Note:  Do not rely on this information. It is very old.


Hydrometer is an instrument for the determination of the density of a, substance. Most hydrometers are for the determination of liquid densities, and many of them have special names. Thus a lactometer is a hydrometer to test milk by observation of its density, u, salinometer is for salt water, a saccharometer for solutions of sugar, and an oleometer for oils. The ordinary hydrometer is very simple. It consists of a glass float with a thin graduated stem. It floats with the stem vertical by reason of a small enclosed quantity of mercury at the lower end. In light liquids the greater portion of the stem is immersed, but in heavier liquids the instrument rises, and less of the stem is immersed. By proper graduations on the stem, the liquid level may give direct readings of the density of the liquid in which the instrument is made to float. Nicholson's hydrometer or areometer is on the same principle, but is arranged for the determination of the densities of solids. The instrument is usually of metal; it is much larger, and only one mark is necessary on the stem. There is a cup supported at the top, and another at the bottom of the instrument, for the estimation of the weight of the solid in air and in water respectively. These two weights are sufficient to determine the density required. [Hydrostatics.]