Amedeo Avogadro (1776-1856), Italian physicist and chemist who proposed a hypothesis that later became known as Avogadro's law. Avogadro was born in Turin and educated as a lawyer. He became interested in mathematics and physics and, after several years of study, was appointed to a professorship at the Royal College of Vercelli. From 1820 until his death, Avogadro was professor of physics at the University of Turin. Although he also conducted research on electricity and the physical properties of liquids, he is best known for his work with gases, which led him to formulate in 1811 the law that now bears his name.

Avogadro's law states that two equal volumes of gas at the same temperature and pressure contain an equal number of gas molecules. Now known to be true, this law was not universally accepted until the 1850s. See also Avogadro's Number.