The procedure includes three methods which use the same principle but differ in that somewhat different apparatus is used. The method A is carried out with a displacement liquid in which the material to be tested is insoluble and which has good wetting properties, a pyknometer of the Gay-Lussac type or another type of pyknometer fitted with a capillary stopper and a vacuum apparatus. For the method B a filter flask is used instead of the vacuum desiccator to carry out the procedure. Method C uses for the displacement of the entrained air the displacement liquid; centrifuge until the entrained air is removed.