Methods for the determination of the density of liquids can be divided into three classes as follows. 1. Methods in which the density is measured in terms of the fundamental physical standards of measurement. 2. Methods in which it is measured relative to the density of a reference liquid or solid. Usually, although not always, the density of the reference liquid or solid will have been determined either directly or via intermediate liquids by a method in category 1. 3. Methods in which the change of density of a liquid, caused by an impressed change of the state of the liquid such as temperature, pressure, magnetic or electric field, etc., is measured. In these methods, either the change of volume is measured relative to the change of volume of the containing vessel or the change of density is measured relative to the change of density of a float or sinker. 4. Methods in which the volume, or more usually the change of volume, is deduced from measurements other than the direct measurement of change.