Liquefaction requires T < T_c, then apply P > P_c. Methods: (1) Linde's process: compress gas, cool by countercurrent exchange, expand through nozzle (Joule-Thomson cooling). Repeated cycles eventually liquefy. (2) Claude's process: adiabatic expansion against piston (does work, cools gas more efficiently). Joule-Thomson effect: when a real gas expands through a porous plug at constant enthalpy, it cools (if T < T_i) or heats (if T > T_i). Inversion temperature T_i = 2a/(Rb) = 2T_B. Most gases: T_i >> room temperature (so they cool on expansion). H2: T_i = 202 K, He: T_i = 40 K — must be pre-cooled below T_i before Joule-Thomson cooling works. Ideal gas: no Joule-Thomson effect (no intermolecular forces).