Mustard gas was first used as an offensive agent by the Germans on July 12-13, 1917, at Ypres. According to an English report, the physiological properties of mustard gas had been tested by them during the summer of 1916. The Anti-Gas Department put forward the suggestion that it should be used for chemical warfare, but at that time its adoption was not approved. This fact enabled the English to quickly and correctly identify the contents of the first Yellow Cross dud received. It is not true, as reported by the Germans, that the material was first diagnosed as diethylsulfide. The tactical value of mustard gas was immediately recognized by the Germans and they used tremendous quantities of it. During ten days of the Fall of 1917, it is calculated that over 1,000,000 shell were fired, containing about 2,500 tons of mustard gas. Zanetti states that the British gas casualties during the month following the introduction of mustard gas were almost as numerous as all gas casualties incurred during the previous years of the war. Pope says that the effects of mustard gas as a military weapon were indeed so devastating that by the early autumn of 1917 the technical advisers of the British, French, and American Governments were occupied upon large scale installations for the manufacture of this material. PREPARATION AND MANUFACTURE The analysis of the first German shell indicated that the mustard gas contained therein had been prepared by the method published by Victor Meyer (1886) and later used by Clark (1912) in England. It was natural, therefore, that attention should be turned to the large scale operation of this method. The following operations are involved: Ethylene is prepared by the dehydration of ethyl alcohol. The interaction of hypochlorous acid (HClO) and ethylene yields ethylene chlorhydrin, ClCH₂CH₂OH. When this is treated with sodium sulfide, dihydroxyethyl sulfide forms, which, heated with hydrochloric acid, yields dichloroethyl sulfide. Chemically, the reactions may be written as follows: CH₃CH₂OH = CH₂ : CH₂ + H₂O CH₂ : CH₂ + HClO = HOCH₂CH₂Cl 2HOCH₂CH₂Cl + Na₂S = (HOCH₂CH₂)₂S + 2NaCl (HOCH₂CH₂)₂S + 2HCl = (ClCH₂CH₂)₂S + 2H₂O Without going into the chemistry of this reaction, which is thoroughly discussed by Gomberg[18] (see also German Manufacture), it may be said that this “procedure proved to be unsuitable for large scale production” (Dorsey). As Pope remarks, “That he (the German) should have been able to produce three hundred tons of mustard gas per month by the large scale installation of the purely academic method (of Meyer) constitutes indeed ‘a significant tribute to the potentialities represented by the large German fine chemical factories.’” It is true that a great deal of experimental work was carried out by the Allies on this method, but further study was dropped as soon as the Pope method was discovered. [Footnote 18: J. Am. Chem. Soc. =41=, 1414 (1919).]