fluid level on the chilling fluid side of heat exchanger 12 being operably connected to a low level switching device 66 having control over valves v a to v d inclusive to close the apparatus down should the chilling fluid level in the boiling passage of the chiller drop below a predetermined valve a further safety feature may be provided in the form of low temperature detector 68 which is operably connected to valve v d for the control thereof should the temperature of the spent chilling fluid c f fall below a pre determined value thereby indicating a system failure a manually operated switch 70 may also be provided to override either or both of these systems and initiate control over the flow of chilling fluid c f waste chilling fluid c f is passed for disposal or further use via line 72 and flow indicator 74 if liquid nitrogen is being used as the chilling fluid the waste gas which is typically at 276k and atmospheric pressure may be compressed in compressor 76 and used as the source of nitrogen rich gas n the apparatus shown in fig1 also includes a pressure relief valve 77 several non return valves 78 and temperature indicators 79 and 80 which respectively display the temperature of the gas mixture and the temperature.