Abstract: The invention provides an apparatus for detecting gases by absorption of infra-red radiation comprising a source of infra-red radiation arranged at one end of an optical path through the apparatus, at the other end of which path there is an infra-red detector. A plurality of cells located within an enclosure housing the cells are arranged to be moved in turn into the optical path in such position that when a cell is positioned on the optical path, the infra-red radiation passes through the cell before reaching the detector. The atmosphere to be tested is introduced into the enclosure and some of the cells are provided with means by which during movement of those cells through the enclosure into the optical path the atmosphere in the enclosure is pumped into the cell.