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 $T_i$ = 2$\frac{a}{Rb}$ = 2$T_B$. 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).