The first step in advance in the manufacture of mustard gas was the discovery that ethylene would react with sulfur dichloride. While American chemists were not very successful in their application of this reaction, either in the laboratory or the plant, it was apparently, according to Zanetti, the only method used by the French (the only one of the Allies that manufactured and fired mustard gas). The plant was that of the Société Chimique des Usines du Rhone and was started early in March, 1918, with a production of two to three tons a day. In July it was producing close to twenty tons a day. The plant was being duplicated at the time of the Armistice, so that probably in December, 1918, the production of mustard gas by the dichloride process would have reached about 40 tons. Zanetti points out, however, that the process involved complicated and costly apparatus and required considerable quantities of carbon tetrachloride as a solvent. It is for this reason that the Levinstein process would have been a tremendous gain, had the war continued. About the end of January, 1918, Pope and Gibson, in a study of the reaction originally used by Guthrie, found that the action of ethylene upon sulfur chloride (S₂Cl₂) at 60° yielded mustard gas and sulfur: 2CH₂ : CH₂ + S₂Cl₂ = (CH₂ClCH₂)₂S + S The reaction at this temperature caused the separation of sulfur; this occurred after the product stood for some time or immediately if it was treated with moist ammonia gas. While this process was put into commercial operation, both in England and America, it offered considerable difficulty from an operating standpoint. The sulfur would often separate out and block the inlet tubes (ethylene). While it is comparatively easy to remove the mustard gas from the separated sulfur by decantation, a certain amount always remains with the sulfur. It is almost impossible to economically remove this, and its presence adds to the difficulty of removing the sulfur from the reactors; the men engaged in this operation almost always become casualties. [Illustration: FIG. 29.—The Levinstein Reactor as Installed at Edgewood Arsenal.]