Avogadro's law (sometimes called Avogadro's principle) states that equal volumes of different gases at the same temperature and pressure contain equal numbers of molecules.Amedeo Avogadro (1776- 1856), an Italian physicist, formulated this law in 1811 as a direct consequence of Gay-Lussac's law which was artic ulated by the French chemist Joseph-Louis Gay-Lussac (1778-1850) in 1802.

Gay-Lussac's law states that gases combine in simple whole number ratios (all volumes measured at the same temperature and pressure). Avogadro's law explains Gay- Lussac's law without violating John Dalton's idea that atoms are indivisible. Avogadro's law also led to the concepts of Avogadro's number and the mole, and t o the ideal gas law. Avogadro's ideas were largely ignored for half a century until Stanislao Cannizarro (1826-1910) began to promote them in the 1850s.