Avogadro’s Law, fundamental law of chemistry stating that under identical conditions of temperature and pressure, equal volumes of gases contain an equal number of molecules. The law was first proposed as a hypothesis by the Italian physicist Amedeo Avogadro in 1811. Italian chemists and physicists continued to develop this hypothesis, and in the 1850s, largely through the efforts of the Italian chemist Stanislao Cannizzaro, Avogadro's law was universally accepted.