not only represents that certain definite weights of hydrogen and oxygen
furnish a certain definite weight of the compound which we term water,
but that if the water in the state of gas, the hydrogen and the oxygen
are all measured at the same temperature and pressure, the volume
occupied by the oxygen is only half that occupied by the hydrogen,
whilst the resulting water-gas will only occupy the same volume as the
hydrogen. In other words, 2 volumes of oxygen and 4 volumes of hydrogen
furnish 4 volumes of water-gas. A simple equation like this, therefore,
when properly interpreted, affords a large amount of information. One
other instance may be given; the equation