 Phosgene, colorless with a more subtle "moldy hay" odor, was introduced by a group of French chemists led by Victor Grignard and first used by the French in 1915.It was also used in a mixture with an equal volume of chlorine, with the chlorine helping to spread the denser phosgene.Phosgene was more potent than chlorine, though some of the symptoms of exposure took 24 hours or more to manifest, meaning the victims were initially still capable of putting up a fight.