A term introduced to explain the apparent existence of very nearly allied but nonseparable forms of chemical elements, the general chemical properties being identical, but exhibiting variation in their atomic weights, or mass, or molecular arrangement. Thus it is supposed that magnesium, silicon, and chlorine, in addition to neon, are mixtures of isotopes; further, that nickel, copper, zinc, mercury, and other of the elements, are really mixtures of isotopes. Ionium and thorium are said to be isotopes. Many of the conclusions of the investigators of these matters are based upon radioactive evidence, and others upon the separation of gases by diffusion, but much remains to be done before the subject and the electron constitution or matter can be regarded as satisfactorily established.

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Definition of  ISOTOPES