Apparatus particularly suited for use in mixing ammonia gas with hard water is disclosed. The apparatus includes an ejector assembly having a throat with a flexible tubular liner which is periodically flexed inwardly to loosen any solid matter which may have formed on the inside of the liner. The liner is flexed by a three-way valve connected to the water supply line and controlled by timers and a pressure switch. If desired, a similarly-lined tubular extension may be mounted downstream of the throat and flexed periodically to loosen any matter deposited therein. An orifice may be mounted in a diaphragm upstream of the throat to move in response to liner flexure for limiting solid matter accumulation adjacent the throat inlet.