This answer was found at John Hanson's Refractometry Webpages at the University of Puget Sound. The speed of light in a substance is slower than in a vacuum because as the light passes through the substance it is being absorbed and reemitted by the atoms of the substance. Since the density of a liquid usually decreases with temperature, the speed of light in a liquid will normally increase as the temperature increases. Thus, the index of refraction normally decreases as the temperature increases for a liquid. A table showing the small changes in refractive index for a number of different liquids is given by Hanson on his site.