Interferometric Determination of Near-Critical Isotherms of Xenon in the Earth's Field

Optical measurements which determine the equation of state in xenon near its critical point are described. The method is based upon an analysis of the Fraunhofer interference pattern produced by a thin slab of near-critical fluid in the earth's field. The top of the coexistence curve is rounded, i.e., Guggenheim's relation is uniformly valid as T→Tc to within ε = ΔT / Tc = 5 × 10−6. The coefficient of isothermal compressibility β0 continues to diverge as T→Tc down to at least ε = 5 × 10−6. The rate of divergence, however, is slower than others have found at larger ΔT. If [εPcβ0]−1 = 2(ε / 10−3)0.4 represents the accepted relation, our measured value of β0 at ε = 10−5 is smaller by a factor of five. Supercritical isotherms ΔP(ρ, ΔT) for 5 × 10−6 < ε < 5 × 10−5 have been measured. No departure from Widom's “scaling law” is detected. Further work to sharpen this observation is under way. The purity of our sample has been assumed. Further work to establish the effect of small impurity concentrations is planned.