The relative weights of gases at the same temperature and pressure compared with hydrogen as the unit, determined cither by ascertaining the weight of a given volume or the volume of a given weight of vapour. These densities or specific gravities of the elements are in some cases identical with their atomic weights, including hydrogen, oxygen, nitrogen, chlorine, bromine, iodine, sulphur and selenium. The exceptions to the rule include mercury, cadmium, zinc, potassium, sodium, arsenic and phosphorus, the first five named of which have densities onehalf of their atomic weights, and the last two twice that of their atomic weights. The general law is that the vapour densities of the elements are onehalf of their molecular weights, and the explanation of the noted exceptions lies in the fact that the molecules of mercury, cadmium, zinc, potassium and sodium, consist of but one atom, so that their atomic and molecular weights are identical, while arsenic and phosphorus contain each four atoms in their molecules, and their smallest part that can take part in a chemical change is onefourth of their molecular weights or onehalf of their densities.

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