The chemical potential and the concentration of dissolved hydrogen in Pd_0.9Ag_0.1 have been measured along fifteen isotherms near the critical point. The critical exponent γ, describing the temperature dependence of the lattice gas compressibility, was determined to 1.02 ± 0.04. For the temperature range investigated ((T – T_c)/T_c > 0.01) the isotherms can very well be fitted by Landau's theory. Quantitative arguments are given in favour of the interpretation that the attractive interaction is the long range elastic dipoledipole interaction. From the critical data the dipole strength is found to P = 3.5 eV. The validity of the mean field approximation so close to the critical point is consistent with the assumption of long range interaction.