The Periodic Table's First Confirmation

Boisbaudran's gallium announcement became historically significant for a reason he had not anticipated: four years earlier, in 1871, Dmitri Mendeleev had predicted from a gap in his periodic table that an undiscovered element, which he called eka-aluminium, must exist with a specific set of properties, including an approximate atomic weight and density. When Boisbaudran's measurements of gallium's density came in lower than Mendeleev had predicted, Mendeleev wrote to him directly, asserting from theory alone that the measurement must be in error; Boisbaudran remeasured his sample after further purification and found a corrected density matching Mendeleev's prediction almost exactly.

That sequence, a theoretical prediction confirmed and even corrected against a fresh experimental measurement, was received by contemporaries as something close to a scientific prophecy fulfilled, and it did more than any other single event to establish the periodic law as a genuine predictive framework rather than a convenient filing system for known elements. The atlas records this specific episode, and the language of prophecy and confirmation contemporaries used to describe it, as the documented historical event it is, without asserting any personal esoteric belief on Boisbaudran's own part, for which no source provides evidence.