<P> On 6 August, German troops used chlorine gas against Russian troops defending the Fortress of Osowiec . Surviving defenders drove back the attack and retained the fortress . </P> <P> Germany used chemical weapons on the eastern front in an attack at Rawka, south of Warsaw . The Russian army took 9,000 casualties, with more than 1,000 fatalities . In response, the artillery branch of the Russian army organised a commission to study the delivery of poison gas in shells . </P> <P> It quickly became evident that the men who stayed in their places suffered less than those who ran away, as any movement worsened the effects of the gas, and that those who stood up on the fire step suffered less--indeed they often escaped any serious effects--than those who lay down or sat at the bottom of a trench . Men who stood on the parapet suffered least, as the gas was denser near the ground . The worst sufferers were the wounded lying on the ground, or on stretchers, and the men who moved back with the cloud . Chlorine was less effective as a weapon than the Germans had hoped, particularly as soon as simple countermeasures were introduced . The gas produced a visible greenish cloud and strong odour, making it easy to detect . It was water - soluble, so the simple expedient of covering the mouth and nose with a damp cloth was effective at reducing the effect of the gas . It was thought to be even more effective to use urine rather than water, as it was known at the time that chlorine reacted with urea (present in urine) to form dichloro urea . </P> <P> Chlorine required a concentration of 1,000 parts per million to be fatal, destroying tissue in the lungs, likely through the formation of hydrochloric acid when dissolved in the water in the lungs (2Cl + 2H O → 4HCl + O). Despite its limitations, chlorine was an effective psychological weapon--the sight of an oncoming cloud of the gas was a continual source of dread for the infantry . </P>

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