Lorenzo Avogadro was an Italian scientist who made fundamental contributions to early atomic theory. In 1811, Avogadro stated that “equal volumes of all gases at the same temperature and pressure contain the same number of molecules regardless of their chemical nature and physical properties,” which is now known as Avogadro's Principle or Law. Avogadro made the distinction between atoms and molecules, which today seems clear, but it was largely ignored at the time because John Dalton rejected it. In recognition of his contribution, the number of molecules in a gram mole of any chemical substance is now called Avogadro's number (6.0221367 x 1023), one of the most fundamental constants in all of science.