Displacement liquids of known density should be selected in which the material to be tested is insoluble, and which has good wetting properties and a low evaporation rate under vacuum. High-boiling aromatic or aliphatic hydrocarbons are suitable. The temperature, t, at which the determination is carried out affects the density of the displacement liquid; each weighing should be made at a constant temperature. The principle consists in weighing into a tared tube a quantity of a dried sample, wetting it with displacement liquid, centrifuging until the occluded air is removed and calculation of the density from the difference of masses.