Rayleigh scattering from methane

The decay rate of spontaneous thermal fluctuations in methane has been determined from Rayleigh scattering experiments. Measurements have been made in a broad region about the critical point, along the coexistence curve, the critical isochore, and an isotherm 0.8 K above the critical temperature. Values of thermal diffusivity obtained from the data are compared with other available data. In the immediate critical region the measured thermal diffusivity is compared with calculated thermal diffusivity values based on a recent prediction of the anomalous thermal conductivity of methane. The comparison involves assumptions concerning the scattered wave vector dependence of the decay rate in the critical region but involves no adjustable parameters. In addition, the critical isochore data are also fitted by other theoretically predicted forms for the decay rate to estimate parameters in these theories. Good agreement between measurement and prediction is found.