to vacuum pump 2 provided with vacuum gauge 4 vacuum pump 2 provided with vacuum gauge 4 creates a vacuum either in high pressure cell 50 via lines 26 and 27 containing valve v3 or in sample holder 1 via line 23 controlled by valve v1 and pressure sensor 5 or in the mercury reservoirs 3 and 8 via lines 26 25 and valve v2 or lines 26 27 28 and valves v3 v8 high pressure cell 50 comprises stainless steel cell body 10 which is adapted to resist very high pressures 4 5 u00d7 10 2 mpa and contains chamber 10a for sample holder 1b closure 11 closes the upper portion of cell body 10 by means of screw 12 and seals not shown the sample holder which is held in chamber 10a by suitable lugs has closure 1d in its upper portion provided with tube 21b mentioned above the upper portion of chamber 10a into which tube 21b opens is connected to transparent expansion chamber 13 via orifice 14b and knurled closure 14 which has a tapered seal and which includes open ended tube 14a this expansion chamber is connected via line 27 controlled by valve v3 to vacuum pump 2 the expansion chamber is also connected to mercury reservoir 8 via line 28 provided with valve v8 in order to prevent oxidation of the mercury during depressurisation to atmospheric pressure nitrogen is fed from reservoir 24.