The law of combining volumes states that when gases chemically react together, they do so in amounts by volume which bear small whole-number ratios (the volumes calculated at the same temperature and pressure).
The ratio between the volumes of the reactant gases and the gaseous products can be expressed in simple whole numbers.
For example, Gay-Lussac found that two volumes of hydrogen react with one volume of oxygen to form two volumes of gaseous water. Expressed concretely, 100 mL of hydrogen combine with 50 mL of oxygen to give 100 mL of water vapor: Hydrogen(100 mL) + Oxygen(50 mL) = Water(100 mL). Thus, the volumes of hydrogen and oxygen which combine (i.e., 100mL and 50mL) bear a simple ratio of 2:1, as also is the case for the ratio of product water vapor to reactant oxygen.
Based on Gay-Lussacs results, Amedeo Avogadro hypothesized in 1811 that, at the same temperature and pressure, equal volumes of gases (of whatever kind) contain equal numbers of molecules (Avogadros law). He pointed out that if this hypothesis is true, then the previously stated result
:2 volumes of hydrogen + 1 volume of oxygen = 2 volume of gaseous water
could also be expressed as
:2 molecules of hydrogen + 1 molecule of oxygen = 2 molecule of water.
The law of combining volumes of gases was announced publicly by Joseph Louis Gay-Lussac on the last day of 1808, and published in 1809. Since there was no direct evidence for Avogadros molecular theory, very few chemists adopted Avogadros hypothesis as generally valid until the Italian chemist Stanislao Cannizzaro argued convincingly for it during the First International Chemical Congress in 1860.