Abstract: A suspension of solid particles in a liquid is produced and maintained by riodically moving agitating elements up and down in an elongated, flat bottom container. The agitating elements may, for example, be baffle plates, grids or the like, which are moved up and down either in a linear or in an angular movement. Preferably, two sets of agitating members are arranged so as to pass through each other in their periodic up and down movements. A carriage having mounted thereon a pump and a spray nozzle is movable alongside the elongated container with an intake of the pump reaching into the container without interfering with the moving agitating elements.