A gas generator is provided which can be selectively switched between reaon and nonreaction modes. The gas generator incudes a reaction chamber which has top and bottom ends. The chamber contains reactive and nonreactive liquids, the nonreactive liquid having a greater specific gravity than the reactive liquid. A solid reactant member is provided. A device is provided for projecting the solid reactant member through the bottom of the chamber, thence through the nonreactive liquid, and thence into the reactive liquid for generating gas. Further, a device is provided for withdrawing the solid reactant member from the reactive liquid into the nonreactive liquid for terminating the generation of gas.