It may seem astonishing that after radioactive substances have been known for over half a century there should still remain important unsolved problems in the domain of natural radioactivity. This field was the first of the great subdivisions of nuclear physics to be explored, and yet much remains to be done here. The reason for this situation becomes evident when we recall that radium is expensive, and that until comparatively recently radioactive substances were not available in any large amounts. The second contributing factor is the great improvement in recent years in instrumentation, an improvement which has rendered many of the classical techniques obsolete.