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Acetylene cylinders are not just a hollow tank with a valve and threaded coupling. In order to make unstable acetylene gas portable Prestolite invented the safety cylinder. These are filled with pumice rock (foamed rock or other fill) and liquid acetone. The acetylene dissolves into the acetone so there is no unstable gas in the cylinder. The rock foam prevents wave propagation necessary for an explosion. The cylinder also has melting and pressure fuses to release gas in a more or less controlled manner in a fire. The liquid fill is one reason you NEVER use cylinders lying on their sides. If you notice the the gas flame turn purplish you are drawing acetone and its time to stop using the cylinder.