Mustard Gas Neutralizing Agent, Batman Wiki

A capsule of chemicals was kept on Batman's Utility Belt in his early career. The contents were capable of mixing with mustard gas and rendering the agent harmless. Batman made use of the vial to escape a trap set up by Sheldon Lenox in Sin Fang's Shop. Detective Comics #35: "The Case of the Ruby Idol" Though the capsule's contents are never specified, it's possible the agent was hydrogen peroxide.

Mustard Gas Neutralizing Agent, Batman Wiki

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