We report interferometric measurements of the frequency dependence of the refractive index n and the dipole polarizability a of Hg vapor. From the measured dispersion of the dipole polarizability the Cauchy moments S(-2) = 33.91(34), S(-4) = 279.8(8,4), and S(-6) = 14 100(2810) have been obtained. The static limit S(-2) is in good accordance with ab initio calculations and results obtained from dielectric constant measurements. The Cauchy moments are used to predict approximations of the dispersion interaction energy constant C-6 = 255 au and the second hyperpolarizability gamma(0) = 2.2 x 10(4) au.