A [https://en.wikipedia.org/wiki/Gas_pycnometer/ pycnometer] is an instrument used to measure the volume of solids based on the [https://en.wikipedia.org/wiki/Archimedes%27_principle/ Archimedes' principle using a gas. There are two chambers of known volume that are pressurized sequentially. The pressure difference compared to that of the empty chamber or calibration volume is used to find the volume of the sample. Helium is the gas most often used since it is inert and the most https://en.wikipedia.org/wiki/Ideal_gas/ ideal gas. The density of the sample can be calculated if the mass is known.
Be aware of hazards associated with compressed gas cylinders.