Atomic theory formula for the water molecule (HO), or 16 if one assumes the modern water formula (HO). The flaw in Dalton's theory was corrected in principle in 1811 by Amedeo Avogadro. Avogadro had proposed that equal volumes of any two gases, at equal temperature and pressure, contain equal numbers of molecules (in other words, the mass of a gas's particles does not affect the volume that it occupies). Avogadro's law allowed him to deduce the diatomic nature of numerous gases by studying the volumes at which they reacted. For instance: since two liters of hydrogen will react with just one liter
History of molecular theory Determining the Relative Masses of the Elementary Molecules of Bodies", he essentially states, i.e. according to Partington's "A Short History of Chemistry", that: Note that this quote is not a literal translation. Avogadro uses the name "molecule" for both atoms and molecules. Specifically, he uses the name "elementary molecule" when referring to atoms and to complicate the matter also speaks of "compound molecules" and "composite molecules". During his stay in Vercelli, Avogadro wrote a concise note ("memoria") in which he declared the hypothesis of what we now call Avogadro's law: "equal volumes of gases, at the same temperature and pressure,