An apparatus used to determine the density of powdered minerals or of substances which react with the heavy liquids normally used in the floating equilibrium method. The specimen is placed in a test chamber, graduated with ‘upper’ and ‘lower’ marks, and fitted with an airtight lid. The base of the chamber is connected by a flexible U-tube to a mercury reservoir and raising or lowering the reservoir brings mercury into the chamber as far as the ‘upper’ or ‘lower’ mark. The volume of compressed air in the chamber is calculated before and after a given weight of a substance is introduced, so the volume of the substance can be determined. The density can then be calculated from the volume and the atomic weight of the substance.