Isotopes—elements having the same atomic number, the same chemical properties, but different atomic weights—were discovered early in the present century. In 1906, Boltwood of Yale isolated an element, ionium, from pitchblende. It was exactly like thorium except in atomic weight. Three kinds of lead, each with an atomic weight of its own, were found. J. J. Thomson found two kinds (weights) of neon. Frederick Soddy and Kasimir Fajans (working independently, be it noted) advanced an hypothesis by which these different forms might be explained. Thomson and F. W. Aston in England and K. T. Bainbridge in the United States examined the whole series of elements in a search for isotopes. They found that many elements have isotopes. The atomic weight of hydrogen— 1.00778 instead of 1.0—indicated the existence of an isotope of this element but neither Aston nor Bainbridge could isolate it with the mass spectrometer. Harold C. Urey thought that separation might be effected by evaporation of liquid hydrogen. It was assumed that the light isotope would evaporate more freely, leaving a concentration of the heavy form in the residue. (White 1949:213)
WHITE, Leslie. 1949. The science of culture: a study of man and civilization. New York: Grove Press.