Avogadro's Law (1811)

Equal volumes of any gases measured under the same conditions of temperature and pressure contain equal numbers of molecules (or atoms if the gas in monatomic).

This law was first known as Avogadro's Hypothesis. It was proposed in explanation of Gay-Lussac's Law of Combining Volumes, in which Avogadro also reasoned that elements could consist of 'molecules'.

The volume of 1 mole of any gas (the molar volume) is therefore a constant at a given temperature and pressure.

See also Avogadro Constant | Mole | Gay-Lussac's Law | Molar Volume.