A device for mixing a substance comprised of two constituents contained in a cartridge (12) and having a cartridge holder (32) thereabout, the device comprising a plunger rod (46) for pushing a closure member (20) to permit the transfer of a diluent in a second chamber (28) through a by-pass channel (24) to mix with a constituent in a first chamber (26), the plunger rod (48) being driven by means of a rack (48) and a pinion gear (50) arrangement to prevent overly rapid movement of the plunger rod (46).
