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