Helium and radio-activity in rare and common minerals

The original discovery of helium in cleveite and other minerals by Ramsay, and the subsequent explanation of its presence as due to productionin situ by radio-active change, are of fundamental importance, and are too well known to need anything more than mention here. No exception has been known hitherto to the rule that helium is found in radio-active minerals exclusively—minerals, that is, in which either uranium, or thorium, or both, are present in appreciable quantity. It seemed, however, that valuable information might be gleaned from a fuller examination of the subject than has yet been made.