Abstract: A feeder device for controlled dissolution of a solid treating agent in a liquid of variable flow rate is designed such that the concentration of treating agent in the effluent liquid can be substantially constant, irrespective of the actual flow rate of the liquid. Further, the value of this concentration may be adjusted to varying levels and adapted to meet specific needs and requirements of the particular liquid system in which this feeder device may be employed. The constancy of concentration of the agent is obtained by the provision of a proportional weir, positioned in connection with a dissolution chamber, useful for maintaining a liquid volume in this dissolution chamber which can be linearly proportional to the flow rate of the liquid therethrough. In this liquid volume, dammed up by the weir, the solid treating agent, generally a substance containing hypochlorite, sulfite, sulphate or permanganate, can be present at least partly as a soluble solid.