28o 



NATURE 



[October 27, 1921 



mination of the amount of radium in the super- 

 ficial parts of the earth which are alone accessible. 

 From radium analysis we can calculate the 

 amount of uranium and other associated sub- 

 stances and the thermal output from them, and 

 the result is to show that if we suppose the same 

 radium content to extend to a depth of some 

 20 miles, the whole output of heat would be 

 accounted for without assuming- that any of it 

 comes from the store of primeval heat as postu- 

 lated by Lord Kelvin. It is without doubt difficult 

 to understand why the output of heat is not 

 greater, for it would certainly be expected that the 

 rocky crust of the earth would be more than 

 20 miles thick, to say nothing- of any radium 

 there might be in the unknown interior. 



Can we at present infer anything definite from 

 the earth's internal heat as to the possible dura- 

 tion of g-eological time? I think practically not. 

 It appears certain that the radio-active materials 

 present in the earth are generating at least as 

 much heat as is now leaking out from the earth 

 into space. If they are generating more than this 

 (and there is evidence to suggest that they are), 

 the temperature must, according to all received 

 views, be rising. In a word, we are puzzled to 

 explain the existing state of things, and cannot 

 use it as a firm basis from which to explore the 

 past. 



Next, as to the sun's heat. Lord Kelvin's argu- 

 ment was that we knew of no possible source at 

 all adequate to supply the existing output of solar 

 energy except secular contraction, and even this 

 source of supply was not enough to account for 

 more than twenty million years of solar heat in the 

 past. It is impossible to condemn on principle 

 arguments of this kind. We often must, and do, 

 rely on them in science as in everyday affairs ; 

 but a certain reserve is always needed on the 

 ground that there are more things in heaven and 

 earth than are dreamt of in our philosophy. 

 Knowledge which has accumulated since Lord 

 Kelvin's time has driven us back on this alterna- 

 tive. 



The sun is only one of the host of stars, and 

 if we find it impossible to account adequately for 

 their radiation by contraction it evidently will not 

 do to assume that the sun is limited to this source 

 of supply. 



Now some of the stars (the giant red stars), 

 though of about the same mass as the sun, are 

 radiating energy at something like one thousand 

 times the rate that the sun does. They ought, 

 according to the contraction theory, to have ex- 

 pended a considerable fraction of their total 

 energy in historical times. No one will maintain 

 that this has occurred, and if not there must be 

 some source of supply other than contraction. It 

 is not necessary for our immediate purpose to 

 inquire what this source is. It is enough to note 

 that its existence invalidates Lord Kelvin's esti- 

 mate of the age of the sun's heat. 



Modern knowledge in radio-activity has given 

 what appears, if separately considered, to be a 

 firm and satisfactorv bas-'; for the estimation of 

 NO. 2713, VOL. 108] 



' geological time. Uranium, for example, goes 

 through a series of changes (radium is one of the 

 stages in its progress), changing eventually into 

 an isotope of lead — that is, an element chemically 

 indistinguishable from lead, except by a slight 

 difference of atomic weight and (practically at 

 least) inseparable from ordinary lead by chemical 

 means if once mixed with it. The isotope of 

 lead in question has probably an atomic weight 

 of 206 exactly, as contrasted with an atomic 

 weight of 207-1 for ordinary lead.^ This is much 

 less than the atomic weight of uranium (2385), 

 and the difference represents approximately the 

 weight of helium atoms, which are the debris shed 

 at the various stages of the transformation. 



Further, it is well established that a gram 

 of uranium as found along with its products in 

 rocks and minerals is now changing at a rate 

 represented by the production of i-SSxio-^^ 

 grams of helium and 1-22 x 10"^° grams of 

 lead isotope per annum. We have not time this 

 morning to consider the methods by which these 

 figures have been reached. It must suffice to say 

 that in the case of helium it amounts practically 

 to direct observation, while in the case of lead 

 isotope the evidence, though less direct, is very 

 strong, and, so far as I am aware, is not con- 

 tested by any student of the subject. I have said 

 that this is the rate at which one gram of uranium 

 as found in the earth is producing helium and 

 lead isotope at present. It is important to inquire 

 whether one gram of uranium did the same in 

 the past. This we cannot, of course, determine 

 directly. It is certain that nothing we can do 

 in a laboratory in the way of change of tempera- 

 ture and pressure can alter the rate sensibly, and 

 enough has been done in this way to make it 

 unlikely that any pressures and temperatures en- 

 countered in the superficial parts of the earth 

 could have such an effect. It has been suggested 

 by Prof. Joly that the absolute age of a gram 

 of uranium may affect its rate of disintegration. 

 All possibilities should be considered, but this 

 suggestion derives no support from the behaviour 

 of the shorter-lived radio-active substances the 

 behaviour of which we can watch. 



Upon the whole, therefore, it would seem that 

 in the disintegration of a gram of uranium we 

 have a process the rate of which can be relied 

 upon to have been the same in the past as we 

 now observe it to be. 



The application is either to individual uranium 

 minerals or to the earth's crust as a whole. 

 Taking first the minerals containing uranium, 

 these are found in all cases to contain helium and 

 lead. The helium in them, which appears to be 

 retained mechanically, may safely be treated as 

 wholly a radio-active product. The lead in some 

 cases"^ conforms closely to the expected atomic 

 weight of 206, about one unit lower than common 

 lead, and in such cases we may safely regard the 

 whole of it as a product of uranium disintegra- 

 tion. 



* Ordinary lead may partly consist of it, but this i-i not yet certain, and 

 not very important for the immediate purpose. 



