The method is based on the measurement of mass of a known volume of gas under definite condition of temperature and pressure. Two glass bulbs, each of 2 L capacity, are first dried, evacuated and suspended from the two arms of a balance (Figure: Regnault’s apparatus for determination of density of a gas).
The bulbs are counterpoised in the balance. One of the bulbs, say A, is then filled with gas under known pressure and again counterpoised against bulb B. adding necessary weights on the pan from which B is suspended.
This additional weight necessary for balancing the arms is the mass of gas taken in bulb A. The temperature of the surroundings is kept uniform. The volume of bulb A is obtained from the mass of water necessary to till the bulb and the density of water at the temperature of the experiment. From the mass and volume of the gas the density may be calculated and knowing the pressure and temperature the molecular mass of the gas is obtained. The bulb B is used as a counterpoise in order to minimize troublesome corrections for air buoyancy and moisture adhering to the surface. Even then a small buoyancy correction is required because the volumes of the evacuated bulbs differ somewhat from that when filled with water. This correction is, however, very small.