Amedeo Avogadro

Lorenzo Romano Amedeo Carlo Avogadro was born in Turin in 1776 into a family of distinguished lawyers, and he trained in law himself, earning a doctorate in ecclesiastical law at twenty. Drawn to mathematics and physics, he abandoned his legal career for science and in 1820 became the first professor of mathematical physics at the University of Turin.

In a paper of 1811 Avogadro put forward the hypothesis that bears his name: equal volumes of all gases, at the same temperature and pressure, contain equal numbers of molecules. To make it work he drew a careful distinction — radical for its time — between atoms and molecules, proposing that the elementary particles of common gases such as hydrogen and oxygen are themselves pairs of atoms. This neatly explained Gay-Lussac's law of combining volumes.

The hypothesis was almost entirely ignored for half a century. Avogadro worked in relative isolation in Turin, publishing in Italian journals of limited reach, and the leading chemists of the age were reluctant to accept his atom–molecule distinction. Only in 1860, at the Karlsruhe Congress — four years after Avogadro's death — did his compatriot Stanislao Cannizzaro show that Avogadro's idea resolved the era's confusion over atomic and molecular weights.

Avogadro lived quietly, holding various public offices in Piedmont and raising a large family. He never knew the scale of his vindication; the constant counting molecules in a mole was named for him only later, and first measured by Perrin in the twentieth century. He died in Turin in 1856.