The term isotopes was coined by Soddy to describe two or more chemical elements of different atomic weights but with identical chemical properties. For the most part, these isotopic elements are also physically identical, except only with regard to the relatively few physical properties that depend on atomic mass directly. The development of knowledge of isotopes represents much of interest not only to the chemist and the physicist but to the investigators in all branches of science. About 253 stable isotopes are known,1 and only a few elements remain without definite data to confirm the existence of all elements in isotopic forms. It is now clear, for example, that chlorine consists of two isotopes of atomic weight 35 and 37, mixed in such proportions as to give the usual atomic weight of 35.46. Some elements may have six isotopes (krypton), while other elements contain even more (xenon and tin).