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A portable apparatus for cooling specimens used in X-ray and neutron diffraction studies is described. Temperatures in the + 50 to -1 185 °C range can be automatically and continuously maintained within 1 ° of the desired temperature for several weeks. Cold gas is obtained from liquid nitrogen which is boiled off at a rate sufficient to maintain a constant pressure in the sealed Dewar. The temperature-determining gas-flow rate is controlled by a pneumatically operated cryogenic valve which responds to a thermocouple signal.