nAvogadro's lawthe principle that equal volumes of all gases (given the same temperature and pressure) contain equal numbers of molecules
Webster's Revised Unabridged Dictionary
Avogadro's law(Chem) a fundamental conception, according to which, under similar conditions of temperature and pressure, all gases and vapors contain in the same volume the same number of ultimate molecules; -- so named after Avogadro, an Italian scientist. Sometimes called Ampère's law.
Like the atomic theory itself, Avogadro's law is an outcome of physical work and of physical reasoning.
"Heroes of Science" by M. M. Pattison Muir
Avogadro's law of gaseous volumes, 28.
"A Century of Science and Other Essays" by John Fiske