Vac'uum, an empty space. The term is usually applied to a space from which air or other gases have been exhausted. An absolute vacuum is impossible, since, however completely the gases may be exhausted from an enclosed space, it will still be filled with ether (See ETHER). However, in the ordinary use of the term, a vacuum is said to be produced when the air is removed from a space as completely as possible by means of an air pump. Such vacuums are sufficiently perfect for common experiments. The most perfect vacuum formed in practice is that above the mercury in a barometer tube, which is produced by first filling the tube with mercury and then allowing this to settle, until the column sustained is equal to the weight of an equal column of atmosphere (See BAROMETER). Other practical applications of the vacuum are found in the vacuum brake and the vacuum pan. See AIR BRAKE; VACUUM PAN.