September io, 1908] 



NATURE 



457 



extracted from well waters. Radium was found in brick- 

 earths, and everywhere in roclcs containing the least trace 

 of demonstrable uranium, and Rutherford calculated that a 

 quantity of radium so minute as 4'6xio-" grams per 

 gram of the earth's mass would compensate for all the heal 

 now passing out through its surface as determined by the 

 average temperature gradients. In iqo6 the Hon. R. J. 

 Strutt, to whom geology owes so much, not only here bur 

 in other lines of advance, was able to announce, from a 

 systematic examination of rocks and minerals from various 

 parts of the world, that the average quantity of radium per 

 gram was many times in excess of what Rutherford esti- 

 mated as adequate to account for terrestrial heat-loss. The 

 only inference possible was that the surface radium was not 

 an indication of what was distributed throughout the mass 

 of the earth, and, as you all know, Strutt suggested a world 

 deriving its internal temperature from a radium jackel 

 some 45 miles in thickness, the interior being free from 

 radium.' 



My own experimental work, begun in 1904, was laid 

 aside until after Mr. Strutt's paper had appeared, and a 

 valued correspondence with its distinguished author was 

 permitted to me. This address will be concerned with the 

 application of my results to questions of geological 

 dynamics. ^ 



Did time permit I would, indeed, like to dwell for a 

 liltle on the practical aspect of measurements as yet so 

 little used or understood ; for the difficulties to be overcome 

 ■ ire considerable, and the precautions to be taken many. 

 The quantities dealt with arc astoundingly minute, and to 

 I'Xtract with completeness a total of a few billionths of a 

 cubic millimetre of the radio-active gas — the emanation — 

 from perhaps half a litre or more of a solution rich in dis- 

 solved substances cannot be regarded as an operation 

 exempt from possibility of error ; and errors of deficiency 

 are accordingly frequently met with. 



Special difficulties, too, arise when dealing with certain 

 classes of rocks. For in some rocks the radium is not 

 uniformly diffused, but is concentrated in radio-active sub- 

 stances. We are in these cases assailed with all the 

 troubles which beset the assayer of gold who is at a loss 

 to determine the average yield of a rock wherein the ore is 

 sporadically distributed. In the case of radium determin- 

 ations this difficulty may be so much the more intensified 

 as the isolated quantities involved are the more minute and 

 yet the more potent to affect the result of any one experi- 

 ment. There is here a source of discrepancy in successive 

 experiments upon those rocks in which, from metamorphic 

 or other actions, a segregation of the uranium has taken 

 place. With such rocks the divergences between successive 

 results are often considerable, and only by multiplying the 

 number of experiments can we hope to obtain fair indica- 

 tions of the average radio-activity. It is noteworthy that 

 these variations do not, so far as my observations extend, 

 present themselves when we deal with a recent marine sedi- 

 ment or with certain unaltered deposits wherein there has 

 been no readjustment of the original fine state of sub- 

 division, and even distribution, which attended the precipi- 

 tation of the uranium in the process of sedimentation. 



But the diflicuUies attending the estimation of radium in 

 rocks and other materials leave still a large balance of cer- 

 tainty — so far as the word is allowable when applied to the 

 ever-widening views of science — upon which to base our 

 deductions. The emanation of radium is most character- 

 istic in behaviour ; knowledge of its peculiarities enables us 

 to distinguish its presence in the electroscope not only from 

 the emanation of other radio-active elements, but from any 

 accidental leakage or inductive disturbance of the instru 

 ment. The method of measurement is purelv comparative. 

 The cardinal facts upon the strength of which we associate 

 radium \vith geological dynamics. Its development of heat 

 and its association with uranium, are founded in the first 

 case directly on observation, and, In the second, on evidence 

 so strong as to be equally convincing. Recent work on 

 the question of the influence of conditions of extreme pres- 

 sures and temperatures on the radio-active properties of 

 radium appear to show that, as would be anticipated, the 

 effect Is small, if indeed existent. As observed- by Makower 

 and Rutherford, the small diminution noticed under very 

 1 Pror. R.S., l.xxvii , p. 472, and lx.\v.ii., p. 150. 



NO. 2028, VOL. 78] 



c'xlreme conditions in the 7 ladiatlon possibly admits of 

 explanation on indirect effects. These observations appear 

 to leave us a free hand as regards radio-thermal effects 

 unless when we pursue speculations into the remoter 

 depths of the earth, and even there while they remain as a 

 reservation, they by no means forbid us to go on. 



