Avogadro’s number — 6.022 times 10 to the 23rd — is one of the most fundamental constants in chemistry and physics. But Amedeo Avogadro himself never calculated it, never heard of it, and spent most of his career as a provincial Italian lawyer and part-time physics professor whose hypothesis was ignored for fifty years. The number that bears his name was calculated by other scientists long after his death, and attached to him as an honor he could not have imagined.

This episode traces Avogadro from his legal career through the molecular hypothesis that nobody believed, the fifty years of scientific neglect, and the posthumous recognition that turned an obscure Italian professor into a household name in every chemistry classroom on earth.

• Avogadro’s legal training and his unlikely pivot to physics in early nineteenth-century Italy
• The molecular hypothesis — that equal volumes of gases contain equal numbers of molecules
• Fifty years of scientific rejection and the Karlsruhe Congress that finally vindicated his idea
• The posthumous calculation of “Avogadro’s number” and the irony of his accidental immortality