Page:Popular Science Monthly Volume 67.djvu/37

Rh to be expected that the rate of change of radium will be always proportional 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 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 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