Law of Gaseous Volumes was given by Gay Lussac in 1808. He observed that when gases combine or are produced in a chemical reaction they do so in a simple ratio by volume provided all gases are at same temperature and pressure.
Thus, 100 mL of hydrogen combine with 50 mL of oxygen to give 100 mL of water vapour.
Hydrogen + Oxygen → Water
100 mL 50 mL 100 mL
Thus, the volumes of hydrogen and oxygen which combine together (i.e. 100 mL and 50 mL) bear a simple ratio of 2:1.
Gay-Lussac’s discovery of integer ratio in volume relationship is actually the law of definite proportions by volume. The law of definite proportions, stated earlier, was with respect to mass. The Gay Lussac’s law was explained properly by the work of Avogadro in 1811.