In the heart of the historic centre of Vercelli, at Via Manzoni 11, the former Royal College is today a building of great architectural and cultural value. But for the international scientific community this place represents much more: it is the physical space in which Amedeo Avogadro (1776–1856) taught and developed the ideas that would lay the foundations of modern chemistry. Between 1809 and 1819, in these classrooms, Avogadro taught courses in physics and mathematics – then grouped under the denomination of “positive philosophy” – and developed the famous hypothesis on gases that bears his name, from which derive the concept of the mole and the so-called “Avogadro’s Number”.