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 to 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.