the further embodiment of the invention represented in fig4 which employs a temperature sensor 8 in the container 2 which sensor indicates by means of an indicating instrument 9 on the control unit 10 the temperature reached by the cooling medium 6 at a particular time as soon as the temperature drops too low corrective heating can be applied by means of a heating element for example a cartridge type heater 11 one embodiment of the invention uses this heating element 11 in conjunction with the sensor 8 to provide a thermostatic temperature control system whereby it is possible to preselect a required value the input to this control system is a setting element for example a calibrated rotary knob 12 on the control unit 10 a further embodiment of the invention involves connecting the assembly to an automatic system for replenishing the liquid nitrogen as shown in fig4 in this case liquid nitrogen is supplied from an adequately sized dewar vessel 14 via a thermally insulated supply line 13 when the valve 15 is closed and the heating element 16 is switched on the elements 15 16 which are required to obtain the necessary filling pressure are controlled by means of level sensors 17 18 and 19 e g heat sensitive diodes and the electronic system of the control unit 10 in this process filling initially takes place up to the level which is.