From 1904 to 1914 Soddy was lecturer in physical chemistry and radioactivity in the University of Glasgow. Here he did much practical chemical work on radioactive materials. During this period he evolved the so-called “Displacement Law”, namely that emission of an alpha-particle from an element causes that element to move back two places in the Periodic Table. His peak was reached in 1913 with his formulation of the concept of isotopes, which stated that certain elements exist in two or more forms which have different atomic weights but which are indistinguishable chemically.