Acetylene, a transparent colorless gas, of a peculiar disagreeable odor, perceptible when coal gas is imperfectly burned in the air. It burns with a bright smoky flame. Berthelot formed it by transmitting olefiant gas or marsh gas through red-hot tubes. When copper service pipes are used for distributing coal gas, a dark-red copper compound is sometimes deposited which detonates powerfully on the application of heat or on receiving a sudden blow. Some serious cases of explosions in New York, where the pipes were undergoing repairs, were traced by Dr. John Torrey to this cause. When mixed with chlorine, acetylene explodes spontaneously; it has not yet been liquefied by cold or pressure.