Abstract: An apparatus for periodically raising a quantity of liquid from a well to the earth's surface comprises a piston assembly above which the liquid collects in a tubing in which the piston assembly slides, the piston assembly including a piston member having an outside diameter slightly smaller than the inside diameter of the tubing to permit liquid to seep about the piston member and accumulate above the piston assembly near the lower end of the tubing. An outlet on the tubing is periodically opened via a timed valve to permit natural gas pressure in the well to drive the piston assembly to the surface. The piston assembly includes a mandrel which carries a slide valve disposed to be closed when the piston assembly is at rest and to be opened by deceleration of the mandrel.