Examples of such cryogenic liquid refrigerant materials include, for example, freon 14 (freezing point 128??? K., boiling point 145??? K.); freon 22 (boiling point 233??? K.); freon 13 (boiling point 192??? K.); liquefied air, normally gaseous hydrocarbons, such as propane and isobutane, helium, argon, etc.