The method of weighing equal volumes is particularly applicable to the determination of the relative densities of liquids.
Morley determined the densities of hydrogen and oxygen in the course of his classical investigation of the composition of water.
The heights of the columns above the surface of junction of the liquids are inversely proportional to the densities of the liquids.
The instrument thus adapted to the determination of densities exceeding that of water was called the hydrometer for salts.
The hydrometer intended for densities less than that of water, or the hydrometer for spirits, is constructed on a similar principle.