Abstract: An immersible lamp for use in a photochemical reactor has an outer bulb (10) surrounding an inner discharge tube (1) which can be cooled by a flow of inert gas such as nitrogen from a supply chamber (23) though hollow guide rods (6, 7) into a remote end (27) of the outer bulb, and back out through an outlet hole (18) and an exhaust chamber (24). In order to facilitate swapping out of the discharge tube (1) in the event of burnout or changes in output power or output spectrum requirements, without the need for welding or glassblowing expertise, the discharge tube (1) is mounted in a pair of sockets (4,5), one of which (4) is axially slidable. The socket is spring-loaded (8), and thus readily adapts to discharge tubes of differing lengths. The socket slides along guide rods (6, 7, 16) which also serve as electrical supply conductors to the sockets (4, 5). Spacer elements (11) help maintain alignment of the guide rods, sockets, and discharge tube with respect to the axis (12) of the outer bulb (10).