X-Ray Study of Critical Opalescence in Argon

The small-angle x-ray scattering from argon has been investigated at the critical pressure and at four other pressures over a range of temperatures above and below the liquid—vapor transition temperature for each pressure. For three of these pressures the angular distribution of the scattering was analyzed in terms of the Ornstein—Zernike theory of critical opalescence and found to be in good agreement with this theory. Values of the short-range correlation length and relative values of the isothermal compressibility were determined for each condition of pressure and temperature. For the conditions for which compressibilities can be found by numerical differentiation of the isotherms of Michels, Levelt, and De Graff, the compressibility values calculated by numerical differentiation are in good agreement with those obtained from the scattering curves.