Abstract

The philosophical significance of Dmitri Mendeleev’s successful predictions of the properties of gallium and scandium vis a vis the acceptance of the Periodic Table 1874–1886 has been debated recently. This author presents evidence that De Boisbaudran and Cleve both respectively predicted the possible existence of gallium and scandium, but on the basis of the old TRIAD methodology. This suggests that these successful Mendeleev predictions were therefore not independent corroboration of the concept of the Periodic System. Instead the significantly independent predictive successes for Mendeleev’s system were the determination of the atomic weight of the known element uranium as 240 instead of the previously accepted 120 in 1874 and the isolation of germanium by Winkler in 1886.