Heat Capacity of Gases by the Adiabatic Expansion Method

The theory of adiabatic expansions is given and an apparatus for the measurement of gaseous heat capacities by this method is described. As here applied the method is relative, involving a comparison with a standard gas, nitrogen. The heat capacities of oxygen, carbon dioxide, boron trifluoride, cyanogen, sulfur dioxide, ethylene, difluorodichloromethane, fluorotrichloromethane, sulfuryl fluoride, and phosphorus trifluoride found at temperatures ranging from 0° to 57° are compared with those reported by earlier investigators and with those derived from molecular data. In general the agreement is good to within 0.5%, but for sulfur dioxide and phosphorus trifluoride there are discrepancies between the observed and calculated values of about 3% and 8%, respectively. The difficulty with sulfur dioxide may possibly arise from an error in the equation of state, but the phosphorus trifluoride discrepancy stands quite without explanation.