One chemist's correct idea about gases was ignored for half a century

In 1811 Amedeo Avogadro proposed that equal volumes of gas, at the same temperature and pressure, contain equal numbers of molecules. Chemists mostly ignored it until 1860, when Stanislao Cannizzaro used it at a landmark conference to finally sort out a century of confused atomic weights.

— Amedeo Avogadro, Essai d'une manière de déterminer les masses relatives des molécules — Journal de Physique, 1811