Avogadro's big idea was ignored until four years after he died
In 1811 a provincial physics teacher proposed that equal volumes of any gas, at the same temperature and pressure, hold equal numbers of molecules. Almost nobody paid attention. Only at the Karlsruhe Congress of 1860, four years after Amedeo Avogadro's death, did Stanislao Cannizzaro show that the idea explained the puzzling exceptions.
Avogadro was born in Turin in 1776 into a noble family of the Kingdom of Sardinia; his father was a lawyer and magistrate. He dutifully qualified in church law at 20 and practised it, but drifted toward physics and mathematics, then grouped together as positive philosophy. In 1809 he began teaching at a secondary school in Vercelli, where his family owned property, and it was from there that he sent his famous essay to a Paris journal.
The paper built on Joseph Louis Gay-Lussac's 1808 finding that gases combine in simple ratios by volume. Avogadro's key step was untangling a muddle of the day: he separated the particles that make up a gas, molecules, from the smaller units within them, what we call atoms. John Dalton had not considered that possibility. Avogadro himself avoided the word atom, speaking of elementary molecules instead, and took care to distinguish mass from weight. André-Marie Ampère proposed something very similar three years later and met the same indifference.
Politics interrupted his career. Appointed professor of physics at Turin in 1820, he joined the revolutionary movement of March 1821 and lost his chair in 1823; the university politely claimed it was giving him a rest from heavy teaching. Recalled in 1833, he taught for another twenty years. He introduced the metric system to Piedmont, worked on statistics and meteorology, and with his wife Felicita Mazzé raised six children in what seems a sober, religious household.
Vindication came slowly, through organic chemistry, Cannizzaro's work, and later Rudolf Clausius's kinetic theory of gases. Johann Josef Loschmidt first estimated how many particles make up one mole, a figure now named the Avogadro constant and fixed at exactly 6.02214076 times ten to the 23rd power. It is one of the seven constants that define the SI system of units, and a crater on the Moon's far side bears his name.
Source: Amedeo Avogadro
