The adiabatic expansion method, due to Clement and Desormes uses the very Simple apparatus shown in Fig. 2. The change in state(6) is carried out by quickly removing and replacing the stopper of a large carboy containing the desired gas at a pressure initially somewhat higher than 1 atm pressure, so that the pressure of gas in the carboy momentarily drops to atmospheric pressure p2. The change in state(7) consists of allowing the gas remaining in the carboy to return to its initial temperature. The initial pressure p1 and the final pressure p3 are read from an open-tube manometer.
The thermodynamic equations(4)-(13 apply only to that part of the gas that remains the carboy after the stopper is replaced. We may imagine the gas initially in the carboy to be divided into two parts by an imaginary surface; the part above the surface leaves the carboy when the stopper is removed and presumably interacts irreversibly with the surroundings.