Thermal expansion of liquids. Laboratory apparatus designed to demonstrate the thermal expansion of liquids. Here, a reservoir of red-coloured liquid is in the round-bottomed flask at bottom. Running upwards from the flask is a microbore tube made of glass. The microbore is the narrow opening running up the centre of the tube. As the liquid is heated or cooled, it will expand or contract and move up or down the microbore. The expansion is relatively small, and it can only be seen when a narrow tube is used. The same principle is used in mercury thermometers to measure temperature changes as the mercury moves up and down a microbore.