A liquid injector for treating timber comprises an accumulator having a first compartment (10) for liquid and a second compartment (12) separated from the first compartment by a wall (4) having therein gas-diffusable means (5), the second compartment receiving in use two or more agents which when mixed generate a gas, one of said agents being contained in a container having a rupturable membrane portion (13) to prevent mixing, and the second compartment has a flexible closure (14) which on depression thereof onto the container, ruptures the membrane portion to allow mixing whereby the agents react to produce a propellant gas, which displaces treatment liquid into the timber directly or into a preservative jacket for the timber.