver\- ancient, certainly not younger than 

 the Devonian period. Similar rings, but not so sharp 

 and easily measured, have been found in the Carbon- 

 iferous granite of Cornwall. In younger granites I 

 have not succeeded in finding them. It would be 

 important to measure this ring in the younger 

 granites, supposing they have been formed in these 

 rocks. Such measurements would make quite clear 

 whether or not the abnormal dimension of this first 

 ring is really due to the former existence of a longer 

 NO. 2493, VOL. 99] 



average range of the rays responsible. If the misfit 

 of the first ring proves to be inexplicable in any 

 deficiency of our knowledge of the ranges of the 

 uranium isotopes, and especially if we are able to 

 get evidence that it is confined to the more ancient 

 rocks, then it will be difficult to escape the direct 

 conclusion that, however brought about, there was a 

 former greater range of the o ray of the parent 

 element of the uranium family. 



There is a certain temptation to accept such a con- 

 clusion, for there is a strange contradiction in the 

 evidence advanced for the duration of geological time. 

 The conclusion that the halo reveals a former greater 

 range for the a ray from Ui carries with it the former 

 more rapid decay of that element.- All the difficulties 

 and contradictions respectmg the age vanish if this 

 indeed occurred. It will only require a few words to 

 state the present position of the matter. 



From measurements of the rates of denudative pro- 

 cesses at the earth's surface, and of the quantities 

 accumulated, the evidence is, with wonderful consist- 

 ency, in favour of a period of about 100 millions of 

 years having elapsed since those processes came into 

 existence. By makirtg certain assumptions some 

 150 millions of years might be claimed, and even, 

 not inconceivably, somewhat more. What other 

 evidence have we? The only major limit which 

 astronomy appears to give us would be in favour of 

 an age even less than 100 millions of years. I refer 

 to the duration of solar heat. It is quite certain that 

 the earth was bathed in abundant sunshine even in 

 Cambrian times ; but solar heat of the present intensity 

 cannot be accounted for on any known source of 

 supply for 100 millions of years. From lunar theory 

 we do not seem able to get a major limit. We must 

 remember that we are not discussing the age of the 

 earth as an astronomical unit. The geological age is 

 the period of denudation only. Well, then, a genera- 

 tion ago verv brilliant work was done by KeKnn 

 on the period since the solidification of the surface 

 rocks. But the thermal data involved became invali- 

 dated in the light of Strutt's discoven,- that heat- 

 producing radio-active elements exist all over the 

 earth's crust. 



But if radio-active science in this wav has close<? 

 one avenue of approach to the age problem, it has 

 opened up another. Rutherford pointed out that the 

 accumqlation of radio-active products of decay in 

 ancient rocks and minerals should afford a measure 

 of the age in much the same manner as, from the 

 amount of sand which has fallen through, we compute 

 time bv the hour-glass. In this connection Strutt's work 

 on the amount of helium accumulated in materials of 

 various geological ages will ever be memorable. The 

 amount of accumulated lead, however, possesses, in 

 some respects, less liability to error. The measure- 

 ment of the ratio of the quantity* of lead to the quan- 

 tity of parent radio-active element in the case of 

 uranium has occupied the attention of several investi- 

 gators. The conclusion as regards the accumulation 

 of lead in uranium-bearing minerals seems to be — 

 although not without conflicting evidence — that the 

 earth's geological age is not less than some 1500 

 millions of vears. 



Now. wliTle w-e must admit the possibility' of con- 

 siderable variations in the rate of denudation over the 

 past, vet the statement that the rivers are now pouring 

 some ten times as much dissolved matter into, and 

 transmitting some ten times as much sediment to, 

 the ocean as they did in past times is, I think, 

 quite inadmissible. All efforts to explain so extra- 

 ordinary an increase — w-hether we suppose it to be tem- 

 porary or permanent — ^have so far failed. 



But the uranium series of radio-active elements is 



