Physical law stating that equal volumes of different ideal gases at the same temperature and pressure contain equal numbers of particles (molecules or atoms). The interpretation of the law lies in the statistical–mechanical theory of equipartition of energy, and the ideal gas law is a consequence of this. The volume occupied by one mole of gas (see Avogadro's number) is the same for all gases (22. 414 L). See also Loschmidt's number.