PRESENT PROBLEMS IN RADIOACTIVITY. 31 



to be expected that the rate of change of radium will be always pro- 

 portional to the amount present. The amount of radium would thus 

 decrease exponentially with the time falling to half value in about 

 1,000 years. On this view, radium behaves in a similar way to the 

 other known products, the only difference being that its rate of change 

 is slower. We have already seen that, in all probability, the product 

 radium D is half transformed in about 40 years and radium E in about 

 one year. In regard to their rate of change, the two substances radium 

 D and E, which are half transformed in about 40 years and one year 

 respectively occupy an intermediate position between 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 sub- 

 stances. 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 substances 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 activity 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 will always be proportional to the amount of uranium 

 contained in the minerals. The recent experiments of Boltwood, 

 Strutt and McKoy indicate that this is very approximately the case. 

 It is not to be expected that the relation will always 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 

 percolating water or other chemical agencies. The results so far 

 obtained strongly support the view that radium is a product of the 

 disintegration 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 be very readily put to experimental 

 trial. This has been done for uranium by Soddy, and for thorium by 



