In the past the chief use of measurements of the velocity of sound in liquids has been in the determination of the adiabatic compressibility from which the ratio of the specific heats may be found provided the isothermal compressibility is known. At the same time certain qualitative facts relating the sound velocity to structure have become well known. These showed that the velocity of sound itself was not an additive and constitutive property, and little application of measurements of the velocity of sound to the elucidation of molecular structure seemed possible, until Rao discovered a function of the velocity which was independent of the temperature. Rao (15,16) showed that in any one liquid the density and the velocity of