What did Lorenzo Romano Amedeo Carlo Avogadro theorize in 1811?

Avogadro's law, which states that equal volumes of gases under the same conditions of temperature and pressure will contain equal numbers of molecules.

He theorized that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules.

Explanation:

In 1811, Avogadro published an article in a scientific journal, where he distinguished between molecules and atoms. He argued (contrary to what was thought) that in the case of water, the hydrogen and oxygen "atoms" were actually "molecules." One molecule of oxygen would react with two molecules of hydrogen (H2O).

Thus he proclaimed his famous hypothesis: "Equal volumes of any gases contain the same number of molecules when measured under the same conditions of temperature and pressure."