What did Amedeo Avogadro propose about gases and molecules?

Amedeo Avogadro proposed two fundamental hypotheses in 1811 to explain the behavior of gases and their chemical combinations:12

• Equal volumes of gases: He stated that under the same conditions of temperature and pressure, equal volumes of all gases contain an equal number of molecules.32
• Molecular nature of elemental gases: He proposed that the fundamental units of elemental gases might exist as molecules composed of multiple atoms, rather than as separate, individual atoms as previously believed by scientists like John Dalton.12

These hypotheses allowed Avogadro to explain Gay-Lussac’s law of combining volumes by assuming that molecules could divide during chemical reactions. Although his ideas were initially ignored, they were later championed by the Italian chemist Stanislao Cannizzaro, who used them to construct a consistent system of atomic weights and molecular formulas that remains the foundation of modern chemistry.1234