We present results for the purely coherent dynamic structure factor of liquid sodium obtained from inelastic X-ray scattering experiments. Measurements were carried out at four temperatures between 388 and 1173 K. The dispersion relations for longitudinal collective excitations in the liquid could be determined for each temperature. It is found that the adiabatic sound velocity given as the slope of the dispersion at lower Q is increased compared to the bulk value. This `positive dispersion' still exists even at the highest temperatures attained in this experiment. From the data an estimate is given for the shear modulus of the liquid along the temperature range covered by our experiment.