Amedeo Avogadro publishes his memoir on the molecular content of gases

Background. Avogadro developed his hypothesis following Joseph Louis Gay-Lussac's 1808 publication on gas volumes. The 1811 memoir, titled 'Essay on a manner of Determining the Relative Masses of the Elementary Molecules of Bodies,' was submitted to Jean-Claude Delamétherie's Journal de Physique. It addressed the incompatibility between Dalton's atomic theory and the idea that gases were composite substances in integer proportions.

The Hypothesis. The core assertion was that equal volumes of all gases, at the same temperature and pressure, have the same number of molecules. This principle, known as Avogadro's law, relates gas volume to the amount of substance. It served as a specific case of the ideal gas law and helped determine both molecular and atomic masses.

Terminology and Distinctions. Avogadro distinguished between 'elementary molecules' (atoms) and composite molecules, though he did not strictly use the word 'atom' as modern science does. He clarified that gases are composed of molecules made of atoms, a distinction John Dalton had not considered. This conceptual separation was crucial for reconciling atomic theory with gas volume observations.