PHYSICAL CONSIDERATIONS A short time after radium was discovered, it was found that objects in the neighborhood of open radium preparations acquired a radioactivity of their own. This activity was very short lived if the object was removed from the vicinity of the radium but could be renewed at will by bringing it back. This mysterious phenomenon is readily explained when the sequence of radioactive changes is known. Radium itself is a metal, belonging to the same chemical group as barium and calcium. The nucleus of the atom is, however, unstable, and at some (entirely unpredictable) time it suddenly ejects a portion of itself. This portion, which is called an alpha ray or alpha particle, is exactly like the nucleus of a helium atom. In any given quantity of radium, every instant a certain percentage of all the atoms present undergo this transformation, the rest remaining completely unchanged. The apparent