Chemical weapons in World War I volatile oily liquid. It was introduced as a vesicant by Germany in July 1917 prior to the Third Battle of Ypres. The Germans marked their shells yellow for mustard gas and green for chlorine and phosgene; hence they called the new gas "Yellow Cross". It was known to the British as "HS" ("Hun Stuff"), and the French called it "Yperite" (named after Ypres). Mustard gas is not an effective killing agent (though in high enough doses it is fatal) but can be used to harass and disable the enemy and pollute the battlefield. Delivered in artillery shells, mustard gas was
German Type IXA submarine The first U-boat to be laid down in the Bremen ship yards was "U-37", whose keel was laid down on 15 March 1937. The last U-boat to be laid down was , whose construction began on 15 September 1937. By November 1939, all eight submarines had been fully constructed and commissioned into the "Kriegsmarine". The Type IXA submarines were the first German submarines to have a double hull. This allowed for a greater chance of survival in the event of an attack and gave them better seaworthiness on the surface. The Type IXA submarines had a test depth of .