110 



DISCOVERY 



atomic weight of about 208, but unfortunately its 

 genesis and accumulation are not yet as well under- 

 stood as in the case of uranium-lead. 



With this brief introduction to the nature and 

 results of radio-active change, we may proceed to con- 

 sider their bearing on the measurement of geological 

 time. The helium atoms discharged as a-rays are 

 explosively emitted with very high velocities, in some 

 atoms even approaching that of hght. The atomic 

 energy thus liberated is transferred to the molecules 

 encountered by the helium projectile in its course, and 

 appears as heat. In 1903 it was discovered that 

 radium is constantly giving out heat ; and evaluating 

 all the contributions from its associated elements, we 

 now know that the total generation of heat from the 

 family represented by i gram of uranium amounts in 

 a year to o"7 calories. Similarly, from i gram of 

 thorium in equilibrium the heat output amounts to 

 o'2 calories per annum. These amounts maj^ appear 

 to be trivial, but they suffice to provide the earth with 

 such an embarrassingly large quantity of heat that it 

 becomes necessary seriously to consider whv the earth 

 is not growing hotter. A simple calculation shows 

 that a distribution of uranium throughout the earth 

 amounting to only 5 parts in 100 million parts of rock 

 would suffice to balance the loss of heat by radiation. 

 The Hon. R. J. Strutt, now Lord Rayleigh, devised 

 an accurate method for determining the uranium con- 

 tent of rocks, and his results and those of others show 

 that the rocks contain 120 times more than is required. 

 When to this the heat effect of thorium is added, it is 

 found that, if the radio-active elements were uniformly 

 distributed, then the amount of heat generated within 

 the earth would be 300 times greater than the amount 

 required to maintain a state of thermal balance. Since 

 it is impossible to believe that the earth has been 

 growing hotter at such a rate, it follows either that 

 radio-active substances cease to give out energy in the 

 deep interior of the earth, or else that they are restricted 

 in their occurrence to the crustal rocks. Both hypo- 

 theses have their adherents ; but whereas there is no 

 experimental support for the former, there is a con- 

 cordant body of evidence favouring the view that 

 the deeper zones of the earth arc free or practically so 

 from these surprisingly energetic elements. The 

 nature of this evidence need not here be discussed. It 

 is sufficient to realise that the necessity of either 

 hypothesis indicates how completely the foundations 

 of the cooling argument have been destroyed. While 

 it still appears probable that the earth's surface was 

 originally in a state of aqueo-igneous fusion, it is certain 

 that the cooling-down process has been extremely slow. 

 The following figures show how the " age " is increased 

 according to the proportion of radio-active heat ad- 

 mitted. 



Proportion of heal lost by 

 mdiatioa attributed to 

 radiotbcnnal cnerKy. 



i or 25 per cent, 



i .. 50 .. .. 



3 ,. 67 „ „ 



i „ 75 .. .. 



f. ,. 80 .. 



J'ctitxi of cooling required 

 to rench the present 

 tempcTuturc gradient. 



20 million years 

 40 .. 

 120 

 580 

 1,600 ,, ,, 



7,200 „ ,, 



The discovery of radio-activity not only revealed with 

 dramatic suddenness the unjustified restrictions which 

 had been placed on geologists, but it led directly to the 

 elaboration of the most elegant and refined method of 

 measuring geological intervals of time that has yet 

 been devised. Every fresh uranium-bearing mineral 

 is now regarded as a natural chronometer, registering 

 time by the atoms of helium and lead that are produced 

 unceasingly within it year after year. 



In 1910 Strutt directly measured the rate of formation 

 of helium in the minerals uraninite and thorianite. 

 He showed that i gram of uranium in equilibrium with 

 aU the members of its family generates i c.c. in 9 million 

 years, and that for thorium the corresponding rate is 

 I c.c. in 32 million years. In the case of the thorianite 

 Strutt found that it originally contained 280 million 

 times the amount of helium that could be generated 

 in a single year, and therefore that the time required 

 for its accumulation must also have been 280 million 

 years. Before asserting that this figure is also the age 

 of the mineral, three questions must be asked and 

 answered : 



(a) Was there any helium present in the mineral at 

 the time of its crystallisation ? 



{b) Has the rate of production of heUum been 

 uniform from year to year ? 



(c) Has any helium escaped from the mineral during 

 the period that has elapsed since its cr^'stallisation. 



The first question is answered by the fact that 

 ordinary rocks and minerals contain only the slightest 

 traces of helium, and that, if a radio-active mineral did 

 contain a little of the gas as an original impurity, its 

 amount would soon become quite negligible compared 

 with the large amounts actually generated. 



The second query presents little difficulty. No 

 physical or chemical conditions that can be imposed on 

 radio-active substances affect their behaviour in the 

 sUghtest degree, and therefore the only variation in 

 rate that can reasonably be considered is that due to 

 the slow decay of the parent element, uranium. Since 

 out of 7,000 million atoms of uranium only one dis- 

 integrates per year, it is clear that the correction to be 

 applied is very slight. 



Finally wc must consider the possibility of leakage. 

 It has been experimentally proved that as soon as a 

 radio-active mineral is exposed to the air it begins to 

 lose helium. On grinding for anal\'sis more is lost, 



