the simplest sort of condenser consists of a single tube wherein the heat of the vapour is conducted to the glass wall, which is only cooled by air; such "air condensers" are often used for condensation of high boiling liquids (for distillations at high temperatures, well above 100 °c), where columns can be used with or without packing (see below). historically, the retort appearing in illustrations of the practice of alchemists is an apparatus that is essentially an unpacked air condenser. liebig condensers are often used as air condensers, with air circulated rather than a liquid coolant (see next section).