333,210. Harrison, C. F. R., Labrow, S., and Imperial Chemical Industries, Ltd. May 4, 1929. Hydraulic accumulators. -In processes in which mixtures of solids and liquids are treated under high pressure, e.g. in the destructive hydrogenation of mixtures of coal and oil, the pressure is gradually released without erosion of the apparatus by causing part of the potential energy of the system to be expended in the performance of external work without allowing any substantial development of kinetic energy of the particles of liquid or solid. The mixture may be passed through an extraction pump of a reversible nature which acts as a motor. Examples are given of the use of screw viscosity, drum, gear, and ram pumps. The mixture of liquids and solids contained under high pressure in a vessel 5 is allowed to flow through a screw viscosity pump comprising a cylindrical casing 1 acting as a bearing for a shaft 2 provided with spiral channels 3 through which the mixture passes.