In 1802 Joseph Gay-Lussac found that gases expand by the same percentage for the same increase in temperature. This had been previously established by Charles in 1787. In 1805 Gay-Lussac showed that water consists of two parts hydrogen and one part oxygen by volume. Three years later he proved that all gases combine chemically in volume ratios of small whole numbers, when measured at the same temperature and pressure. This is often called the Law of Combining Volumes. From this Avogadro made the assumption that equal volumes of gases at the same temperature and pressure contain an equal number of molecules.