|--Otto Dix Sturmtruppe gehen unter Gas vor (1924)|
100 years ago this week, the small but once a powerful Belgian city called Ypres will mark a grim centenary: the first military use of chlorine gas.
The  fighting at Ypres left the British saddled with a most unfavorable defensive position. It was a salient some six miles deep, with the town of Ypres [Ieper in Flemish] at the center of the base. The Germans held positions on low hills that gave them excellent observation over the entire salient. The area was so low that the construction of entrenchments was difficult, ground water being struck in many places at a depth of a one foot. A far better defensive position would have been one extending north and south just behind Ypres--giving up the salient and the town. Ypres, however, had become a symbol of Allied resistance; and the effect on public opinion of pulling back would have been unfortunate. Therefore, the troops held onto it, year after year. The Ypres salient hosted perennial WW I battles. The first was in 1914, then again in 1915, and last and worst of all, in 1917. But the second one--100 years ago this week--saw the first use of chlorine gas by the Germans.
On the morning of 22 April 1915 the Germans commenced to shell the Ypres salient. At 1730 a strange green vapor (chlorine gas) drifted toward the Allied lines from some 5,000 gas cylinders in the forward German trenches. The cloud engulfed the portion of the line held by some French Territorials and African troops, who promptly fled and left an undefended gap of over four miles. Not only did the Germans lack reserves to push through the gap but the darkness and the fear of their own gas by the German troops prevented them from realizing the extent of the rout. Even with only the few reserves available, the Germans could have accomplished more than they did. 
Chlorine gas had been known since 1630 or so. Sir Humphry Davy first recognized its elemental nature in 1810 and he was allowed to name it and he did so after its color: chloros: yellowish or light green (cf. chlorophyll). The use of chlorine as a bleach and a disinfectant dates back to the late 1700's. By the time of WW I, chlorine was being made on an industrial scale throughout the world.
So why were the Germans the first to use chlorine gas in war? The French had used tear gas the year before. One reason was an acute shortage of ammunition and a desire to end the carnage. The latter reasoning is essentially the same used to justify Hiroshima. But technological prowess and a dedicated mastermind also led its use as a WMD by the Germans.
Rudolf Knietsch's work at BASF before the war made it possible to liquefy chlorine using compressors lubricated with petroleum. [Fritz] Haber's idea, explained in detail to [Field Marshall Erich von] Falkenhayn and the top military leadership, was to put siphon tubes into steel cylinders, each containing about 20 kilograms of gas. These were then brought to the trenches, where they were positioned and buried in the ground at intervals of about one meter along a continuous front. A lead pipe was screwed into the opening of the cylinders, then brought up over the top of the trench and aimed in the direction of the enemy. When the wind direction was favorable, that is, blowing toward the enemy, all cylinders were opened simultaneously, and the chlorine was released. The liquid chlorine immediately turned into a gas and mixed with air, forming a yellow-green to white cloud containing about 0.5 percent chlorine at a distance of 50 to 100 meters from the cylinders. Since chlorine is two and a half times as heavy as air, this cloud rolled forward into the enemy trenches and foxholes and forced the enemy troops to flee their positions rapidly. The gas also damaged their weapons with corrosion. The German troops could follow the cloud and penetrate into enemy positions.
Chlorine liquefaction was key because it allowed the transport and positioning of sufficient amounts of the gas. British and especially French industrial counterparts had no such capacity for producing liquid chlorine. British production went from 5 to 150 tons per week after Ypres.
This map shows the geographic extent of the chlorine gas dispersion. The gas was released and rolled south across the French lines and towards the town of Ypres [Ieper]:
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Both sides were unprepared for the consequences. The French had no protections in place. Within days and before the next gas attack on Canadian troops, word had spread that urine-soaked rags or cloth held against the mouth and nose afforded some protection: urea, a base, neutralized some of the HCl and HOCl. As already mentioned, the Germans failed to follow through with troops.
Two days later the Germans gassed British and Canadian troops. Retaliation occurred in kind the following year.