Avogadro's Law proposed by Amedeo Avogadro

States that under equal conditions of temperature and pressure, equal volumes of gases contain an equal number of molecules.

This hypothesis was not acknowledged in Avogadro's lifetime and it wasn't until Stanislao Cannizzaro, in 1860, demonstrated that it was the solution to the problem of atomic and molecular weights that Avogadro's Law became widely accepted. The number of particles in one mole of a substance was named Avogadro's number in his honour, and is numerically equal to 6.02252 x 10 23


Further info:

Avogadro's Law -
http://dbhs.wvusd.k12.ca.us/webdocs/GasLaw/Gas-Avogadro.html