Abstract: A pressure feeder for controlled addition of dissolvable agent into a flowing liquid has been designed which provides constant concentration of the agent in the treated liquid. This concentration is independent from the flow rate and the pressure of the liquid and also slugging of the agent at zero flow rate is avoided. A feeder according to the invention comprises a dissolution chamber connectable to a point of slightly increased pressure in the main circuit in which dissolution chamber the liquid is brought in controlled contact with stacked tablets of the agent. A linear weir controls the height of the liquid level in the dissolution chamber such that is proportional to the flow rate. The provision of a holding chamber for treated liquid which is connected to the dissolution chamber incorporates a gas volume that automatically adjusts for pressure differences and enables the liquid level in the dissolution chamber as well as the concentration of the agent in the treated liquid to be pressure independent.