The precise quantity of heat to which radium gives rise, 

 or, rather, which its presence entails, cannot be said to be 

 known to within a small percentage, for the thermal 

 equivalent of the radio-active energy of uranium, actinium, 

 and Ionium, and of those members of the radium family 

 which are slow in changing, has not been measured 

 directly. Prof. Rutherford has supplied me, however, with 

 the calculated amount of the aggregate heat energy 

 liberated per second by all these bodies. In the applications 

 to which I shall presently have to refer I take his estimate 

 of 5'6xio-" calories per second as the constant of heat- 

 production attending the presence of one gram of 

 elemental radium. 



To these words of introduction I have to add the re- 

 mark, perhaps obvious, that the full and ultimate analysis 

 of the many geological questions arising out of the 

 presence of radium in the earth's surface materials will 

 require to be founded upon a broader basis than is afforded 

 by even a few hundred experiments. The whole sequence 

 of sediments has to be systematically examined ; the 

 various classes of Igneous materials, more especially the 

 successive ejecta of volcanoes, fully investigated. The 

 conditions of entry of uranium Into the oceanic deposits 

 have to be studied, and observations on sea-water and 

 deep-sea sediments multiplied. All this work Is for the 

 future; as yet but little has been accomplished. 



The Radium in the Rocks atid in the Ocean. 



The fact first established by Strutt that the radium 

 distributed through the rock materials of the earth's 

 surface greatly exceeds any permissible estimate of its 

 internal radio-activity has not as yet received any explana- 

 tion. It might Indeed be truly said that the concentra- 

 tion of the heaviest element known to us (uranium) at 

 the surface of the earth is just what we should not have 

 expected. Yet a simple enough explanation may be at 

 hand in the heat-producing capacity of that substance. 

 If it was originally scattered through the earth-stuff, not 

 in a uniform distribution, but to some extent concentrated 

 fortuitously In a manner depending on the origin of 

 terrestrial ingredients, then these radio-active nuclei heat- 

 ing and expanding beyond the capacity of surrounding 

 materials would rise to the surface of a world in which 

 convective actions were still possible and, very conceiv- 

 ably, even after such conditions had ceased to be general ; 

 and in this way the surface materials would become richer 

 than the Interior. For Instance, the extruded mass of 

 the Deccan basalt would fill a sphere 36 miles in radius. 

 Imagine such a sphere located originally somewhere deep 

 beneath the surface of the earth surrounded by materials 

 of like density. The ultimate excess of temperature, due 

 to its uranium, attained at the central parts would amount 

 to about 1000° C, or such lesser temperature as con- 

 vective effects within the mass would permit. This might 

 take some thirty million years to come about, but before 

 so great an excess of temperature was reached the force 

 of buoyancy developed in virtue of Its thermal expansion 

 must inevitably bring the entire mass to the surface. 

 This reasoning would, at any rate, apply to material 

 situated at a considerable distance Inwards, and may 

 possibly be connected with vulcanlcity and other crustal 

 disturbances observed at the surface.' The other view, 

 that the addition of uranium to the earth was mainly an 

 event subsequent to Its formation in bulk, so that radio- 

 active substances were added from without and, possibly, 

 from a solar or cosmic source, has not the same d priori 

 probabllitv in Its favour.^ 



I have In this part of my address briefly to place before 

 you an account of my experiments on the amounts of 

 radium distributed in surface materials. Here, indeed, 

 direct knowledge is attainable ; but this knowledge takes - 

 us but a very few miles inwards towards the centre of 

 the earth. 



■ J See .Appendix A, - Nature, Ixxv., p. 294. 



