<P> The first methods of employing gas was by releasing it from a cylinder when the wind was favourable . This was prone to miscarry if the direction of the wind was misjudged . Also, the cylinders needed to be positioned in the front trenches where they were likely to be ruptured by enemy bombardment . Later, gas was delivered directly to enemy trenches by artillery or mortar shell, reducing friendly casualties significantly . Lingering agents could still affect friendly troops that advanced to enemy trenches following its use . </P> <P> Early on, soldiers made improvised gas masks by urinating on a handkerchief and putting it over their nose and mouth so the urea would disable the poison . Armies rushed to issue regulation gas masks as regular equipment for front line troops . Anti-gas equipment and procedures improved significantly during the war, to the point that gas attacks had become less devastating at the war's end . </P> <P> Several different gas agents were used . Tear gas was first employed in August 1914 by the French, but this could only temporarily disable the enemy . In April 1915, chlorine gas was first used by Germany at the Second Battle of Ypres . Exposure to a large dose could kill, and those not killed could suffer permanent lung damage . But the gas was easy to detect by scent and sight . Phosgene, first used in December 1915, was the most lethal killing gas of World War I; it was 18 times more powerful than chlorine and much more difficult to detect . </P> <P> However, the most effective gas was mustard gas, introduced by Germany in July 1917 . Mustard gas was not as fatal as phosgene, but it was hard to detect and lingered on the surface of the battlefield, so could inflict casualties over a long period . Even if not inhaled, it could slowly burn the skin, but quickly burned via the eyes or any wounds, causing blindness and intense suffering . Casualties from mustard gas were unlikely to be fit to fight again, yet only 2% of mustard gas casualties died . The added burden of long - term care of casualties from mustard gas actually increased its overall effectiveness compared to more immediately lethal gas agents . </P>

Describe trench warfare and how it came to define the wwi experience