Measuring instrument (grch. pykno = densely packed) for determining the relative density of solids or liquids by weighing. The first instrument of this kind is attributed to the Arab universal scholar Abu Raihan Muhammad al-Biruni (973-1048). It consists of a glass flask with a thinly perforated glass stopper and must be calibrated to a certain volume. The density can be determined by successively filling it with the liquid to be examined and with water and weighing both. Using a pycnometer, the must weight and alcohol content can be determined. See also under refractometer as well as regarding other methods or measuring instruments under analytical testing.