Atomic Weight. This is usually expressed by the phrase the atomic weight of oxygen is 16; for the atomic weight of hydrogen, being the smallest known, was taken as the unit.
The relative weight of a molecule can also be calculated; as an instance, let us calculate the molecular weight of water-gas. Experiment shows that a given volume of water-gas is 9 times as heavy as an equal volume of hydro gen at the same temperature and pressure ; again, equal volumes of gases contain equal numbers of molecules ; therefore a molecule of water-gas is 9 times as heavy as a molecule of hydrogen. But a molecule of hydrogen consists of two atoms ; consequently a molecule of water gas is 1 8 times as heavy as an atom of hydrogen.
