A chemical warfare agent, C₄H₈Cl₂S; r.d. 1.27; m.p. 14.4°C; b.p. 217°C. It is a potent blistering agent, first used in 1915 by Germany in World War I. It is often known simply as mustard gas, although it is an oily liquid, which can be dispersed as an aerosol. It was one of the early chemicals used in chemotherapy treatment of cancer. The systematic name is bis(2-chlorethyl) sulphide. See also nitrogen mustard.