Thermal conductivity of a material may be defined as the
* quantity of heat flowing in one second through a slab of the material of area 1 cm² and thickness 1 cm when its faces differ in temperature by 1 Degree Centigrade
* quantity of heat flowing in one second through one cm of material when opposite faces are maintained at a temperature difference of 1 Degree Centigrade
* heat conducted in unit time across unit area through unit thickness when a temperature difference of unity is maintained between opposite faces

Gas liquefaction is physical conversion of a gas into a liquid state (condensation). The liquefaction of gases is a complicated process that uses various compressions and expansions to achieve high pressures and very low temperatures. Air refrigeration cycle is used in gas liquefaction process.