NOVELTY - A liquefaction system comprises insulated source tank containing cryogenic liquid supply, heat exchanger, pump for transferring cryogenic liquid in circuit between source tank and liquid inlet of heat exchanger, insulated destination tank connected with liquid outlet of heat exchanger, process system connected to gas outlet of heat exchanger, and vaporizer for vaporizing cryogenic liquid.    USE - For liquefying cryogenic gas, e.g. natural gas, using free cold from cryogenic liquid, e.g. nitrogen or argon (claimed).    ADVANTAGE - Minimizes the amount of specialized treating equipment required. Minimizes the consumption of nitrogen or other cryogenic substances in the provision of refrigeration. Is economical to construct and operate.    DETAILED DESCRIPTION - A liquefaction system consists of an insulated source tank (14) containing cryogenic liquid supply; a heat exchanger (34) having a gas inlet for receiving feed of cryogenic gas, a liquid outlet, a liquid inlet connected with the source tank, and a gas outlet; a pump (19) for transferring cryogenic liquid in circuit between source tank and liquid inlet of heat exchanger; an insulated destination tank (96) connected with the liquid outlet of heat exchanger; a process system connected to the gas outlet of heat exchanger and including cylinder(s) (40); and a vaporizer (26) for selectively receiving cryogenic liquid from source tank and vaporizing it so that resulting gas flows to the process system.    Cryogenic liquid from source tank flows to the heat exchanger when the pump is activated, and cryogenic gas from the feed flows to the heat exchanger so that the cryogenic liquid is vaporized in the heat exchanger by heat from the cryogenic gas and the cryogenic gas is liquefied in the heat exchanger by the free cold from the cryogenic liquid, and the resulting vaporized cryogen flows to the process system and the resulting liquefied cryogenic gas flows to the destination tank.    An INDEPENDENT CLAIM is also included for a method of liquefying cryogenic gas with free cold from vaporizing cryogenic liquid by providing a liquefaction system, purifying cryogenic gas in purifier, directing cryogenic gas to heat exchanger, transferring the cryogenic liquid to heat exchanger, warming cryogenic liquid in heat exchanger with heat from the cryogenic gas so that the cryogenic liquid is vaporized into resulting gas and the cryogenic gas is liquefied with free cold into resulting liquid, filling destination tank with resulting liquid, and directing resulting gas to the process system.    DESCRIPTION OF DRAWING(S) - The figure is a schematic diagram of the inventive liquefaction system.    Source tank (14)    Pump (19)    Vaporizer (26)    Heat exchanger (34)    Cylinders (40)    Manifold (42)    Purifier (54, 56)    Destination tank (96)