He was professor of mathematical physics at the University of Turin 1820–3 and 1833–50. In 1811 he proposed that, at the same temperature and pressure, equal volumes of all gases contain equal numbers of molecules. This is now known as Avogadro’s law and it follows from it that one mole of any substance contains a constant number of particles, usually atoms or molecules. Thus the mass of one mole of a substance can be related to that of a standard sample, later defined as the number of atoms in 12 grams of carbon-12: 6.022140857(74) × 10²³. This is known as the Avogadro constant.