Gaseous Heat Capacities I. The Method and the Heat Capacities of C2H6 and C2D6

Abstract

An apparatus is described for the adiabatic expansion method of Lummer and Pringsheim, new features of which are a device for accurate and rapid pressure measurements and an effective design of the expansion vessel. The Wollaston wires, serving as a resistance thermometer, are subjected to extensive heat treatment which stabilizes their electrical characteristics so well that resistance changes accompanying gas expansion can be translated into temperature changes by means of separately determined temperature coefficients of resistance. The method here described is thus an absolute one and is not dependent on comparison with other gases of known heat capacity. Causes of imperfect temperature constancy after expansion are quantitatively discussed and traced to two processes: the heat conduction by the gas and the absorption of thermal radiation from the walls by the gas. Experimental data on air and carbon dioxide are presented and are compared with the theoretical values. Heat capacities of C₂H₆ and C₂D₆ are reported for the interval 0 to 90°C.