Relative density
A pycnometer (from Greek: πυκνός (puknos) meaning "dense"), also called pyknometer or specific gravity bottle, is a device used to determine the density of a liquid. A pycnometer is usually made of glass, with a close-fitting ground glass stopper with a capillary tube through it, so that air bubbles may escape from the apparatus. This device enables a liquid's density to be measured accurately by reference to an appropriate working fluid, such as water or mercury, using an analytical balance.
Liquid
The speed of sound in a fluid is given by
formula_4 where "K" is the bulk modulus of the fluid, and "ρ" the density. To give a typical value, in fresh water "c"=1497 m/s at 25 °C.