The early idea of gas warfare was that a material, to be of value as a war gas, should have a relatively high vapor pressure. This would, of course, provide a concentration sufficiently high to cause casualties through inhalation of the gas-ladened air. The introduction of “mustard gas” (dichloroethylsulfide) was probably the greatest single development of gas warfare, in that it marked a departure from this early idea, for mustard gas is a liquid boiling at about 220° C., and having a very low vapor pressure. But mustard gas has, in addition, a characteristic property which, combined with its high persistency, makes it the most valuable war gas known at the present time. This peculiar property is its blistering effect upon the skin. Very low concentrations of vapor are capable of “burning” the skin and of producing casualties which require from three weeks to three months for recovery. The combination of these properties removed the necessity for a surprise attack, or the building up of a high concentration in the first few bursts of fire. A few shell, fired over a given area, were sufficient to produce casualties hours and even days afterwards. Mustard gas, chemically, is dichloroethylsulfide (ClCH₂CH₂)₂S. The name originated with the British Tommy because the crude material first used by the Germans was suggestive of mustard or garlic. Various other names were given the compound, such as “Yellow Cross,” from the shell markings of the Germans; “Yperite,” a name used by the French, because the compound was first used at Ypres; and “blistering gas,” because of its peculiar effect upon the skin. HISTORICAL It seems probable that an impure form of mustard gas was obtained by Richie (1854) by the action of chlorine upon ethyl sulfide. The substance was first described by Guthrie (1860), who recognized its peculiar and powerful physiological effects. It is interesting in this connection to note that Guthrie studied the effect of ethylene upon the sulfur chlorides, since this reaction was the basis of the method finally adopted by the Allies. The first careful investigation of mustard gas, which was then only known as dichloroethylsulfide, was carried out by Victor Meyer (1886). Meyer used the reaction between ethylene chlorhydrin and sodium sulfide, with the subsequent treatment with hydrochloric acid. All the German mustard gas used during 1917 and 1918 was apparently made by the use of these reactions, and all the early experimental work of the Allies was in this direction.