PRESENT PROBLEMS OF RADIOACTIVITY 183 



years and 1 year respectively, occupy an intermediate position be- 

 tween the rapidly changing substances like radium A, B, and C and 

 the slowly changing parent substance radium. 



If the earth were supposed to have been initially composed of pure 

 radium, the activity 20,000 years later would not be greater than the 

 activity observed in pitchblende to-day. Since there is no doubt that 

 the earth is much older than this, in order to account for the existence 

 of radium at all in the earth, it is necessary to suppose that radium 

 is continuously produced from some other substance or substances. 

 On this view, the present supply of radium represents a condition of 

 approximate equilibrium where the rate of production of fresh radium 

 balances the rate of transformation of the radium already present. In 

 looking for a possible source of radium, it is natural to look to the sub- 

 stances which are always found associated with radium in pitchblende. 

 Uranium and thorium both fulfill the conditions necessary to be a 

 source of radium, for both are found associated with radium and both 

 have a rate of change slow compared with radium. At the present 

 time, uranium seems the most probable source of radium. The activ- 

 ity observed in a good specimen of pitchblende is about what is to 

 be expected, if uranium breaks up into radium. If uranium is the 

 parent of radium, it is to be expected that the amount of radium 

 present in different varieties of pitchblende obtained from different 

 sources should always be proportional to the amount of uranium con- 

 tained in the minerals. The recent experiments of Boltwood, Strutt, 

 and McCoy indicate that this is very approximately the case. It is 

 not to be expected that the relation should, in all cases, be very exact, 

 since it is not improbable, in some cases, that a portion of the active 

 material may be removed from the mineral, by the action of perco- 

 lating water or other chemical agencies. The results so far obtained 

 strongly support the view that radium is a product of the disinte- 

 gration of uranium. It should be possible to obtain direct evidence 

 on this question by examining whether radium appears in uranium 

 compounds which have been initially freed from radium. On account 

 of the delicacy of the electric test of radium by means of its emanation, 

 the question can very readily be put to experimental trial. This has 

 been done for uranium by Soddy and for thorium by the writer, but 

 the results, so far obtained, are negative in character, although, if 

 radium were produced at the rate to be expected from theory, it 

 should very readily have been detected. Such experiments, however, 

 taken over a period of a few months, are not decisive, for it is by no 

 means improbable that the parent element may pass through several 

 slow changes, possibly of a "rayless" character, before it is trans- 

 formed into radium. In such a case, if these intermediate products 

 are removed by the same chemical process from the parent element, 

 there may be a long period of apparent retardation before the radium 



