A gas comparison pycnometer is disclosed which provides a method and apparatus for determining and checking the accuracy of the volume of a solid substice. The pycnometer is also outfitted with a unique cap assembly (15) which fixes the volume in the pycnometer's sample chamber (20) from run to run, and a flow limiting system (33, 35) to prevent entrainment of fine sample particles in the gas stream without prolonging the analysis. The pycnometer also employs a series of purges with a suitable gas to carry unwanted moisture and vapors out of the system. This pycnometer solves many problems in the prior art by quickly and accurately determining the volume of a solid substance.